KoritheMan's WunderBlog

Tropical Cyclone Report (TCR): Hurricane Michael

By: KoritheMan, 4:51 AM GMT on February 28, 2013

Tropical Cyclone Report
Hurricane Michael
(AL132012)
3 September-11 September 2012

Michael was a small mid-latitude hurricane that briefly reached category 3 strength on the Saffir-Simpson Hurricane Scale. Michael was the first major hurricane to develop from baroclinic sources since Claudette in 1991, and was one of only five major hurricanes to develop from a non-tropical source since the advent of the satellite era.


a. Synoptic History


Michael's origin was non-tropical. A weak mid- to upper-level  low in the vicinity of the Azores was first noted in water vapor imagery on 29 August. This low was likely produced by the same trough that recurved Hurricane Kirk. The low deepened considerably over the next couple of days as it moved eastward, and was closed by around midday 30 August. Over the next couple of days, surface data from the Azores and satellite pictures indicate that the low gradually worked down through the troposphere, leading to the formation of a broad surface trough on 1 September. Some weak convection persisted along the trough over the next day or so in ill-defined bands as the overall system moved southwestward. The small system became nearly stationary the next day as any shortwave perturbations in the westerlies remained firmly to the north. The circulation became better defined throughout the day as small curved bands became better established. The low maintained a small area of deep convection near the center, and it is presumed that it became a tropical depression around 1800 UTC 3 September, centered a few hundred miles southwest of the Azores. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively. (to be added in the final rendition) The best track positions and intensities are listed in Table 1 (to be added in the final rendition). The depression intensified to a tropical storm about twelve hours later. Michael slowly gained latitude while steadily intensifying. The cyclone turned to the northwest on 5 September as a shortwave trough amplified to the north of the tropical storm. Based on the appearance of a ragged eye in geostationary satellite images as well as the cyclone's impressive microwave appearance at that time, Michael is estimated to have become a hurricane around 1800 UTC that day. Michael's intensification subsequently became even more rapid, and it became a major hurricane near 0600 UTC 6 September. At that time, Michael was located about 650 miles east-southeast of Bermuda. The 100-kt intensity at this time turned out to be Michael's peak intensity. It is interesting to note that this intensification occurred over comparatively marginal sea surface temperatures and objectively minimal oceanic heat content.

Michael maintained category three intensity for about 12 hours before it weakened to a category two hurricane. Since there appeared to be little vertical shear over the area, it appears that this weakening was primarily induced by internal structural changes within the eyewall, with a large outer wind maxima noted on microwave imagery, along with a concurrent collapse of the inner eyewall. At the same time, the upstream trough had deepened and extended farther south, which allowed the small hurricane to turn northward between the trough and the Atlantic subtropical high. Michael weakened to an intensity of about 80 kt by 0000 UTC 8 September. Subsequently, the hurricane began another period of intensification and reached a secondary peak of 90 kt around 1200 UTC that day. The eye became considerably larger over the next 24 hours, which allowed Michael to maintain its category two intensity. The shortwave trough that was located to the north of the cyclone had bypassed the system, and Michael turned westward around 0600 UTC 9 September in response to a building lower- to middle-tropospheric ridge that had built to the north in the wake of the trough. Michael gradually turned northward as it came under the influence of Hurricane Leslie and the upper trough that recurved it. Meanwhile, northwesterly to northerly shear associated with outflow from Leslie increased substantially, and the small hurricane quickly weakened, dropping below hurricane strength by 0000 UTC 11 September. The shear continued, and Michael weakened to a tropical depression by 1200 UTC that day, with satellite images showing the low-level center completely exposed to the north of the residual shower activity. Six hours later, Michael degenerated into a remnant low, its center devoid of significant deep convection for about 18 hours.


b. Meteorological Statistics


Observations in Michael (Figs 2 and 3, to be added  in the final rendition) include the satellite-based Dvorak intensity technique from the Tropical Analysis Forecast and Branch (TAFB) and the Satellite Analysis Branch (SAB). The Advanced Dvorak Technique (ADT) from the University of Wisconsin CIMSS laboratory was also used to assess the intensity of Michael. As always, scatterometer data was useful in determining the extent of tropical storm force winds with Michael, as well as for center fixes when a well-defined eye was not apparent in conventional or microwave satellite data. Michael is an example of a very small hurricane that would have likely been missed prior to the advent of satellite and scatterometer data. Various microwave data were used throughout Michael's life cycle to assess the inner core structure. In general, an eye was present in the imagery for much of the cyclone's lifespan, even when this feature was not readily apparent using conventional imagery.

Michael's peak intensity as a 100-kt major hurricane at 0600 UTC 6 September is based on a blend of the various Dvorak numbers. It should be noted that objective CIMSS ADT estimates gave an estimate closer to 105 kt. Dvorak numbers began to oscillate upward again on 8 September, when satellite images suggest that Michael had begun to intensify again. Predicated on these estimates is the assigned secondary peak intensity of 90 kt at 1200 UTC that day. It should be noted that when the eye grew considerably in size on 9 September, satellite estimates generally trended upward. This was probably more reflective of the large eye structure, which was annular in nature, and not representative of additional strengthening. Finally, due to Dvorak constraints, satellite estimates generally lagged the satellite signature during the early stages of Michael's final weakening phase. Thus, Michael's intensity is considered to be somewhat lower than the satellite estimates during that time.

Based on historical records, Michael is only the fifth known major hurricane to have developed from a non-tropical source during the satellite era. The others were: Alicia (1983), Diana (1984), Bob (1991), and Claudette (1991).

There were no ship reports of tropical storm force winds associated with Michael.


c. Casualty and Damage Statistics


Since Michael was a small hurricane that remained well out at sea, no damage or fatalities have been received in connection with the storm.


d. Forecast and Warning Critique


The genesis of Michael was not anticipated, probably because there was little signal in the global models, and because the system formed in an unusual manner. Even after genesis, the models in general had difficulty following the vortex, especially in the Mean Sea Level Pressure (MSLP) fields, probably because of its small horizontal scope.

Average track errors with Michael were considerably low, and indeed, these sets of forecasts were among the best in the season.

Intensity forecast errors were considerably larger. Initial forecasts had a substantial low bias, with the first forecast indicating cessation as a tropical cyclone in three days, with complete dissipation by day five. It appears that these forecasts were caused primarily by two reasons: moderate to strong westerly shear seen in the 850-200 mb GFS upper-level forecast fields, which was not considered to be a favorable upper flow pattern for strengthening given Michael's initial westward component along with its small size. Second, the inability of the global models to accurately initialize and track the Michael vortex likely played a large role in the low bias of the various forecasts. Even when Michael was strengthening and it was noted in the forecast schemes, the rapid intensification of the hurricane on 5 and 6 September was not anticipated, perhaps again because the models, global and statistical, failed to accurately convey this incident. Michael's second phase of intensification that occurred on 8 September was also not well predicted, with weakening expected when Michael actually strengthened. It appears that inner core dynamics played the dominant role in the intensity of Michael during the period of 6 through 9 September, which is likely why forecasts failed to identify and successful predict this.

There were no watches and warnings issued with Michael.



Infrared satellite image of Hurricane Michael at peak intensity.

2012 Atlantic hurricane season

Updated: 8:14 AM GMT on February 28, 2013

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Tropical Cyclone Report (TCR): Hurricane Leslie

By: KoritheMan, 8:36 AM GMT on February 27, 2013

Tropical Cyclone Report
Hurricane Leslie
(AL122012)
30 August-11 September 2012

Leslie was a long-lived tropical cyclone that meandered north of the Leeward Islands, reached hurricane intensity twice, and made landfall along the southeast coast of Newfoundland as a hurricane-force extratropical cyclone.


a. Synoptic History

Leslie developed from a vigorous tropical wave that moved off the coast of Africa on 26 August. The wave was convectively active, with the associated cloud pattern first showing signs of organization around midday 28 August, at which time a broad surface area of low pressure appears to have formed along the wave axis. Curved bands formed the next day as the system moved westward to the south of a mid-level ridge. Situated beneath an anticyclone, the low pressure area continued to become better organized, and it is estimated that the system became a tropical depression around 0600 UTC 30 August, centered approximately 1200 miles east of the Lesser Antilles. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively ( to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added  in the final rendition). Under light shear and very warm waters, the depression quickly became a tropical storm about 12 h later. Northerly shear increased over Leslie, which caused the center to become partially exposed along the northern edge of the convective cloud shield the next day. Leslie also moved very slowly during this time as a departing trough over the western Atlantic gave way to increased ridging north of the tropical storm. Despite the shear, Leslie steadily strengthened, and it reached an initial peak of 60 kt, just under hurricane strength, for a short time near 0000 UTC 1 September. At that time, Leslie was located about 425 miles east of the Leeward Islands. Leslie began to weaken thereafter as the shear increased dramatically, leaving the center well-removed from the shower activity. Leslie slowed down and moved slowly west-northwestward well to the northeast of the northern Leeward Islands.

A large burst of convection blossomed in association with the Leslie vortex late on 2 September, which expanded into 3 September. The cyclone appeared to become a little better organized during this time, although microwave data suggests that the low-level center was still straddling along the edge of the convective canopy -- west this time due to increasing westerly shear. The cloud pattern abruptly deteriorated after 0600 UTC 4 September. Leslie weakened during this time, likely due to the westerly shear mentioned above, as well as dry air entering the western portion of the large circulation as seen by arc clouds emanating westward away from the center on satellite animations (not shown). Convection began to ignite again just after 1800 UTC that day, but scatterometer ambiguities indicate that the low-level center had lost some definition relative to previous days, probably symptomatic of the westerly shear that had penetrated the inner core region. Synoptic scale steering in the region surrounding Leslie weakened further as broad upscale troughing began to develop over the eastern seaboard, and Leslie responded by becoming stationary on 5 September. The system also became a hurricane at 1200 UTC that day, still not moving. Leslie maintained hurricane strength for about a day, when it weakened back to a tropical storm. Since the upper tropospheric shear over the area appeared to be relatively light, it seems that the primary cause of weakening was cold water upwelling caused by the quasi-stationary motion of the system; analysis of areal sea surface temperatures in that area indicates that the hurricane cooled the surrounding waters by an average of 3C during the 72-hour period from 4 September through 7 September, with localized cooling of near 6C.

Convection diminished considerably shortly after Leslie lost hurricane status, and the cyclone weakened to an intensity of 45 kt by 1200 UTC 7 September. Leslie turned northwestward late on 7 September, and turned northward early on 8 September as the surrounding steering regime became a little more concentrated. As the former hurricane gradually increased its forward speed in the mid-latitude southerly flow associated with the east coast trough, it left its cold water wake. As this occurred, Leslie made several unsuccessful attempts at redeveloping an inner core, but it was never able to do so, likely because the outer wind maxima was expanding. In fact, surface and satellite data indicate that the circulation actually became larger during this time as the primary convection was focused in large bands well-removed from the center. Nevertheless, aircraft data during a flight into the system early on 8 September indicate that Leslie's winds had increased back to 50 kt. Leslie turned toward the north-northeast late the next day as it continued to accelerate, its center passing about 150 miles east of Bermuda around 1800 UTC that day. Probably due to baroclinic forcing from the approaching negatively-tilted upper trough, Leslie's winds continued to increase, and the cyclone is estimated to have regained hurricane status around 0600 UTC 10 September, centered about 600 miles south-southwest of Cape Race, Newfoundland.

AMSU vertical temperature profiles and surface observations indicate that a well-defined warm front was approaching Leslie from the west, and Leslie transitioned to an extratropical cyclone a few hours prior to landfall, which occurred along the southern coast of Newfoundland about 10 miles north of Placentia around 1215 UTC 11 September with an intensity of 65 kt, although given the highly baroclinic environment, most of the associated weather was well-removed from the center and reached the coast hours prior to the arrival of the center. The post-tropical remnants of Leslie were absorbed by a frontal zone over the Labrador Sea later that day.


b. Meteorological Statistics

Observations in Leslie (Figs 2 and 3, to be added in the final rendition) include the Dvorak satellite intensity intepretation technique used by the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the Advanced Dvorak technique (ADT) used by the University of Wisconsin CIMSS. Several ASCAT passes were useful in determining the tropical storm force wind radii associated with Leslie, as well as for estimating the horizontal extent of its surface circulation. Indeed, the circulation appeared to have become considerably larger after Leslie pulled away from the Leeward Islands. Various microwave data were also used to track the center of Leslie. In addition, several drifting buoys owned and operated by Environment Canada were useful in ascertaining the intensity of Leslie when it made landfall.

The peak intensity of Leslie, estimated at 70 kt, is estimated to have occurred at 1800 UTC 5 September when the satellite signature appeared to be at its most impressive, and is based on a blend of the Dvorak numbers, which supported an intensity of about 65 kt from TAFB/SAB, and which supported an intensity closer to 75 kt by the UW-CIMSS ADT estimates. It should be noted that aerial reconnaissance was unavailable at the time of peak intensity, and the first flight into the system was not until 8 September, when Leslie had already weakened to a tropical storm.

Leslie's restrengthening to a hurricane again at 0600 UTC 10 September is difficult to pinpoint, since the satellite signature was not one of a typical hurricane. The rapidly cooling (but still sufficiently warm for tropical cyclogenesis/convective sustenance) sea surface temperatures along the track of the hurricane along with increasing southwesterly shear associated with the western Atlantic cold front, would definitely not argue for classical tropical intensification. However, AMSU microwave data still showed a solid warm core associated with Leslie at that point. Also, extrapolaton of data from a reconnaissance flight into the cyclone on 8 September suggest that the maximum surface winds associated with Leslie had increased to about 50 kt as the cyclone pulled away from its cold water wake it had generated north of the Leeward Islands. Since the convection actually became deeper -- albeit not concentrated near the center -- after departure of the aircraft, it is presumed that Leslie was eventually able to bring sustained hurricane force winds -- in small patches -- down to the surface, the vigor of the circulation and baroclinic forcing allowing these winds to persist even after the convection had dissipated. Finally, the acceleration of the hurricane as it approached Bermuda would argue for increasing winds, particularly in the eastern semicircle. Given the marginal environment over that portion of the Atlantic during 10 September, the previously alluded to upper-level trough, and the asymmetric cloud pattern and distribution of the wind field, and the lack of a well-defined inner core, it is presumed that a large portion of Leslie's intensification that day was related to baroclinic, rather than tropical processes. However, nature produces a wide spectrum of cyclones, tropical and extratropical, and without reconnaissance observations to contradict the aforementioned observations, Leslie is deemed to have reached hurricane strength near 0600 UTC 10 September, as it was pulling away from Bermuda.

Sustained tropical storm force winds occurred on Bermuda, where a sustained wind of 34 kt was reported on Saint David's on at 1631 UTC local time on 9 September; this occurred concurrently with a gust of 46 kt.

Leslie produced strong winds over portions of Atlantic Canada. St. John's, located on the southeastern tip of Newfoundland, reported a sustained wind of 51 kt with a gust to 71 kt around 1430 UTC local time on 11 September. Additionally, a sustained wind of 43 kt with a gust to 66 kt was recorded at Cape Race at 1203 UTC local time, also on 11 September.

Heavy rains also accompanied Leslie, with the Bermuda airport recording a storm maximum rainfall of 5.18 inches during the 48-hour period from 8 to 10 September. Leslie produced heavy rainfall over portions of Atlantic Canada, although its acceleration in the mid-latitude westerlies prevented a significant flood disaster. A storm maximum of 6.50 inches was observed at Shubenacadie in Nova Scotia.

Storm surge flooding was minimal since storm hit at low tide.

The lowest observed central pressure in Atlantic Canada was 973.1 mb at Sagona Island, which was noted at 1100 UTC local time 11 September.

Some rip currents and high waves were noted across portions of the eastern United States coast due to Leslie's large wind field. Rough surf also impacted portions of the northern Leeward Islands as the tropical cyclone passed by to the north.


c. Casualty and Damage Statistics

No fatalities have been attributed to Leslie, but the storm is estimated to have caused around $10 million in damage (2012 USD). Damage on Bermuda was minor and limited primarily to downed tree limbs and power lines, which caused some localized power outages. Leslie passed farther east of Bermuda than originally thought, which kept the island on the weaker side of the tropical cyclone.

Damage was more extensive across Atlantic Canada, where an extensive power outage was reported along the Avalon Peninsula in Newfoundland, where as much as 45,000 homes lost electricity for about six hours. In Pouch Cove, which is located on the Avalon Peninsula, Leslie destroyed several partially constructed homes in Pleasantville, which is a neighborhood located on the east side of St. John's on the far eastern tip of Newfoundland. In addition, some localized flooding occurred in the region, which caused some road and bridge closures.


d. Forecast and Warning Critique

Track forecasts associated with Leslie were fairly good, except for when the storm had begun to move and was approaching Bermuda. During that period, forecasts continuously called for Leslie to pass west and very near the island, based primarily on the GFS model, which was consistent on a track that brought Leslie to the west of Bermuda as opposed to east like actually occurred. It should be noted that there large discrepancies within the various computer model forecasts during that time. Leslie's smooth recurvature into the westerlies was correctly forecast, as was its slow motion and eventual trajectory into Atlantic Canada.

Intensity forecasts errors were considerably large, especially beyond 48 hours, where they exhibited a high bias. Several of the forecasts for Leslie called for the system to reach the threshold of Category 2/3 intensity as the cyclone made its closest approach to Bermuda. There were fears that Leslie would be a repeat of Hurricane Fabian in 2003, which directly struck the island at category 3 intensity. Leslie's intensity when it was meandering in the western Atlantic, and when it was approaching Newfoundland, was well predicted.

A tropical storm watch was issued for Bermuda at 2100 UTC 6 September, which was nearly four days before the arrival of tropical storm force winds. A tropical storm warning was issued for the island about 36 hours later. A hurricane watch was issued for sections of Atlantic Canada at 0600 UTC 10 September by Environment Canada.

A summary of the watches and warnings given with Leslie are indicated in Table 3 (to be added in the final rendition).






Visible satellite image of Leslie at peak intensity near 1800 UTC 5 September.

2012 Atlantic hurricane season

Updated: 11:24 AM GMT on February 27, 2013

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Tropical weather analysis - February 26, 2013

By: KoritheMan, 5:41 AM GMT on February 27, 2013

Rusty

Tropical Cyclone Rusty is closing in on the northwest coast of Australia. As of the latest JTWC advisory, the following information was available on the storm:

Wind: 110 mph, with higher gusts
Location: 19.3°S 119.1°E
Movement: Stationary
Category: 2 (Saffir-Simpson Hurricane Scale)

And the Australian Bureau of Meteorology had to this say:

Sustained wind: 90 kt (10-minute average)
Wind gusts: 125 kt (3-second average)
Movement: Southeast at 1 mph (2 km/h)
Pressure: 945 mb
Category: 4 (Australian scale)

The cloud pattern associated with Rusty is improving as the cyclone approaches the coast, with the last few visible satellite images showing the eye becoming more distinct, although it is still a little ragged. Doppler radar from Port Hedland shows little change to the convective structure or the relative definition of the eye. The radar data indicates that the eyewall is fragmented and poorly-defined to the east.



Figure 1. Latest infrared satellite image of Tropical Cyclone Rusty. Image credit: NOAA's Satellite Services Division (SSD).

While the synoptic steering encompassing Rusty is still ill-defined, there are some signals that the mid- to upper-level flow is beginning to strengthen as a strong ridge over central Australia deepens. The global models show Rusty moving inland in later today (Australian local time), probably around 10:00 PM local time, which be roughly nine hours from now. Given current motion trends finally showing some solid movement, this seems a little slow. I'd say closer to 6 or 7:00 PM local time. It should be noted that the eye is likely to come ashore very Pardoo, and thus they are most likely to the strongest winds and heaviest rains.

Environmental conditions remain conducive to intensification in the hours remaining prior to landfall, but most likely the cyclone will remain at about the same intensity, but there could of course be fluctuations in either direction. Objective SST analyses in this region indicate that Rusty has cooled the waters underneath it by about a degree or two (C) over the last couple of days, but since these waters are deep, this is not expected to be a critical factor toward future intensity. One possible factor that could weaken Rusty a little, especially given the storm's large size and thus inherent capacity to entrain it, is the presence of dry continental air to the west, which had eroded the convection in the western portion of the cyclone circulation earlier; this dry air showed up nicely on the University of Wisconsin CIMSS MIMIC-Total Precipitable Water (TPW) product.

In any event, Rusty is running out of time to strengthen or weaken appreciably, and interests in the warning area should carefully monitor the progress of this tropical cyclone. Damage will be lessened relative to what could have occurred had the storm struck Port Hedland directly.

Rusty is expected to produce significant storm surges near and to the east of where the center makes landfall, accompanied by large and dangerous waves. Areas west of the center will see mostly offshore flow, since tropical cyclones rotate clockwise in the Southern Hemisphere, which is opposite to the Northern Hemisphere.

Heavy rainfall and associated flash flooding is also anticipated, and given the slow movement and large size of the storm circulation, rainfall amounts could be quite prodigious, on the order of 8 to 12 inches, with local amounts of 20 up to inches, especially in areas of higher terrain farther south along the track of the center after it moves inland.

Port Hedland recently reported a sustained wind of 40 mph. Although they will escape the dangerous storm surge that will be experienced in areas farther east, they are on the most dangerous side of the circulation where convection is deepest. Thus, wind gusts to hurricane force, probably up to about 80 mph, will be possible, and residents there should take Rusty extremely seriously. Rainfall will also be a serious concern.

5-day intensity forecast

INITIAL 02/26 0300Z 95 KT 110 MPH
12 hour 02/26 1200Z 95 KT 110 MPH...AT THE COAST
24 hour 02/27 0000Z 80 KT 90 MPH...INLAND
36 hour 02/27 1200Z 60 KT 70 MPH...INLAND
48 hour 02/28 0000Z 35 KT 40 MPH...INLAND
72 hour 03/01 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 03/02 0000Z...DISSIPATED

Storm information (Bureau of Meteorology)

IDW24100
Australian Government Bureau of Meteorology
Western Australia
Tropical Cyclone Warning Centre

Media: Transmitters serving the area between Wallal and Whim Creek are
requested to USE the Standard Emergency Warning Signal before broadcasting the
following warning.

TOP PRIORITY FOR IMMEDIATE BROADCAST

TROPICAL CYCLONE ADVICE NUMBER 32
Issued at 11:54 am WST on Wednesday 27 February 2013

A Cyclone WARNING is current for coastal areas from Bidyadanga to Dampier
including Port Hedland, Karratha and Dampier, and adjacent inland areas of the
Pilbara, including Marble Bar, Nullagine, Millstream, Tom Price, Telfer and
Newman.
A Cyclone WATCH is current for remaining central and eastern areas of the
Pilbara including Paraburdoo, Three Rivers and adjacent parts of the Gascoyne
district and the Interior.

At 11:00 am WST Severe Tropical Cyclone Rusty, Category 4 was estimated to be
125 kilometres northeast of Port Hedland and
190 kilometres north of Marble Bar and is moving southeast at 5 kilometres per
hour towards the coast.

Severe Tropical Cyclone Rusty has started to take a southeasterly track over
the last few hours. During the afternoon it is expected to take a more
southerly track as it moves towards the Pilbara coast.

Gales are occuring on the coast between Sandfire Roadhouse and Whim Creek and
will extend inland towards Marble Bar during today, and may reach Nullagine
overnight.

DESTRUCTIVE winds with gusts in excess of 125 kilometres per hour are occuring
along the coast in the vicinity of De Grey and Pardoo, and could reach Marble
Bar overnight.

VERY DESTRUCTIVE winds with gusts in excess of 165 kilometres per hour are
likely in the vicinity of De Grey and Pardoo during the afternoon and evening
as Severe Tropical Cyclone Rusty approaches the coast.

Widespread very heavy rainfall today and on Thursday is likely to lead to MAJOR
FLOODING in the De Grey catchment. Significant flooding is also possible in the
Fortescue catchment and in Pilbara coastal streams.

People on the coast between Wallal and De Grey are warned of the potential for
a VERY DANGEROUS STORM TIDE as the system approaches the coast. Tides are
likely to rise significantly above the normal high tide mark with DAMAGING
WAVES and VERY DANGEROUS COASTAL INUNDATION.



DFES State Emergency Service (SES) advises of the following community alerts:
RED ALERT: People in or near communities between Pardoo and Whim Creek,
including Port Hedland and South Hedland should remain in shelter.
YELLOW ALERT: People in communities between Wallal and Pardoo, extending inland
to Marble Bar and Nullagine need to take action and get ready to shelter from a
cyclone.
BLUE ALERT: People in communities between Bidyadanga and Wallal and between
Whim Creek and Dampier, including Karratha and extending to inland areas
including Millstream, need to prepare for cyclonic weather and organise an
emergency kit including first aid kit, torch, portable radio, spare batteries,
food and water.
ALL CLEAR: People in communities from Mardie to Dampier are advised that the
cyclone danger has passed.


Details of Severe Tropical Cyclone Rusty at 11:00 am WST:
.Centre located near...... 19.5 degrees South 119.4 degrees East
.Location accuracy........ within 30 kilometres
.Recent movement.......... towards the southeast at 5 kilometres per hour
.Wind gusts near centre... 230 kilometres per hour and weakening
.Severity category........ 4
.Central pressure......... 945 hectoPascals

The next advice will be issued by 3:00 pm WST Wednesday 27 February.

Cyclone advices and State Emergency Service Community Alerts are available by
dialling 1300 659 210

A map showing the track of the cyclone is available at:
http://www.bom.gov.au/cyclone

Watches and warnings

A Cyclone WARNING is current for coastal areas from Bidyadanga to Dampier
including Port Hedland, Karratha and Dampier, and adjacent inland areas of the
Pilbara, including Marble Bar, Nullagine, Millstream, Tom Price, Telfer and
Newman.
A Cyclone WATCH is current for remaining central and eastern areas of the
Pilbara including Paraburdoo, Three Rivers and adjacent parts of the Gascoyne
district and the Interior.

2012-2013 Australian tropical cyclone season Tropical Cyclone Rusty

Updated: 5:51 AM GMT on February 27, 2013

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Tropical weather analysis - February 25, 2013

By: KoritheMan, 8:34 AM GMT on February 25, 2013

Tropical Cyclone Rusty

Tropical Cyclone Rusty is taking aim on the northwestern Australia coast. As of the most recent advisory from the Bureau of Meteorology, the following information was available on the cyclone:

Wind gusts: 140 kmh
Location: 17.7°S 119.0°E
Movement: Near stationary
Pressure: 970 mb
Category: 2 (Australian scale)

Rusty looks about the same as it did six hours ago. Convection is trying to wrap around a cloud-free area that is not yet deserving of being called an eye. Recent satellite and microwave fixes suggest that Rusty has moved very little over the last few hours, save perhaps a small nudge toward the east-southeast. CIMSS total precipitable water data, along with the actual satellite signature, suggest that Rusty may have entrained some dry continental air from northwestern Australia.



Figure 1. Latest infrared satellite image of Tropical Cyclone Rusty. Image credit: NOAA Satellite Services Division (SSD).

The intensity forecast is rather tricky, and depends almost entirely on the evolution of the cyclone circulation. Rusty is an unusually large tropical cyclone, and experience has shown that these cyclones tend to respond rather slowly to seemingly favorable atmospheric dynamics. Given this, and my positive bias for Hurricane Isaac last year, I will choose to err on the side of caution, forecasting a somewhat weaker cyclone than the JTWC in their most recent (6z) forecast for the system, where they are showing a Category 3 (SSHS). While this type of intensification is possible if Rusty can develop an inner core, I would like to see some more evidence of such before I go along with such a forecast. One positive for strengthening is that the most recent AMSUB image shows a somewhat better defined cyclone signature, with the cloud free spot alluded to showing somewhat better definition, perhaps the beginnings of an inner core. There is no obvious reason why Rusty should not strengthen up to landfall, as there is little vertical shear and there is plenty of high oceanic content in the region, even toward the coast.



Figure 2. Southeast Indian Ocean Tropical Cyclone Heat Potential (TCHP) as of February 23, 2013. Values of 80 or over are considered to be conducive for rapid intensification. Image credit: AOML NOAA.

Following the typical progression with landfalling tropical cyclones, Rusty should weaken rapidly after landfall, although the inland decay rate will probably be a little minimized with this storm relative to an average tropical cyclone given the enormity of the storm circulation.

5-day intensity forecast

INITIAL 02/25 0600Z 60 KT 70 MPH
12 hour 02/25 1800Z 65 KT 75 MPH
24 hour 02/26 0600Z 70 KT 80 MPH
36 hour 02/26 1800Z 85 KT 100 MPH
48 hour 02/27 0000Z 95 KT 110 MPH...AT THE COAST
72 hour 02/28 0000Z 70 KT 80 MPH...INLAND
96 hour 03/01 0000Z 25 KT 30 MPH...INLAND POST-TROPICAL/REMNANT LOW
120 hour 03/02 0000Z...DISSIPATED

Note that the forecast intensities given above are based on the Saffir-Simpson Hurricane Scale, which utilizes maximum 1-minute sustained wind speeds. The Australian scale is quite different, which bases the severity category on maximum wind gusts. Initial wind speed is based off JTWC.

Also, the times provided within that table are based on Eastern Standard Time.

Since Rusty is a slow mover, determining when and where the system will make landfall is a little muddled. In general, the most reliable guidance suggests that it will strike a little to the east of Port Hedland, but given the erratic nature of the track, this is uncertain. In fact, the cyclone warning area covers quite a large stretch of coastline, effectively detailing the uncertainty associated with this storm. Perhaps the greater challenge is predicting the timing of landfall. The GFS and ECMWF, which have historically been very reliable, show landfall just after 48 hours, and that's what I will go with.

CIMSS steering data and upper-level wind analyses show that a weakness trails between a developing tropical low to the west and Rusty. This sort of synoptic evolution tends to favor erratic motion, and Rusty has been behaving in precisely that fashion. The cyclone is still forecast to begin a more definitive southward movement later today. Little deviation from that general motion is expected up to landfall, with the possibility of a more southwestward movement by day three and four. Virtually all of the global models lose the circulation by 120 hours as Rusty moves well inland.

Interests along the coast within the warning area should begin their cyclone preparations, as Rusty's large circulation will begin to impart rainbands over coastal areas later today as the synoptic steering increases and drives the system southward. Interests within the cyclone watch area should carefully monitor the progress of the tropical cyclone in case any significant track deviation occurs. Unfortunately, I haven't been able to find a decent geographical map to draw an actual track forecast on, so if you guys know how to find one, feel free to elucidate.

Given its slow motion, Rusty poses a heavy rainfall/flash flood threat. An earlier TMI image showed rainfall rates up to 1 inch per hour to the north of the center over water. Rainfall could reach 10 to 20 inches, with localized areas of 25 to 30 inches. I am seeing a similar flood situation that occurred with Hurricane Isaac in southeast Louisiana last year.


TROPICAL CYCLONE ADVICE NUMBER 15
Issued at 2:43 pm WST on Monday 25 February 2013

A Cyclone WARNING is current for coastal areas from Cape Leveque to Mardie.
A Cyclone WATCH is current for coastal areas for adjacent inland areas of the
Pilbara including Marble Bar, Nullagine and Millstream

At 2:00 pm WST Tropical Cyclone Rusty, Category 2 was estimated to be
295 kilometres north of Port Hedland and
345 kilometres west of Broome and
was near stationary.

Tropical Cyclone Rusty is moving slowly towards the coast. It has recently
moved slowly to the east southeast but it is likely to resume a more southward
track later this afternoon or this evening.

Gales are expected to develop on the coast between Wallal and Whim Creek during
this afternoon, possibly extending north towards Broome this evening or
overnight. During Tuesday afternoon gales could extend west to Karratha and
begin to extend inland towards Marble Bar and Millstream.

Further intensification is likely as the cyclone approaches the coast. There is
a HIGH RISK THAT RUSTY WILL CROSS THE COAST AS A SEVERE TROPICAL CYCLONE.
However, the slow motion of the cyclone means that the crossing time and
location is uncertain.

Rusty is a large tropical cyclone and its slow movement is likely to result in
rainfall that is heavier than that associated with a typical tropical cyclone.
Very heavy rainfall is expected in near coastal parts of the eastern Pilbara
and western Kimberley over the next few days. During Tuesday and Wednesday
widespread very heavy rainfall is likely to lead to MAJOR FLOODING in the De
Grey catchment. Significant flooding is also likely in the Fortescue catchment
and in Pilbara coastal streams.

Rusty's intensity, size and slow movement is also likely to lead to a VERY
DANGEROUS STORM TIDE as the cyclone centre nears the coast. Tides are likely
to rise significantly above the normal high tide mark with DAMAGING WAVES and
VERY DANGEROUS COASTAL INNUNDATION.

DFES State Emergency Service (SES) advises of the following community alerts:
BLUE ALERT: People in communities between Cape Leveque and Mardie, including
Port Hedland and Karratha and extending to adjacent inland areas including
Marble Bar and Millstream, need to prepare for cyclonic weather and organise an
emergency kit including first aid kit, torch, portable radio, spare batteries,
food and water.

Details of Tropical Cyclone Rusty at 2:00 pm WST:
.Centre located near...... 17.7 degrees South 119.0 degrees East
.Location accuracy........ within 55 kilometres
.Recent movement.......... near stationary
.Wind gusts near centre... 140 kilometres per hour
.Severity category........ 2
.Central pressure......... 970 hectoPascals

The next advice will be issued by 6:00 pm WST Monday 25 February.

Cyclone advices and State Emergency Service Community Alerts are available by
dialling 1300 659 210

A map showing the track of the cyclone is available at:
http://www.bom.gov.au/cyclone

2012-2013 Australian tropical cyclone season Tropical Cyclone Rusty

Updated: 10:41 AM GMT on February 25, 2013

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Tropical Cyclone Report (TCR): Hurricane Kirk

By: KoritheMan, 12:06 AM GMT on February 19, 2013

Tropical Cyclone Report
Hurricane Kirk
(AL112012)
28 August-2 September 2012

Kirk was a small category two hurricane (on the Saffir-Simpson Hurricane Scale) that remained over the central Atlantic without affecting land.


a. Synoptic History

Kirk formed from a large tropical wave that left the west coast of Africa on 22 August. The wave passed south of the Cape Verde Islands the next day before moving slowly west-northwestward toward a weakness in the Atlantic subtropical ridge partially left behind by Tropical Storm Joyce. The wave was poorly-defined on satellite images, and did not generate much deep convection due to a stable airmass. An area of low pressure developed along the wave axis on 24 August as the wave was passing a few hundred miles to the west of the western Cape Verde Islands and the cloud pattern exhibited some small banding features. While deep convection was sporadic and not well organized due to the dry environment, upper tropospheric shear over the area was fairly light, which allowed the wave to maintain a vigorous lower-tropospheric circulation as it moved into higher latitudes. The circulation began to become better defined in the surface wind field around midday on 25 August, but dry air continued to inhibit development despite progressively warmer sea surface temperatures. Zonal flow over the north Atlantic enhanced the outflow over the northern semicircle. The wave slowly became better organized as it moved into an increasingly moist environment, and it became a tropical depression near 1800 UTC 28 August, while centered about 1000 miles southwest of the Azores. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added in the final rendition) The best track positions and intensities are listed in Table 1 (to be added in the final rendition). Some moderate southwesterly shear affected the tropical cyclone beneath the outflow layer due to a weak upper trough over the central Atlantic. The shear appeared to lessen a bit thereafter, and the cyclone became a tropical storm, albeit one with minimal deep convection, near 0600 UTC 29 August. Kirk intensified only slightly over the next day or so as the cyclone moved northwestward between an amplifying mid-level trough to the west and a mid-level ridge to the east.

The cyclone intensified rather quickly beginning around 0600 UTC 30 August, and a ragged eye began to become apparent on geostationary satellite images a few hours later. Kirk reached hurricane status near 1200 UTC that day, while centered about 900 miles east-southeast of Bermuda. Kirk's rapid intensification continued, and the hurricane assumed its peak intensity of 90 kt at approximately 0600 UTC the next day; by that point, the small hurricane had already turned northward ahead of a large mid-latitude trough. The satellite signature of the hurricane deteriorated only a short time later, as Kirk entered an environment of increasing westerly shear. Kirk turned north-northeastward late that day as it became fully captured by the aforementioned trough. The small cyclone weakened as quickly as it strengthened, becoming a tropical storm near 0000 UTC 1 September. Kirk gradually turned more toward the northeast and weakened as it moved into increasingly hostile conditions. The cyclone's acceleration over the cold north Atlantic waters soon caused it to encounter a highly baroclinic environment as a large cold front approached the storm from the west. Due to interaction with this front, Kirk became extratropical near 1800 UTC that same day while it was racing northeastward over the north Atlantic at about 30 kt. At the time of extratropical transition, post-tropical Kirk was located about 750 miles northwest of Flores in the southwestern Azores.


b. Meteorological Statistics

Observations in Kirk (Figs 1 and 2, to be added in the final rendition) include the satellite-based Dvorak method from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). ADT satellite estimates from the University of Wisconsin CIMSS were also used. Kirk was also surveyed by various microwave data, along with scatterometer data, specifically the ASCAT. Most notably, the Advanced Microwave Sounding Unit (AMSU) was particularly useful in depicting the point at which Kirk had lost its tropical characteristics and become a post-tropical cyclone over the north Atlantic. This is estimated to have occurred around 1800 UTC 2 September, when the unit, along with satellite and water vapor imagery, indicated that Kirk had become fully involved with a frontal zone. At that time, ASCAT data and satellite animations suggested that the low-level circulation had become elongated in a southwest to northeast direction and well-separated from an isolated band of showers well to the north of the center.

The peak intensity of Kirk, estimated to be 90 kt, was reached on 0600 UTC 31 August. This was based on Dvorak estimates, as well as microwave data showing a well-defined eye surrounded by a ring of deep convection. It should be noted that some of the objective CIMSS-ADT estimates exceeded the major hurricane threshold during that time, but given the abrupt deterioration in the cloud pattern experienced thereafter, along with lower estimates elsewhere, Kirk's peak intensity is set at 90 kt as a compromise. It also interesting to note that the circulation of Kirk was extremely small, with scatterometer data indicating that 34-kt winds extenteded out no more than about 80 nmi from the center at the time Kirk was at its largest. Also, it is still a bit unclear as to how such a large tropical wave as the one that produced Kirk went on to generate such a small hurricane.

There were no ship or buoy reports of tropical storm or hurricane force winds in association with Kirk.


c. Casualty and Damage Statistics

There were no reports of casualities or damage association with Kirk.


d.  Forecast and Warning Critique

Tropical Weather Outlooks (TWOs) were not generated for Kirk. Track and intensity forecasts were also unavailable. This was due to the author having been struck by Hurricane Isaac around the time genesis occurred, which caused widespread disruption to the regional electric grid and prevented the production of forecasts.






Infrared satellite image of Hurricane Kirk at peak intensity.

2012 Atlantic hurricane season

Updated: 12:09 AM GMT on February 19, 2013

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Tropical Cyclone Report (TCR): Tropical Storm Joyce

By: KoritheMan, 12:13 AM GMT on February 14, 2013

Tropical Cyclone Report
Tropical Storm Joyce (AL102012)
22-24 August 2012

Joyce was a brief tropical storm that formed over the central Atlantic and quickly succumbed to strong vertical shear.

a. Synoptic History

A tropical wave moved off the coast of Africa late on 18 August, and is believed to have been the precursor to Joyce. This wave trailed immediately behind the one that would spawn Hurricane Isaac. The associated shower activity showed signs of organization as the wave moved quickly westward across the eastern Atlantic, although satellite pictures during the following few days suggested that the strongest of the associated convection was located to the west of the wave axis, probably as a result of moderate easterly shear which was also evident in the overall outflow pattern; this sort of shear pattern is not uncommon in the deep tropics in the Atlantic basin, particularly with easterly waves that propagate off Africa and move across the Atlantic east of 35W, since such systems are generally closer to the center of the Bermuda-Azores ridge. Nevertheless, satellite and scatterometer data indicated that the wave had a well-defined lower- to middle- tropospheric circulation, and this feature quickly showed signs of closing off at the surface. A low pressure area developed within the wave on 20 August. Organization continued, and it is assumed that the system acquired enough organization to be considered a tropical depression near 1200 UTC 22 August, while centered approximately 900 miles west-southwest of the Cape Verde Islands. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added in the final rendition) An upper low over the central Atlantic, the same one that was initially responsible for shearing Isaac, produced strong southerly shear over the tropical cyclone, which prevented significant development. Under the influence of a synoptic scale ridge to the north, the depression moved generally west-northwestward. Although the shear never relaxed, the depression was able to generate a persistent burst of convection which led to the formation of a tropical storm around 1200 UTC the next day.

Joyce began to weaken almost immediately after becoming a tropical storm, as southerly shear increased over the cyclone as the upper low strengthened and moved east. Joyce turned northwestward and weakened to a tropical depression around 0000 UTC 24 August as the deep convection sputtered. Lacking any significant convection near its center, Joyce degenerated into an elongated trough at approximately 1200 UTC, at which time microwave, satellite, and scatterometer ambiguities indicated that the low-level center was no longer closed. The remnant circulation lost its identity shortly after 1800 UTC that day.


b. Meteorological Statistics

Observations in Tropical Storm Joyce include include the Dvorak-based satellite intensity technique, which were introduced by the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB).

Joyce's peak intensity of 35 kt near 1200 UTC 23 August is based on unanimous Dvorak estimates from TAFB and SAB, when they reached 2.5 near 1200 UTC. This was also based on an AMSUB microwave overpass at 1230 UTC 23 August, which showed the center tucked within the southern edge of a robust central dense overcast (CDO). Geostationary satellite images were used to authenticate the peak intensity as well, at which time Joyce was exhibiting a curved band pattern.

There were no ship reports of tropical storm force winds in association with Joyce.


c. Casualty and Damage Statistics


No reports of damage or casualties have been received in association with Joyce.


d. Forecast and Warning Critique


The genesis of Joyce was fairly well anticipated. The tropical wave that would spawn Joyce was first introduced in the Tropical Weather Outlook (TWO) at 0600 UTC 20 August, and was assigned a "low" (10%) chance of development.  Genesis forecasts reached the "high" probability (60%) about 48 hours later, approximately 12 hours prior to genesis.

Since Joyce lasted for only 48 hours, it is impossible to meaningfully verify any of the forecasts that were issued on the storm.









Visible satellite photo of Joyce at its peak intensity at 1200 UTC 23 August.

2012 Atlantic hurricane season

Updated: 12:19 AM GMT on February 14, 2013

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Tropical Cyclone Report (TCR): Hurricane Isaac

By: KoritheMan, 6:29 AM GMT on February 12, 2013

Tropical Cyclone Report
Hurricane Isaac
(AL092012)
21 August-1 September 2012

Isaac made landfall in southeastern Haiti as a strong tropical storm. It then traversed eastern Cuba and the Gulf of Mexico before making landfall in southeast Louisiana as a category 1 hurricane (on the Saffir-Simpson Hurricane Wind Scale).

a. Synoptic History

Isaac developed from a vigorous tropical wave that emerged from west Africa on 16 August. The wave was accompanied by a rather broad cyclonic circulation that extended horizontally for several hundred miles as indicated by satellite, scatterometer, and microwave data. A broad area of low of pressure developed in association with the tropical wave on 17 August as it moved slowly westward. The wave showed signs of organization beginning on 18 August but strong easterly shear initially prevented development. The circulation began to become better defined the next day as the wave continued to move westward but convection remained minimal. The system slowly organized as the vertical shear decreased, and based on the exhibition of the convective cloud pattern, the low is estimated to have become a tropical depression near 0600 UTC 21 August while centered approximately 600 miles east of the Lesser Antilles. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added in the final rendition). Northwesterly shear produced by an upper low over the central Atlantic prevented the depression from strengthening immediately after formation, with visible satellite images during the day showing the center to be exposed along the northern edge of some rather disorganized convection. Nevertheless, the cyclone became a tropical storm near 1800 UTC 22 August.

The shear soon changed to westerly, which continued to halt Isaac's intensification. The cyclone circulation was broad and disorganized, with reconnaissance and scatterometer data indicating that winds near the center were relatively light, with all of the tropical storm force winds confined to the southwestern quadrant of the circulation in a band of deep convection over the Lesser Antilles prior to the storm entering the Caribbean Sea. Isaac remained quite disorgranized as it traversed the eastern and central Caribbean Sea, with reconnaissance and satellite observations suggesting that there were at least two competing areas of vorticity within the larger circulation. It appears that a combination of the unusually large size of the cyclone circulation combined with strong low-level shear caused the storm to struggle. Nevertheless, surface and reconnaissance observations indicate that Isaac maintained a well-defined, albeit broad, surface circulation during this time. Isaac began to become a little better organized late on 24 August as it passed to the south of Haiti, with satellite and microwave data suggesting a developing inner core. Based on these data and wind observations from a reconnaissance aircraft, Isaac is estimated to have strengthened to a 60-kt tropical storm around 0000 UTC 25 August. Possibly due to land interaction, the cyclone weakened slightly before it made landfall along the southeast peninsula of Haiti near Jacmel near 0700 UTC 25 August; maximum 1-minute sustained winds are estimated to be about 55 kt when Isaac made landfall in Haiti.

Isaac spent only a short time over Hispaniola, emerging into the Gulf of Gonave around 0900 UTC as a 45-kt tropical storm. Despite the relatively short duration overland, Isaac's inner core was severely disrupted by the mountains of Hispaniola, with microwave data subsequent to the cyclone moving offshore showing a complete lack of an eye, which had been previously observed by aircraft, microwave, and satellite data just several hours earlier. Approximately six hours later, Isaac made another landfall along the eastern coast of Cuba near Baracoa, still at 45 kt. Isaac moved back into the Atlantic waters just a couple hours later and strengthened slightly, back to 50 kt, although the southern portion of the cyclone remained largely overland as the storm tracked west-northwestward toward the Gulf of Mexico; along with moderate southwesterly shear and dry air over the western Caribbean Sea on the subsident side of a decaying trough over the western Atlantic and Isaac's cyclonic circulation, this combination apparently inhibited significant strengthening. Despite some brief attempts at inner core organization per aircraft and radar data from Casablanca and Camaguey, Cuba, Isaac generally changed little in strength as it passed through the Florida Straits over the next 24 hours. Isaac emerged into the extreme southeastern Gulf of Mexico near 0000 UTC 27 August after passing through the Florida Keys. Satellite, radar, and aircraft data indicate that Isaac began to turn northwestward later that day, likely in response to an upstream trough that was developing over the southern United States.

The central pressure steadily fell during the following couple of days leading up to landfall, but Isaac's winds only very gradually increased during this time. It appears that the extremely large size of the circulation, which spanned over 500 miles, inhibited strengthening even as Isaac entered a more favorable upper-level environment in the Gulf of Mexico; in particular, the outer wind maxima was continually expanding as Isaac approached the coast and while the central pressure was falling, a situation that has not been shown to be particularly conducive for hurricane intensification in the past (i.e. Hurricane Ike in 2008, and, more recently, Hurricane Irene in 2011). In addition, analysis of the oceanic heat content in that region (not shown) suggest that Isaac missed the culmination of the Loop Current, which would have likely mitigated the large size of the storm and allowed it to intensify at a somewhat steadier rate. Also, water vapor images during the period suggested that the aforementioned dry airmass in the western Caribbean continued to linger, and eventually gave way to more dry air near the coast. Based on reconnaissance data, Isaac is assumed to have become a hurricane near 1200 UTC 28 August while centered roughly 75 miles southeast of the Mouth of the Mississippi River. As Isaac continued slowly toward the coast of southeastern Louisiana, it assumed its peak intensity of 70 kt near 1800 UTC 28 August; it is estimated that the lowest minimum pressure of 965 mb was attained several hours later, just prior to Isaac's second and final Gulf Coast landfall.

Isaac made its first United States landfall along the extreme lower end of Plaquemines Parish near the Southwest Pass of the Mississippi River near 2345 UTC 28 August with winds of 70 kt. Isaac spent only a brief period of time overland, and because that particular strip of land is rather swampy and narrow, Isaac did not weaken; in fact, the satellite and radar signature actually became better defined subsequent to this landfall; Hurricane Danny in July of 1997 had a similar evolution when passing through this area. After about 0230 UTC 29 August, the convective pattern abruptly eroded, likely due to entrainment of dry air from Texas as well as coastal upwelling caused by the slow movement of the hurricane. Satellite and radar data indicate that Isaac made a second and final landfall along the Louisiana Gulf Coast just west of Port Fourchon at approximately 0800 UTC 29 August, with winds still estimated to be about 70 kt. As Isaac continued inland, it weakened to a tropical storm near 1800 UTC that day while centered about 20 miles northwest of Houma. Isaac dropped to tropical depression status around 0000 UTC 31 August while moving into southern Arkansas. Six hours later, satellite images suggested that Isaac was quickly becoming a remnant low as it accelerated northward toward southern Missouri. The remnant low completely lost its identity early on 1 September. Operationally, it was thought that Isaac's mid-level remnants were the primary focus for an area of disturbed weather that formed across the central Gulf of Mexico several days later; posthumously, however, it was decided that Isaac's primary remnant continued northeast in the prevailing mid-latitude westerlies, with perhaps only a small fragment of the residual energy eventually reemerging into the Gulf of Mexico. This low would ultimately become inconsequential, and did not develop into a tropical cyclone.


b.Meteorological Statistics

General Observations

Observations in Hurricane Isaac (Figs 2 and 3) include the Dvorak-based satellite intensity technique. A total of 34 aircraft missions were flown into Isaac, providing crucial data in assessing the structure and intensity of the tropical cyclone. These missions were particularly helpful when Isaac was traversing the eastern Caribbean Sea, since subsequent flight data indicated that the center had reformed farther to the south as the storm approached Haiti. In addition, several coastal radars, both in the Caribbean and the United States, were used to track Isaac. These include the Martinique, San Juan, Gran Piedra, Camaguey, and Casablanca radars in the Caribbean. Coastal radars in the United States include the Key West, Mobile, and New Orleans radars. Various microwave data from the Navy Research Laboratory (NRL) website were also useful in depicting the structure and location of Isaac.

Isaac's peak intensity of 70 kt is estimated to have occurred around 1800 UTC 29 August, and is based on a slew of data, including the New Orleans radar, reconnaissance data, and satellite imagery. It should be noted that the aircraft reported flight-level winds late on 28 August generally supported surface winds a little higher than 70 kt, but these were conflicted by nearby surface observations. It is likely that because the convection was focused in primary bands rather than a definite eyewall, attributable to the large size of the hurricane, there was not enough momentum to bring the strongest winds down to the surface from flight-level. Aircraft reports indicate that the central pressure continued to fall after Isaac moved back into the Gulf of Mexico subsequent to its first landfall, but the data also indicate that the maximum winds were not increasing during this process. So while Isaac reached its lowest minimum pressure of 965 mb a few hours later, the maximum winds are not assumed to have increased from the assigned peak intensity of 70 kt at 1800 UTC 28 August.

Isaac is analyzed to have strengthened into a tropical storm near 1800 UTC 22 August based on SFMR data from an aircraft mission, which generally measured believable surface winds of at least 35 kt. In addition, ASCAT data during that time showed similar evidence. It should be noted that at the time of upgrade as denoted in post-season analysis, Isaac's cloud pattern was rather disorganized, as northwesterly shear was keeping the heaviest precipitation (and thus the strongest winds) downstream -- or south -- of the analyzed low-level center. This was due to an upper tropospheric vortex over the central Atlantic.

While Isaac was forecast to strengthen to a hurricane throughout virtually every single forecast, especially across the Caribbean, this did not occur until the cyclone had moved into the northern Gulf of Mexico and neared the Louisiana coast. Isaac is assumed to have -- finally -- reached hurricane status at approximately 1200 UTC 28 August, when the first indication of hurricane-force winds were found from a reconnaissance aircraft. It should be noted that both the central pressure and reported flight-level winds were consistent with hurricane status about 12-18 hours prior to the time Isaac is estimated to have reached hurricane status in the northern Gulf of Mexico. It appears that the lack of an inner core and the large outer core, which was expanding as the hurricane approached the coast, prevented significant strengthening. While unusual, similar evolutions have occurred with landfalling hurricanes in the past in regards to the winds lagging behind the central pressure (i..e. hurricanes Isidore and Ike in 2002 and 2008, respectively). The mechanisms by which this occurs is still unclear, but is likely related to a broad and uneven distribution of the synoptic pressure fields. It should be noted that this is a more common occurrence in the Western North Pacific, where the winds found in a typhoon are frequently lower than the central pressure would suggest; given the vast expanse of seemingly inexhaustible oceanic heat content in that basin, this can probably be attributed to a broader pressure gradient, as alluded to above.


Wind and Pressure

Isaac produced tropical storm force winds over portions of the Lesser Antilles when it moved through that area as a tropical storm. The highest sustained winds reported in the archipelago was 41 kt gusting to 48 kt on Desirade; this occurred around 0700 UTC 23 August. Tropical storm force winds, especially in gusts, spread elsewhere across the Lesser Antilles.

Reliable surface observations along the coast of Haiti are difficult to come by given the dismal economy of that nation which generally inhibits the operation of observation sites or local weather offices with relevant equipment. A wind gust of 27 kt was reported at Port-au-Prince on 24 August at 2000 UTC local time; this was about six hours before Isaac made landfall along the southeast coast of that country.

A sustained wind of 39 kt with a gust to 53 kt occurred at the Guantanamo Naval Air Station (NAS) when Isaac made landfall in eastern Cuba later on 25 August. Northwesterly winds at this site suggest that Isaac made landfall just to the east; winds later veered around to the southwest as the center of Isaac moved off toward the west. The aforementioned sustained wind/gust combination occurred at 2225 UTC in a convective band as Isaac moved toward the Florida Straits.

A sustained wind of 33 kt with a gust to 46 kt was reported in Key West on as Isaac moved through on 26 August. Surface observations at the Key West airport suggest that wind gusts at or near tropical storm strength lashed the city until around 1900 UTC 27 August, by which time Isaac was trudging closer to Louisiana.

Hurricane-force winds were reported along and inland from the southeast coast of Louisiana, with the highest sustained wind report being 65 kt at the Southwest Pass of the Mississippi River. A sustained wind of 58 kt was reported in New Orleans, while a sustained wind of 51 kt occurred at Baton Rouge. It should be noted that Isaac had an unusually large capability to bring down strong wind gusts from the lower and middle troposphere to the surface in strong convective downdrafts, with numerous sites and stations offshore, along the coast, and inland across southeast Louisiana reporting wind gusts to hurricane force, even long after the hurricane had moved inland.

A pressure of 1002 mb was recorded at Port-au-Prince and Guantanamo Bay; in both instances, however, the center of Isaac was west or east of the respective locations, giving credence to lower central pressures in areas closer to the center in both cases.

Select surface observations for land and buoys are given in Table 3 (to be added in the final rendition).

Storm surge

According to island reports, storm surges were minimal across the Lesser Antilles and Puerto Rico, the highest being a report of 1.23 water rise in Puerto Rico. Storm surge estimates are not available for Haiti or Cuba.

Along the west coast of Florida, storm surges were generally between 1 to 3 feet. Inundations of about 3 feet occurred along the panhandle, with Pensacola reporting a maximum storm surge of 3.47 feet; this was the highest reported in the state of Florida.

Primarily due to the large size of the hurricane, Isaac produced devastating storm surges across much of the northern Gulf Coast; a NOS tide gauge at Shell Beach, Louisiana located along the southern end of Lake Borgne reported a maximum storm surge of 11.03 feet. Additionally, a storm surge of 6.35 ft was observed at the New Orleans New Canal Station along the southern shore of Lake Pontchartrain. The storm surge produced considerable flooding across sections of Plaquemines Parish, with estimates as high as 17 feet in some areas. While the highest water levels associated with Isaac stayed away from populated areas, it is presumed that storm surges of up to about 10 feet inundated certain residential areas within the parish. There was also extensive flooding in unprotected areas of Orleans, St. Bernand, and St. Tammany parishes. In St. Bernard Parish alone, Isaac produced water level rises of 8 to 12 feet outside the federal levee system based on tide gauge surveys from several USGS tide sensors in Wood Lake, Delacroix, and Alluvial City. Across eastern sections of Orleans Parish near Rigolets and Lake St. Catherine, storm surge estimates are in the 4 to 8 feet range based on further tide surveys and high water marks. Finally, Isaac is assumed to have produced a total storm surge of 4 to 7 ft across sections of the northeastern shore of Lake Pontchartain. Water from the lake was able to permeate inland, flooding portions of Slidell and other nearby communities. Storm surge from the western portion of Lake Pontchartrain produced 1 to 3 feet of surge in Laplace as indicated by USGS tide gauges in the area.

Isaac also produced widespread river flooding, and the USGS reported that the hurricane actually forced the Mississippi River to flow backward for almost 24 hours. The river rose as much as 8 feet in Baton Rouge.

Mississippi and Alabama did not escape water level rises from Isaac. Mississippi saw the highest water rises between the two states, with USGS tide gauges indicating water level rises between 5 and 8 ft as occurring in Hancock and Harrison Counties; additionally, it appears based on areal high water marks that the surge moved as far inland as north of Interstate 10. Surge values exceeded 6 feet farther east in Jackson County. In Alabama, a maximum storm surge estimated at 4.5 ft was observed at Dauphin Island, which lies offshore from the mainland. Areal sensors on the mainland suggest that storm surges of 1 to 3 feet were experienced along Mobile Bay. A maximum storm surge value of 8.00 ft was recorded by a NOS tide gauge along the coast of Mississippi at the Bay Waveland Yacht Club. A storm surge of 4.63 feet occurred in Mobile, Alabama, at the Coast Guard Sector Mobile Facility.

Select storm surge observations are given in Table 4 (to be added in the final rendition).

Rainfall

While it was moving through the Caribbean Sea as a tropical storm, Isaac produced heavy rainfall across Puerto Rico and Hispaniola, with a maximum storm total of 9.28 inches reported near Ponce. 16.18 inches of rain was reported at Barahona in the Dominican Republic. As of this writing, there have been no reliable rainfall totals received from Haiti. Isaac produced torrential rains across eastern Cuba as well, with 14.08 inches reported at San Antonio del Sur, Guantanamo. Finally, a total of 10.80 inches was reported in El Plátano in the Granma province.

In addition to the aforementioned, Isaac produced considerable rainfall along much of Florida and the northern Gulf Coast due to its slow movement around a blocking high pressure system over the central United States. In Florida, Isaac produced severe flooding in Palm Beach County, with certain neighborhoods in Wellington, Royal Palm Beach, Loxahatchee, and Acreage being isolated due to several feet of water. The highest reported storm rainfall total in the state of Florida from Isaac was 15.86 inches at the Lion County Safari in Loxahatchee. Significant rainfall totals occurred elsewhere across the state, and totals of 10 inches or more were not uncommon across southeast Florida in a narrow band east of Lake Okeechobee. This band included the counties of Martin, St. Lucie, Indian River, and the extreme eastern end of Okeechobee County.

Isaac produced considerable rainfall and flooding over the northern Gulf Coast due to its prolonged slow movement. The highest total reported in Alabama from Isaac was 13.99 inches at Grand Bay located in Mobile County, 22.20 in Pasacagoula, and finally, in Louisiana, the highest was 20.66 in New Orleans. The torrential rainfall caused local river rises, record in some cases. A river crest of 8.5 feet was observed near Gulfport, Mississippi, while a river crest of 4.4 ft was noted near Landon. In addition, Isaac's heavy rains contributed to a 6.5 ft river crest at the East Hobolochitto Creek near Caesar, Louisiana. Major flooding also occurred along the Tangipahoa River in Louisiana near Robert, plateauing at 9.0 ft above flood stage, and 14.8 ft at the Bogue Chitto River near Tylertown, Mississippi.

Tornadoes

As is typical for landfalling hurricanes, there were some tornadoes associated with Isaac. A total of 26 tornadoes appear to have been spawned by Isaac, 17 of which occurred while Isaac was a tropical cyclone. While most of these were relatively weak and short-lived, there was a report of two EF2s. Five tornadoes touched down in Florida, all of which were rated EF0. Four tornadoes occurred in Alabama, the strongest being an EF1 that struck near the city of Samson. Six occurred in Mississippi, with an EF2 touching down in Pascagoula; this tornado almost completely removed the roof from a residence, and it also downed large trees. An EF1 tornado that was generated near Skyes, Mississippi injured three people, and had a path length of 6.1 miles. Isaac produced two additional tornadoes across Arkansas, Missouri, and Illinois on 1 September, after the system had dissipated as a tropical cyclone; one of the tornadoes injured a person near the village of Franklin, Illinois. An EF2 tornado produced considerable damage to two hangars and some aircraft at the airport in Corning, Arkansas.


c. Casualty and Damage Statistics

Isaac is blamed directly for 35 deaths across the Caribbean and the United States. It is difficult to distinguish which deaths were direct or indirect in Haiti due to conflicting reports from various agencies within that country. A 51-year old woman was killed in the coastal town of Marigot along the southern coast when a tree fell on her home, while a 10-year old girl was killed in the village of Thomazeau when a wall collapsed on her. Isaac produced five direct deaths in the Dominican Republic, two of which can be attributed to two adult males being swept away in two different river flood events.

Six direct deaths were caused by the hurricane in the United States, two of which occurred in Mississippi. A 62-year old woman in Lexington, and a 52-year old man in Picayune were killed by high winds that toppled trees onto their cars. A 75-year old man was killed in Slidell, Louisiana when he drove his car off an on-ramp on Interstate 10 and into a ditch that was flooded with up to 9 ft of water. In Vermilion Parish, a 36-year old man was killed when he fell 18 feet from a tree while attempting to help his friends move a vehicle before the storm. There were reports of a man dying in a restaurant fire, and a couple drowned in their home in Braithwaite.

Some minor damage occurred in the Caribbean, primarily associated with heavy rains. The rains caused flooding and landslides that caused damage to several homes in Trinidad and Tobago. Heavy rains caused damage in Puerto Rico, where mudslides caused by the heavy rains caused mudslides that caused some road closures and washed out a bridge in the southern part of the island.

In the Dominican Republic, nearly 13,000 people evacuated their homes, while an estimated 864 homes were damaged. A total of 90 towns were isolated by the hurricane due to flooding produced by the hurricane. Roughly 10% of the banana and coconut crops were damaged on the island, and total damage is estimated to be about $30 million (USD).

The United Nations Office for the Coordination of Humanitarian Affairs (OCHA) reported that Isaac killed 24 people across Haiti, and caused 42 injuries and 3 missing persons. Isaac produced storm surge across the southern half of the peninsula from Jacmel to Port-au-Prince. The hurricane destroyed about 6,000 shelters in shelter camps set up after the January 2010 earthquake that struck the region. In addition, Isaac destroyed about 1,000 homes in the country.

In Cuba, nearly 50,000 people evacuated. Isaac destroyed 4 homes and damaged 19 in Baracoa, and 72 homes were flooded by the Sagua de Tánamo River in Holguín province. Strong winds blew down power lines and caused electrical disruptions, although the extent of this damage is not known. Isaac also damaged crops in the region, including the cocoa and coconut crops, spanning over 1,100 acres.

Isaac caused about $970 million in insured damage across the United States according to insurance estimates, with total uninsured losses presumed to be about $2.35 billion. Minor to moderate flooding and some power outages were experienced in Florida, but the majority of damage came from Isaac's impact to the northern Gulf Coast. Isaac produced considerable storm surge flooding across the northern Gulf Coast, particularly in Louisiana. For the first time, the Hurricane and Storm Surge Risk Reduction (HSDRRS), which was implemented subsequent to the widespread economic damage wrought by hurricanes Katrina and Rita in 2005 was tested for the first time by a serious hurricane. The project proved to be successful in protecting most areas within the system, and storm surge flooding appears to have occurred primarily in areas outside the protection of the federal levee system. Around 59,000 homes were damaged by the hurricane. Although the majority of houses that were damaged were located in Jefferson Parish, the most extensive damage occurred in St. John the Baptist and Plaquemines Parish. Severe flooding from storm surge was observed in Plaquemines Parish, where a non-federal levee situated on the east bank was overtopped by storm surge; the resulting flooding inundated areas within the levee system from Braithwaite southward to White Ditch. The force of the water was strong enough in Braithwaite to pick up cars, homes, and various debris and plant it atop Highway 93 and onto the Mississippi River levee. Over 100 people had to be rescued from flooded homes and rooftops within the community. Later on, city officials intentionally breached the non-federal levee to allow water to escape from the town. The city of Laplace in St. John the Baptist Parish experienced storm surge from nearby Lake Pontchartrain, which caused nearly 4,000 people to be rescued, and also a closure of Interstate 10. 1,000 people had to be rescued in Lafourche Parish due to storm surge that overtopped the Bayou Boeuf levee. Additional flooding occurred elsewhere within adjacent parishes, both from rainfall and primarily storm surge.

A total of 901,000 homes and businesses lost power in Louisiana, which is approximately 47% of all customers in the state. Isaac caused considerable crop damage, with about 90% of the sugarcane crop destroyed by the hurricane. In addition, there were reports from state officials that about 565,000 pounds of oily debris from the Deepwater Horizon disaster site were brought to the surface and deposited on the coast.

Damage also occurred in sections of Mississippi and Alabama. Around 6,000 homes in south Mississippi appear to have sustained some degree of damage according to the state governor, which resulted in about 70 road closures. Storm surge from Isaac deposited 4 feet of sand on the western end of Dauphin Island, but overall damage in the state was minor. Isaac unearthed the shipwreck of The Rachael, a vessel from the early 20th century that ran aground in Gulf Shores, Alabama, purportedly due to rough seas in 1930.

Isaac also caused damage across portions of Arkansas. Numerous streets, homes, and other structures were flooded in Pine Bluff. Heavy rains from the once powerful Isaac caused damage to crops in the state, including the soybean, cotton, corn, and rice. Isaac's winds also caused some power outages in the state, and around 20,000 customers lost power during the storm. Total damage in the state is estimated to be about $30.5 million (USD).


d. Forecast and Warning Critique

The formation of Isaac was anticipated very well. The tropical wave that would eventually give birth to Isaac was first mentioned in a Tropical Weather Outlook that was issued early on 18 August, about three days prior to formation. At the time, the system was assigned a low chance (less than 30%) of developing into a tropical cyclone within 48 hr. Probabilities were increased to "high" (exceeding 50%) early on 19 August, and remained in this category until genesis. In fact, genesis probabilities reached 100% at 0600 UTC 21 August, the time of genesis as denoted in post-season analysis. This type of strong wording is extremely rare in developing systems.

Isaac proved extremely difficult to track. While track forecasts up to the 96 hr mark were generally good, the 120 hr forecasts proved most challenging. Isaac was consistently forecast to strike well to the east of the Mississippi River Delta; this was due to, in a similar situation to Tropical Storm Debby three months earlier, there was a notable contrast between the two most reliable global computer model systems: the GFS and the ECMWF regarding Isaac's ultimate trajectory. The former continually anticipated that Isaac would make landfall in Florida somewhere between the Big Bend and Pensacola, while the ECMWF was more insistent on a landfall along the northern and central Gulf Coast, generally in Mississippi or Alabama. As it turns out, the GFS gradually shifted westward with time, joining the ECMWF's initial prognostications of a Louisiana landfall. As this happened, confidence increased within official forecasts regarding the threat to the northern Gulf Coast. Isaac was forecast to make landfall along the Louisiana coast about 48 hours before landfall actually occurred; this gave satisfactory time for hurricane watches and warnings.

Intensity forecasts fared a little better, although there was still a notable high bias, particularly in the early forecasts, which generally showed Isaac becoming a hurricane over the Caribbean Sea. In addition, although it was correctly forecast that Isaac would become a hurricane over the Gulf of Mexico, the timing and onset of this process was difficult to predict. In particular, the rate of intensification over the Gulf of Mexico was overestimated; in fact, a forecast that was issued around 0600 UTC 26 August, two days before landfall, showed Isaac becoming a 100 kt major hurricane and making landfall along the extreme western Florida panhandle. This was due, in part, to model prognostications, especially from the GFDL and HWRF, of a large and powerful hurricane striking the United States Gulf Coast. It appears that these models and the official forecasts failed to fully appreciate the negative role that the enormous size of the Isaac vortex would play in regards to intensification. This is still a subject of considerable research in the meteorological community.

Watches and warnings associated with Isaac are given in Table 5 (to be added in the final rendition).






Figure 3. Visible satellite image of Hurricane Isaac at peak intensity at 1800 UTC 28 August. It should be noted that the maximum winds were attained several hours prior to the lowest central pressure.






Figure 4. Rainfall totals from Hurricane Isaac beginning 25 August and ending 3 September, 2012. Image courtesy of the Hydrometeorological Prediction Center (HPC).




Acknowledgments

The National Hurricane Center (NHC) provided a fair bit of the data detailing the damage and fatalities associated with Isaac, especially across the Caribbean. Data from the National Data Buoy Center (NDBC) was used to compile the marine observations provided in Table 3 (to be added in the final rendition). The rainfall data given in Figure 4 was provided by the Hydrometeorological Prediction Center (HPC).

2012 Atlantic hurricane season

Updated: 6:41 AM GMT on February 12, 2013

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