KoritheMan's WunderBlog

Tropical Cyclone Report (TCR); Tropical Storm Aletta

By: KoritheMan, 2:47 PM GMT on March 19, 2013

Tropical Cyclone Report
Tropical Storm Aletta
(EP012012)
14 May-19 May 2012

Aletta was a rare out of season tropical storm that formed about a day before the official start of the eastern North Pacific hurricane season. Since reliable satellite measurements were taken in 1966, there have only been two other documented tropical storms that formed before the official start of the season.


a. Synoptic History


As is typical for most early season tropical cyclones in the eastern North Pacific, the exact system from which Aletta originated is difficult to determine. Satellite, scatterometer, and lower- to middle- tropospheric vorticity analysis from University of Wisconsin CIMSS suggest that a tropical wave may have crossed the coast of west Africa on 1 May. This wave was very poorly-defined on satellite pictures and did not generate deep convection across its journey through the Atlantic. There were some indications in surface observations from Trinidad and Tobago of a poorly-defined cyclonic wind shift on 8 May that could have been related to this wave; however, the available data in the area is inconclusive since no marked pressure drop occurred accompanying the passage of the purported wave. The Intertropical Convergence Zone (ITCZ) was very active over the eastern North Pacific during the second week of May, and three separate disturbances developed within the gyre. The central disturbance, which was located about 1000 miles southeast of the southern tip of Baja California, became the dominant system and began to acquire central banding features on 11 May. The first Dvorak classifications were initiated at this time. Meanwhile, the tropical wave entered the basin early the next day, and may have contributed to the formation of a broad surface low within the central gyre later that day. The disturbance moved slowly to the west-northwest, then turned west as a weak mid-level trough over northern Mexico moved off to the east. Convection began increasing around the center beginning near 0600 UTC 14 May, and the low became a tropical depression near 1800 UTC that day while located about 500 miles south-southwest of Acapulco, Mexico. 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). The best track positions and intensities are listed in Table 1 (to be added). The cyclone became a tropical storm about 6 h later.

Aletta strengthened slightly to reach a peak intensity of 40 kt near 1200 UTC 15 May, when the center appeared to be embedded in a small area of deep convection that was surrounded by a well-defined curved band to the east. Easterly shear increased as Aletta began to execute a sharp clockwise loop, and it weakened to a tropical depression around 0000 UTC 17 May. Aletta generated several bursts of convection over the next couple days, at times garnering satellite classifications that were of tropical storm strength. The bursts were transient enough that the cyclone was unable to regain that strength. Overcome by southerly to southwesterly shear, Aletta's circulation continued to wind down, and it degenerated into a non-convective remnant low pressure area around 0000 UTC 19 May, centered about 650 miles south of the southern tip of Baja California. Aletta's remnants continued southeastward into an area of very strong southeasterly upper-level winds and quickly lost their identity by 20 May.


b. Meteorological Statistics

Observations in Aletta (Figs 2 and 3, to be added) include the satellite-based Dvorak intensity technique from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the University of Wisconsin CIMSS' Advanced Dvorak Technique (ADT). Various microwave units, along with scatterometer data, were also useful in tracking the center of Aletta.

The peak intensity of Aletta is estimated to be 40 kt and occurred at 1200 UTC 15 May. This was based on Dvorak estimates which supported an intensity from 35 to 45 kt during that time.


c. Casualty and Damage Statistics

No fatalities or damages were reported with Aletta.


d. Forecast and Warning Critique

The formation of Aletta was well anticipated in off-season Special Tropical Weather Outlooks (STWOs). The precursor disturbance was mentioned on 0000 UTC 12 May and given a "low" chance of development. About 30 hours later, genesis probabilities reached the "high" category, with explicit mention of tropical cyclone formation during that time.

Average track and intensity forecasts associated with Aletta were generally small, with intensification into a low to moderate tropical storm expected, which is concurrent with what occurred. Aletta's small clockwise loop prior to dissipation was also well predicted, largely due to the uniformity of the global models in recognizing it.






Infrared satellite image of Aletta at peak intensity.

2012 East Pacific hurricane season

Updated: 2:50 PM GMT on March 19, 2013

Permalink

Tropical Cyclone Report (TCR); Tropical Storm Tony

By: KoritheMan, 2:45 PM GMT on March 19, 2013

Tropical Cyclone Report
Tropical Storm Tony
(AL192012)
22 October-25 October 2012

Tony was a short-lived tropical storm that formed over the central Atlantic about midway between the northern Leeward Islands and the Azores. Tony had no impact on land.


a. Synoptic History

Tony developed from a tropical wave that moved off the coast of Africa on 11 October. The southern portion of the wave moved westward and spawned Hurricane Sandy in the Caribbean Sea. The wave was ill-defined and not easily trackable in satellite images. At the same time, a sharp and very well-defined upper cold low and accompanying mid- to upper-level trough extending its cyclonic vorticity down to below 500 mb was in existence over the eastern Atlantic between the Cape Verde Islands and the Lesser Antilles. This feature moved westward roughly in tandem with the tropical wave for the next several days. The northern portion of the wave fractured and began to move west-northwest on 13 October. The interaction of these two features resulted in a large area of disturbed weather over the eastern Atlantic. The northern portion of the tropical wave moved very slowly over the next several days while continuing to move in tandem with the upper-level system. The latter system weakened and slowly became more elongated during this time, likely due to a loss of upper air support and warm air advection associated with the tropical wave. The tropical wave became the dominant feature, and a broad surface low formed in connection with it on 19 October. The trough continued to weaken and move westward, which resulted in a less hostile environment. At the same time, another upper-level trough was in the process of undergoing amplification over the western Atlantic, which caused the budding disturbance to turn toward the northwest toward a weakness in the ridge. Concentrated bands of convection persisted around the center, and, within an environment of relatively light vertical wind shear, the low became a tropical depression around 1800 UTC 22 October while centered midway between the northern Leeward Islands and 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). The best track positions and intensities are listed in Table 1 (to be added). The depression initially failed to strengthen, possibly due to a relatively dry environment. A burst of deep convection occurred over the low-level center around 1200 UTC the next day, and it is estimated that the cyclone reached tropical storm strength at that time.

Tony quickly turned northward and then northeastward as southwesterly mid-level flow associated with an upper trough to the west began to impinge on the system. The cyclone gradually strengthened, reaching its peak intensity of 50 kt near 0600 UTC 24 October. Available microwave data suggested a mid-level eye feature at that time. The satellite presentation and convective pattern began to degrade after 1200 UTC as Tony entered an environment of increasing southwesterly shear and cooler waters, but winds were slow to decrease even though the associated convection was relatively shallow. This could have been due to a strong pressure gradient between Tony and the Bermuda-Azores ridge, which is undulatingly stronger in this region of the Atlantic. Its circulation elongated east-northeast to south-southwest, Tony became an extratropical remnant low pressure system near 1800 UTC 25 October while located about 600 miles southwest of the Azores.


b. Meteorological Statistics

Observations in Tony (Figs 2 and 3, to be added) include the satellite-based Dvorak technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). Objective ADT estimates from the University of Wisconsin CIMSS was also used. Scatterometer and microwave data were useful in determining when Tony's circulation had become distorted on 25 October.

Tony's peak intensity of 50 kt is a little higher than the Dvorak consensus (although CI numbers using the CIMSS ADT were as high as 55 kt during the time of peak intensity)., and is based primarily on several microwave passes over the cyclone from about 0300 UTC to 0900 UTC showing a well-defined mid-level eye feature, which is normally characteristic of strong tropical storms. The general Dvorak consensus was at 3.0 during that time, which translates to 45 kt, so this report isn't in too large a conflict with the available satellite estimates. An image of the closest microwave pass to peak intensity is shown below in Figure 5.


c. Casualty and Damage Statistics

Since Tony was a marine interest, there were no reports of damage or fatalities.


d. Forecast and Warning Critique


The genesis of Tony was not very well predicted. The precursor disturbance was first mentioned around 0000 UTC 21 October and given a "medium" chance (30%) of development. This was about 36 hours prior to genesis. Genesis probabilities increased to 40% about 24 hours later, or about less than a day prior to formation.

Average track forecast errors with Tony were fairly small, although it was a little too short-lived to garner a meaningful forecast evaluation. Even prior to formation, the global models were in excellent agreement on the future path of Tony, which was controlled primarily by an upper-level trough over the central Atlantic.

Intensity forecast errors were fairly small as well, although the timing of Tony's peak intensity was off by about a day.

No watches or warnings were issued with Tony since it remained far away from land.






Infrared satellite image of Tony at peak intensity.







AMSUB microwave pass of Tony showing a well-defined mid-level eye feature. The pass was taken at 0446 UTC 24 October, right around the time Tony is assumed to have reached its peak intensity.

2012 Atlantic hurricane season

Updated: 4:49 PM GMT on March 19, 2013

Permalink

Tropical Cyclone Report (TCR): Hurricane Rafael

By: KoritheMan, 5:31 AM GMT on March 15, 2013

Tropical Cyclone Report
Hurricane Rafael
(AL172012)
12 October-17 October 2012

Rafael impacted portions of the Leeward Islands and Puerto Rico as a tropical storm. It became a category 1 hurricane (on the Saffir-Simpson Hurricane Scale) to the south of Bermuda before becoming extratropical.


a. Synoptic History


Satellite images suggest that Rafael developed from a tropical wave that entered the eastern Atlantic from Africa late on 4 October. The wave emerged from the coast at a rather southerly latitude, embedded within an active Intertropical Convergence Zone (ITCZ) over the eastern Atlantic. The wave generated a large area of disturbed weather over the next couple of days, and a broad area of low pressure formed along the tropical wave axis on 6 October while the system was located about 1000 miles east-southeast of the southern Windward Islands. A large upper-level trough was in the process of amplifying over the western Atlantic, which allowed the tropical wave to turn more toward the west-northwest over the ensuing several days. Westerly shear initially prevented development, but the circulation gradually became better defined. The system entered the eastern Caribbean Sea on 12 October, spreading heavy rain and strong gusty winds. Based on scatterometer, buoy, and local island reports indicating the existence of tropical storm force winds within the now closed surface circulation, the designation of the system as a tropical cyclone near 1800 UTC that day begins immediately as a tropical storm, centered about 250 miles southeast of San Juan, Puerto Rico. 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). Rafael initially consisted of a rather broad circulation as denoted by local island and drifting buoy reports, and these reports suggest that there was little wind near the center, likely due to continued southwesterly shear induced a longwave trough over the western Atlantic. The disorganized structure of the tropical cyclone was confirmed by subsequent reconnaissance observations which showed the strongest winds in bands east of the center; also, microwave data around the time of formation indicated that Rafael's mid- and upper-level circulations were displaced by about 15 to 20 nautical miles due to the persistent upper tropospheric shear pattern.

Rafael moved slowly northwest and north between the British Virgin Islands and the island of Antigua as it gradually came under the influence of the mid- to upper-level trough, which had now moved into the central Atlantic. Shear over Rafael was slow to decrease, and the cyclone only slowly intensified over the next couple of days. The center appeared to reform a little to the north on 14 October in a region where vertical shear was less. The shear began to decrease more steadily early the next day, and Rafael responded by strengthening at a quicker pace. A small eye began to develop in microwave imagery around 0600 UTC that same day and made occasional attempts to manifest in conventional satellite images as well. Based on analysis of aircraft and satellite data, Rafael is estimated to have become a hurricane near 1200 UTC 15 October while centered about 500 miles south of Bermuda. Additional strengthening occurred under relatively light shear, and Rafael reached its peak intensity of 80 kt near 0215 UTC 16 October. The hurricane began to accelerate north-northeastward after that as it became captured by the approaching frontal trough. It also began to weaken as southwesterly shear returned and the circulation encountered cooler waters, with satellite estimates indicating an intensity below hurricane strength was achieved by 0000 UTC 17 October, at which time Rafael was located about 100 miles east of Bermuda. AMSU temperaure data showed a gradual deterioraton of the warm core associated with the cyclone which started in the upper troposphere. Rafael became extratropical by 1200 UTC that day while centered about 700 miles southwest of Cape Race, Newfoundland.

Post-tropical Rafael remained a potent extratropical gale center for over a week before moving inland over western Europe.


b. Meteorological Statistics

Observations in Rafael (Figs 2 and 3, to be added in the final rendition) include the satellite-based Dvorak technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). Objective ADT estimates from the University of Wisconsin CIMSS were also used to help assess the intensity of Rafael during its life. As is typically the case, microwave and scatterometer data were also helpful in tracking the hurricane. Several buoy reports were helpful in determining the tropical storm force wind radii with Rafael, as well as for determining the evolution and gradual tightening of the surface circulation during the early stages of the cyclone. Additionally, since Rafael formed west of 50W, it was within the accepted flight range of reconnaissance aircraft observations. These observations were naturally very useful in structuring the intensity of the hurricane. Several reports from the Lesser Antilles and Puerto Rico were also useful in tracking Rafael.

The timing of Rafael's peak intensity is somewhat difficult to assess. The satellite signature looked to be at its best around 0215 UTC 16 October, which is when this report assigns the peak intensity of 80 kt to Rafael. However, subsequent AMSUB microwave data just after 0600 UTC showed a better defined eyewall structure, where it was more banded during the time of peak intensity. In both cases, however, satellite images contradicted the microwave data, with Rafael seemingly looking better closer to 0000 UTC than 0600 UTC, when the microwave data showed a better internal structure and more definition to the eye. While microwave data has a well-documented capacity to see the inner core structure of tropical cyclones that conventional satellite data lacks, it is difficult to ignore the deteriorating cloud pattern that occurred in satellite pictures during the time of the AMSUB pass, where the central dense overcast appeared to have become a little distorted; additionally, upper-level outflow was being inhibited in the southwest quadrant as southwesterly shear began to increase around that time. Thus, the final best track of Rafael shows the hurricane assuming its peak intensity closer to 0000 UTC. It should be noted that the CIMSS ADT estimates had a high bias for Rafael during this period, with the Adjusted T Number climbing to 5.5 around 0600 UTC 16 October, which is normally considered characteristic of a lower end category 3 hurricane, perhaps due to an incorrect scene type that could have identified a pinhole eye structure, of which there was little evidence. Rafael's peak intensity was based on a blend of aircraft data and satellite intensity estimates.

While Rafael produced widespread tropical storm force wind gusts across portions of the Lesser Antilles, there have been no reports of sustained winds across any of the islands. A sustained wind of 19 kt with a gust to 32 kt was reported at the V.C. Bird International Airport on Antigua at 1523 UTC local time 12 October. Sustained winds of 21 kt with a gust to 32 kt occurred at the airport in San Juan, Puerto Rico on that same day. Rafael also produced tropical storm force wind gusts on Bermuda as it brushed by to the east.

There were several buoy reports of wind gusts near tropical storm force associated with Rafael.

In addition to the wind, widespread heavy rainfall occurred in association with Rafael. According to Meteo France, up to 5.9 inches of rain fell in Basse-Terre on the island of Guadeloupe, with a maximum of up to 12 inches reported from 13 to 14 October; 7.9 inches fell in Grand-Terre during that same period. St. Kitts reported 11.87 inches of rain within a 30-hour period, which caused extensive flooding near Basseterre.


c. Casualty and Damage Statistics

According to media reports, Rafael killed one person on the island of Guadeloupe when a woman attempted to drive her vehicle through a flooded roadway.

Rainfall from Rafael caused some localized flooding across portions of the Lesser Antilles. The resulting flooding that sprung off from these rains cause extensive crop damage to vegetable crops on Guadeloupe. Strong thunderstorms produced by Rafael generated a lightning strike that caused fires and resulted in some power outages on the island.

Elsewhere, significant wave damage and overwash occurred across sections of the Avalon Peninsula in Newfoundland. According to locals in the region, the waves produced by Rafael were worse than those generated by Hurricane Igor in 2010, which was touted as Newfoundland's most damaging hurricane.


d. Forecast and Warning Critique

The genesis of Rafael was well anticipated in Tropical Weather Outlooks (TWOs). The precursor disturbance was mentioned around 0600 UTC 9 October, and was assigned a "low" (less than 30%) chance of becoming a tropical cyclone within 48 hr. Probabilities reached the "medium" (up to 50%) about 48 hours later. Genesis forecasts reached the "high" category (greater than 50%) about 12 hours prior to formation.

An objective evaluation of the track and intensity forecasts issued for Rafael show that the hurricane was a relatively easy storm to predict. In fact, average intensity errors were about 5 kt during most forecast periods, and other than an initial westward track that brought the center over the Avalon Peninsula at longer ranges, average track forecast errors were remarkably consistent with what actually occurred.

Watches and warnings associated with Rafael are given in Table 3 (to be added in the final rendition).






Infrared satellite image of Rafael at peak intensity.

2012 Atlantic hurricane season

Permalink

Tropical Cyclone Report (TCR): Tropical Storm Patty

By: KoritheMan, 5:28 AM GMT on March 15, 2013

Tropical Cyclone Report
Tropical Storm Patty
(AL162012)
11 October-13 October 2012

Patty was a short-lived tropical storm that formed near the central Bahamas. Patty had little impact on the islands.


a. Synoptic History


A surface trough was first noted on satellite pictures about 150 miles south of Bermuda early on 5 October. The exact origin of this trough is a bit unclear, but could have possibly been related to the large upper low which supported the frontal zone that absorbed Tropical Storm Oscar. Upper air data indicate that a rather strong mid-level ridge had built across the Bahamas, Florida, and the eastern Gulf of Mexico in the wake of the aforementioned trough. The small surface trough moved quickly southwestward steered primarily by this ridge. The disturbance became quasi-stationary near Crooked Island in the central Bahamas. At that time, there was an upper tropospheric cold low to the north of the Leeward Islands, which in combination with the western Atlantic high pressure area was apparently enough to inhibit much movement of the disturbance. A large mid-latitude trough amplified over the eastern United States, but was of relatively low amplitude and southward extent; consequently, the surface trough moved only slowly northward at speeds of about 5 kt. The trough quickly lifted out, leaving behind additional low- to mid-level ridging. As a result, the disturbance abruptly ceased its slow northward motion while convection became more persistent near the axis of the low pressure area. The low turned back to the southwest again under the influence of the ridge early on 10 October. A small upper low had developed over the eastern Gulf of Mexico and moved across the central Florida peninsula during the early morning hours of the 11th. This low contributed to persistent southwesterly shear over the disturbance and caused the low-level center to remain along the west side of the associated shower activity. Notwithstanding, the low maintained enough organized convection within 50 miles of the center to be designated a tropical depression near 0600 UTC that day while centered about 130 miles east-northeast of Eleuthera Island in the central Bahamas. 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 (to be added in the final rendition), respectively. The best track positions and intensities are listed in Table 1 (to be added in the final rendition). The depression became a tropical storm about six hours later.

Patty moved less than 75 miles from its systematic point of origin as it remained within an environment of weak steering lacking any appreciable synoptic forcing mechanisms. Patty reached its peak intensity of 40 kt near 0000 UTC 12 October. The cyclone dropped below tropical storm strength just after 1200 UTC that same day as strong vertical shear continued to remove the convection from the center. Other than a small episodic burst of relatively cold-topped convection that was quickly sheared away, Patty remained devoid of significant deep convection for about 18 hours, and it degenerated into a remnant low pressure system around 0600 UTC 13 October, when a low-level center could no longer be identified.


b. Meteorological Statistics

Observations in Patty (Figs 2 and 3, to be added  in the final rendition) include the satellite-based Dvorak technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). Objective ADT estimates from the University of Wisconsin CIMSS were also used. Various microwave instruments, most notably the AMSUB unit, were also useful in tracking the center of Patty, as was Advanced Scatterometer (ASCAT) data.

The peak intensity of Patty is estimated to be 40 kt, and is based on Dvorak estimates ranging from 2.5 to 3.0. Using a shear pattern, it is possible to assign a 40 kt peak intensity to Patty around 0000 UTC 12 October, when the satellite presentation and convective vigor was most prominent. It should be noted that the Dvorak estimates escalated back to the low end of tropical storm strength in conjunction with one final convective burst which occurred shortly before 0000 UTC 13 October. While this burst generated cloud tops of -70 to -80C at a distance relatively close to the cyclone center, convection had diminished several hours before the burst began. In addition, the burst only appeared to last for about three hours, and the convection associated with this burst was rather elongated. Thus, the final best track of the storm does not show Patty regaining tropical storm strength.


c. Casualty and Damage Statistics


Despite its close proximity to the Bahamas, there were no reports of casualty or damages, as strong shear kept most of the convection downstream of the center, and with it, all of the strong wind and heavy rains.


d. Forecast and Warning Critique


The formation of Patty was not anticipated. The incipient disturbance was first mentioned about 96 hours prior to genesis. At the time, the disturbance was given a "low" (10%) chance of becoming a tropical cyclone within 48 hours. Probabilities remained in that category until genesis occurred.

Since Patty was a short-lived cyclone, there is no way to verify the track and intensity forecasts issued for the storm.

No watches or warnings were required with Patty, as westerly shear kept all of the tropical storm force winds well east of the low-level center.






Infrared satellite image of Patty at peak intensity.

2012 Atlantic hurricane season

Permalink

Tropical Cyclone Report (TCR); Tropical Storm Oscar

By: KoritheMan, 12:08 AM GMT on March 12, 2013

Tropical Cyclone Report
Tropical Storm Oscar
(AL152012)
3 October-5 October 2012

Oscar was a short-lived tropical storm that formed unclimatologically over the eastern Atlantic via Cape Verde origin in early October. Oscar did not threaten land.


a. Synoptic History


Oscar developed from a poorly-defined tropical wave. It is difficult to distinguish the date of this wave's exit from the African coast, but based on a couple of surface observations from the Cape Verde Islands and extrapolation, it is estimated to have entered the eastern Atlantic waters on 27 September. Scatterometer data and surface observations showed that this wave produced a slight cyclonic wind shift in the vicinity of the Cape Verde Islands the next day, but there was little organization to the cloud pattern, and the associated convection was rather poorly-organized. Another tropical wave that moved off the coast of Africa on 28 September may have also played a role in the genesis of Oscar by merging with a preexisting area of cloudiness and showers generated by the other wave, although there was little evidence of this wave in the surface wind field. Regardless, satellite pictures showed that a rather large area of disturbed weather had formed a few hundred miles west of the Cape Verde Islands on 29 September. A broad area of low pressure developed in association with the large disturbance that day. The system was subsequently slow to organize, likely due to the unusually large size of the disturbance as well as moderate southwesterly upper-level shear formulated by a mid- to upper-level trough over the central Atlantic. The wave slowed and turned northward on 3 October. Based on satellite pictures, the wave is presumed to have developed into a tropical depression around 1800 UTC that day, centered about 800 miles west 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). At the time of formation, the low-level center of the depression was exposed well to the west of a rather vigorous cluster of deep convection due strong upper-level westerly winds. Based on an ASCAT pass indicating tropical storm force winds in the eastern portion of the circulation, the cyclone strengthened into a tropical storm around 0600 UTC 4 October.

Oscar steadily strengthened in an environment of strong westerly upper tropospheric vertical wind shear induced by an amplifying trough and associated cold front over the central Atlantic; this was due to the persistent convection to the east of the low-level center, which was likely enhanced by upper divergent flow associated with the eastern portion of the trough. Oscar turned northeast ahead of the cold front the next day and reached its peak intensity of 45 kt around 0200 UTC 5 October. All the while, the circulation was elongating, and Oscar became absorbed into the cold front near 1800 UTC that day.


b. Meteorological Statistics

Observations in Oscar (Figs 2 and 3, to be added in the final rendition) include the satellite-based Dvorak technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). In addition, objective ADT estimates from University of Wisconsin CIMSS (UW-CIMSS) were also used to ascertain the intensity of Oscar, especially in its early stages. Various microwave data was useful in tracking Oscar's center, as was certain scatterometer data.

The peak intensity assigned to Oscar at 0200 UTC 5 October is 45 kt, and was based primarily upon an ASCAT pass that morning showing a couple of believable 40 kt wind vectors in the convection to the east. Given the well-documented low bias of that particular instrument, it is reasonable to assume that slightly stronger winds were occurring within the circulation at that time. Oscar began to become distorted by the frontal zone shortly thereafter.


c. Casualty and Damage Statistics

There were no damage reports or fatalities associated with Oscar.


d. Forecast and Warning Critique

While Oscar was expected to develop, the precursor disturbance was not identified until 0000 UTC 2 October, at which time development probabilities were immediately assigned to the "high" category, and the disturbance was given a corresponding 60% chance of developing into a tropical cyclone within 48 hours. Genesis expectations reached 80% approximately 24 hours later.

Since Oscar was a short-lived tropical storm, there were very few forecasts to verify, making their evaluation meaningless.







Infrared satellite image of Oscar at peak intensity.

2012 Atlantic hurricane season

Permalink

Tropical Cyclone Report (TCR): Hurricane Nadine

By: KoritheMan, 12:06 AM GMT on March 12, 2013

Tropical Cyclone Report
Hurricane Nadine
(AL142012)
10 September-3 October 2012

Nadine was an exceptionally long-lived hurricane whose trajectory was generally confined to the eastern Atlantic. Nadine was the fourth longest-lasting tropical cyclone in the historical record dating back to 1851, and the longest-lasting since Inga of 1969.


a. Synoptic History

Nadine originated from a tropical wave that moved off the west coast of Africa on 7 September. The wave exhibited a broad Inverted-V pattern on satellite images and was accompanied by ample lower- to middle-tropospheric rotation. A weakness in the mid-tropospheric ridge existed over the central Atlantic between Leslie and Michael, which allowed the wave to progressively gain latitude. The circulation became better defined over the next couple of days, but convection was not very persistent, possibly due to mid-level dry air as denoted by water vapor imagery. Finally, the thunderstorm activity became more concentrated on 10 September, and the system became a tropical depression around 1200 UTC that day, while centered about 850 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). Convection diminished considerably after genesis, again due to subsident flow aloft. The thunderstorm activity rebounded a bit later that evening, and it is estimated that the depression strengthened to a tropcial storm around 0000 UTC 11 September. Nadine initially showed signs of strengthening, with upper tropospheric outflow becoming well-established in the western semicircle. Nadine reached an intensity of 55 kt by 1800 UTC 12 September, during which time Dvorak estimates were unanimously 3.5, and when an AMSUB microwave pass showed what appeared to be a mid-level eye feature. Shortly afterward, southwesterly shear increased over the circulation as Nadine neared the axis of a cutoff upper low over the central Atlantic a few hundred miles east-northeast of Puerto Rico. Nadine turned northwestward during this time as it came under the influence of the low and a longwave trough upstream from the low.

Nadine turned northward late on 13 September as the central Atlantic trough became the dominant steering mechanism. Nadine strengthened further and became a minimal hurricane with an intensity of 65 kt around 1800 UTC the next day, still continuing northward. It is worth noting that the convective pattern was somewhat disorganized at that time as southwesterly shear continued. However, it has been observed in past hurricanes, such as Lili in 2002, that the inner core of a tropical cyclone can still be mature and strengthen even while the outer core is being buffeted by strong upper-level winds; this appears to have been the case with Nadine. Nadine strengthened to an initial peak of 70 kt at 0000 UTC 14 September. Afterward, the shear gradually increased, and Nadine's center became exposed to the southwest of the thunderstorm activity. Nadine maintained hurricane status for about two days before the shear contributed to weakening back to a tropical storm near 1800 UTC 16 September. Nadine turned toward the east-northeast under the combined influences of a large upper-level trough over the central Atlantic and a developing upper low north of the Azores. Nadine weakened to an intensity of 45 kt around 0000 UTC 18 September as it moved across cooler waters, entrained drier air, and continued to battle strong vertical shear. Zonal flow developed over the north Atlantic, which allowed the trough to bypass Nadine while steering currents gradually weakened. Nadine's northeastward motion toward the Azores abruptly ceased later that day, and during the following two days Nadine made a tight cyclonic loop to the south of the Azores under the influence of a weak mid-level ridge.

The ridge strengthened a little on 20 September, and Nadine responded by turning southward with a slight increase in forward speed, although the cyclone was still moving rather slowly. Despite heading into slightly warmer sea surface temperatures concurrent with this motion, analysis of satellite imagery suggests that Nadine entrained a large area of dry air within a large stratocumulus cloud deck over the eastern Atlantic. Nadine continued to wind down, with deep convection practically vanishing near 0000 UTC 21 September; in addition, ASCAT data during this time suggested that the radius of maximum winds had broadened. Residual shower activity limited to a shallow band well-removed from the center in the southern quadrant of the circulation, Nadine degenerated into a non-convective remnant low around 1200 UTC that same day, located about 175 miles south-southwest of Sao Miguel Island in the eastern Azores. A well-defined surface circulation remained, however, and Nadine gradually headed into warmer waters, turning southwestward early the next day. Convection increased, primarily in bands north of the center as post-tropical Nadine turned westward. It is estimated that the system reacquired the organization necessary to be designated a tropical storm again near 0600 UTC 23 September, centered about 400 miles west-northwest of Isla de La Palma in the Canary Islands. Convection was limited and not very persistent over the next couple of days as Nadine continued moving toward the west, likely due to still relatively cold underlying water temperatures (while warmer than in earlier days, they were not quite at the minimum threshold typically needed for tropical cyclone formation) and a stable airmass.

Heights began to rise to the north of Nadine as a deep-layer trough moved off the coast of North America, and the cyclone responded by turning southwestward again around midday on 26 September under a mid-level ridge over the north Atlantic. Nadine began to strengthen again as the waters warmed to about 26 to 26.5C and any remaining vertical wind shear decreased. A mid-level eye feature became apparent in microwave data around 1200 UTC the next day, but this feature was not apparent in conventional satellite pictures until around 0600 UTC 28 September. Approximately six hours later, the cyclone became a hurricane again. Nadine gradually turned northward under the influence of the central Atlantic trough while strengthening, and the cyclone reached its final -- and greatest -- peak intensity of 80 kt around 0600 UTC 30 September. Northerly shear gradually increased over Nadine as the upper trough encroached on the cyclone circulation from the west, with satellite and microwave data showing an erosion of the eyewall in the northern semicircle, along with a concurrent loss of convection. However, the shear was initially not strong enough to fully penetrate the core, and Nadine weakened only gradually at first. Based on satellite intensity estimates and microwave data, Nadine is presumed to have dropped below hurricane strength around 1200 UTC 1 October. An eye feature briefly became apparent again in satellite imagery about 18 hours later, but it persisted for less than 12 hours before the shear increased further.

Nadine turned eastward on 2 October to the north of the subtropical ridge toward the general vicinity of the Azores once again. The shear and rapidly cooling waters ultimately took their toll, and Nadine is estimated to have become a remnant low for the second and final time near 1800 UTC 3 October while centered about 250 miles southwest of the island of Flores in the Azores. At the time of dissipation, ASCAT data still indicated winds of tropical storm force, and these winds lashed the Azores even after Nadine had dissipated as a tropical cyclone.


b. Meteorological Statistics

Observations in Nadine (Figs 2 and 3, to be added in the final rendition) include the Dvorak-based satellite intensity technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). Objective ADT estimates from University of Wisconsin CIMSS were unavailable to the writer. Scatterometer data was particularly useful, most notably the ASCAT. Various microwave data were also used to corroborate the best track of Nadine, as well as to assess its internal convective structure. The AMSU temperature unit courtesy of CIMSS was useful in determining Nadine's thermal structure, especially when it was in the vicinity of the cold water near the Azores. Surface observations from the Azores were useful in determining the intensity of Nadine as it hung around those islands, and especially for determining the existence of tropical storm force winds within the circulation after Nadine had dissipated for a final time on 3 October.

Nadine's peak intensity of 80 kt at 0600 UTC 30 September is based on consensus Dvorak estimates of 4.5.

Nadine produced tropical storm force winds over portions of the Azores as its circulation meandered in the vicinity of the islands for several days. A sustained wind of 54 kt with a gust of 70 kt was observed at 0110 UTC 21 September at Horta on the island of Faial, located in the central Azores. A sustained wind of 36 kt was reported on Santa Maria Island at 0400 UTC 22 September, while a wind gust of 40 kt was reported on the island of Flores at 1830 UTC local time 20 September.

The lowest minimum pressure observed in the Azores was 993 mb which occurred in Pico in the central Azores at 0740 UTC 4 October after Nadine had ceased to exist as a tropical cyclone.

Nadine produced tropical storm force winds across the islands even after it dissipated as a tropical cyclone. Sustained winds of 33 kt were observed at Sao Miguel Island at 1520 UTC 4 October, with a gust 49 kt at Nordeste, which is located on that island. An elevated observing site reported a wind gust of 76 kt at the Wind Power Plant on Santa Maria Island.

Nadine persisted as a tropical cyclone for approximately 22 non-consecutive days, making it the fourth longest-lasting Atlantic hurricane in the historical database. Nadine spent 20.50 days at tropical storm or hurricane intensity, which makes it the third longest-lasting Atlantic basin tropical cyclone with at least an intensity of tropical storm force. Nadine was the longest-lived Atlantic hurricane since Inga of 1969. However, it is interesting to note that, while Nadine lasted longer in terms of total duration, its measured trajectory in terms of nautical miles was very small considering the duration. Indeed, there have been several storms in the past that dissipated several days in advance of Nadine's total lifespan and whose tracks spanned over 2,000 miles. These include but are not limited to: Alberto (2000), Ivan (2004), and Bertha (2008).


c. Casualty and Damage Statistics

While Nadine brought tropical storm conditions to portions of the Azores, there were no reported cases of any damages or fatalities in connection with the storm.


d. Forecast and Warning Critique

Nadine's formation was well anticipated. The precursor disturbance which ultimately spawned the hurricane was mentioned in Tropical Weather Outlooks (TWOs) beginning at 0600 UTC 7 September. At the time, genesis probabilities were assigned to 10%, or the "low" category. The wave had just emerged from west Africa at this time. Development probabilities were increased to the "medium" range (40%) at the 0000 UTC 9 September TWO. Development probabilities reached the "high" category (greater than 50%) just before 0600 UTC 10 September, or about 12 hours prior to tropical cyclone formation as denoted in post-storm analysis. After it had dissipated as a tropical cyclone, Nadine's regeneration was very well forecast. Early on 22 September, genesis probabilities were immediately assigned to the "medium" category, at which time Nadine was given a 30% chance of regenerating within 48 hours. About 24 hours later, genesis expectations reached the "high" category, and Nadine was given a 70% chance of regenerating within a 48 hour timespan. However, it should be noted that the post-storm analysis of Nadine indicates that the system had regenerated into a tropical storm right around that time.

Track forecasts with Nadine were generally fairly good, although, possibly due to poor timing in the forecast of the looping motion that eventually occurred, Nadine was originally predicted to track much farther west of the Azores, by as much as 150 to 300 miles depending on the forecast. However, once the forecasts locked onto it, the manner of Nadine's looping was well predicted.

Intensity forecasts with Nadine were, on average, very good. The largest errors occurred after Nadine had regenerated into a tropical cyclone and was entering its second and final intensification phase. At that time, Nadine was not anticipated to reach hurricane strength, even while the strengthening episode was apparent in satellite images. This was more than likely due to the overestimated influence of cool waters, which do not apply as well to mid-latitude systems. In addition, Nadine's initial cessation as a tropical cyclone was generally well predicted, although the specifics and timing of this event were not, and forecasts varied considerably as to where, if, and when extratropical transition would occur. As it turns out, Nadine did not become extratropical. Nadine's subsequent regenation was well predicted.

Tropical storm watches and warnings were in effect for the Azores on many occasions as Nadine meandered near those islands. A summary of those advisories are listed in Table 3 (to be added in the final rendition).








Infrared satellite image of Nadine at peak intensity.

2012 Atlantic hurricane season

Updated: 5:14 AM GMT on March 12, 2013

Permalink

About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

Local Weather

Scattered Clouds
93 °F
Scattered Clouds