KoritheMan's WunderBlog

Tropical weather analysis - November 30, 2012

By: KoritheMan, 4:35 AM GMT on December 01, 2012

Invest 91L

Unfitting considering the calender date, an area of low pressure has formed in the eastern Atlantic between the Cape Verde Islands and Bermuda. Like many lows in this part of the world this time of year, Invest 91L is of non-tropical origin.

The satellite signature is less than impressive, as the storm is experiencing westerly vertical wind shear. Consequently, nighttime infrared satellite pictures show an exposed circulation center near 25N 43W; this position is also consistent with an earlier AMSUB microwave overpass just after 0z, and also with a recent ASCAT pass.

Figure 1. Latest infrared satellite image of Invest 91L. Image credit: NOAA

The GFS develops a narrow area of weak shear over the system as it moves northward over the next couple of days. However, I should note that the denoted flow is not at all anticyclonic, and suggests only weak ridging aloft which will probably break down with any sudden increase in wind shear. The 0z SHIPS takes 91L to near hurricane strength prior to extratropical transition in about 72 hours, but this seems awfully suspect in light of the large-scale environment, and this model is not designed to handle non-tropical entities particularly well. That being said, some slow development of this disturbance is possible as it moves northward at 10 to 15 mph through Sunday. Thereafter, a turn to the north-northeast or northeast with some acceleration is expected as synoptic mid-level southwesterly flow increases over the system. This agrees well with the available model guidance.

It should be noted that the upper-level circulation appears less well-defined than the middle and lower-tropospheric circulations, which suggests that this low is on its way to working the associated circulation down to the surface. Notwithstanding, the aforementioned ASCAT pass showed only a sharp wind shift, not a closed circulation. Satellite imagery also supports this idea.

This low is forecast to become extratropical in about 72 hours, leaving only a short window for development.

Probability of development in 48 hours: 30%


Elsewhere in the tropics, dangerous Typhoon Bopha is heading toward the Philippines, and is likely to deliver a substantial blow to that nation. I will have a blog post on Bopha in a day or so, and will continue to follow it until dissipation.

Also, the global models, including the GFS and ECMWF, have been consistent over the last few days in developing another area of low pressure on the heels of Invest 91L. Such a system should be expected to take a northeastward path out to sea in response to a mid-level trough forecast to set up over the central Atlantic. The culprit for such a pattern is probably going to be the developing trough over the western United States/four corners region, which is showing up nicely on water vapor imagery. This low also has the potential for subtropical or tropical development.

2012 Atlantic hurricane season

Updated: 4:36 AM GMT on December 01, 2012


Tropical Cyclone Report (TCR): Hurricane Ernesto

By: KoritheMan, 4:13 AM GMT on November 27, 2012

Tropical Cyclone Report
Hurricane Ernesto
1-10 August

Ernesto moved uneventfully through the Caribbean Sea as a tropical storm before becoming a hurricane as it approached the Yucatan Peninsula. Ernesto then made a second landfall over extreme southeast Mexico as a tropical storm. Ernesto's remnants contributed to the formation of Tropical Storm Hector in the Eastern North Pacific.

a. Storm History

Ernesto's development began when a vigorous tropical wave emerged from the coast of Africa on 23 July. While in situ observations are relatively scarce in this area of the Atlantic, ASCAT ambiguities suggest that the wave was of fairly high amplitude, and it contained a vigorous lower- to middle tropospheric cyclonic circulation envelope which appeared to be closed at times. Notwithstanding, there was little overall development for the next several days as the wave marched westward, possibly in response to a large dust-laden airmass associated with the Saharan Air Layer (SAL) that prevailed over much of the tropical Atlantic during this time. Possibly enhanced by the Intertropical Convergence Zone (ITCZ), the wave began showing signs of organization on 28 July, when a broad low pressure area is estimated to have developed along the wave axis. Microwave data (not shown) showed the inner structure beginning to look less skeletal around midday 30 July, which was concurrent with satellite images showing developing banding features.

Subsequently, the tropical wave began to increase in forward speed while gradually turning toward the west-northwest. Development continued, and it is estimated that the system became a tropical depression near 1800 UTC 1 August while centered approximately 700 miles east-southeast of Barbados. 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. The best track positions and intensities are listed in Table 1. The depression failed to strengthen initially, and the cloud pattern as a whole was disorganized. Based on a flight from a reconnaissance aircraft and a slightly improved convective presentation in satellite images, the cyclone is assumed to have become a tropical storm around 0000 UTC that day. Ernesto appears to have strengthened a little as it moved through the eastern Caribbean on 4 August, but the low-level center remained displaced to the west of the convection due to westerly speed shear caused by the fast forward motion of the tropical storm as it was steered to the north of a strong subtropical ridge. The cyclone started to lose organization again late that same day, and there is some doubt as to the veracity of a closed circulation during that time.

Little change in strength was noted over the next couple of days as Ernesto continued westward. As the system approached the western Caribbean late on 6 August, however, it began to strengthen, possibly in response to a relative decrease in the forward speed in response to an upper-level trough that was moving into the southern United States. Based on satellite pictures and data from a reconnaissance aircraft, it is estimated that Ernesto became a hurricane just before 1800 UTC 7 August, centered about 250 miles east-northeast of Belize City. Ernesto continued to strengthen up until landfall, reaching its maximum estimated intensity of 75 kt near 0000 UTC 8 August while located about 60 miles east of Chetumal, Mexico. The hurricane made landfall along the southern coast of the Yucatan in a remote area just south of Chetumal just after 0200 UTC at peak intensity. At that time, satellite and aircraft data indicated that Ernesto was strengthening, and could have been going through a rapid deepening phase. The eye briefly became better defined after landfall near 0600 UTC, but the inner core quickly collapsed as the cyclone continued inland. Ernesto weakened below hurricane status near 1200 UTC that day while moving overland.

The cyclone entered the Bay of Campeche a little after 1800 UTC 8 August and began to gradually turn southwestward under a building low- to mid-level ridge over the Gulf of Mexico. Despite the weakening while over Yucatan, Ernesto's cloud pattern remained well-organized. Based on surface observations and satellite data, Ernesto made a second landfall along the coast of Mexico along the southeast portion of the coast near Coatzacoalcos near 1530 UTC 9 August as a 50-kt tropical storm. The tropical cyclone weakened after landfall, and is estimated to have dissipated over the Sierra Madre Oriental near 0600 UTC 10 August. The remnants -- apparently a mid-level circulation -- continued moving southwestward, where they entered the Eastern North Pacific and contributed to the formation of a large area of disturbed weather which ultimately spawned Tropical Storm Hector.

b. Meteorological Statistics

Observations in Ernesto include the satellite-based Dvorak intensity technique, although to a lesser extent since reconnaissance observations in the storm were widespread. Various microwave data were used to track the tropical cyclone as well, most notably the SSMI/S unit.

2012 Atlantic hurricane season

Updated: 5:16 AM GMT on November 28, 2012


Tropical Cyclone Report (TCR): Tropical Storm Debby

By: KoritheMan, 4:10 AM GMT on November 27, 2012

Tropical Cyclone Report
Tropical Storm Debby
23-27 June

Debby moved erratically over the Gulf of Mexico before finally making landfall in northwest Florida as a minimal tropical storm. Its rains produced flash floods over interior portions of the state. Forming on 23 June, Debby is the earliest known occurrence of the fourth named storm in the Atlantic hurricane database, eclipsing the previous record set by Hurricane Dennis in 2005.

a. Storm History

Debby was spawned by a complex meteorological pattern involving three distinct parameters. Satellite, water vapor, and scatterometer data suggest that a poorly-defined tropical wave emerged from the coast of Africa late on 9 June. The wave entered the eastern Caribbean on 15 June. Around this time, the tropical wave began to approach a large area of cloudiness and showers stretching from the eastern Pacific to the western Caribbean. This weather was partially related to Hurricane Carlotta, which made landfall on the southern coast of Mexico that same day. The cyclone dissipated the next day over the rugged terrain of the Sierra Madre.

However, its mid-level circulation moved eastward and merged with the aforementioned cloudiness, which in itself can probably be attributed to the passage of the upward motion of the Madden Julian Oscillation (MJO). Over the following several days, a broad cyclonic circulation began developing between Cuba and the Yucatan Peninsula, but the system as a whole remained disorganized. A broad area of low pressure developed in association with the system on 20 June. As it moved across the Yucatan Peninsula the next day, modest pressure falls were noted. The low emerged off the north coast of the peninsula on 21 June. The large disturbance was slow to organize as it drifted slowly northward and encountered strong upper-level shear associated with an upper low over the western Gulf of Mexico that was lingering near the Texas coast.

The upper low was slow to pull away, and westerly to southwesterly flow on the south side of it produced vertical shear over the disturbance, keeping the wind field broad and the associated convection well east of the surface low. The system remained disorganized, with little evidence of a central gyre. Instead, the system appeared to consist of a broad area of low pressure with multiple centers embedded in a larger cyclonic envelope, a fairly common pattern in sheared disturbances or tropical cyclones. Throughout the following couple of days subsequent to emerging from the Yucatan, the low produced strong winds in bands of showers east of the center. A reconnaissance flight into the system late on 23 June confirmed the existence of a tropical storm about 350 miles southeast of the Mouth of the Mississippi River, and Debby's "best track" (shown below) begins at 2100 UTC that day. Various coordinates, including six-hourly position, pressure, and intensity estimates, respectively, are given.

Debby was highly sheared at the time of classification due to the persistent upper tropospheric cold low, which by that time had moved to a position between Galveston and Corpus Christi and intensified. The combination of westerly shear and dry air from the low would prevent significant strengthening of the tropical cyclone. Debby reached its peak intensity of 50 kt around 1200 UTC 24 June while centered about 185 miles southeast of Pensacola, Florida. Afterward, the cyclone began to weaken as southwesterly shear increased; this shear also appeared to cause the tropical storm to entrain dry air into its circulation, and this combination seems to have dismantled Debby before it had a chance to develop an inner core. Debby was initially situated between a building ridge over the central United States, and a departing trough over the eastern United States. The latter feature was reinforced by the passage of a shortwave trough over the northeastern United States that had dropped out of Quebec. As a result, Debby gradually turned eastward toward the west coast of Florida, its center becoming fully exposed at times.

Satellite and radar data suggests that Debby crossed the Big Bend region of Florida near 2100 UTC 25 June in the southern portion of Taylor County about 20 miles south of Perry as a 35 kt tropical storm. Debby's center became poorly organized while crossing the northern Florida peninsula, with radar data indicating that the primary convection was confined to a frontal-like band north of the center. Debby emerged into the western Atlantic near St. Augustine near 0600 UTC 27 June, and dissipated shortly thereafter.

b. Meteorological Statistics

Observations in Tropical Storm Debby include the satellite-based Dvorak intensity technique, Advanced Scatterometer (ASCAT) wind ambiguities, surface observations from land and oceanic stations (i.e. oil rigs and drifting buoys). In addition, several United States NEXRAD radars were useful in tracking Debby while it was over the Gulf of Mexico, primarily the Tampa (TBW) and Tallahassee (TLH) radars. Aircraft reconnaissance data was also used routinely.

Debby's peak intensity of 50 kt near 1200 UTC 24 June is based on reconnaissance aircraft reported flight-level winds, which were representative of a 50 kt cyclone throughout several points on 24 June; perhaps not surprisingly, this was more or less just before the onslaught of strong westerly shear that Debby would encounter. Given that the winds reported by the aircraft were coincident with the period of minimum vertical shear, it seems reasonable to conclude that Debby's peak 1-minute wind speed was somewhere in the realm of 50 kt for several hours on 24 June.

The highest sustained wind reported on land was a 33 kt 1-minute wind speed with a gust of 46 kt at the National Weather Service forecast office in Tampa, Florida, which occurred on 24 June. Since the satellite signature changed little from the time of this observation to the time of landfall, Debby is estimated to have made landfall as a 35 kt tropical storm.

The lowest reported barometric pressure associated with Tropical Storm Debby was a 998 mb reading at Tallahassee near 0000 UTC 25 June, which occurred at the time of the cyclone's landfall. In addition, a coastal observing buoy just offshore Keaton Beach in Taylor County, where Debby made landfall, reported a minimum pressure of 999 mb for several hours beginning in that general timeframe. It's interesting to note that the former observation occurred in a location that was well away from the cyclone center; thorough examination of satellite pictures around that time suggest that the deepest convection was occurring in a strong but relatively shapeless ball of convection well to the north of the center directly over the Tallahassee metro area, which makes this reading not considered suspect. Since it is possible that more remote areas outside the observational jurisdiction of the Tallahassee National Weather Service forecast office experienced slightly lower pressure readings in the deeper thunderstorms, the lowest minimum central pressure estimated for Debby at landfall is considered to be 997 mb.

Debby produced a significant tornado outbreak across the state of Florida beginning on 23 June and persisting into 26 June. As is the case with the majority of tropical cyclones, many of these tornadoes were weak; however, they were relatively long-lived. The longest-lived tornado occurred in Collier County east of Goodland, where a 16 mile path was noted. Fortunately, this tornado was rated EF0, and its damage swath consisted of downed trees. The strongest tornadoes produced by Debby were a pair of EF2 tornadoes: the first of these was reported near Venus in Highlands County, where a woman was thrown 200 feet from her mobile home; in addition to the aforementioned death, this tornado injured one other person. The second EF2 tornado was spawned in Polk County east of Winter Haven. This particular tornado reportedly caused significant structural damage, including damage to 17 homes, mobile homes, and a retail store. Additionally, one person is purported to have been injured from this tornado.

c. Casualty and Damage Statistics

A total of nine deaths have been attributed to Debby, seven of which were direct. All but two of these occurred in Florida.

Overall property damage in the United States from Debby is estimated to be in the relam of $308 million, mostly from flooding and tornadoes. In Florida, water was reported to be head deep in certain sections of Pasco County. In addition, Debby's tornado outbreak, which was rather atypical for a tropical cyclone, caused minor to major structural damage in some areas. All in all, damage from the storm was fairly minor.

d. Forecast Verification

Debby's formation was well predicted. The precursor disturbance was first noted on 20 June and given a low probability (under 30%). Development probabilities reached the high category (above 50%) on 22 June, with explicit mention of a tropical depression beginning on that day as well. Probabilities remained in the high category thereafter until the time of genesis.

Initial prognostications were predicated on a false assumption that the tropical storm would slide toward the Texas coast underneath a building ridge over the Ohio Valley. In reality, what actually occurred was much different, although later forecasts captured Debby's eventual motion toward peninsular Florida quite nicely.

The initial errors, while indisputably large, were perhaps warranted, as there was a sharp dichotomy between the two most reliable global models, the Global Forecast System (GFS) and European Center for Medium-Range Weather Forecasts (ECMWF); the former consistently called for a landfall along the western peninsula of Florida, while the latter, which has shown to be a historically reliable model, postulated that Debby would head toward the northern Gulf Coast instead. For this reason, tropical storm warnings were issued for a portion of that coast on 23 June. The warnings and the forecast track were smoothly shifted eastward as the GFS ensemble members came more attuned with the deterministic solution, and as the ECMWF gradually trended toward that solution as well.

Intensity forecast errors with Debby were rather high, with the first couple forecasts calling for the storm to become a hurricane over the Gulf under the premise of more favorable upper-level winds. As it turns out, the upper tropospheric flow never quite relaxed over the storm.

2012 Atlantic hurricane season

Updated: 8:44 AM GMT on November 30, 2012


Tropical Cyclone Report (TCR): Hurricane Chris

By: KoritheMan, 4:04 AM GMT on November 27, 2012

Tropical Cyclone Report
Hurricane Chris
19-22 June

Chris was a high latitude hurricane that remained well out at sea over the north Atlantic shipping lanes.

a. Storm history

Chris developed from a weak cold front. Synoptic data indicate this front entered the western Atlantic on 13 June. An area of showers developed along the front as it progressed steadily eastward. An area of low pressure formed along the front early on 17 June while the system was centered about 250 miles southwest of Bermuda. As the low accelerated northeastward in lower- to middle tropospheric southwesterly flow, the overall satellite presentation and wind structure began to become better defined, with an ASCAT pass just after 0000 UTC 18 June indicating a broad surface circulation and increasing winds. Moving across the relatively warm waters of the Gulf Stream and through an environment of weak upper tropospheric shear, the system gradually developed symmetrical and relatively deep convection across roughly 75% of the center. This evolution led to the formation of a tropical storm around or just after 1800 UTC 19 June, when the system was located approximately 400 miles south-southwest of Cape Race, Newfoundland.

After genesis, Chris moved to the east-northeast under the influence of an upper low and its associated baroclinic zone located over Atlantic Canada. The cyclone turned northward early on 21 June, moving in a broad cyclonic fashion around the upper low. Just before 0000 UTC 21 June, an eye-feature began to develop in satellite and microwave data (not shown). As Chris turned northward, it became a hurricane near 1200 UTC while centered about 625 miles east-southeast of Cape Race, or about 1000 miles south of the southern tip of Greenland. Chris's strengthening over relatively cold underlying sea surface temperatures can probably be attributed to several meteorological phenomena: first, the chilly ocean temperatures, which are normally considered unfavorable for tropical cyclone formation and intensification, were likely mitigated by the very cold upper tropospheric temperatures in the surrounding environment as denoted on AMSU imagery at the time Chris became a hurricane (shown below). Second, Chris was located in close proximity to a large extratropical low pressure system to its south; northeasterly flow on the back side of this low probably acted to produce upper-level diffluence over the hurricane, a process that has been shown to aid in the development of convection. Lastly, the mean vertical shear pattern over eastward-moving Chris was nearly a uniform westerly when it began developing the eye. Finally, when Chris turned northward on 21 June, the upper tropospheric flow over the storm became more southerly.

This is another atmospheric process that has been shown to be conducive to the intensification of a tropical cyclone.When the two entities conflate and move in the same direction, the magnitude of the shear is generally reduced. Shortly after becoming a hurricane, Chris began to weaken. The eye became indistinct just after 1800 UTC, at which time Chris weakened to a tropical storm. Devoid of deep convection, the cyclone lost all tropical characteristics near 1200 UTC 22 June while centered about 350 miles east-southeast of Cape Race. Chris became absorbed into the extratropical low a few hours later.

b. Meteorological Statistics

Observations in Hurricane Chris include the satellite-based Dvorak intensity technique, AMSU microwave data, the Advanced Scatterometer (ASCAT), which was highly useful for tracking this marine cyclone, both in terms of track and intensity.

Chris's peak intensity of 65 kt is based on the incorporation of the Dvorak satellite interpretation method, where satellite estimates were firmly in the 4.0 range, considered the minimum threshold for hurricane intensity using the Dvorak method. Various microwave data were also used to assess the inner core structure of Chris. Fig 1 shows an AMSU microwave pass taken at 1825 UTC 21 June, after Chris had weakened back to a tropical storm. Fig 2 shows an SSMIS image shortly before 1200 UTC 21 June showing well the broad eye associated with the tropical cyclone. Fig 3 is a geostationary satellite picture of Chris at peak strength.

There have been no ship reports of tropical storm or hurricane force winds received in relation to Chris.

c. Casualty and Damage Statistics

Since it remained out at sea throughout its lifetime, there were no reports of damage or fatalities associated with this cyclone.

d.Forecast Verification

The track errors associated with Chris were exceptionally small, although the hurricane was too short-lived to gain a meaningful quantitative forecast evaluation.

Initial forecasts called for much less strengthening than actually occurred, likely due to the baroclinic environment in which the storm was embedded. When it became apparent that Chris would strengthen significantly, later intensity forecasts were more bullish.

2012 Atlantic hurricane season

Updated: 4:05 AM GMT on November 27, 2012


Tropical Cyclone Report (TCR): Tropical Storm Beryl

By: KoritheMan, 2:51 AM GMT on November 27, 2012

Tropical Storm Beryl


26 May - 30 May

Beryl was the second out of season tropical storm to form in the Atlantic during 2012. Its formation marked the first time since 1887 that two tropical storms formed during the month of May. In addition, Beryl's landfall near Jacksonville as a 60 kt tropical storm makes it the strongest pre-June tropical cyclone landfall on record for the United States. The previous record was held by Subtropical Storm Alpha, which struck the Georgia coast with 50 kt winds in May of 1972.

a. Storm history

A tropical wave moved off the coast of Africa on 12 May, and is believed to have been the precursor to Beryl. The wave was accompanied by some deep convection as it emerged from the coast. Thereafter, the wave become largely indistinct as it marched across the Atlantic, and its entrance into the western Caribbean on 21 May is based largely on extrapolation and continuity. Subsequently, the wave began to interact with a preexisting area of anomalous southwesterly flow encompassing a distance from the far eastern Pacific to Bermuda. This large-scale flow pattern, which has shown to be quite favorable for the initiation of thunderstorm development, could have been triggered by the upward phase of the Madden Julian Oscillation (MJO), which was making its way across the far eastern Pacific and adjacent Caribbean Sea at that time. Strong westerly shear prevented significant development while the system was in the Caribbean. In the wake of Alberto, a small trough became established over the eastern seaboard, which forced the disorganized disturbance toward the east-northeast. The system crossed eastern Cuba early on 24 May, and entered the western Atlantic in the vicinity of the Bahamas later that day. Surface observations from Grand Bahama indicated westerly winds as early as 1200 UTC 25 May as the disturbance passed to the east, providing evidence of a closed circulation at the surface.

The low moved northeastward at about 15 mph during this time, embedded in broad southwesterly flow associated with the trough. Under the influence of the warm waters of the Gulf Stream, the system began to acquire organized deep convection near the center. While such an evolution would typically presage tropical cyclone formation, analysis of satellite and water vapor imagery indicates that the surface low was collocated with an upper low that had apparently been spawned from the same trough that recurved Alberto. This prevented the convection from forming directly over the center. In addition, the system initially lacked upper-level outflow, another distinct characteristic of a tropical cyclone.

Late on 25 May, the trough weakened, leaving the system in a region of weak steering. Owing to the influence of the upper low, the designation of the system as a cyclone at 0000 UTC 26 May is considered to be subtropical. The "best track" of the cyclone (listed below) begins at this time. Other coordinates, including six-hourly position, pressure, and intensity estimates, respectively, are also given.

Beryl was initially trapped in a region of weak steering, and moved only slowly southwest. A large blocking pattern began to amplify over the eastern United States at this time, which caused the cyclone to gradually accelerate. Based on microwave data and satellite imagery, Beryl is estimated to have transformed into a tropical cyclone near 1800 UTC 27 May while centered about 110 miles east of Jacksonville, Florida. The cyclone's winds are estimated to have been around 55 kt at this time. As it rounded the southern periphery of the ridge, Beryl turned westward throughout much of the 27th. Under light shear and warm waters, the cyclone strengthened, reaching a peak of 60 kt shortly before landfall along the northeastern Florida coast near Jacksonville Beach just after 0400 UTC 28 May. The cyclone appeared to be on the verge of becoming a hurricane, as doppler visuals indicated a developing eyewall. Following the typical progression, Beryl began to weaken as it moved inland. Concurrently, the cyclone slowed significantly, which was followed by a gradual turn to the north and northeast around the western periphery of the subtropical ridge.

Indeed, the forward speed had decreased so much that the cyclone became essentially stationary near the Georgia/Florida border for about twelve hours beginning around 0000 UTC 29 May. Beryl weakened to a tropical depression near 1200 UTC 29 May while still over northern Florida about 15 miles south of the Georgia border. It should be noted that it took Beryl roughly 38 hours to weaken to a tropical depression after landfall -- a rather anomalous inland decay rate, especially for a system of Beryl's strength.The slow weakening of the tropical cyclone while moving overland is likely attributable to Beryl's close proximity to the Gulf of Mexico and western Atlantic. Its slow movement enabled it to develop a well-defined and persistent inflow band to the east, which helped to continue mixing the strong winds aloft to the surface. Interestingly, water vapor imagery shortly after landfall indicated dry air and some westerly shear impinging on the cyclone's western periphery, which kept much of the associated precipitation in well-defined bands to the east of the center. By around 1200 UTC 29 May, Beryl began to accelerate. As Beryl neared the South Carolina near 0600 UTC 30 May, the cloud pattern began to become more suggestive of an extratropical cyclone as the storm began to interact with a cold front moving across the northeastern United States. Synoptic data suggest that this process was complete six hours later, when the system was very near the North Carolina coast.

b. Meteorological Statistics

Observations in Tropical Storm Beryl include the satellite-based Dvorak intensity technique, as well as reconnaissance aircraft data. Several NEXRAD doppler radars over the southeastern United States were also helpful in tracking the tropical cyclone, most notably the JAX (Jacksonville) and VAX (Valdosta) radars. The Advanced Scatterometer (ASCAT) was also used to help determine the wind structure of Beryl when it was away from land, particularly before the first reconnaissance flight into the system.

Beryl's peak intensity of 60 kt is based on several meteorological instruments and observations, including the Jacksonville radar, which showed areal velocities as high as 80 kt in the final hours before landfall (not shown). This would generally translate to winds of about 60 kt at the surface. At around that time, a reconnaissance aircraft reported 700 mb flight-level winds of about 80 kt, along with an SFMR measurement of 62 kt surface winds. While this would ordinarily support minimal hurricane intensity, other data suggests this value was rain-inflated. For instance, Dvorak numbers were not quite supportive of hurricane strength during the hours preceding Beryl's landfall, and the doppler radar signature was not suggestive of a well-defined eye or eyewall, but rather, broken fragments of each. In addition, a careful study of areal surface observations near the landfall point in northern Florida indicate that the most likely peak intensity of Beryl was 60 kt, achieved shortly before the tropical storm's landfall. It is likely that had the cyclone had a few more hours over water, it would have become a hurricane.

As it moved inland, Beryl produced tropical storm force winds and heavy rainfall. The National Weather Service in Jacksonville, Florida reported sustained winds of 30 kt, along with a wind gust to 45 kt just before landfall. Closer to the landfall point, the Mayport Naval Air Station (NAS) reported sustained winds of 40 kt with a gust to 50 kt about two hours before the time Beryl is estimated to have made landfall.

The lowest observed central pressure in Florida associated with Beryl was 29.40 (995 mb) at the Mayport NAS at the time of landfall, suggesting that the cyclone center passed very near this location. However, since winds in the 20 to 25 kt range were reported concurrently, it is doubtful that this was the lowest central pressure in association with Beryl. Based on extrapolation of the areal surface observations, it is presumed that Beryl passed just south of this location, where the lowest central pressure is estimated to have occurred (993 mb), as well as the strongest winds. The maximum reported surface wind and pressure readings across select locations are given in Table 3. Rainfall totals are also given within that table.

In addition to damaging winds, Beryl produced heavy rainfall across much of the southeastern United States, particularly northern Florida and southern Georgia, where a 24-hour stall appears to have occurred after landfall. A 48-hour rainfall total of near 3 inches was reported at Jacksonville, while a storm total of 15 inches was reported at Wellborn. Interestingly, the majority of Beryl's rainfall, including the previously cited observation at Wellborn, appears to have fallen to the south of the storm center.

A total of four tornadoes have been attributed to Beryl, two in Florida, one in South Carolina, and one in North Carolina. There was minor damage reported with two of the tornadoes: one (which happened to be Beryl's first) which occurred near Port Saint Lucie caused minor damage to two homes, and another was an EF1 in North Carolina that damaged 67 homes and destroyed 3 near Peletier in Carteret County.

c. Casualty and Damage Statistics

A total of six fatalities have been attributed to Tropical Storm Beryl; four of these were direct, while two of those deaths were due to freshwater flooding in central Cuba while the precursor disturbance to the cyclone was traversing the western Caribbean. Two deaths also occurred in Florida while Beryl was a tropical cyclone: a motorist in Taylor County was killed when the vehicle of another hydroplaned on the wet roads and hit him head on, and a teenager drowned in rough surf in Orlando.

Property damage estimates in the United States are at $148,000 (2012 USD). Overall damage from Beryl was relatively minor, limited to downed trees and power lines as well as minor structural damage in the affected areas.

d. Forecast Verification

Beryl's genesis was not well anticipated. The precursor disturbance was first given mentioned on 23 May, and given a low chance of development ( 50%) of development.

Track forecast errors associated with Beryl were remarkably small from the time the storm formed to the time it dissipated. There was only a slight eastward bias at first since the global models were still unresolved in how far Beryl would move inland after landfall.

Intensity forecast errors were also fairly low, with Beryl predicted to make landfall as a tropical storm after completing the transition into a warm-core tropical cyclone. However, reintensification was expected over the western Atlantic, which does not appear to have occurred, since the storm completed extratropical transition much sooner than predicted.

2012 Atlantic hurricane season

Updated: 5:33 AM GMT on November 27, 2012


Tropical Cyclone Report (TCR): Tropical Storm Alberto

By: KoritheMan, 2:26 AM GMT on November 27, 2012

Tropical Cyclone Report
Tropical Storm Alberto
May 19 - May 22

Alberto was an out of season tropical storm, the first of two, that developed in the month of May in the north Atlantic. Alberto did not affect land.

a. Storm history

Alberto's origins appear to have begun as early as 10 May. During this time, satellite and water vapor imagery images showed that a well-defined upper-tropospheric cold low, accompanied by a well-marked cold front, entered west Texas. The front entered the Gulf of Mexico early on 12 May. Although the front gradually decayed, it became quasi-stationary over the central Gulf of Mexico, possibly in response to being sandwiched between two high pressure areas. During this time, the front produced intermittent clusters of showers and thunderstorms. The preexisting large-scale cyclonic flow was reinforced in this area by the passage of several shortwave perturbations in the semipermanent mid-latitude pressure belt. The associated cloudiness moved across the Florida peninsula, and entered the western Atlantic on 16 May. The activity moved steadily northeastward and soon became entangled with an approaching trough.The southern portion of this activity became stationary over the western Atlantic waters, while the northern portion of the trough continued moving northward. Around 1200 UTC 17 May, satellite and radar animations showed that a cloud mass formed over central South Carolina, possibly associated with a weak mesoscale convective system (MCS). This system moved offshore shortly after 0000 UTC 18 May, and later ASCAT data indicated the presence of a small surface circulation. The small low continued to become better organized, and it is estimated that a tropical depression formed from it around 1200 UTC 18 May, while centered about 100 miles south of Cape Fear, North Carolina. The "best track" of the cyclone (listed below) begins at this time. Other coordinates, including six-hourly position, pressure, and intensity estimates, respectively, are also given. The depression became a tropical storm about 6 hr later.

Initially, Alberto was embedded in a region of weak steering currents, and drifted slowly southwest. Based on a nearby ship report, the cyclone reached its estimated peak intensity of 50 kt around 2100 UTC. Soon thereafter, the tropical storm began to weaken under increasing southwesterly shear. In addition, water vapor imagery during this time suggests that Alberto was ingesting a very dry airmass over the southeastern United States, which likely counteracted the otherwise favorable sea surface temperature regime of the Gulf Stream. Synoptic steering currents gradually became more defined as a weak upper-level trough moved through the Ohio Valley, and Alberto responded with a gradual turn to the south and southeast, on a track well offshore the southeastern United States coast.Continuously battered by marginal atmospheric and thermodynamic parameters -- namely dry air and wind shear, Alberto weakened to a tropical depression near 0000 UTC 22 May. At that time, the center became almost completely exposed to the west of a diminishing area of showers. Convection subsequently increased, but this activity was disorganized, and is not assumed to have been sufficient to bring Alberto back to a tropical storm. Later that day, around 1200 UTC, the cyclone became a remnant low while located approximately 160 miles southeast of Cape Hatteras, North Carolina.

Moving northeastward, the remnant low lost its identity within a broad and nearly-stationary trough that extended from the northwestern Caribbean Sea to Bermuda. This same trough would soon assist in the formation of Tropical Storm Beryl.

b. Meteorological Statistics

Observations in Tropical Storm Alberto include the satellite-based Dvorak technique, various microwave data, ASCAT data, drifting buoy observations, and various National Weather Service (NWS) doppler radar stations, mostly CAE (Columbia) and CLX (Charleston). The NOAA Air Force Reserve Hurricane Hunter squadron was also useful in tracking the tropical cyclone.

While it was developing over South Carolina, the precursor disturbance produced locally heavy rain and gusty winds, but there were no reports of tropical storm force winds overland in association with Alberto. While the satellite appearance at the time of peak intensity was certainly not a conventional one, a ship that passed through the center shortly after 2100 UTC 19 May suggested that the central pressure was lower than operationally estimated. This results in a peak intensity of 50 kt at that time.Several yachts passed through the center of Alberto on 21 May and reported tropical storm force winds. No land areas reported tropical storm force winds while Alberto was a tropical cyclone, although wind gusts between 25 and 30 kt were not uncommon with the precursor disturbance.

c. Casualty and Damage Statistics

There have been no deaths or damage reported in association with Alberto, which was primarily a marine interest.

d. Forecast Verification

Alberto's genesis was not anticipated, with the incipient system never being mentioned in any of the forecasts prior to formation. There are a couple of factors that likely resulted in the lack of recognition: first, while the global models, particularly the GFS, had suggested the potential for tropical cyclone formation somewhere in the western Atlantic, they were not consistent in the timing or the placement of this feature; some runs showed a tropical cyclone and/or low pressure area forming in the western Caribbean, while others indicated that such formation would occur off the eastern seaboard, over the open waters of the western Atlantic. Secondly, while there was a general consensus on a broad area of disturbed weather developing, Alberto's horizontal vortex was rather small, which is a feature not well resolved by global models at this time. Finally, when the precursor to Alberto finally did manifest, the rather rapid transition to a tropical cyclone likely resulted in a negligent mention of the disturbance.

A verification of the track forecasts suggests that there was a substantial westward bias during the first day of Alberto's existence; initial forecasts called for the system to move inland along the coast of South Carolina before accelerating toward the northeast. While Alberto did come relatively close to the United States east coast, there was apparently enough amplitude in the trough over the Ohio Valley to recurve the cyclone before it was able to reach the coast.

Intensity forecasts with Alberto were generally good, although it was too short-lived to gain a meaningful forecast quantification.

A tropical storm watch was issued for the South Carolina coast from the Savannah River to the South Santee River beginning at 0300 UTC 20 May, primarily in response to a southward shift in the model consensus at that time. This watch was canceled at 2100 UTC that same day when it became apparent that Alberto would remain offshore. A summary of the watches and warnings given in association with Alberto are indicated below.

Watch and warning table

Date/Time (UTC): 20/0300
Action: Tropical Storm Watch issued
Location: Savannah River to South Santee River

Date/Time (UTC): 20/2100
Action: Tropical Storm Watch discontinued
Location: Savannah River to South Santee River

Peak intensity: 50 kt 995 mb - 2100 UTC 19 May

2012 Atlantic hurricane season

Updated: 4:27 AM GMT on November 27, 2012


Tropical weather analysis - November 9, 2012

By: KoritheMan, 6:17 AM GMT on November 09, 2012

Eastern Atlantic disturbance

An area of disturbed weather has formed in the eastern Atlantic about 500 miles west-northwest of the Cape Verde Islands. This activity is associated with a surface trough interacting with a large upper low.

Figure 1. Latest infrared satellite image of the eastern Atlantic disturbance, soon to be designated Invest 91L.

Upper-level winds are currently not favorable for significant development, but the GFS suggests that the mid-level flow will relax before the upper tropospheric flow does. In fact, according to that model, upper-level winds never quite become optimal for tropical cyclogenesis, with only a brief period of relatively light northwesterly flow from about 72-96 hours that still looks more like shear than diffluence. Subtropical entities do not generally have especially cold cloud tops, which is why the upper-level winds don't really affect them.

This system is not currently showing signs of organization, although it is producing sustained winds to near gale force as per earlier scatterometer data. However, the aforementioned data, along with surface observations, suggest that the associated wind field is rather broad, which is characteristic of a non-tropical low. Buoy 41139, located about two degrees west of the disturbance recently reported a sustained wind of 30 mph. Since this observation occurred outside the deepest thunderstorms, it is presumed that stronger winds are occurring closer to the convection, particularly in the well-defined band east of the estimated center; it also suggests that the gale force winds seen on the ASCAT pass were not rain-contaminated. Should this system ultimately acquire a name, it will likely skip the depression stage.

All available data suggests that the system currently lacks a well-defined surface wind field, with all of the closed winds found in the middle and upper levels of the atmosphere. Additionally, surface pressures are not falling.

While this system could ultimately attempt to develop, I expect that any significant development will wait until the mid-level shear starts to relax, in about 24-36 hours. Complicating matters further is a possible low pressure system that the global models (including the GFS, and to a lesser extent the Euro) forecast to from from the vigorous cold front over the western Atlantic ahead of this disturbance. While not explicitly shown in the global model forecast fields, this sort of evolution could lead to large errors in the track later in the period due to a binary interaction between the two entities.

This disturbance is expected to move slowly westward over the next day or two, before turning toward the west-northwest thereafter. This system is not expected to threaten any land areas.

Probability of development in 48 hours: 20%

2012 Atlantic hurricane season

Updated: 6:19 AM GMT on November 09, 2012


Tropical weather analysis - November 4, 2012

By: KoritheMan, 6:06 AM GMT on November 04, 2012

Invest 90E

An area of low pressure off over the eastern Pacific about 380 miles west-southwest of Manzanillo is moving little. The low has a well-defined surface circulation as denoted by scatterometer, satellite, and microwave data, but strong southwesterly shear is keeping the small center completely exposed. The low is producing a large area of showers and thunderstorms to the northeast of the center for several hundred miles.

Figure 1. Latest infrared satellite image of Invest 90E. Image credit: NOAA

This activity is affecting the Mexican coast between Puerto Vallarta and Manzanillo, but surface observations suggest that there is little wind accompanying the thunderstorms. Upper-level winds are expected to remain highly unfavorable for additional development to occur as the low drifts slowly northwestward, then gradually turns west-northwestward away from Mexico. If the system were deeper, it would gravitate toward the mid-level flow on a path towards Mexico, but since it is devoid of convection, it will tend to follow the lower-tropospheric steering.

Probability of development in 48 hours: 20%

2012 East Pacific hurricane season Invest 90E


Tropical weather analysis - November 3, 2012

By: KoritheMan, 5:07 AM GMT on November 03, 2012


Wind: 45 mph, with higher gusts
Location: 12.7°N 119.4°W
Movement: WSW at 5 mph
Pressure: 1005 mb

After becoming completely exposed earlier in the day, a burst of deep convection has developed over the northeastern portion of the circulation. Satellite estimates support an initial intensity of 40 kt, and a 0200 UTC OSCAT pass indicated a large area of 30 kt wind vectors outside the deepest convection. Thus, it is reasonable to assume that stronger winds are probably occurring in lieu of the recent convective burst.

Figure 1. Latest infrared satellite image of Tropical Storm Rosa. Image credit: NOAA

Westerly shear is still affecting Rosa, and it appears to be slowly increasing. Thus, the cyclone is forecast to progressively weaken subsequent to this point, although the intensity was held at 40 kt for the first 12 hours due to the possibility of an expansion/persistence of the ongoing convective burst during the upcoming diurnal convective maximum period overnight tonight. The SHIPS and GFS continue to suggest that this shear will increase to over 30 kt in about 24 hours, which is coincidentally when the tropical storm is expected to weaken at a somewhat quicker rate. Remnant low status is shown at day two, followed by dissipation at day four.

Satellite fixes suggest that Rosa has turned toward the west this evening, although it may be hesitating again. The presumed slower motion could be a symptom of the convective resurgence, as the low-level center attempts to tuck underneath the thunderstorms. It is my expectation that this will be the general motif for the next 6-12 hours, so little movement is indicated at that time. Thereafter, Rosa is forecast to gradually accelerate as the surrounding steering currents strengthen due to an amplifying mid- to upper-level trough to the north of the tropical cyclone. However, water vapor imagery suggests that this trough is fairly weak, and should this continue, Rosa could track farther westward than forecast, particularly at longer ranges.

I have little apparent reason to disagree with the National Hurricane Center forecast track, and the track I signify below is a reflector of that.

5-day intensity forecast

INITIAL 11/03 0300Z 40 KT 45 MPH
12 hour 11/03 1200Z 40 KT 45 MPH
24 hour 11/04 0000Z 35 KT 40 MPH
36 hour 11/04 1200Z 30 KT 35 MPH
48 hour 11/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
72 hour 11/06 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 11/07 0000Z...DISSIPATED

5-day track forecast

Figure 2. My 5-day forecast track for Rosa.

2012 East Pacific hurricane season Tropical Storm Rosa


Tropical weather analysis - November 2, 2012

By: KoritheMan, 5:52 AM GMT on November 02, 2012


Tropical Storm Rosa continues its crawl over the open Eastern Pacific. As of the latest NHC advisory, the following was available on the cyclone:

Wind: 50 mph, with higher gusts
Location: 13.7°N 118.4°W
Movement: Stationary
Pressure: 1003 mb

The satellite signature of the tropical cyclone has changed little over the past few hours. The central convection continues to pulsate, but remains quite cold.

Figure 1. Latest infrared satellite image of Tropical Storm Rosa. Image credit: NOAA

Analysis of water vapor imagery suggests that the core of the westerlies remain to the north of Rosa, which is likely the reason the cyclone has not weakened as quickly as original prognostications indicated. In fact, the outflow pattern looks better defined than earlier. Since these unfavorable upper-level winds appear to be sagging only very slowly southward, and since Rosa has been losing latitude, an abrupt weakening is not anticipated in the short-term. In about 48 hours, the GFS and SHIPS still insist on sharply increasing the shear over Rosa as the cyclone begins to gain some latitude. Since the synoptic pattern does appear consistent with a slow increase in latitude by this time, and the fact that there is evidence of the shear moving slowly southward, this seems like a reasonable expectation. A quicker rate of weakening is anticipated beyond 48 hours, and Rosa is forecast to degenerate into a remnant low by 72 hours, although it is likely to maintain its identity throughout the forecast period. It should be noted that the SHIPS shows Rosa approaching cooler waters Tuesday evening, so termination of the system at that time may need to be considered in my next forecast.

Rosa remains entrenched within a weak steering regime between a large anticyclone over the central Pacific and weak cyclonic flow to the north. Recent satellite fixes and comparison of evening microwave data suggest that little movement has occurred. 0z upper air data and water vapor images suggests that a well-established weakness has begun to manifest across southern California and the desert southwest as a mid- to upper-level shortwave trough amplifies across that region. This trough is forecast to dig southward according to the global models, but they realize that it will be a slow process. Thus, Rosa's latitude gain will not be met without opposition; the central Pacific high appears to be the dominant steering mechanism at this time, and although that's not saying much, it is expected that this will prove sufficient to keep Rosa in a very weak steering environment, with perhaps a slow southwestward drift. This is consistent with a blend of the GFS and ECMWF, and in fact, the forecast track as a whole is pretty close to this.

5-day intensity forecast

INITIAL 11/02 0300Z 45 KT 50 MPH
12 hour 11/02 1200Z 45 KT 50 MPH
24 hour 11/03 0000Z 40 KT 45 MPH
36 hour 11/03 1200Z 40 KT 45 MPH
48 hour 11/04 0000Z 35 KT 40 MPH
72 hour 11/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 11/06 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
120 hour 11/07 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast

Figure 2. My 5-day forecast track for Rosa.

2012 East Pacific hurricane season Tropical Storm Rosa

Updated: 5:54 AM GMT on November 02, 2012


Tropical weather analysis - November 1, 2012

By: KoritheMan, 6:11 AM GMT on November 01, 2012


Tropical Storm Rosa continues moving across the open Pacific. As of the latest NHC, the following information was available on the storm:

Wind: 60 mph, with higher gusts
Location: 14.1°N 118.1°W
Movement: W at 2 mph
Pressure: 1000 mb

The cloud pattern associated with Rosa has changed very little, and consists of an amorphous mass of very deep convection. Satellite estimates have not appreciably changed, and the surface winds are probably still about 50 kt. Earlier microwave data suggested that the center was on the far western edge of the convection, and this signature is confirmed by analysis of nighttime satellite imagery. Upper-level outflow has become restricted to the west due to about 15 kt of westerly shear, as diagnosed by UW-CIMSS and the SHIPS model.

Figure 1. Latest infrared satellite image of Tropical Storm Rosa. Image credit: NOAA

Since the shear appears to be increasing, Rosa's brief intensification on Wednesday is likely to be its last. Beyond 48 hours, the shear is forecast to increase even further as an upper-level trough forces the westerlies to move southward toward the tropical cyclone. Ergo, the expected symptom of all this is a steady decay of the tropical storm subsequent to this point. Remnant low status is shown by day four, although it is possible that this could occur a little sooner if the shear is quick to increase. A rapid demise is not anticipated since Rosa is expected to remain over warm waters throughout the forecast period.

The track forecast remains tricky, owing to the inherent complexity of the synoptic pattern over the Pacific. While the models are in relatively good agreement on the first 48 hours of the track, there continues to be significant diversity after that. After reviewing the National Hurricane Center forecast discussion for the storm yesterday (Wednesday), and carefully analyzing water vapor imagery and the 500 mb forecast fields of the global models, it appears that the primary reason for the disagreement is that they seemingly cannot agree on the evolution and amplitude of a deep-layer trough developing off the western United States. The models that show recurvature show a more negatively-tilted trough, similar to what we saw with Sandy. Conversely, the models that show a more positively-tilted regime suggest a more zonal flow to the north of Rosa that is more apt to shunt the system westward, well south of the mean mid-latitude westerly flow. Since water vapor images show a trough that is more on the positively-tilted spectrum, my forecast track continues to be south of the GFS/GFDL/HWRF trio, and closer to the ECMWF, which appears to be handling the evolution of the synoptic pattern better.

My forecast is a little to the right of the NHC forecast track in the latter portion of the forecast period, in anticipation that the trough will cause at least some poleward bend, especially given the sizable weakness that exists north of the storm.

5-day intensity forecast

INITIAL 11/01 0300Z 50 KT 60 MPH
12 hour 11/01 1200Z 45 KT 50 MPH
24 hour 11/02 0000Z 45 KT 50 MPH
36 hour 11/02 1200Z 40 KT 45 MPH
48 hour 11/03 0000Z 35 KT 40 MPH
72 hour 11/04 0000Z 30 KT 35 MPH
96 hour 11/05 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 11/06 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast

Figure 2. My 5-day forecast track for Rosa.

2012 East Pacific hurricane season Tropical Storm Rosa


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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