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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.

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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4:13 AM GMT on November 27, 2012
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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