Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
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
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