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Subtropical Storm Alberto Discussion Number   2
NWS National Hurricane Center Miami FL       AL012018
400 PM CDT Fri May 25 2018

The inner-core low-level wind field of Alberto has changed little
since the previous advisory based on recent data from an Air Force
Reserve reconnaissance aircraft, along with land and ship
observations. However, the convective structure of the cyclone has
degraded over the past several hours due a pronounced intrusion of
dry mid-/upper-level air and the cloud pattern continues to exhibit
the structure of a subtropical cyclone. The lowest pressure measured
thus far by the aircraft has been 1006 mb. The initial position is a
little north of the recon position of a pronounced swirl due to the
broad overall nature of the low-level circulation.

The initial motion estimate is 090/02 kt, but this is considered to
be a short-term motion. A gradual turn toward the north should begin
later tonight due to a strong ridge located to the east across the
Greater Antilles. A steadier northward motion is forecast to occur
by Saturday evening and continue into Sunday as a sharp
mid/upper-level trough digging southward into the central Gulf of
Mexico combines with southerly flow around the western portion of a
large subtropical ridge to produce deep-layer southerly flow across
Alberto. By 48 hours and continuing through 72 hours, the developing
mid/upper-level low over the central Gulf should cause the cyclone
to turn northwestward and accelerate until it nears the Gulf Coast
by Monday night. After that, steering currents are forecast to
collapse as a broad weakness develops in the subtropical ridge axis
located along the Gulf coast. Slow but steady recurvature into the
westerlies across the Deep South is expected to begin by 96-120 h.
The new NHC forecast track is similar to the previous advisory,
and closely follows the consensus models TVCN and HCCA, and the
Florida State Superensemble (FSSE) model.

The broad nature of the inner-core wind field, along with strong
westerly wind shear in excess of 20 kt is forecast to continue for
the next 24 hours. The latest model runs actually decrease the shear
sooner than previously forecast, but the ragged nature of the wind
field should prevent any significant intensification until after 48
hours. As a result, only slow but steady strengthening is expected
for the next 3 days, and Alberto could peak around 60 kt around 60
hours when the storm will be in a low wind shear regime and
over SSTs greater than 28 deg C. However, proximity to dry mid-
level air around landfall could hinder any additional strengthening,
and the NHC intensity forecast remains similar to the previous
advisory, closely following a blend of the HCCA and ICON consensus
models and the FSSE model.

The new NHC forecast necessitates the issuance of tropical storm
and storm surge watches for portions of the U.S. Gulf Coast at
this time. Note that if the intensity forecast increases with later
advisories, a hurricane watch could be needed for a portion of the
Gulf Coast.

KEY MESSAGES:

1. Regardless of its exact track and intensity, Alberto is expected
to produce heavy rainfall and flash flooding over the northeaster
Yucatan Peninsula of Mexico, western Cuba, southern Florida and the
Florida Keys. Rainfall and flooding potential will increase across
the central U.S. Gulf Coast region and the southeastern United
States later this weekend and early next week when Alberto is
expected to slow down after it moves inland.

2. Tropical-storm-force winds and hazardous storm surge are
possible along portions of the central and eastern U.S. Gulf Coast
beginning on Sunday, including areas well east of the track of
Alberto's center, and tropical storm and storm surge watches have
been issued for portions of these areas. Residents in the watch
areas are encouraged not to focus on the details of the forecast
track of Alberto and should follow any guidance given by their local
government officials.

3. Dangerous surf and rip current conditions are affecting portions
of the Yucatan Peninsula and western Cuba and will likely spread
along the eastern and central U.S. Gulf Coast later this weekend.

FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 19.4N  86.3W   35 KT  40 MPH
 12H  26/0600Z 20.7N  86.0W   40 KT  45 MPH
 24H  26/1800Z 22.5N  85.7W   45 KT  50 MPH
 36H  27/0600Z 24.8N  85.6W   50 KT  60 MPH
 48H  27/1800Z 26.8N  86.2W   55 KT  65 MPH...TROPICAL STORM
 72H  28/1800Z 29.0N  87.8W   55 KT  65 MPH...TROPICAL STORM
 96H  29/1800Z 31.7N  88.6W   30 KT  35 MPH...INLAND
120H  30/1800Z 35.2N  87.6W   20 KT  25 MPH...INLAND

$$
Forecaster Stewart

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