Tropical Depression Four Forms

By: National Hurricane Center
By: National Hurricane Center

TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042014
500 PM AST SAT AUG 23 2014

The Air Force Reserve Hurricane Hunter aircraft investigating the
disturbance near the southeastern Bahamas found a well-defined
circulation this afternoon. Even though the convective presentation
is somewhat ragged, there is enough organization to classify the
system as a tropical cyclone. The intensity is set to 30 kt based
on flight level wind data from the aircraft and an earlier ASCAT
pass, and the central pressure of 1005 mb is based on a recent
surface observation from Providenciales in the Turks and Caicos. The
SHIPS model shows light to moderate shear over the cyclone for the
next couple of days, which should allow for intensification given
that the cyclone will be moving over waters of 29 to 30C. The NHC
forecast shows slow strengthening in the first 24 to 36 hours as
the cyclone organizes. Global models show more favorable upper-
level winds over the system in a couple of days, which should allow
for strengthening to a hurricane in by 3 days. The official forecast
is close to the IVCN intensity consensus through the period.

The initial motion estimate is a relatively uncertain 310/10 given
the recent formation of the center. The cyclone should continue
moving northwestward and north-northwestward into a developing break in the subtropical ridge cause by a mid/upper-level trough off the
U.S. east coast. As the trough begins to lift out and steering
currents weaken, a slow motion is expected in 36 to 48 hours. Then
as a subtropical ridge rebuilds to the east of the cyclone later in
the period, the system is expected to turn northward and then
northeastward. However, there is significant disagreement in
the timing and sharpness of the turn in this and previous dynamical
model cycles. Currently the GFDL and GFS are along the western
edge of the guidance envelope showing a more westward track and
a slower motion at day 5. The ECMWF, GEFS mean, and HWRF show a sharper turn and a more northeastward motion by day 5 on the east
side of the guidance envelope. The NHC forecast is near the TVCA
multi-model consensus through most of the period and a little to the
left of it at day 5. Given the spread in the guidance and the recent
formation of the system, confidence in the details of the track
forecast is lower than normal.


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