[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 020257
TCDAT5

Tropical Storm Elsa Discussion Number   6
NWS National Hurricane Center Miami FL       AL052021
1100 PM AST Thu Jul 01 2021

The convective structure with Elsa tonight appears to be somewhat
better organized than earlier, with a bursting type pattern of cold
-75 to -80 C cloud top temperatures just to the northeast of the
estimated center. However, a SSMIS microwave pass at 2130 UTC
revealed that, underneath the cirrus, the deeper convection is still
struggling to rotate up-shear as the system moves quickly to the
west-northwest. Despite that fact, an ASCAT-A pass clipped the
northeastern edge of Elsa and showed several wind retrievals of
44-46 kt. In addition, the most recent subjective Dvorak satellite
classification from TAFB was T3.0/45 kt. In support of these data,
the current estimated intensity was raised to 45 kt for this
advisory.

Elsa continues to move quickly to the west-northwest at 290/23 kt. A
continued rapid motion to the west-northwest is expected for the
next 36 hours as the storm remains steered by a strong subtropical
ridge to its north, and the guidance has trended a bit faster once
again tonight. Thereafter, Elsa will reach the western extent of
this ridge which will be eroded by a strong mid-latitude trough
centered off the eastern US. Once again, the guidance spread
increases greatly by this time, with the GFS/UKMET on the slow and
left side of the guidance envelope, the ECMWF and its ensembles on
the fast and right side, and the Canadian roughly in the middle.
Interestingly, the latest GFS ensembles show some bifurcation within
the larger guidance envelope, with the strongest members further
south and west. The latest NHC track forecast is close to the
previous track early on but somewhat faster, and in the latter
period was nudged just slightly eastward towards the TVCN
consensus. However, the track forecast in the latter time period
remains low confidence.

The intensity forecast with Elsa also continues to be challenging
this evening. While the GFS-based SHIPS guidance indicates that the
current 200-850-hPa vertical wind shear is only 5-10 kt, the strong
east-southeasterly low-level flow Elsa is embedded in is resulting
in stronger 15-20 kt of west-northwesterly mid-level shear. This
mid-level shear has thus far prevented deep convection from wrapping
around the circulation and helping to align the low- and mid-level
vortex like the GFS/HWRF models have been forecasting over the past
day. Despite this convective structure, the fast east-southeasterly
low-level flow will likely continue to enhance the winds on the
north side of the circulation. For this reason, the intensity
forecast still shows intensification in the short term to 55 kt.
However, additional intensification beyond that will likely require
a better vertically aligned vortex. This structure may be difficult
to achieve as moderate mid-level shear continues, counter to the
motion vector of the storm. After 48 hours, the intensity forecast
shows slight weakening given the possibility of land interaction
over the Greater Antilles. The latest intensity forecast continues
remain on the conservative side relative to the guidance, especially
the HWRF/HMON regional hurricane models, and is also low
confidence.


Key Messages:

1. Tropical storm conditions are expected to begin early Friday in
portions of the Windward and southern Leeward Islands, and are
possible over portions of southern Hispaniola on Saturday, and
are also possible over Jamaica beginning Saturday night.

2. Heavy rainfall from Elsa will move quickly across the Windward
and southern Leeward Islands, including Barbados, on Friday. Outer
rain bands will impact Puerto Rico on Friday and southern Hispaniola
by early Saturday. Flooding and mudslides are possible.

3. There is a risk of wind and rainfall impacts in portions of
Cuba, the Turks and Caicos, and the Bahamas through early next
week. Interests in these areas should monitor Elsa's progress
and updates to the forecast.

4. There is a risk of storm surge, wind, and rainfall impacts in the
Florida Keys and portions of the Florida Peninsula early next week.
However, the forecast uncertainty remains larger than usual due to
Elsa's potential interaction with the Greater Antilles this weekend.
Interests in Florida should monitor Elsa's progress and updates to
the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  02/0300Z 11.8N  55.9W   45 KT  50 MPH
 12H  02/1200Z 12.8N  59.7W   55 KT  65 MPH
 24H  03/0000Z 14.5N  64.8W   55 KT  65 MPH
 36H  03/1200Z 16.2N  69.8W   55 KT  65 MPH
 48H  04/0000Z 17.7N  73.3W   55 KT  65 MPH
 60H  04/1200Z 19.4N  76.5W   50 KT  60 MPH
 72H  05/0000Z 21.2N  78.8W   50 KT  60 MPH
 96H  06/0000Z 23.9N  81.7W   50 KT  60 MPH
120H  07/0000Z 28.2N  83.1W   50 KT  60 MPH

$$
Forecaster Papin/Brown
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