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Tropical Depression SEVEN

Tropical Depression Seven Discussion Number   3
NWS National Hurricane Center Miami FL       AL072021
1100 PM AST Fri Aug 13 2021
While Tropical Depression Seven has maintained a irregularly shaped 
cirrus canopy of deep cold cloud tops near its estimated center, the 
convection does not appear well organized. Several SSMIS microwave 
passes between 2011 UTC and 2205 UTC did not reveal much 
organization under the cirrus, with just a few patches of deeper 
convection contributing to the larger stratiform region. A helpful 
ASCAT-B pass at 0030 UTC showed that the center was near the 
southeastern end of this cirrus canopy, and found peak winds lower 
than earlier today at only 27 kt. The 0000 UTC subjective Dvorak 
estimates were T2.5/35 kt from SAB and T2.5/30 kt from TAFB and the 
most recent objective ADT estimate was in between at T2.2/32 kt. A 
blend of these data support keeping the initial intensity at 30-kt 
for this advisory.
The small cyclone continues to move quickly off to the west at 
280/18 kt. A large low- to mid-level ridge draped across the central 
and western North Atlantic is expected to maintain the system on a 
general west-northwest heading, though with gradual deceleration as 
the ridge is eroded some by a mid- to upper-tropospheric trough. The 
latest track guidance remains tightly clustered but a bit more 
poleward through the first 72 hours. Afterwards, more track guidance 
spread becomes apparent. A quick look at the latest ECMWF ensemble 
guidance suggest that some of this spread is driven by the forecast 
intensity of the system, with stronger members taking the cyclone on 
a more poleward track. For now, the latest NHC track forecast is 
fairly close to, but a little poleward of the previous track. This 
track remains close to the HCCA and TVCN consensus aids, and roughly 
splits the difference between the deterministic GFS and ECMWF model 
The intensity forecast is somewhat conflicting. Even though both the 
GFS & ECMWF based SHIPS guidance depict low 200-850 hPa vertical 
wind shear between 5-10 knots over the next 48 hours, the depression 
is also embedded in very dry mid-level air, with 700-500 hPa layer 
mean relative humidity as low as 44 percent currently in the 
ECMWF-SHIPS.  In addition, the system is moving rapidly westward, 
and a continued fast motion in the short-term may result in higher 
westerly mid-level shear which may have a larger than normal effect 
to a small tropical cyclone in a very dry environment. After 48 
hours, vertical wind shear out of the northwest is expected to 
increase, ahead of a large upper-level trough digging southwestward, 
upstream of the cyclone. Moreover, land interaction with both Puerto 
Rico and Hispaniola remains a distinct possibility, especially if 
the cyclone tracks left of the current forecast track. It is worth 
noting the latest HWRF run continues to be a extreme outlier with a 
much higher intensity than the remaining guidance. In fact, much of 
global model guidance and COAMPS-TC regional hurricane model barely 
maintains a closed circulation over the next 36-48 hours. I have 
elected to maintain a very similar forecast to the previous 
advisory, with peak winds of only 45 kt in 48-60 hours. This 
forecast remains conservative and is still lower than the SHIPS and 
HCCA intensity guidance.
Key Messages:
1.  Tropical storm conditions are expected in portions of the 
Leeward Islands late Saturday or early Sunday, and are possible over 
the Virgin Islands and Puerto Rico on Sunday.  The risk of strong 
winds will then spread westward to the Dominican Republic Sunday 
night and Monday.
2.  Heavy rainfall could lead to flash and urban flooding over the 
Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may 
lead to flash, urban and small stream flooding, along with the 
potential for mudslides.
3.  There is a risk of wind and rainfall impacts across Haiti, the
Turks and Caicos Islands, the southeastern Bahamas, and Cuba next
week, and interests in those areas should monitor the progress of
this system.
INIT  14/0300Z 15.5N  53.8W   30 KT  35 MPH
 12H  14/1200Z 16.0N  56.8W   35 KT  40 MPH
 24H  15/0000Z 16.7N  60.3W   35 KT  40 MPH
 36H  15/1200Z 17.5N  63.5W   40 KT  45 MPH
 48H  16/0000Z 18.3N  66.2W   45 KT  50 MPH...INLAND
 60H  16/1200Z 18.9N  68.2W   45 KT  50 MPH...OVER WATER
 72H  17/0000Z 19.6N  70.4W   35 KT  40 MPH...INLAND
 96H  18/0000Z 21.7N  74.3W   40 KT  45 MPH...OVER WATER
120H  19/0000Z 24.3N  78.6W   40 KT  45 MPH
Forecaster Papin/Brown