[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 012052
TCDAT4

Tropical Storm Isaias Discussion Number  19
NWS National Hurricane Center Miami FL       AL092020
500 PM EDT Sat Aug 01 2020

The last data received from a previous Air Force Reserve
reconnaissance aircraft, along with recent satellite and radar
imagery, indicate that Isaias has weakened to a tropical storm due
to a combination of shear, dry air and interaction with Andros
Island earlier today. The initial intensity of 60 kt is based on
SFMR surface wind speeds of near 60 kt in the northeastern quadrant
on the last flight. A new reconnaissance mission into the cyclone is
currently ongoing and will provide new data concerning the Isaias'
intensity.

The initial motion estimate is 315/09 kt. After making a slight
west-northwestward jog a few hours ago after convection
significantly weakened, Isaias appears to have returned to its base
northwestward course. The new NHC model guidance is tightly packed
but has shifted slightly westward, with some of the more reliable
models now showing landfall along the east-central Florida coast in
about 24 hours. Earlier NOAA G-IV jet dropsonde data and 12Z
upper-air data reveal that the surface to 700 mb ridge extending
east-west across central and northern Florida remains intact whereas
the 500-300 mb ridge has completely eroded. The result is that lower
level ridge will cause Isaias to slow its forward motion to
northwestward at 6-8 kt during the next 36 hours. By 48 hours, the
erosion of the ridge due to an approaching shortwave trough will
allow the cyclone to move northward, followed by a gradual increase
in forward speed toward the northeast on days 3-5. The new NHC track
forecast was nudged slightly closer to the Florida east-central
coast through 24 hours, with no significant changes made to the
previous forecast after 36 hours.

A combination of Isaias moving over the warm Gulfstream waters
during the convective maximum period and increasing frictional
convergence due to land interaction with Florida should lead to an
increase in deep convection near and over the center, as shown by
simulated satellite imagery from the ECMWF model. As a result,
Isaias is forecast to regain hurricane status tonight, as shown by
the HWRF and HMON model fields. By 36 hours and beyond, the global
models are in good agreement that an approaching mid- to upper-level
trough will increase southwesterly vertical wind shear, which
should result in gradual weakening until Isais becomes an
extratropical cyclone in about 96 hours. The new NHC intensity
forecast is closest to the HMON in 12 hours and closely follows the
IVCN and HCCA consensus models after 36 hours.

Key Messages:

1. Hurricane conditions and dangerous storm surge will continue in
portions of the northwest Bahamas tonight.

2. Hurricane conditions are expected along portions of the Florida
east coast on Sunday with tropical storm conditions expected to
begin tonight. Preparations to protect life and property should be
rushed to completion.

3. Dangerous storm surge is possible along the Florida east coast
from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4
feet above ground level are possible along the immediate coastline
and adjacent waterways. Residents there should follow advice given
by local emergency officials.

4. Isaias will produce heavy rains and potentially life-threatening
flash flooding in the Bahamas, and flash and urban flooding,
especially in low-lying and poorly drained areas, along the East
Coast of the United States. Minor river flooding and isolated
moderate river flooding is possible across portions of the Carolinas
and Virginia early next week.

5. Tropical storm watches and warnings are in effect for portions of
the United States East Coast from northeast Florida to South
Carolina. Additional watches and warnings will likely be issued
tonight and Sunday as Isaias is expected to move northward near or
over the southeast and mid-Atlantic coasts early next week.

FORECAST POSITIONS AND MAX WINDS

INIT  01/2100Z 25.1N  78.7W   60 KT  70 MPH
 12H  02/0600Z 26.1N  79.4W   65 KT  75 MPH
 24H  02/1800Z 27.4N  80.1W   65 KT  75 MPH
 36H  03/0600Z 28.8N  80.7W   60 KT  70 MPH
 48H  03/1800Z 30.7N  80.6W   55 KT  65 MPH
 60H  04/0600Z 33.4N  79.1W   50 KT  60 MPH
 72H  04/1800Z 37.4N  76.3W   45 KT  50 MPH...INLAND
 96H  05/1800Z 45.4N  68.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  06/1800Z 50.7N  57.9W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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