Tropical Storm Isaias Forecast Discussion

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019 
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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