Tropical Storm Isaias Forecast Discussion Number 22

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Issued at 1100 AM EDT Sun Aug 02 2020


000
WTNT44 KNHC 021458
TCDAT4

Tropical Storm Isaias Discussion Number 22
NWS National Hurricane Center Miami FL AL092020
1100 AM EDT Sun Aug 02 2020

Another recent burst of deep convection has recently developed
northeast of the center, which has resulted in Isaias making a
slight northward jog. An impressive mid-level circulation has
developed within the strongest thunderstorm cluster, along with rare
reflectivity values of 55-60 dBZ for a tropical cyclone. The
Melbourne, Florida, Doppler weather radar has been indicating
patches of velocity values of 65-66 kt at around 10,000 ft just
north of the center, which equates to 58-59 kt surface winds. For
now, the intensity will remain at 55 kt since previous convective
bursts have not persisted for more than a couple of hours at best.
Another Air Force Reserve Hurricane Hunter aircraft is scheduled to
investigate Isaias later today.

Radar and aircraft fixes indicate that Isaias is now moving toward
the north-northwest or 340/07 kt. A slow north-northwestward motion
should continue for the next 24 hours or so as Isaias moves into a
weakness that has developed in the Bermuda-Azores ridge over north
Florida and off the Georgia coast seen in 02/1200Z upper-air data. A
turn toward the north is forecast to occur by all of the global
models by Monday morning, followed by a faster forward motion toward
the northeast by Monday afternoon and evening when the cyclone will
be influenced by southwesterly steering flow ahead of a strong
eastward-moving mid- to upper-level trough. Continued northeastward
acceleration across the mid-Atlantic and New England states is
expected on days 3 and 4. The global models continue to show little
cross-track difference, but still have significant along-track
differences with the GFS being the fastest and the ECMWF being
the slowest . Since the preponderance of the model guidance is
slower than the GFS solution, the new official forecast track
closely follows the consensus models TVCA and HCCA, and lies near
the previous advisory track after 12 h.

Isaias will remain over warm Gulfstream waters where water
temperatures are near 30 deg C. Despite very unfavorable vertical
shear conditions of 25-30 kt the past couple of days, the cyclone
has managed to hold together, which is an indication that the system
has a deep, well-formed vertical circulation. While some slight
intensification is possible if the shear decreases, the official
forecast calls for the intensity to remain steady until landfall
occurs in the Carolinas in 36 hours or so. Some baroclinic
interaction with the right-rear quadrant of an anticyclonically
curved jet streak is expected to hold the intensity a little above
what would normally be expected for a post-landfall tropical cyclone
in 48-72 hours. The NHC intensity forecast is similar to the HFIP
corrected consensus model and the IVCN intensity consensus model.

Key Messages:

1. Tropical storm conditions will spread northward along the east
coast of Florida within the warning area through early Monday and
will reach the coasts of Georgia, South Carolina, and southern North
Carolina within the warning area Monday and early Tuesday.

2. Dangerous storm surge is possible from Edisto Beach South
Carolina to Cape Fear North Carolina where water rises of 2 to 4
feet above ground level are possible along the immediate coastline
and adjacent waterways. A Storm Surge Warning may be needed for a
portion of this area later today, and residents there should follow
advice given by local emergency officials.

3. Heavy rainfall from Isaias will continue to result in potentially
life-threatening flash flooding in the Northwest Bahamas through
tonight. Flash and urban flooding, some of which may be significant
in the coastal Carolinas and Virginia, is expected through midweek
along and near the path of Isaias along the U.S. East Coast.
Widespread minor to isolated moderate river flooding is possible
across portions of the Carolinas and Mid-Atlantic.

4. A Tropical Storm Watch is in effect for portions of the North
Carolina coast, where tropical storm conditions are possible on
Tuesday. Additional watches and warnings will likely be issued later
today as Isaias is expected to move northward near or over the
mid-Atlantic and northeast coasts Tuesday and Wednesday.

FORECAST POSITIONS AND MAX WINDS

INIT 02/1500Z 26.9N 79.6W 55 KT 65 MPH
12H 03/0000Z 28.0N 80.1W 55 KT 65 MPH
24H 03/1200Z 29.6N 80.5W 55 KT 65 MPH
36H 04/0000Z 32.2N 79.8W 55 KT 65 MPH
48H 04/1200Z 35.7N 77.7W 50 KT 60 MPH...INLAND
60H 05/0000Z 40.1N 74.3W 45 KT 50 MPH...INLAND
72H 05/1200Z 44.6N 70.4W 40 KT 45 MPH...INLAND
96H 06/1200Z 51.7N 63.2W 30 KT 35 MPH...POST-TROP/EXTRATROP
120H 07/1200Z...DISSIPATED

$$
Forecaster Stewart

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