[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 022057
TCDAT4

Tropical Storm Isaias Discussion Number  23
NWS National Hurricane Center Miami FL       AL092020
500 PM EDT Sun Aug 02 2020

The earlier intense recent burst has waned since this morning, but
the large convective cell has persisted. At the peak of the activity
around 1500Z, several small patches of Doppler velocities of 90-96
kt were co-located with the pronounced mid-level circulation that
was evident in the Melbourne radar reflectivity data. However,
these intense wind speed values were short-lived for only about 20
minutes and, thus, were not considered to representative of Isaias'
tangential wind field. Since that time, the cyclone has become more
steady state with Doppler radar and Air Force Reserve aircraft data
indicating surface winds in the 56-63 kt range. Therefore, the
initial intensity of 60 kt is an average of these values.

Radar and aircraft fixes indicate that Isaias is still moving
toward the north-northwest or 345/08 kt. The latest model guidance
remains in excellent agreement on Isaias moving north-northwestward
through a break in the subtropical ridge tonight and turning
northward by Monday morning, all the while remaining offshore of the
coast from east-central Florida to Georgia. By Monday night, Isaias
is forecast to turn northeastward and accelerate toward the
Carolinas, reaching the mid-Atlantic states on Tuesday and New
England by early Wednesday. The new NHC forecast track during the
first 24 hours lies a little east of the previous one, but is
essentially just an extension of the previous advisory track
thereafter, and lies close to the various consensus models, which
are lightly packed around the previous NHC foreast.

Isaias will continue to move slowly over the warm Gulfstream waters
for the next 36 h or so. Despite unfavorable vertical shear
conditions of around 25 kt, Isaias is expected to maintain its
current intensity until landfall, and could restrengthen to
hurricane status in 24-36 h when the vertical shear vector is
forecast to switch from westerly to southwesterly which would align
the shear along the direction of storm motion. Some baroclinic
interaction is expected on days 2-3 when Isaias will move into the
right-rear quadrant of an anticyclonically curved jet streak, which
is expected to hold the intensity a little above what would normally
be expected for a post-landfall tropical cyclone. The NHC intensity
forecast is similar to the HFIP corrected consensus model and the
IVCN intensity consensus model, which agree well with the GFS and
ECMWF model intensity forecasts.

Key Messages:

1. There is the danger of life-threatening storm surge inundation of
2 to 4 feet above ground level from Edisto Beach South Carolina to
Cape Fear North Carolina along the immediate coastline and adjacent
waterways. Life-threatening storm surge is possible along the
North Carolina coast from Cape Fear to Duck.  Residents in these
areas should follow advice given by local emergency officials.

2. Tropical storm conditions will spread northward within the
Tropical Storm Warning area from Florida to North Carolina through
Monday night. Isaias is expected to be near hurricane strength when
it reaches the coast of northern South Carolina and southern North
Carolina Monday night, and strong tropical storm force winds are
likely with hurricane conditions possible in the Hurricane Watch
area.

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 eastern Carolinas and the mid-Atlantic, are expected through
midweek along and near the path of Isaias across the U.S. East
Coast. Widespread minor to isolated moderate river flooding is
possible across portions of the Carolinas and the mid-Atlantic.

4. A tropical storm watch is in effect for the U.S. mid-Atlantic
coast, including the Chesapeake Bay, Delaware Bay, and Long Island
Sound, as tropical storm force winds are possible in these areas on
Tuesday and Tuesday night. Additional watches and warnings will
likely be issued tonight and Monday as Isaias is expected to move
northward near or over the mid-Atlantic and New England states
Tuesday and Wednesday.

FORECAST POSITIONS AND MAX WINDS

INIT  02/2100Z 27.8N  79.8W   60 KT  70 MPH
 12H  03/0600Z 29.0N  80.1W   60 KT  70 MPH
 24H  03/1800Z 30.9N  79.9W   60 KT  70 MPH
 36H  04/0600Z 33.7N  78.8W   60 KT  70 MPH
 48H  04/1800Z 38.1N  75.8W   50 KT  60 MPH...INLAND
 60H  05/0600Z 42.5N  72.2W   45 KT  50 MPH...INLAND
 72H  05/1800Z 46.3N  68.4W   40 KT  45 MPH...INLAND
 96H  06/1800Z 54.2N  58.4W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  07/1800Z...DISSIPATED

$$
Forecaster Stewart
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