Tropical Storm Arthur Forecast Discussion

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127 
WTNT41 KNHC 170253
TCDAT1

Tropical Storm Arthur Discussion Number   2
NWS National Hurricane Center Miami FL       AL012020
1100 PM EDT Sat May 16 2020

An Air Force Reserve reconnaissance aircraft that has been
investigating the tropical cyclone east of Florida has recently
found maximum 925-mb flight-level winds of 45-46 kt to the
southeast and east of the center, which equates to surface winds
of 34-35 kt, along with uncontaminated SFMR surface wind speeds of
33-36 kt. Based on these data, the depression has been upgraded to
Tropical Storm Arthur, the first named storm of the 2020 Atlantic
hurricane season.

The initial motion estimate remains north-northeastward or 030/11
kt. Arthur made a slight northeastward jog earlier this evening,
but now appears to have returned to its previous base course. A
motion toward the north-northeast is forecast to continue for the
next 24 hours or so, keeping the cyclone well offshore the coasts
of Florida and Georgia. A sharp shortwave trough currently moving
across the southern Plains is forecast to dig east-southeast to
southeastward over the next 48 hours, which will act to accelerate
and eject Arthur more poleward. The more the shortwave trough digs
and loses latitude, the more Arthur could get pulled closer to
the North Carolina Outer Banks as per the GFS and HWRF scenarios.
In contrast, the ECMWF and UKMET models show the shortwave losing
amplitude quickly and lifting out, which acts to push Arthur
farther away from the United States east coast. For now, the new
NHC forecast track closely follows the various consensus models,
which are about midway between the GFS-HWRF and ECMWF-UKMET
solutions. However, the track was adjusted slightly to the right of
the previous advisory track due mainly to the more eastward initial
position. It should be noted that forecast track uncertainty is 
typically larger for weak systems like Arthur.

Arthur has moved off of the warm waters of the Gulfstream current
and currently is passing over a cold pool with SSTs near 24.5 deg
C. These cooler waters should prevent any significant strengthening
in the very near term. By 24 hours however, the cyclone is forecast 
to pass back over the warmer waters of the Gulfstream while moving 
into a very low vertical wind shear regime. These conditions, 
coupled with some cooler air aloft, should allow more vigorous 
convection to develop near the center, resulting in more 
strengthening as Arthur passes near the North Carolina coast. 
Extratropical transition should occur in about 48-60 hours over the 
much cooler waters of the North Atlantic. The NHC intensity 
forecast follows a blend of the consensus models HCCA and IVCN, and 
is similar to the previous intensity forecast.

Key Messages:

1. A tropical storm watch is in effect for a portion of the
North Carolina coast.  Tropical-storm-force winds and heavy rains
are possible there on Monday.

2. Dangerous coastal surf conditions and rip currents are expected
to spread northward from Florida to the mid-Atlantic states during
the next few days.  See products from your local National Weather
Service Forecast Office for more details.


FORECAST POSITIONS AND MAX WINDS

INIT  17/0300Z 29.4N  77.7W   35 KT  40 MPH
 12H  17/1200Z 30.5N  77.2W   35 KT  40 MPH
 24H  18/0000Z 32.2N  76.4W   40 KT  45 MPH
 36H  18/1200Z 34.1N  75.2W   45 KT  50 MPH
 48H  19/0000Z 36.2N  73.0W   45 KT  50 MPH
 60H  19/1200Z 37.5N  70.6W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  20/0000Z 38.0N  69.1W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  21/0000Z 38.0N  67.6W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  22/0000Z...DISSIPATED

$$
Forecaster Stewart




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