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Tropical Storm Arthur Discussion Number   4
NWS National Hurricane Center Miami FL       AL012020
1100 AM EDT Sun May 17 2020
 
Visible satellite imagery reveals a couple of convective bands over 
the eastern semicircle of the tropical storm, but convection is 
sparse over the western portion of the circulation.  The latest 
reports from an Air Force reconnaissance aircraft show peak 925-mb 
flight-level winds of 48 kt, and believable SFMR winds of 35-40 kt.  
On this basis, the initial wind speed has been increased to 40 kt. 
The plane has reported a minimum pressure of 1002-1003 mb, which is 
down a few millibars from the previous flight.
 
Arthur has a little more than 24 hours in which to gradually
strengthen.  The storm will be traversing the relatively warm
waters of the Gulf stream, and the vertical shear is forecast to
remain low through early Monday.  After that time, increasing shear
and cooler sea surface temperatures should put a halt to the
tropical cyclone strengthening processes.  Baroclinic forcing is
likely to help the post-tropical cyclone maintain its strengthen
through extratropical transition.  Later in the forecast period,
the global model guidance shows weakening as the frontal gradients
decrease.
 
The storm is moving north-northeastward or 015/8 kt.  Arthur is
expected to begin moving a little faster later today and tonight as
a mid-level trough approaches the eastern United States.  Later in
the forecast period, the cyclone should turn eastward within the 
westerly steering flow. Although the dynamical models are in 
agreement on the overall scenario, there remains some spread as to 
how close the center of Arthur will track to the North Carolina 
Outer Bands.  The GFS and HWRF remain along the western side of the 
guidance while the ECMWF and UKMET bracket the eastern side.  The 
NHC track lies near the model consensus and little change was 
required to the previous track through 36-48 hours.  After that 
time, the track guidance spread increases with the ECMWF showing a 
much faster east-southeastward motion than the GFS.  The NHC 
forecast remains near the consensus after 48 hours, but there is 
less confidence in that portion of the track prediction.

 
Key Messages:
 
1. A Tropical Storm Warning is in effect for a portion of the
North Carolina coast.  Tropical-storm-force winds and heavy rains
are expected there on Monday.
 
2. Dangerous coastal surf conditions and rip currents are expected
to spread northward along the southeast U.S. coast to the 
mid-Atlantic states during the next couple of days.  See products 
from your local National Weather Service Forecast Office for more 
details.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  17/1500Z 30.5N  77.4W   40 KT  45 MPH
 12H  18/0000Z 32.0N  76.8W   45 KT  50 MPH
 24H  18/1200Z 34.3N  75.5W   50 KT  60 MPH
 36H  19/0000Z 36.4N  73.2W   50 KT  60 MPH
 48H  19/1200Z 37.3N  70.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  20/0000Z 37.1N  68.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  20/1200Z 36.5N  66.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  21/1200Z 35.5N  64.5W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  22/1200Z...DISSIPATED
 
$$
Forecaster Brown
 
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