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Tropical Storm Bill


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First discussion.

Tropical Depression Two Discussion Number   1
NWS National Hurricane Center Miami FL       AL022021
1100 AM EDT Mon Jun 14 2021

Satellite and radar images show that the low pressure area that NHC 
has been following since yesterday off the coast of North Carolina 
has become better organized, with a small central dense overcast 
over the center and more prominent banding features.  The low also 
has advanced ahead of a nearby diffuse stationary front, with that 
boundary lying northwest of the center.  Considering the small core 
of the low, ample deep convection, satellite pattern, and that the 
low is feeding off of the thunderstorm activity (and not the front) 
-- it is now classified as a tropical depression.  The initial 
intensity is set to 30 kt, which is our best estimate assuming 
strengthening from the 20-25 kt overnight ASCAT and surface 
observations.

The depression is moving northeastward at about 18 kt.  A large 
mid-latitude trough digging across eastern North America should 
cause the cyclone to continue moving generally northeastward, but 
faster, over the next couple of days.  In about 48 hours, the system 
is forecast to dissipate near Newfoundland as it is absorbed by a 
larger extratropical low.  Model guidance is in very good agreement 
on this scenario, and the NHC forecast lies near the track 
consensus.

The depression has about 24 hours over marginally warm waters in
low-to-moderate shear to strengthen before it moves north of the 
Gulf Stream and decays.  Almost all of the intensity guidance shows 
the system becoming a tropical storm tonight, and considering the
healthy initial structure, the official forecast follows that
guidance.  The low should lose tropical characteristics in about 36
hours due to very cold waters and dissipate near Newfoundland in 
about 2 days.  

FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 35.0N  73.7W   30 KT  35 MPH
 12H  15/0000Z 36.6N  70.9W   40 KT  45 MPH
 24H  15/1200Z 39.8N  65.3W   45 KT  50 MPH
 36H  16/0000Z 43.6N  59.7W   40 KT  45 MPH...POST-TROP/EXTRATROP
 48H  16/1200Z...DISSIPATED

$$
Forecaster Blake

 

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Tropical Storm Bill Discussion Number   3
NWS National Hurricane Center Miami FL       AL022021
1100 PM AST Mon Jun 14 2021

Deep convection has increased in a cluster to the northeast of the 
estimated center.  Scatterometer measurements showed a couple of 
40-41 kt vectors over the southeastern quadrant of the cyclone.  On 
this basis, the estimated intensity is increased to 40 kt, which 
makes this the second tropical storm of the season, albeit a rather 
high-latitude one.  The system is in an environment of fairly 
strong southwesterly shear, but the diffluent upper-level flow has 
apparently contributed to some strengthening in a seemingly hostile 
environment.  Some additional short-term strengthening could occur 
but by 36 hours, the cyclone should merge with a baroclinic zone 
while approaching Newfoundland and become extratropical.  This 
transition is also shown by the FSU cyclone phase analyses of the 
GFS model fields.

Bill is moving fairly swiftly northeastward, or 055/20 kt.  The 
track forecast appears to be straightforward.  The flow ahead of a 
large mid-tropospheric trough near the United States east coast 
should accelerate the system northeastward for the next day or two, 
and until the system dissipates.  The NHC track forecast is similar 
to the previous one and not far from the model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0300Z 36.7N  69.8W   40 KT  45 MPH
 12H  15/1200Z 38.9N  65.1W   45 KT  50 MPH
 24H  16/0000Z 42.6N  59.6W   40 KT  45 MPH
 36H  16/1200Z 46.5N  54.5W   35 KT  40 MPH...POST-TROP/EXTRATROP
 48H  17/0000Z...DISSIPATED

$$
Forecaster Pasch

 

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