Friday, July 04, 2014

Here comes Hurricane Arthur (Cat. 1) Update Six - Becoming Extratropical

WTNT41 KNHC 050237

1100 PM EDT FRI JUL 04 2014

Satellite and radar data indicates that Arthur has begun
extratropical transition.  The eye feature seen previously has
dissipated, and the remaining convection is now displaced somewhat
to the northeast of the low-level center.  In addition, microwave
total precipitable water data from a recent SSM/IS overpass shows
very dry air entraining into the west and south sides of the
cyclone.  SFMR data from the previous Air Force Reserve Hurricane
Hunter flight showed maximum winds of 60-65 kt, and based on this
the initial intensity is reduced to a possibly generous 65 kt.

The initial motion is now 045/27 kt.  Arthur should continue to move
northeastward during the next 24-36 hours in deep-layer
southwesterly flow ahead of a baroclinic trough over the
northeastern United States.  After that time, the post-tropical
cyclone is forecast to slow down and eventually turn
east-northeastward.  Based on the current motion and model trends,
this part of the forecast has been nudged a little to the east of
the previous track.  There remains a large amount of spread in the
track guidance after 48 hours on whether the former Arthur will turn
more northward or turn east-northeastward out into the Atlantic. The
new official forecast during that period has been revised based on
a blend of input from the Ocean Prediction Center along with the
GFS and ECMWF models.

Arthur is expected to complete extratropical transition in about 12
hours.  After transition, the former tropical cyclone should
steadily weaken, with winds dropping below gale force by 96 hours.
The intensity forecast has been revised downward based on input from
the OPC, as well as from the GFS and ECMWF models.

Winds in excess of gale/tropical storm force are expected over
portions of eastern Maine on Saturday.  Because Arthur is expected
to be extratropical by Saturday morning, and because the strong
winds in Maine will be post-frontal, this threat is being handled by
high wind warnings issued by local NWS offices.


INIT  05/0300Z 41.2N  68.7W   65 KT  75 MPH
12H  05/1200Z 44.0N  66.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
24H  06/0000Z 46.5N  63.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
36H  06/1200Z 48.5N  59.3W   40 KT  45 MPH...POST-TROP/EXTRATROP
48H  07/0000Z 51.5N  55.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
72H  08/0000Z 58.0N  51.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
96H  09/0000Z 60.0N  49.0W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  10/0000Z 61.5N  44.5W   25 KT  30 MPH...POST-TROP/EXTRATROP

Forecaster Beven


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