Friday, July 04, 2014

Here comes Hurricane Arthur (Cat. 1) Update Four

WTNT41 KNHC 042056

500 PM EDT FRI JUL 04 2014

The cloud pattern of Arthur remains very symmetric with a hint of an
eye remaining in visibile and infrared satellite imagery, although
the cloud tops have continued to gradually warm during the day.
Recent reconnaissance data show that the radius of maximum winds has
increased, which is likely the first sign that the hurricane is
beginning the transition to a post-tropical cyclone. The aircraft
data also indicate that the winds aloft are not mixing down as
efficiently as they were overnight and this morning. In fact, during
the latest pass through the southeastern quadrant the aircraft
measured a peak 700-mb flight level wind of 103 kt, but the surface
winds underneath were only 63 kt. Based on these data, the initial
intensity is lowered to 70 kt.  Arthur is now located over sea
surface temperatures of around 24C. The hurricane will be moving
over progressively colder waters and into an area of increasing
southwesterly shear, which should result in continued weakening.
Global model guidance indicates that Arthur should become
post-tropical tomorrow morning.  After the extratropical transition,
the cyclone is forecast to continue weakening as it moves over the
North Atlantic.

The initial motion estimate is 040/23 kt. The hurricane should
continue to move northeastward during the next 24 to 36 hours in
deep-layer southwesterly flow ahead of the approaching deep-layer
trough. After that time, the post-tropical cyclone is forecast to
slow down and eventually turn east-northeastward.  There is still
a large amount of spread in the track guidance after 48 hours. The
updated NHC forecast is between the ECWMF and GFS models, and leans
toward guidance provided by the NOAA Ocean Prediction Center.

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  04/2100Z 39.4N  71.1W   70 KT  80 MPH
12H  05/0600Z 42.2N  68.2W   60 KT  70 MPH
24H  05/1800Z 45.3N  65.3W   55 KT  65 MPH...POST-TROP/EXTRATROP
36H  06/0600Z 47.0N  61.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
48H  06/1800Z 49.4N  57.2W   40 KT  45 MPH...POST-TROP/EXTRATROP
72H  07/1800Z 56.0N  51.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
96H  08/1800Z 60.0N  49.0W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  09/1800Z 61.0N  42.5W   25 KT  30 MPH...POST-TROP/EXTRATROP

Forecaster Brown


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