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Tropical Storm GERT


Tropical Storm Gert Discussion Number   7
NWS National Hurricane Center Miami FL       AL082017
1100 AM EDT Mon Aug 14 2017

Gert has become better organized this morning.  A recent 1336 UTC
GMI overpass shows a large band of convection wrapping around the
circulation from the southwest to the northwest quadrant.  However,
imagery from the GMI overpass, as well as an earlier SSMIS overpass,
suggest that the mid-level center is displaced somewhat south or
southeast of the low-level center, due to continued northerly shear
of about 15 kt. Based on the improved structure and increased
subjective Dvorak classifications from TAFB and SAB, the initial
intensity has been raised to 50 kt.

Gert is now moving toward the north, and the initial motion estimate
is 360/07 kt.  Very little change has been made to the track
forecast.  Gert should continue to move around the western
periphery of the subtropical ridge for the next day or so, before
rapidly accelerating eastward ahead of a trough.  Since Gert has
already turned northward, the NHC forecast is on the eastern side
of the guidance envelope for the first 48 h, but very near the
multi-model consensus after that time.

Despite the presence of moderate northerly vertical wind shear,
there is good agreement among the intensity guidance that Gert will
continue to strengthen for the next 48 h.  The HWRF is an outlier
in showing Gert becoming a major hurricane around that time, while
DSHP, LGEM, and CTCI only forecast modest intensification.  Since
SHIPS diagnostics from the GFS and ECMWF suggest that the shear will
never decrease below 10 kt, the NHC forecast gives more weight to
the modest intensification scenario.  All of the global models show
extratropical transition (ET) beginning around 48 h, and completing
by about 72 h.  After ET completes, Gert is still expected to
steadily weaken before being absorbed by a larger extratropical low
in about 4 or 5 days.

The 72 h and 96 h track, intensity, and wind radii forecasts
incorporated guidance from the Ocean Prediction Center.


INIT  14/1500Z 30.3N  72.2W   50 KT  60 MPH
 12H  15/0000Z 31.5N  72.2W   55 KT  65 MPH
 24H  15/1200Z 33.4N  71.3W   60 KT  70 MPH
 36H  16/0000Z 35.8N  68.8W   65 KT  75 MPH
 48H  16/1200Z 38.4N  64.0W   70 KT  80 MPH
 72H  17/1200Z 43.0N  50.0W   65 KT  75 MPH...POST-TROP/EXTRATROP
 96H  18/1200Z 50.0N  36.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  19/1200Z...DISSIPATED

Forecaster Zelinsky