[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 210837
TCDAT4

HURRICANE DANNY DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
500 AM AST FRI AUG 21 2015

Danny has strengthened a little overnight.  The eye of the compact
hurricane has become more distinct recently and the cloud tops have
cooled in the eyewall.  The Dvorak classifications from TAFB and SAB
were both T4.5 at 0600 UTC, and the initial wind speed has been
nudged upward to 75 kt accordingly.  The eye of Danny passed about
75 n mi south of NOAA buoy 41041 a few hours ago and winds at that
location were only around 20 kt, indicative of the very small size
of the wind field of this hurricane.

The cyclone continues to move west-northwestward at about 9 kt.
The relatively slow forward speed of the system is due to a trough
over the western Atlantic, which has weakened the subtropical
ridge.  This trough is expected to lift northward in a couple of
days, allowing the ridge to build westward and strengthen.  The
change in the steering flow should cause Danny to turn westward and
speed up this weekend and early next week.  The track guidance
remains in good agreement, and the NHC forecast lies closest to the
consensus of the GFS and ECMWF models.  The new forecast track is
just a tad to the north of the previous one at days 4 and 5, but is
otherwise just an update.

Danny is currently in a very low wind shear environment and over
warm water, and it is expected to remain in these favorable
conditions for about another 12-24 hours.  Therefore, additional
strengthening is possible in the short term.  Beyond that time,
however, the system is expected to move into an area of increased
southwesterly shear and drier air.  These factors should induce a
weakening trend.  The NHC intensity forecast is in best agreement
with the intensity model consensus IVCN.  As mentioned in previous
discussions, the small size of the hurricane makes it susceptible to
sudden changes in intensity, which are difficult to predict.

A NOAA aircraft will be conducting a research mission in and around
Danny this afternoon, and an Air Force Reserve Unit reconnaissance
aircraft will investigate Danny Saturday afternoon.  These data
will provide a better assessment of the intensity and structure of
the hurricane.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 13.7N  47.4W   75 KT  85 MPH
 12H  21/1800Z 14.3N  48.8W   80 KT  90 MPH
 24H  22/0600Z 14.9N  50.8W   75 KT  85 MPH
 36H  22/1800Z 15.5N  53.0W   65 KT  75 MPH
 48H  23/0600Z 15.9N  55.3W   60 KT  70 MPH
 72H  24/0600Z 16.7N  60.5W   50 KT  60 MPH
 96H  25/0600Z 17.8N  65.7W   45 KT  50 MPH
120H  26/0600Z 18.5N  70.5W   40 KT  45 MPH...INLAND

$$
Forecaster Cangialosi
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