[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 200238
TCDAT4

TROPICAL STORM DANNY DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
1100 PM AST WED AUG 19 2015

Convection associated with Danny has increased in both coverage and
organization during the past several hours.  Overall, though, the
storm still looks a little less organized than 24 hours ago.  The
various satellite intensity estimates are unchanged since the last
advisory, so the initial intensity remains 45 kt.  The cirrus
outflow is good in the northern semicircle and poor elsewhere.

Danny has turned to the right over the past several hours, and the
initial motion is now 295/9.  A weak subtropical ridge north of the
cyclone is expected to steer Danny generally west-northwestward for
the next couple of days.  After that, the ridge is forecast to
strengthen as a mid- to upper-level low currently near Bermuda
lifts out to the north.  This should cause Danny to turn more
westward at a faster forward speed, with the center forecast to be
near the Leeward Islands by 120 hours.  There is some spread in
the guidance during this time, with the UKMET, GFDN, GFDL and NAVGEM
models showing a more northward motion than the other guidance.  One
change since the previous advisory is that the new GFS has shifted
southward and now lies closer to the ECMWF forecast on the south
side of the guidance envelope.  The new track forecast, which is on
the south side of the guidance near the ECMWF/GFS/HWRF/GFS Ensemble
Mean solutions, is similar to the previous forecast.

Earlier Windsat data shows that Danny has a well-defined inner
core, and this, combined with a light-vertical wind shear
environment, should allow strengthening for the next three days or
so.  However, experimental Saharan Air Layer (SAL) Meteosat
imagery continues to show abundant dry air not far from the storm
in the northern semicircle.  Entrainment of this air is expected to
limit intensification, and this is reflected by a downward trend in
the intensity guidance.  After 72 hours, the cyclone is expected to
encounter even drier air and southwesterly shear, which should cause
weakening.  The new intensity forecast is identical to the previous
forecast through 48 hours and then shows slightly lower intensities
thereafter.  It should be noted that the ECMWF and Canadian models
forecast Danny to weaken to a tropical wave by 120 hours, which is a
plausible alternative scenario given the forecast environment and
the small size of the cyclone.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 12.1N  42.7W   45 KT  50 MPH
 12H  20/1200Z 12.6N  43.9W   50 KT  60 MPH
 24H  21/0000Z 13.1N  45.4W   55 KT  65 MPH
 36H  21/1200Z 13.7N  47.0W   60 KT  70 MPH
 48H  22/0000Z 14.4N  48.6W   65 KT  75 MPH
 72H  23/0000Z 15.5N  53.0W   70 KT  80 MPH
 96H  24/0000Z 16.0N  57.5W   65 KT  75 MPH
120H  25/0000Z 17.0N  62.5W   55 KT  65 MPH

$$
Forecaster Beven
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