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Tropical Storm DANNY


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TROPICAL STORM DANNY DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
1100 PM AST TUE AUG 18 2015

Danny has become a little better organized since the last advisory,
with a central convective feature and some outer banding in the
southeastern semicircle.  At 2200 UTC, PIRATA buoy 41026 reported
42-kt sustained winds just north of the center, and an ASCAT-B
overpass near 2330 UTC showed many 40-45 kt vectors.  Based on
these data, the initial intensity is increased to 45 kt.  The ASCAT
data also showed that the storm was a little larger than previously
thought.

The initial motion is now 285/12.  Danny is on the south side of
the subtropical ridge.  The dynamical models forecast the ridge to
weaken during the next 48-72 hours due to developing mid- to
upper-level troughs near Bermuda and over the northeastern
Atlantic.  This should result in Danny continuing on a
west-northwestward track with a decrease in forward speed.  After
72 hours, the trough near Bermuda should move northward and allow
the subtropical ridge to intensify.  In response, Danny is expected
to turn westward with an increase in forward speed.  The GFS had a
better initialization of Danny on the 1800 UTC run, and it now
forecasts a faster forward motion.  Otherwise, there are no
significant changes in the track guidance since the last advisory.
Thus, the new forecast track is an update of the previous track and
lies near the consensus models and the center of the guidance
envelope.

Danny is forecast to be in an environment of light vertical wind
shear through the forecast period, and earlier Windsat data
suggest the storm has tight convective banding near the center.
In addition, the Rapid Intensification Index of the SHIPS model has
about a 50 percent chance of 25 kt of strengthening during the next
24 hours.  However, there is abundant dry air near Danny,
particularly to the north of the storm, and the dynamical models
forecast Danny to encounter increasingly drier air through the
forecast period.  The intensity forecast, which follows the overall
trend of the intensity guidance, calls for continued strengthening
through 96 hours, followed by a slight weakening due to dry air.
The new forecast has a higher peak intensity than that of the
previous advisory, but is not as intense as the SHIPS and LGEM
models.  There are two alternative possibilities for this forecast.
The first is that rapid intensification occurs in the next 24 hours
before dry air entrains into Danny's core.  The second is that
enough dry air starts entraining during the next 24 hours to cause a
slower rate of development than currently forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 11.2N  38.8W   45 KT  50 MPH
 12H  19/1200Z 11.6N  40.2W   55 KT  65 MPH
 24H  20/0000Z 11.9N  41.7W   60 KT  70 MPH
 36H  20/1200Z 12.3N  42.9W   70 KT  80 MPH
 48H  21/0000Z 12.8N  44.1W   75 KT  85 MPH
 72H  22/0000Z 13.5N  47.5W   85 KT 100 MPH
 96H  23/0000Z 14.5N  51.5W   90 KT 105 MPH
120H  24/0000Z 15.0N  56.5W   85 KT 100 MPH

$$
Forecaster Beven

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