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TROPICAL DEPRESSION IDA DISCUSSION NUMBER  28
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST FRI SEP 25 2015

A few convective cells have been forming near Ida's center during
the past several hours, but most of the thunderstorm activity
remains displaced more than 150 n mi to the east-northeast due to
20 kt of shear.  Maximum winds remain 30 kt based on the latest
Dvorak CI number from TAFB, but it's clear from satellite imagery
that the shear and dry air has caused a steady degradation of the
cyclone's structure during the past few days.  With moderate
vertical shear expected to continue and the mid-level relative
humidity forecast to drop to between 30 and 40 percent, additional
weakening is anticipated during the next couple of days, and Ida
could degenerate to a remnant low by 48 hours.  The NHC intensity
forecast shows the possibility of the maximum winds increasing again
by days 3 and 4, but this is the result of a tight pressure gradient
between the remnant low and strong high pressure over the north
Atlantic--not a restrengthening of Ida's circulation.  Based on
global model guidance, the remnant low could open up into a trough
by day 5, if not sooner.

Ida is being steered generally north-northwestward, or 335/3 kt, by
a low-level ridge over the eastern Atlantic.  The depression should
move northwestward through 48 hours and then abruptly turn westward
or west-southwestward during its remnant low stage when strong
low-level ridging develops over the north Atlantic.  The NHC track
forecast is close to the various consensus aids and shows a slightly
faster motion of the remnant low after 72 hours compared to the
previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 21.7N  45.3W   30 KT  35 MPH
 12H  25/1800Z 22.4N  45.6W   25 KT  30 MPH
 24H  26/0600Z 23.4N  46.4W   25 KT  30 MPH
 36H  26/1800Z 24.4N  47.2W   25 KT  30 MPH
 48H  27/0600Z 25.0N  47.9W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  28/0600Z 24.1N  49.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  29/0600Z 23.2N  52.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  30/0600Z...DISSIPATED

$$
Forecaster Berg

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