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TROPICAL STORM IDA DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST MON SEP 21 2015

A significant increase in the amount and organization of deep
convection has occurred during the past 6 hours. A large CDO-like
feature with a connecting curved band in the southern quadrant has
developed, and cloud tops have cooled significantly to colder than
-80C. However, the low-level circulation center is not embedded in
the core of the main convective cloud mass, and is instead located
about 30 n mi inside the western portion of the cloud shield. This
suggests that some westerly vertical shear still exists. The initial
intensity has been increased to 45 kt based on a Dvorak satellite
intensity estimate of T3.0/45 kt from TAFB using a shear pattern.

Recent AMSU and AMSR-2 microwave satellite fixes indicate that Ida's
forward motion has continued to decrease and is now 330/07 kt.
Global models indicate that the subtropical ridge to the north of
Ida is rapidly eroding as a broad mid-/upper-level trough to the
northeast and east of the cyclone slowly retrogrades westward.
Steering currents are forecast to collapse within the next 12-24
hours, resulting in Ida becoming nearly stationary during that time.
By 36-48 hours, Ida is expected to become embedded within the
western portion of the aforementioned east-west oriented trough and
move slowly southeastward through 72 hours or so. After that time,
the trough is forecast to weaken, allowing the cyclone to move
slowly north-northwestward to northward. The resultant model
guidance more closely resembles the proverbial squashed spider
pattern, which is usually indicative of slow and meandering motion.
The NHC official track forecast follows suit, lying close to the
previous one and the consensus model TVCA.

Once the low-level center becomes juxtaposed with the mid-/and
upper-level circulations, significant strengthening should occur.
Given that vertical wind shear values are forecast by the GFS and
ECMWF models to decrease to near 5 kt within the next 12 hours or
so, at least slow and steady intensification should continue for the
next 24-36 hours. After that time, the vertical shear is forecast to
increase from the west and northwest as the aforementioned trough
begins to interact with Ida. The intensity is forecast to decrease
from 48-72 hours, followed by re-strengthening as the trough and its
associated upper-level shear lifts out to the northeast, leaving Ida
in a weak col region. Given that Ida will be over SSTs of at least
29C and within an upper-level cold pool by days 4 and 5, instability
should increase and act to regenerate vigorous convection despite
mid-level humidity values of near 50 percent. The official intensity
forecast is slightly higher than the previous forecast and the
intensity consensus model IVCN, and is similar to a blend of the
SHIPS/LGEM, GFDL, and ECMWF intensity forecasts.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 19.8N  47.3W   45 KT  50 MPH
 12H  21/1800Z 20.7N  47.9W   50 KT  60 MPH
 24H  22/0600Z 21.3N  48.1W   60 KT  70 MPH
 36H  22/1800Z 21.4N  48.0W   60 KT  70 MPH
 48H  23/0600Z 21.1N  47.7W   55 KT  65 MPH
 72H  24/0600Z 20.8N  47.2W   50 KT  60 MPH
 96H  25/0600Z 21.8N  47.6W   60 KT  70 MPH
120H  26/0600Z 23.3N  48.5W   65 KT  75 MPH

$$
Forecaster Stewart

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