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TROPICAL STORM NORBERT DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP142014
200 AM PDT WED SEP 03 2014

The convective cloud pattern of Norbert has improved significantly
since the previous advisory. A pronounced CDO feature has developed
with two distinct convective bands wrapping into the center. A 0432
UTC AMSU microwave indicated that a small mid-level eye feature
could be trying to form. Satellite intensity estimates from TAFB
and SAB were a consensus T3.0/45 kt at 0600 UTC, and UW-CIMSS ADT
values have been steadily increasing and are now T3.7/59 kt. Based
on these intensity estimates, the continued improvement in the cloud
pattern, and the small radius of maximum winds, the advisory
intensity is conservatively set at 50 kt.

The initial motion estimate is still northwestward or 305/07 kt.
Norbert is forecast to move in a general northwestward direction
around the southwestern periphery of a deep-layer subtropical ridge
that stretches westward across northern Mexico and Baja California.
This motion, accompanied by a slight decrease in forward speed, is
expected to persist throughout the forecast period. The NHC model
guidance has shifted slightly to the right again, but the size of
the guidance envelope has decreased, showing much less spread on
this cycle. The official forecast track is similar to the previous
advisory track through 36 hours, and then is somewhat to right of
the previous forecast after that.

The combination of the small radius of maximum winds, 29.5C SSTs, a
moist mid-level environment, and vertical shear decreasing to
around 10 kt suggests that Norbert should at least intensity at the
typical rate of 20-25 kt per day for the next 36 hours or so.
However, if thew shear decrease more than currently expected, then
rapid intensification during the next 24 hours is a very distinct
possibility. By 96 hours and beyond, Norbert will be moving over
SSTs less than 26.5C, which should induce slow weakening. NHC
intensity forecast is above the previous forecast, and closely
follows the SHIPS intensity model.

FORECAST POSITIONS AND MAX WINDS

INIT  03/0900Z 19.6N 107.6W   50 KT  60 MPH
 12H  03/1800Z 20.0N 108.5W   55 KT  65 MPH
 24H  04/0600Z 20.6N 109.4W   65 KT  75 MPH
 36H  04/1800Z 21.4N 110.1W   75 KT  85 MPH
 48H  05/0600Z 22.3N 110.9W   75 KT  85 MPH
 72H  06/0600Z 24.0N 112.7W   65 KT  75 MPH
 96H  07/0600Z 25.1N 115.1W   60 KT  70 MPH
120H  08/0600Z 26.2N 117.0W   55 KT  65 MPH

$$
Forecaster Stewart

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