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Tropical Storm Alvin Discussion Number   7
NWS National Hurricane Center Miami FL       EP012019
300 AM MDT Thu Jun 27 2019

Alvin's satellite presentation is much improved since this time
yesterday, consisting of a small but distinct CDO with extremely
cold overshooting cloud tops of about -90C near the well-defined
center. Outflow is fairly symmetrical due to the low vertical wind
shear conditions affecting the compact cyclone. A 0435Z ASCAT-A pass
revealed that tropical-storm-force winds only extended 15-20 nmi
away from the center in the northern semicircle along with one peak
wind vector of 46 kt located in the northwestern quadrant near the
overshooting cloud tops. Given Alvin's small size and likely
undersampling by the scatterometer instrument, the intensity is
raised to 50 kt, which is consistent with ADT and SATCON estimates.

The initial position and motion vector of 300/11 kt are based on
ASCAT-A/-C scatterometer wind data and passive microwave fixes.
Although the new track has been shifted north of the previous
advisory track, there otherwise are no significant changes to the
previous forecast track reasoning. Alvin is expected to move
steadily west-northwestward around the southwestern periphery of a
deep-layer subtropical ridge that extends westward from northwestern
Mexico. The latest track guidance has again shifted northward and
the new NHC track has likewise been shifted northward, but not quite
as far the consensus models out of the respect for the lower
latitude ECMWF and FSSE models.

Additional strengthening is expected during the next 12 to 24 hours
while Alvin remains over SSTs greater than 26 deg C and within a low
vertical wind shear regime. By 36 hours, however, the combination of
cooling SSTs and a more stable airmass is forecast to induce steady
to rapid weakening. Alvin is expected to degenerate into a remnant
low by late Saturday, and dissipate by Monday. The NHC intensity
forecast is similar to a blend of the consensus models IVCN. HCCA,
and FSSE.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 15.1N 111.9W   50 KT  60 MPH
 12H  27/1800Z 15.9N 113.5W   55 KT  65 MPH
 24H  28/0600Z 17.3N 116.0W   55 KT  65 MPH
 36H  28/1800Z 18.7N 118.1W   45 KT  50 MPH
 48H  29/0600Z 19.6N 119.9W   35 KT  40 MPH
 72H  30/0600Z 20.7N 122.7W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  01/0600Z...DISSIPATED

$$
Forecaster Stewart

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