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Tropical Storm Lorena Forecast Discussion


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000
WTPZ45 KNHC 171456
TCDEP5

Tropical Storm Lorena Discussion Number   1
NWS National Hurricane Center Miami FL       EP152019
1000 AM CDT Tue Sep 17 2019

First-light 1-minute visible imagery from GOES-W revealed that the
disturbance just south of Mexico has developed a well-defined
surface center. The system has also developed an extensive
convective band stretching from the southwest to the northeast
quadrant of the cyclone. A pair of ASCAT passes overnight showed
winds were already at tropical-storm strength, so the system has
been designated as Tropical Storm Lorena. The initial intensity is
set at 35 kt, though the ASCAT data showed winds could actually
already be higher than that, so that may be conservative.

Lorena is moving quickly toward the northwest, or 305/13 kt. A
mid-level ridge centered over the Gulf of Mexico should keep it
moving generally northwestward for the next few days, at a slightly
slower speed. Most of the dynamical models show this general
scenario, however some of the typically reliable track models,
including the GFS, seem to have trouble resolving the small
cyclone, and show the cyclone moving more west-northwestward. This
small difference is important and could be the difference between
Lorena directly impacting Mexico or passing just to the southwest.
The NHC forecast is on the far right side of the guidance envelope,
favoring the ECMWF and UKMET solutions, which seem to have a more
realistic current depiction of the tropical cyclone. The NHC
forecast is on the far right side of the standard guidance
envelope, but is near the middle of the ECMWF ensemble. This brings
Lorena very near the Mexico coast in about 2 days and it is possible
that the storm could make landfall. At longer ranges the GFS depicts
the beginning of a binary interaction between Lorena and the
newly-formed depression to the west, however at the moment this does
not seem particularly likely, and the NHC forecast favors the ECMWF
through day 5 which shows no such interaction.

Moderate easterly shear appears to be affecting the tropical storm
for now, and this will likely be the primary moderating factor for
Lorena's intensity. SSTs are very warm and there is ample moisture,
but only slow intensification is likely as long as the shear
persists. The NHC forecast therefore shows slow strengthening for
the next couple of days. Beyond that time, if the cyclone moves
inland, it should quickly weaken or possibly dissipate, but if it
stays farther offshore it may have an opportunity to strengthen
further. The NHC forecast holds the cyclone nearly steady-state, but
it is well below the consensus at day 4 and 5, since most of the
intensity models are based on tracks that stay well offshore of
Mexico.

FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z 13.7N 100.0W   35 KT  40 MPH
 12H  18/0000Z 14.8N 101.0W   40 KT  45 MPH
 24H  18/1200Z 16.3N 102.4W   45 KT  50 MPH
 36H  19/0000Z 17.5N 103.6W   50 KT  60 MPH
 48H  19/1200Z 18.5N 104.6W   55 KT  65 MPH
 72H  20/1200Z 20.0N 106.3W   55 KT  65 MPH
 96H  21/1200Z 21.0N 108.0W   55 KT  65 MPH
120H  22/1200Z 22.0N 110.0W   50 KT  60 MPH

$$
Forecaster Zelinsky