[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 250858
TCDAT3

Tropical Storm Laura Discussion Number  22
NWS National Hurricane Center Miami FL       AL132020
400 AM CDT Tue Aug 25 2020

Satellite images show that Laura is becoming better organized.  Now
that the center is clear from Cuba, very deep convection has
developed into a ragged, pulsing central dense overcast, with a
large curved band on the southern side of the circulation.  The
intensity is kept at 55 kt, matching the satellite estimates and a
blend of the earlier flight-level and SFMR reconnaissance data.
Hurricane Hunter missions from both the Air Force and NOAA should be
in the storm within a couple hours to help obtain a new estimate.

After a westward jog earlier, Laura is estimated to be moving
west-northwestward again or 290/15.  The synoptic situation consists
over a large ridge near the southeastern United States and a
weakness in the ridge over Central Texas due to an inverted trough.
Laura should gradually gain latitude and turn to the northwest and
north-northwest over the next two days while it is steered between
those two features, move northward late this week through the
southern United States, then move quickly eastward across the
eastern U.S. over the weekend as it encounters the mid-latitude
westerlies.  The majority of the guidance has shifted a notable
distance to the west on this run, perhaps due to a weaker trough
over Texas and a more westward initial position of Laura (possibly
due to persistent northerly mid-level shear).  The new NHC
prediction is at the eastern edge of the new guidance envelope since
I don't want to bite off on such a large change on just one set of
model runs. But since the storm has been tracking west of forecast
expectations for quite some time, future westward track adjustments
could be required later today.

Laura is forecast to move over the very warm and deep waters of the
Gulf of Mexico, with similar or lighter shear conditions through
the next couple of days.  Now that an inner core appears to be
trying to form, conditions appear ripe for at least steady
intensification, and rapid intensification is becoming more likely
before landfall.  In fact, almost all of the explicit guidance
models, save the statistical-dynamical models, are showing a period
of rapid strengthening at some point during the next couple of days.
Thus, the new NHC forecast is higher than the last one, but not as
high as the most of the regional hurricane models since shear could
increase just before landfall.

Users are again reminded not to focus on the exact details of the
track or intensity forecasts as the average NHC track error at 48 h
is around 80 miles and the average intensity error is close to 15
mph. In addition, wind, storm surge, and rainfall hazards will
extend far from the center.

Key Messages:

1. Laura is forecast to reach the northwestern Gulf Coast as a
hurricane late Wednesday and early Thursday. Do not focus
on the details of the official forecast given the typical
uncertainty in NHC's 2-to-3 day track and intensity predictions. In
addition, storm surge, wind, and rainfall hazards will extend well
away from Laura's center along the Gulf Coast.

2. There is a risk of life-threatening storm surge from San Luis
Pass, Texas, to Ocean Springs, Mississippi, within the next 48
hours, and a storm surge watch is in effect for these areas
outside of the southeast Louisiana Hurricane and Storm Damage Risk
Reduction System. Residents in these areas should follow any advice
given by local officials.

3. Hurricane conditions are possible by late Wednesday from San
Luis Pass, Texas, to west of Morgan City, Louisiana, with tropical
storm conditions possible by Wednesday afternoon, and a hurricane
watch is in effect. Hurricane Warnings will likely be issued for a
portion of that area later today.

4. The threat of widespread flash and urban flooding, along with
small streams overflowing their banks, will be increasing Wednesday
night into Thursday from far eastern Texas, across Louisiana, and
Arkansas.  This will also lead to minor-to-isolated moderate river
flooding.  The heavy rainfall threat will spread northeastward into
the middle-Mississippi, lower Ohio and Tennessee Valleys Friday and
Saturday.


FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 22.9N  85.7W   55 KT  65 MPH
 12H  25/1800Z 23.8N  87.9W   65 KT  75 MPH
 24H  26/0600Z 25.1N  90.6W   80 KT  90 MPH
 36H  26/1800Z 26.8N  92.7W   95 KT 110 MPH
 48H  27/0600Z 29.3N  93.7W  100 KT 115 MPH
 60H  27/1800Z 32.1N  93.7W   55 KT  65 MPH...INLAND
 72H  28/0600Z 34.5N  93.0W   30 KT  35 MPH...INLAND
 96H  29/0600Z 37.0N  86.5W   25 KT  30 MPH...INLAND
120H  30/0600Z 39.0N  73.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake
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