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Hurricane Maria Discussion Number  23
NWS National Hurricane Center Miami FL       AL152017
500 PM AST Thu Sep 21 2017

The last pass of today's Air Force reconnaissance flight found
maximum flight-level winds of 117 kt in the northeastern eyewall,
which supported increasing the intensity to 105 kt in the 2 PM
intermediate advisory.  The SFMR instrument continued to report
higher winds, as was mentioned in the previous discussion.  However,
Maria has been moving over the Navidad and Silver Banks to the
north of the Dominican Republic, where water depths are less than
30 meters deep in some places, and it is likely that shoaling
effects inflated some of these numbers.

Maria continues to move northwestward, or 315/8 kt, around the
southwestern periphery of a mid-level high centered south of
Bermuda.  As Maria moves around this high, and toward an elongated
trough extending from Tropical Storm Jose southwestward into the
Gulf of Mexico, it is expected to turn north-northwestward within
24 hours and then north-northeastward by the end of the forecast
period.  This motion will take Maria's center very close to the
Turks and Caicos Islands and the southeastern Bahamas during the
next 24 hours, but the hurricane should then stay over the waters
of the western Atlantic through day 5.  The track models remain
stable, and there is very little cross-track spread even at day 5.
The guidance has slowed down a bit by the end of the forecast
period due to Maria possibly interacting with a shortwave trough
moving off the southeastern U.S. coast, and the new NHC forecast is
therefore a little slower than the previous one at that time.

SHIPS diagnostics indicate that southwesterly shear will be
increasing over Maria during the next 12-24 hours, even though the
hurricane will begin to move over an area of higher oceanic heat
content.  While it can't be ruled out that Maria might still
strengthen a bit, the preponderance of the guidance suggests that
the cyclone will only maintain its intensity for the next 12 hours
and then begin a gradual decrease in strength on Friday.  That
trend should continue through the end of the forecast period, but
Maria is expected to remain a hurricane through day 5.  For now,
the NHC forecast remains just above the intensity consensus,
hedging toward the slower decay shown by the SHIPS model.


KEY MESSAGES:

1. Flash flood warnings continue in portions of Puerto Rico due
to persistent heavy rainfall from Maria's trailing rainbands.
Catastrophic flooding is occurring on the island, especially in
areas of mountainous terrain, and everyone in Puerto Rico should
continue to follow advice from local officials to avoid these
life-threatening flooding conditions.

2. A Hurricane Warning is in effect for the northern coast of the
Dominican Republic, the Turks and Caicos Islands, and the
southeastern Bahamas, where Maria is expected to bring dangerous
wind, storm surge, and heavy rainfall.

3. Swells from Maria are expected to begin reaching the coast of
the southeastern United States on Friday.  These swells are likely
to cause dangerous surf and life-threatening rip currents along the
coast for the next several days, even with Maria forecast to remain
well offshore over the western Atlantic Ocean.

FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 20.8N  69.8W  105 KT 120 MPH
 12H  22/0600Z 21.6N  70.4W  105 KT 120 MPH
 24H  22/1800Z 23.0N  71.2W  100 KT 115 MPH
 36H  23/0600Z 24.5N  71.7W  100 KT 115 MPH
 48H  23/1800Z 26.1N  72.1W   95 KT 110 MPH
 72H  24/1800Z 28.9N  72.1W   95 KT 110 MPH
 96H  25/1800Z 31.0N  71.0W   85 KT 100 MPH
120H  26/1800Z 33.0N  70.0W   75 KT  85 MPH

$$
Forecaster Berg

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