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#899537 (Received by flhurricane at: 10:57 AM 25.Aug.2017)
TCDAT4

Hurricane Harvey Discussion Number 21
NWS National Hurricane Center Miami FL AL092017
1000 AM CDT Fri Aug 25 2017

Multiple observing platforms indicate that Harvey's structure is
evolving this morning. The hurricane has developed concentric
eyewalls, as observed in data from the WSR-88D Doppler radars in
Brownsville and Corpus Christi, and an Air Force Reserve Hurricane
Hunter plane reported double wind maxima with diameters of 12 and
28 n mi. The aircraft data indicate that the central pressure has
continued to fall--now down to 947 mb--but the maximum winds have
not increased further. This discrepancy is not surprising given
hurricane's current structure.

Intensity forecasts become complicated once a hurricane develops
concentric eyewalls, and fluctuations in intensity become more
likely. If an eyewall replacement occurs, then Harvey's intensity
could decrease a bit while at the same time the overall wind field
increases in size. However, the hurricane remains in an environment
for intensification, and strengthening beyond the current intensity
is still possible before the center reaches land. But in the end,
the hurricane's exact intensity at landfall does not change the
fact that catastrophic flooding due to a prolonged period of heavy
rainfall and/or storm surge is expected at the coast and well inland
across much of southern and southeastern Texas. Slow weakening is
expected after Harvey crosses the coast since the center is not
expected to move very far inland, and the cyclone is likely to
maintain tropical storm status through Wednesday.

Harvey has not quite begun to slow down, and the initial motion
estimate is 315/9 kt. Strong mid-level ridging building over the
western United States is still expected to impede Harvey's forward
motion in the coming days, and the track guidance continues to show
the hurricane meandering or stalling near or just inland of the
Texas coast in 36-48 hours. Harvey could begin moving slowly
eastward on days 4 and 5 due to the influence of a mid-level
shortwave trough digging southward over the upper Midwest, but at
this time it is too early to say whether the center will definitely
re-emerge over the northwestern Gulf of Mexico.

Key Messages:

1. Harvey is expected to be a major hurricane when it makes landfall
tonight, bringing life-threatening storm surge, rainfall, and wind
hazards to portions of the Texas coast. Preparations should be
rushed to completion in the warning areas as tropical-storm-force
winds are arriving on the coast, and conditions will continue to
deteriorate through the rest of today and tonight.

2. A Storm Surge Warning is in effect for much of the Texas coast.
Life-threatening storm surge flooding could reach heights of 6 to 12
feet above ground level at the coast between the north entrance of
the Padre Island National Seashore and Sargent. For a depiction of
areas at risk, see the Storm Surge Watch/Warning Graphic at
hurricanes.gov. Due to the slow motion of Harvey and a prolonged
period of onshore flow, water levels will remain elevated for
several days.

3. Catastrophic and life-threatening flooding is expected across the
middle and upper Texas coast from heavy rainfall of 15 to 25 inches,
with isolated amounts as high as 35 inches, through Wednesday.
Please refer to products from your local National Weather Service
office and the NOAA Weather Prediction Center for more information
on the flooding hazard.

4. The Potential Storm Surge Flooding Map is available on the NHC
website. This product depicts a reasonable worst-case scenario -
the amount of inundation that has a 10 percent chance of being
exceeded at each individual location. This map best represents
the flooding potential in those locations within the watch and
warning areas.

FORECAST POSITIONS AND MAX WINDS

INIT 25/1500Z 26.7N 96.0W 95 KT 110 MPH
12H 26/0000Z 27.6N 96.8W 105 KT 120 MPH
24H 26/1200Z 28.4N 97.3W 90 KT 105 MPH...INLAND
36H 27/0000Z 28.8N 97.5W 70 KT 80 MPH...INLAND
48H 27/1200Z 28.9N 97.6W 55 KT 65 MPH...INLAND
72H 28/1200Z 28.3N 96.8W 35 KT 40 MPH...INLAND
96H 29/1200Z 28.5N 95.5W 40 KT 45 MPH...OVER WATER
120H 30/1200Z 30.0N 94.5W 40 KT 45 MPH...INLAND

$$
Forecaster Berg