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Hurricane Harvey Discussion Number  19
NWS National Hurricane Center Miami FL       AL092017
1000 PM CDT Thu Aug 24 2017

Harvey's rapid intensification seems to have slowed for the moment,
as an eye seen earlier in satellite imagery has lost definition
during the past few hours.  In addition, reports from an Air Force
Reserve Hurricane Hunter aircraft show that 700-mb flight level
winds are in the 75-80 kt range, with reliable surface wind
estimates from the Stepped Frequency Microwave Radiometer remaining
near 75 kt.  The aircraft also reported that the central pressure
has slowly fallen to 973 mb inside the 16 n mi wide eye.  Based on
these data, the initial intensity remains 75 kt, and this could be a
little conservative.

Harvey has turned a little to the left since the last advisory with
the initial motion now 315/9.  A mid-level anticyclone over the
eastern Gulf of Mexico is expected to steer Harvey generally
northwestward with a decreasing forward speed for the next 36-48 h,
with the center now forecast to make landfall on the middle Texas
coast in about 36 h.  This part of the new forecast track has been
nudged a little to the left based mainly on the initial position
and motion.  After landfall, the cyclone is likely to get stuck
between the Gulf anticyclone and a stronger anticyclone over the
western United States, with little motion likely from 48-96 h.
A slow eastward motion appears likely by 120 h due to the influence
of a trough in the westerlies digging into the eastern United
States.  There is some spread in the guidance at that time, with
the GFS showing Harvey drifting slowly eastward and the ECMWF
showing a faster motion.  The new forecast track splits this
difference of 5-day forecasts and lies near the consensus models.

It is unclear why the intensification has slowed, although it is
possibly due to some dry air seen earlier wrapping around the north
side of the core convection.  Otherwise, Harvey remains in an
favorable environment for further strengthening with low vertical
shear and high oceanic heat content.  The intensity forecast will
use the scenario that rapid intensification will resume tonight,
with Harvey becoming a major hurricane before landfall in Texas.
After landfall, the intensity forecast is based on the scenario that
Harvey will weaken over land, but it will remain close enough to
the coast so that the weakening will be slower than normal.  Thus,
the forecast keeps Harvey as a tropical storm from 72-120 h.

It is critical that users not focus on the exact forecast track
of Harvey, since cycle-to-cycle adjustments are likely.  All
locations within the hurricane and storm surge warning areas should
be preparing for the possibility of major hurricane-force winds and
life-threatening storm surge.

Key Messages:

1. Harvey is expected to be a major hurricane at landfall, bringing
life-threatening storm surge, rainfall, and wind hazards to portions
of the Texas coast. Preparations to protect life and property should
be completed by tonight, as tropical-storm-force winds will first
arrive in the hurricane and storm surge warning areas on Friday.

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.

3. Devastating 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, from Friday through next
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  25/0300Z 25.2N  94.6W   75 KT  85 MPH
 12H  25/1200Z 26.1N  95.6W   95 KT 110 MPH
 24H  26/0000Z 27.2N  96.5W  110 KT 125 MPH
 36H  26/1200Z 28.1N  97.1W  100 KT 115 MPH...INLAND
 48H  27/0000Z 28.6N  97.3W   70 KT  80 MPH...INLAND
 72H  28/0000Z 28.5N  97.5W   35 KT  40 MPH...INLAND
 96H  29/0000Z 28.5N  97.0W   35 KT  40 MPH...INLAND
120H  30/0000Z 29.5N  95.0W   35 KT  40 MPH...INLAND

$$
Forecaster Beven

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