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Tropical Depression Harvey Discussion Number  14
NWS National Hurricane Center Miami FL       AL092017
1000 PM CDT Wed Aug 23 2017

Reports from an Air Force Reserve Hurricane Hunter aircraft suggest
that Harvey's structure has consolidated a little bit, with the
pressure falling to 1002 mb and the radius of maximum winds
shrinking to 60 n mi.  However, there have been no reliable reports
of winds indicating that the cyclone has reached tropical-storm
strength, and the convective cloud pattern remains ragged. Based on
these data, the initial intensity is held at 30 kt. Perhaps the most
significant finding of the aircraft is that the center was to the
east of the previous advisory position.

The initial motion is a rather uncertain 325/2.  A weak mid-level
ridge to the northeast of Harvey should cause the cyclone to move
on a northwestward or north-northwestward track through 48-72 h,
and this motion should bring the center inland over southern Texas.
Later, steering currents weaken as a ridge builds over the
southwestern United States and a trough drops down from the Plains.
As a result, Harvey should decelerate while making landfall and
move very slowly just inland of the coast.  There is a major spread
amongst the large-scale models from 72-120 h, with the UKMET
forecasting a slow northward motion into eastern Texas, the GFS a
southwestward motion over Texas, and the ECMWF a cyclonic loop that
moves the system back over the Gulf of Mexico.  Given the
uncertainty, the new forecast track shows a slow motion over
southern Texas during that time, with a change from the small right
turn of the previous forecast to a small left turn.  Overall, the
track lies a little to the right of the consensus models through 72
h.

An upper-level low seen in water vapor imagery over the
northwestern Gulf of Mexico appears to be weakening, and as this
happens Harvey should end up in a low shear environment.  This
should allow strengthening over deep warm water in the western Gulf
of Mexico.  The new intensity forecast is adjusted slightly to show
Harvey explicitly becoming a hurricane in 48 h, and some additional
strengthening could occur between the 48 h point and landfall.
After landfall, Harvey is expected to weaken.  However, the
forecast track keeps the system close enough to the Texas coast
that this weakening is likely to be slower than normal.

Since Harvey has moved slower than anticipated during the past
6-12 h, warnings are not yet required for portions of the Texas
coast.  Warnings are likely to be issued sometime on Thursday.

Key Messages:

1. Harvey is likely to bring multiple hazards, including heavy
rainfall, storm surge, and possible hurricane conditions to portions
of the Texas coast beginning on Friday.

2. Heavy rainfall is likely to spread across portions of eastern
Texas, Louisiana, and the lower Mississippi Valley from Friday
through early next week and could cause life-threatening flooding.
Please refer to products from your local National Weather Service
office and the NOAA Weather Prediction Center for more information
on the flooding hazard.

3. A Storm Surge Watch is in effect from Port Mansfield to High
Island, Texas, indicating the possibility of life-threatening
inundation from rising water moving inland from the coast during the
next 48 hours.  For a depiction of areas at risk, see the Storm
Surge Watch/Warning Graphic at hurricanes.gov.

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.  Because the Flooding Map is
based on inputs that extend out only to about 72 hours, it best
represents the flooding potential in those locations within the
watch area.


FORECAST POSITIONS AND MAX WINDS

INIT  24/0300Z 21.9N  92.6W   30 KT  35 MPH
 12H  24/1200Z 22.8N  93.1W   35 KT  40 MPH
 24H  25/0000Z 24.1N  94.1W   40 KT  45 MPH
 36H  25/1200Z 25.4N  95.4W   50 KT  60 MPH
 48H  26/0000Z 26.6N  96.3W   65 KT  75 MPH
 72H  27/0000Z 28.5N  97.5W   50 KT  60 MPH...INLAND
 96H  28/0000Z 28.5N  98.0W   35 KT  40 MPH...INLAND
120H  29/0000Z 28.0N  98.0W   30 KT  35 MPH...INLAND

$$
Forecaster Beven

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