FXUS64 KHGX 232112

Area Forecast Discussion
National Weather Service Houston/Galveston TX
412 PM CDT Wed Aug 23 2017

...Monitoring for a Significant Multi-Day Heavy Rainfall Event...

The ongoing concern is the impacts of Harvey over the next several
days. Of course, this will depend upon the eventual track of the
storm. The weakness in the upper level ridge currently over
Southeast Texas is not expected to change over the next several
days. This could lead to rainfall impacts from Harvey over a
multi-day period from late Friday through Monday. If the storm or
its remnants slow down after landfall the heavy rainfall potential
could continue into Tuesday.

The flood threat could be heightened by higher than normal tides
along the coast combined with high seas will keep the rivers and
bayou system from draining as it would normally drain. Because of
this, there is a potential for event-total rainfall to range from
10 to 15 inches with isolated locations receiving 20 inches or
more. The locations for the greatest rainfall still looks to be
from the Interstate 10 corridor southward to the coast.

Timing of the event will depend upon the track of Harvey. As of
now, it does not appear that widespread rainfall will occur
tonight or Thursday. However, an unsettle airmass due to the
shearing out upper low over the far northwestern Gulf this
afternoon will keep at scattered showers and thunderstorms in
place on Thursday. The coverage will then become more widespread
on Friday particularly over the southern half of the forecast
area. Widespread rain chances are then expected on Saturday,
Sunday, and Monday. The forecast after Monday will depend upon
where Harvey or its remnants travel. The GFS and ECMWF show that
the system will be in between two upper level high pressure
areas--a rather concerning pattern for Southeast Texas.



Marine fcst is obviously tied to the evolution of Harvey. We should
start seeing some swell arrive in the coastal waters Thursday,
probably building to SCA criteria Thurs night...followed by
a continuing wind/sea/surge/tide increase and wx conditions into
landfall. Water levels will remain elevated & hamper inland precip
drainage probably thru the weekend into early next week.

Best estimate of marine related impacts & decision points based on
what we currently know:

* Onset of 7+ foot seas: Thurs night
* Onset of tropical storm force winds within 20 NM - Friday
* Onset of hurricane force winds (if any) - Fri night
* Onset of coastal flood threat - Thurs night
* Reasonable worst case surge: 4-6 feet above ground near landfall,
2-4 feet further away
* High risk of rip currents and building surf thru the day Thursday.
* Discharge from elevated rivers after landfall may continue to
impact navigation.



College Station (CLL) 76 95 74 89 74 / 20 40 30 40 60
Houston (IAH) 77 93 77 85 75 / 20 40 30 60 70
Galveston (GLS) 81 88 79 85 80 / 30 50 60 70 70


TX...Hurricane Watch for the following zones: Brazoria...Jackson...

Tropical Storm Watch for the following zones: Austin...
Chambers...Colorado...Fort Bend...Galveston...Harris...

GM...Hurricane Watch for the following zones: Coastal waters from
Freeport to the Matagorda Ship Channel out 20 NM...
Matagorda Bay...Waters from Freeport to the Matagorda Ship
Channel from 20 to 60 NM.

Tropical Storm Watch for the following zones: Coastal waters
from High Island to Freeport out 20 NM...Galveston Bay...
Waters from High Island to Freeport from 20 to 60 NM.



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