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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 162345
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
645 PM CDT Mon Sep 16 2019

.PREV DISCUSSION... /ISSUED 359 PM CDT Mon Sep 16 2019/ 

..Potential heavy rain and flash flood event setting up Tue 
night thru Thurs...

SHORT TERM [Through Tuesday]...
Showers and isolated thunderstorms have remained south of a line 
from Colorado to Conroe this afternoon, with gusty winds in and 
around stronger storms. Short term models have struggled to 
initialize well with the placement of the convection, with the RAP13 
probably doing the best as of late. Trends in radar imagery east of 
the region over W LA has pushed convection further inland than what 
we have see over our area this afternoon. With winds out of the 
east, expect we could see further development of precipitation over 
our northeastern zones through the remainder of the afternoon hours. 
Convection should continue to shift further inland, moving into 
Trinity and Polk counties over the next hour or so. In general, 
expect most of the convection to wane shortly after sunset this 
evening, with a lull in activity overnight and into early tomorrow. 
Redevelopment of showers and thunderstorms will be possible early 
Tuesday morning, once again starting over the Gulf waters before 
pushing further inland by mid-morning. The region of disturbed 
weather associated the upper-level low pressure system will continue 
to shift to the northwest over the next few days and with it will 
drag ample tropical moisture across the region. That said, 
precipitable water values of 2.2 to 2.4 inches should advect inland 
across the bays and Gulf waters and along the coastal counties by 
the afternoon hours tomorrow. Therefore, anticipate more widespread 
showers and thunderstorms tomorrow in comparison to today as the 
upper level low tracks closer to the upper Texas coast. Main impacts 
associated with this precipitation will be the potential for 
localized heavy downpours and gusty winds associated with stronger 
storms. The threat for periods of heavy rainfall will be possible 
starting late Tuesday as this system shifts closer to SE TX. This 
threat should continue over the next few days...more details in the 
long term section below.  Hathaway

MID TERM [Tuesday Night Through Thursday]...
What we know:
- Ground will gradually become increasingly saturated with periods
  of rainfall and more prone to runoff.
- Upper low will make its way northward into southeast Texas, but
  likely stall and meander over the area due to higher pressure to
  the north.
- Moisture levels will be near record peak values recorded this
  time of year. Rain rates of 1-3"+ per hour are possible at
  times. Heavy rain in a short amount of time will be the primary  
  trigger for street flooding and rises on area creeks, bayous and
  rivers. 
- A weak surface low will slowly move northward from the Coastal
  Bend to near the Matagorda Bay area Tuesday night and Wed 
  morning setting the stage for the first potential round of heavy
  rainfall...possibly in the form of training convective bands 
  closer to the circulation itself...gradually spreading out thru 
  the day with heating. 
- In very general terms, the surface low will meander northward
  Wed & Thur...possibly situated between the Brazos Valley and 
  I-45 late in the period.
- Multiple rounds of precipitation are expected with some
  intermittent breaks in between.
- There will be changing, potentially large run-to-run swings of
  model solutions. Don't focus on just one. Just know the overall  
  conditions are favorable for heavy rain which could cause 
  flooding.

What are some of the unknowns:
- Just how organized or unorganized atmospheric conditions line-
  up for prime and/or significant amounts of rain.
- Specific rain amounts. There will be several inches spread out
  over several days. Focus on the impacts! 
- Specific timing/location of the periods of heaviest rainfall.
  Can occur at any time and any place. They'll be dependent on
  several variables which can be difficult to determine even 6-12
  hours out. Just know there are signals there for significant
  rain and associated flash flood impacts.
- Specific river/creek/bayou impacts. It'll be dependent on where
  and how much rain falls. Rises and flooding impacts can be 
  expected where the heaviest rain occurs. 

LONG TERM [Thursday night Through Monday]...
Look for this system to eventually head further north and away
from se Tx as the ridge just to the north slightly erodes. Moisture
levels and rain chances will diminish and temperatures go back 
up.  47

&&

MARINE...
A slow area of disturbed weather associated with an upper level low 
pressure system is continuing to shift towards the northwest this 
afternoon, bringing with it tropical moisture. Winds out of the east 
associated this system remain between 15 to 20 knots with gusts up 
to 25 knots at times in and around storms. Small craft should 
continue to exercise caution through at least tomorrow morning 
across the Gulf waters and both Matagorda and Galveston Bay. Winds 
will continue to turn more out south to southeast tomorrow, and 
eventually more out of the south to southwest by Wednesday. The SCEC 
currently in effect will likely need be extended through at least 
Wednesday. The risk for strong rip currents also remains possible. 
Seas in the offshore waters look to range between 4 to 6 feet, but 
elevated seas will be possible in and around stronger storms. By 
Friday and into the weekend, both winds and seas will lower as the 
pressure gradient weakens across the region. 

Additionally, tide levels are continuing to run about 1.5 to 2.0 
feet above normal. At Galveston Bay Entrance, tide levels could peak 
at 3.0 to 3.4 feet above MLLW during high tide. High tide values of 
this nature will continue at least through tomorrow and Wednesday 
before slowly lowering beneath the 3.0 feet threshold. A Beach 
Hazards statement is in effect to cover both the risk for strong rip 
currents, as well as the elevated tide levels and impacts that are 
possible. Hathaway

&&

.AVIATION... /ISSUED 645 PM CDT Mon Sep 16 2019/
For the 17/00Z TAFS, quiet this evening as daytime activity continues
to weaken/dissipate. Overnight, could see some MVFR ceiling development,
and anticipate the next batch of SHRA/TSRA to form in/around the coastal
counties. These storms will work their way inland during the day tomorrow
with the best chances near the coast in the morning and further inland
as the day progresses. Lower ceilings/visibilities can be expected in
association with the activity. As of now, it is looking like some TAF
sites could see training of SHRA/TSRA setting up some time tomorrow
evening or tomorrow night and continuing on into Wednesday.  42 
&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      74  92  73  89  74 /  20  60  50  80  60 
Houston (IAH)              76  89  76  86  75 /  50  60  80  90  90 
Galveston (GLS)            80  87  79  86  79 /  80  80  90 100  80 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 1 AM CDT Tuesday for 
     the following zones: Coastal waters from Freeport to 
     Matagorda Ship Channel TX out 20 NM...Coastal waters from 
     High Island to Freeport TX out 20 NM...Galveston Bay...
     Matagorda Bay...Waters from Freeport to Matagorda Ship 
     Channel TX from 20 to 60 NM...Waters from High Island to 
     Freeport TX from 20 to 60 NM.

&&

$$