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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 170906

Area Forecast Discussion
National Weather Service Houston/Galveston TX
406 AM CDT Tue Sep 17 2019

.SHORT TERM [Tonight Through Wednesday]...

...Locally Heavy Rain Possible Today...

.Short Term...
A weak coastal trough is currently extended from about Cameron LA
to KGLS and then down toward the middle TX coast where a weak 
area of low pressure was trying to develop. Fcst soundings show PW
values this afternoon between 2.30 and 2.70 and this seems 
reasonable as GOES data shows 2.55 inches just off of Galveston at
08z. The trough should serve as a focus for additional shra/tsra 
this morning and the developing sfc low pressure coupled with a 
500 mb low will serve as a focus for more storms late today 
through Wednesday. The cyclonic flow/strong 850 mb flow around the
low will continue to pull deep tropical moisture to the NW and 
target areas from northern Brazoria/Galveston/Chambers into SE 
Harris counties today. Have issued a Flash Flood Watch beginning 
at 18z for areas along and south of a Wharton to Houston to 
Cleveland line. WPC has outlooked areas along and SE of US Highway
59 in a Slight Risk for excessive rainfall and areas around 
Galveston Bay in a Moderate Risk for excessive rainfall through 
12z Wednesday. Rainfall in the Watch area will average 1-3 inches 
by 12z Wednesday with isolated totals between 4-6 inches if/where 
training develops. Temperatures will trend cooler today but are 
strongly dependent on when rain/clouds develop. 

Rain will likely continue tonight and Wednesday as the weak surface 
low treks slowly north. The sfc low and upper level system become 
nearly vertically stacked and these systems generally move very 
slowly. PW values on Wednesday remain between 2.40 and 2.70 inches 
and the inflow around the sfc low would again favor the Galveston 
Bay region and the adjacent coastal counties. However, the strongest 
850 mb convergence begins to shift east as a developing 850 mb low 
over SE TX pushes NE. The current Flash Flood Watch will expire at 
18z and it will probably need to be extended but also reconfigured 
to include a few counties to the NNE of the current Watch and 
possibly remove a few counties to the W-SW in the Watch. Rain and 
cloud cover should keep temps on the cool side and some locations 
may not reach 80 degrees. 

Lastly, what could go wrong? If the developing low over the Gulf 
becomes better organized, the deeper moisture will be confined to 
the immediate coast before the system approaches areas to the east 
of SE TX Wednesday night. This scenario would focus the heavier rain 
into LA. Both the HiRes ARW and the German model ICON are showing 
this and the HRRR is hinting at this as well. All the global models 
have shifted their QPF in this direction as well so it is something 
to consider when making decisions. 43

.LONG TERM [Wednesday Night Through Tuesday]...

The potential for heavy rain continues Wednesday night through
Thursday with some possible lingering showers on Friday as the
system responsible for this event moves inland very slowly.
There is still quite a bit of uncertainty in the location and 
strength of the system as it moves inland. The EC has trended 
slightly east in the 00z run compared to the previous runs and is 
very similar location wise to the GFS. However, the GFS does 
remain more conservative in precipitation totals. The NAM remains 
the more westerly and wettest solution. Until the system gets 
better organized, model solution will continue to vary widely 
between runs and each other. 

Storm total QPF currently shows 8 to 12" across much of the 
central and southern portions of the region, however the exact 
amounts will be widely variable depending exactly where the 
heaviest rains falls and it remains too far out to pin-point 
those locations. the difference of just a few miles could be the
difference of several inches of rain. Rainfall rates of 1 to 3"+ 
per hour and potentially saturated soils from the precipitation 
that falls today will continue the threat of flash flooding into 
Thursday. The potential for flash flooding will continue Wednesday
night into Thursday, and please remember to not drive through 
flooded roadways and underpasses. The heaviest rains could fall 
during the overnight hours, so take extra caution as it can be 
hard to tell if a road is flooded or not. 

Showers and thunderstorms will continue to linger on Friday as the
system gets pulled to the north by a passing upper level trough.
The activity should remain mostly in the northern and eastern
portion of the area. Subsidence on the back side of this system
will bring a return to above normal daytime highs by the weekend. 




A weak coastal low off the middle Texas coast will slowly deepen 
as it moves toward Matagorda Bay. A moderate to occasionally 
strong onshore flow is expected today in advance of this feature. 
The gradient will weaken toward Matagorda Bay and winds there 
should be lighter. Small craft should continue to exercise caution
through at for the Gulf waters as winds remain stronger and 
strong convection over the waters will also lead to strong gusty 
winds and higher seas. Winds will begin to decrease tonight as the
low moves inland. If the low stays over the water, considerably 
stronger onshore winds will be possible. Moderate onshore winds 
will persist on Friday into Saturday and Cautions flags may once 
again be required. The pressure gradient finally begins to weaken 
on Sunday with lighter winds and decreasing seas.

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 Wednesday before staying 
below 3.0 feet. 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. 43


.PREV DISCUSSION /Issued 1146 PM CDT Mon Sep 16 2019/...

AVIATION... /ISSUED 1146 PM CDT Mon Sep 16 2019/

For the 17/06Z TAFS, look for periods of SHRA/TSRA as hard to time
bands move generally to the W and NW across the area. Lower ceilings/
visibilities can be expected in association with the stronger activity.

It still looks like SHRA/TSRA training could be setting up across parts
of the area beginning tomorrow evening or tomorrow night and continuing
on into Wednesday. 42 



College Station (CLL)  92  73  89  73  86 /  50  60  70  60  80 
Houston (IAH)          88  76  85  75  84 /  70  80  90  80  90 
Galveston (GLS)        86  78  84  80  87 /  90  90  90  80  80 



TX...Flash Flood Watch from 1 PM CDT this afternoon through Wednesday 
     afternoon for the following zones: Brazoria 
     Islands...Chambers...Coastal Brazoria...Coastal 
     Galveston...Coastal Harris...Coastal Matagorda...Fort 
     Bend...Galveston Island and Bolivar Peninsula...Inland 
     Brazoria...Inland Galveston...Inland Harris...Inland 
     Matagorda...Matagorda Islands...Northern Liberty...Southern 

GM...SMALL CRAFT SHOULD EXERCISE CAUTION through this evening 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.