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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 182028
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
328 PM CDT Wed Sep 18 2019

...FLASH FLOOD WATCH IN EFFECT UNTIL 7AM CDT THURSDAY...
...FLOODING IMPACTS STILL POSSIBLE TONIGHT INTO THURSDAY MORNING
FOR AREAS EAST OF I-45...

.DISCUSSION...

Remnants of Tropical Depression Imelda should continue to wobble
between Conroe and Livingston with a slow track to the north
tonight into Thursday. Heavy rainfall leading to flooding impacts
will still be possible tonight into Thursday morning. Flash flood
watch may need to be extended in time depending upon the extent of
flooding impacts over the watch area.

In the near term this evening, we are watching an area that has
cleared out on visible satellite imagery from Wharton to League
City that could see enhanced convergence and an axis of
instability for storms to develop. We are already seeing scattered
activity in this axis of instability and clearing with day time
heating. There certainly could be some brief heavy rainfall that
could lead to street flooding and rises on area creeks/bayous. Hi
res model guidance has a lot of this activity ending before 03Z
this evening. We will need to watch to see if any more robust band
can form which could lead to training of storms and increase the
threat for more significant flooding impacts.

Overall model guidance continues a signal for the highest rainfall
totals and potential for higher rainfall rates over east Texas.
This includes areas east of a Houston to Huntsville line along
I-45 particularly Chambers, Liberty, San Jacinto and Polk
counties. We cannot rule out eastern Montgomery and eastern Harris
county from flooding impacts as well. There will be a tight
gradient in rainfall amounts from 6 to 10 inches on the high end
to as little as 1 to 3 inches to the west. We caution the use of
some hi res guidance, especially cherry picking a particular run 
for rainfall forecasting. Trends in the RAP and HRRR will need to 
be watched and use a time lagged ensemble approach to get a better
understanding of the trends in these models. Case in point, the 
18Z HRRR from 12Z Thursday to 18Z Thursday raises some eye brows 
but again just one model run. It will be interesting to see future
trends in the model before looking to impacts. That said, we know
how tropical systems can re-develop convection overnight into the
morning hours so we have to be careful.

Should the rainfall we are forecasting occur in the locations
outlined above, we could see flooding on Long King Creek, Menard
Creek, Peach Creek and East fork of San Jacinto. These basins will
be a little more fast responding than the Trinity and perhaps the
west fork of the San Jacinto. Farther south there is still
flooding on Clear Creek and Dickinson Bayou so any additional
rainfall will prolong flooding there or cause secondary rises.
Houston Bayou system will be watched but with there being a large
gradient in rainfall so far, most areas can still handle
additional rainfall. Rainfall rates will be a big component in
flooding impacts tonight into Thursday for these areas.

Overpeck

&&



&&

.PRELIMINARY POINT TEMPS/POPS...

College Station (CLL)  74  89  74  90  73 /  50  70  50  50  10 
Houston (IAH)          77  85  77  87  76 /  80  90  40  50  10 
Galveston (GLS)        81  85  81  87  81 /  80  90  30  50  10 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...

TX...Flash Flood Watch through Thursday morning for the following 
     zones: Brazoria Islands...Chambers...Coastal 
     Brazoria...Coastal Galveston...Coastal Harris...Fort 
     Bend...Galveston Island and Bolivar Peninsula...Inland 
     Brazoria...Inland Galveston...Inland 
     Harris...Montgomery...Northern Liberty...Polk...San 
     Jacinto...Southern Liberty...Trinity...Walker.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 7 PM CDT this evening 
     for the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until 7 PM CDT 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...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.

     SMALL CRAFT SHOULD EXERCISE CAUTION from 7 PM CDT this evening 
     through Thursday morning 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...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.


&&

$$

DISCUSSION...Overpeck