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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 172045
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
345 PM CDT Tue Sep 17 2019

...FLASH FLOOD WATCH IN EFFECT THROUGH WEDNESDAY AFTERNOON...

.Short Term [Tonight Through Wednesday Evening]...
The center of Tropical Storm Imelda pushed onshore earlier this 
afternoon near the city of Freeport. Surface analysis shows a 
surface low pressure of 1010 mb along the coast slowly moving north 
and further inland. Between 2 to nearly 4 inches of rain has fallen 
over southern Harris and northern Galveston counties over the past 
24 hours. The hazards associated with this area of disturbed weather 
has not changed, with heavy rainfall still in the forecast over the 
next few days through early Friday morning. WPC is continuing to 
outlook portions of southeastern Matagorda, a majority of Brazoria 
County, southern Fort Bend and Harris County, Galveston County, and 
the western reaches of Chambers County, with a moderate risk for 
excessive rainfall. Heavy rainfall will come in rounds depending on 
how the convective bands organize and push inland. Where the 
rainfall piles up and just how much over the next few days 
critically depends on how the center of this system tracks. What we 
do know for sure is that there will be ample tropical moisture in 
place and strong signals for heavy rainfall, stretching from 
Matagorda County up through Harris into Liberty County and south
to the coast, where the best chance for heavy rainfall is 
expected to accumulate. With 850 mb winds pumping between 25 to 30
knots, low level moisture will continue to be advected across SE 
TX through the overnight hours. Forecast soundings show 
precipitable water values in this region reaching up to 2.5 to 2.8
inches later this evening, with still decent moisture to the 
north, with 2.3 inches forecast at College Station. In terms of 
the short term models over the next 24 hours, there is decent 
agreement that there should be a lull in activity later this 
afternoon into the early evening hours. Late this evening, showers
with isolated thunderstorms should redevelop once again starting 
along the coast and spreading further inland. At this time, the 
next wave of heavy rainfall looks to occur sometime between 3 AM 
and noon tomorrow, mainly impacting portions of Matagorda to 
Harris and Liberty County and southward. The timing of this next 
round of heavy rain could cause impacts to tomorrow's morning 
commute/rush hour. If you expect to be traveling on the roadways 
tomorrow morning during this time, please plan accordingly! Make 
sure to check drivetexas.org for any road closures due to high 
water on roadways before traveling, and remember to never drive 
through flooded underpasses or roadways. High water or impassible 
roadways may be harder to determine in the dark, so use caution! 

The Flash Flood Watch is in effect through Wednesday 1 PM. We 
will keep a close eye on both model and radar trends, but I would 
assume the Flash Flood Watch will likely need to be extended into 
portions of Thursday based off the latest guidance. Make sure you 
keep an eye on the forecast and have multiple ways to receive 
warnings over the next 24 to 48 hours. Hathaway

&&

.LONG TERM [Thursday Through Tuesday]...
Mid/upper low, and possibly remnant circulation of Imelda and 
deep moisture will linger across the region on Thursday and 
continue the threat of periods of locally heavy rainfall and 
banding across a good portion of southeast Texas. The ground 
should be increasing saturated and more prone to quick runoff with
high rainfall rates, so don't let your guard down.

The low will eventually begin to fill, lift north, and become
absorbed in the flow between the lee side trof and eastern ridge
Friday and into the weekend. Rain chances will begin to slowly
taper off each off as that occurs. Ridge builds in early next week
which should produce hot and humid conditions.  47

&&

.MARINE...
The area of disturbed weather that formed into Tropical Storm Imelda 
pushed onshore near Freeport this afternoon. Radar imagery shows 
widespread showers and a few isolated thunderstorms across the Gulf 
waters and bays this afternoon, with heavy downpours, gusty winds 
and elevated seas possible in and around stronger storms. A Tropical 
Storm Warning only remains in effect for the eastern nearshore Gulf 
waters and Galveston Bay, and a Small Craft Advisory is in effect 
for the eastern offshore Gulf waters. Small craft should continue to 
exercise caution elsewhere across the western Gulf waters. Moderate 
onshore flow continues to the east of the center of the system while 
winds remain out of the northwest east of the center. Wind speeds 
will continue to lower tonight as the system moves further inland, 
but moderate onshore flow will prevail the end of the week. 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 are still 
forecast to peak at 3.0 to 3.4 feet above MLLW during high tide. 
High tide values of this nature could continue at least through 
Thursday. Additionally, the risk for strong rip currents remains 
along Gulf facing beaches.   Hathaway

&&

.HYDROLOGY...
Any rises on area bayous, creeks & rivers will be dependent on 
where the highest rain falls and their hourly rates. We'll 
obviously monitor them all, but the heaviest rainfall in the next 
24 hours should be southeast of the Highway 69/59 corridor.  47

&&

.TROPICAL...
NHC will evaluate trends, but the hope is that TS Imelda can be
reclassified as a TD later this evening or tonight. Hazards remain
the same regardless...heavy rainfall & potential for flash 
flooding. May see some minor coastal flooding near times of high 
tide and 35-40kt wind gusts in bands of precip.  47 

&&

.PRELIMINARY POINT TEMPS/POPS...

College Station (CLL)  73  83  73  86  73 /  40  60  60  70  50 
Houston (IAH)          73  80  75  84  76 /  80  80  90  90  40 
Galveston (GLS)        78  82  80  87  81 / 100 100  90  90  30 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...

TX...Tropical Storm Warning for the following zones: Brazoria 
     Islands...Coastal Brazoria...Coastal Galveston...Coastal 
     Harris...Galveston Island and Bolivar Peninsula. 

     Flash Flood Watch 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 Liberty...Wharton.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 1 AM CDT Wednesday for 
     the following zones: Matagorda Bay...Waters from Freeport 
     to Matagorda Ship Channel TX from 20 to 60 NM.

     Tropical Storm Warning 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...Waters from High Island to Freeport TX 
     from 20 to 60 NM. 


&&

$$