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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 172347

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


With Advisory 2A for Imelda, the tropical storm warnings are 
being canceled for all of Southeast Texas and its coastal waters. 
The tropical storm warning over the water will be replaced with a 
small craft advisory for at least this evening.

Though the storm's winds are weakening post-landfall and the
tropical warnings are being dropped, THE MAIN THREAT OF THIS STORM
tropical storm warnings are a formality tied to the lower winds,
and SHOULD NOT be considered a sign that the threat of this storm
is weakening. Indeed, the heaviest rains from this event likely



.PREV DISCUSSION /Issued 345 PM CDT Tue Sep 17 2019/...


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 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

A mixture of ceilings across the terminals this afternoon ranging
from LIFR to VFR ceilings, with those lower ceilings stretched
across IAH and southward. Visibilities have also lowered to 
MVFR/IFR criteria at TAF sites experiencing showers this 
afternoon. These showers and very isolated thunderstorms are 
associated with now Tropical Depression 11, which should continue 
to push onshore later this afternoon. The main impact associated 
with this system remains the threat for heavy rainfall. Terminals
along the coast and as far north as IAH can expect heavy downpours
reducing visibilities periodically through out the TAF period. 
Short term guidance is hinting at a lull in development during the
early evening hours before ramping back up later this evening 
along the coast and spreading further inland during the early 
morning hours. Winds will become gusty, especially along the 
coast, associated with these bands of showers. Ceilings and 
visibilities will again be reduced similar to last night, lowering
to LIFR to MVFR criteria. 

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

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



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 



TX...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 Advisory until 7 AM CDT Wednesday for the following 
     zones: 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.

     SMALL CRAFT SHOULD EXERCISE CAUTION through Wednesday morning 
     for the following zones: Coastal waters from Freeport to 
     Matagorda Ship Channel TX out 20 NM...Waters from Freeport 
     to Matagorda Ship Channel TX from 20 to 60 NM.