Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 190907

Area Forecast Discussion
National Weather Service Houston/Galveston TX
407 AM CDT Thu Oct 19 2017

A plume of Gulf moisture is evident on GOES-16 Total Precipitable
Water imagery moving towards the Middle Texas Coast early this
morning. Height falls associated with a series of shortwave
troughs lifting across New Mexico and western Texas today will
result in a deepening lee surface trough over the High Plains,
increasing easterly/southeasterly surface winds across the coastal
waters and pushing this moisture plume closer to the coast. The
main questions over the next 24 hours will be how far north and
east this moisture will make it into the region... and what that
will mean for rain chances today and tonight.

Light west to southwest 925-850 MB winds near the Texas coast
will help the moisture plume begin to spread north in the region
later this morning, pushing into the southwestern counties and
Matagorda Bay area. As winds back to the southeast by this
afternoon and strengthen with the deepening lee trough, this
moisture gets pushed farther north towards the Brazos Valley. 00Z
guidance differs by a few hours on when this backing actually
happens, however, and this will affect how close rain chances
actually make it to the Interstate 45 corridor today. Given some
uncertainty with this timing, have pulled a mention of isolated
showers and thunderstorms (15 PoPs) as far west as a Snook-Katy-
Liverpool line. However, expect best rain chances today to remain
much farther south and west. Isolated showers developing across
the western coastal waters later this morning should spread
inland as scattered showers and thunderstorms this afternoon as
deeper moisture moves onshore. Expect rain chances to remain
mainly relegated to the coast and adjacent waters tonight with
loss of heating but cannot rule out a few showers spreading
farther inland as a second wave of moisture arrives overnight.
Otherwise, expect highs today in the low to mid 80s with lows
tonight in the low to mid 60s north of Interstate 10 and mid to
upper 70s south.

Continued moisture advection and another shortwave trough lifting
across the region on Friday is expected to result in the
development of scattered to numerous showers and thunderstorms.
Best chances on Friday are expected to be along and west of
Interstate 45 in a corridor of low level warm air advection.
Forecast precipitable water values surge to 1.7 to 1.9 during the
afternoon hours and brief heavy rainfall will be possible Friday
through the weekend as atmospheric moisture levels remain
elevated. Rain chances are expected to focus closer to the coast
Friday night as speed convergence develops. Expect cloud cover and
rain on Friday to keep highs a few degrees cooler than today in
the lower 80s with lows Friday night in the upper 60s to mid 70s.

As a potent upper trough approaches the region on Saturday, a
60-70 knot jet streak looks to nose across the region ahead of it
and provide enough lift for scattered thunderstorms to again
develop across the region. This jet streak, combined with mid-
level lapse rates steepening to near 7 C/km across the Brazos
Valley and 0-6 km shear increasing to 25-30 knots, may result in
enough instability and/or organization for a strong thunderstorm
or two on Saturday across the Brazos Valley and remaining western
counties. As this upper trough swings across the Southern Great
Plains Sunday into Monday, it will send a cold front surging south
across the region. The upper jet structure as the cold front
enters the region appears to develop into more of a split-jet,
indicating the potential for strong to possibly severe
thunderstorms will continue into Sunday as the cold front arrives
with a band of thunderstorms developing along the cold front as it
pushes across the region. Model guidance still offers slightly
different solutions regarding if/when a closed low may develop,
but appear to be converging on a more consistent solution east of
the region and confidence is beginning to increase that the front
may clear the region Sunday.

Drier conditions are expected behind the cold front on Monday as
the upper trough (or low) exits the region, with a reinforcing
cold front arriving sometime Tuesday allowing for slightly below
normal temperatures in the mid to upper 70s by the middle of next



High pressure over the eastern U.S. and lower pressures over the
southern Gulf of Mexico will produce an easterly flow across the
coastal waters today. The gradient will tighten and stronger
onshore winds are expected. A SCEC will be issued for the Gulf
waters today and will likely need to be extended tonight. A
moderate to occasionally strong onshore flow will persist through
Saturday. Water levels and tides are currently progged to exceed
3.0 feet Thursday afternoon and could exceed 3.5 feet on Friday
near times of high tide. A cold front, and associated showers and
thunderstorms, will cross the coastal waters on Sunday afternoon
and a moderate to strong offshore flow will develop Sunday night
and persist into the first half of next week. A reinforcing front
is penciled in for Tuesday. Small craft advisories are likely
behind both fronts. 43


Satellite imagery not showing any hot spots this morning but not
sure if cirrus is blocking the view or the fires have been
extinguished. Moisture levels should increase today and RH values
will only fall to around 50 percent by afternoon. Precip will also
begin to develop south of I-10 later this afternoon so the fire
weather threat is greatly reduced today and will remain low
through the weekend. 43


College Station (CLL)      85  65  81  68  86 /  10  20  60  30  40
Houston (IAH)              84  68  80  72  85 /  10  20  60  30  40
Galveston (GLS)            82  75  81  76  84 /  10  20  60  30  40


GM...SMALL CRAFT SHOULD EXERCISE CAUTION through this evening for the
     following zones: Coastal waters from Freeport to the
     Matagorda Ship Channel out 20 NM...Coastal waters from High
     Island to Freeport out 20 NM...Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.



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