Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 180319
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
919 PM CST Wed Jan 17 2018

.DISCUSSION...
Kind of a tough temperature forecast tonight as clouds try to move
into the region from the southwest but the cloud shield continues
to contract as it encounters significantly drier air to the
northeast. The air mass to the NE is dry with surface dew points
between 10-15 degrees and fcst soundings and x-sections show a dry
profile from the sfc to 500 mb. Winds are light so coupled with
the dry air mass in place, ideal radiational cooling should
occur. Since the cloud shield is not expected to arrive until
after 12z, have decided to add both Austin and Colorado counties
to the Hard Freeze Warning. Colorado county may be a bit of a
stretch but Columbus is currently 25 so even if clouds move in,
the criteria will probably still be met. Temps at Houston have
been slow to fall this evening but parts of Harris County will
fall below 25 by morning. Over the southwest part of the area,
fcst soundings show some saturation between 6-7000 feet but the
sub-cloud layer is very dry. Temps remain aoa freezing where the
clouds are located. Leaned toward the ECMWF cloud product for
Thursday and would expect widespread cloudiness by mid afternoon.
The cloud cover will likely retard heating and afternoon temps
are trending a bit cooler. Should make the lower 40s and thats
about it. Won`t be much of a temperature drop tomorrow night as
onshore winds and cloud cover help to insulate. 43

&&

.PREV DISCUSSION... /ISSUED 538 PM CST Wed Jan 17 2018/

AVIATION...
VFR conditions can be expected across all TAF sites through the
period. Winds will be light overnight out of the northeast,
turning more easterly by Thursday morning and increasing to 5-7
kts shortly after sunrise. The scattered to broken cloud deck
sitting at 8,000 feet to our southwest will attempt to push
towards the northeast, eventually reaching the coastal TAF sites
first around 15-16Z and then northern TAF sites such as UTS/CXO
closer to 18Z. Will need to continue to monitor the movement of
these mid level clouds, as they are pinned against a very dry
airmass currently draped across all TAF sites.

Hathaway

PREV DISCUSSION... /ISSUED 333 PM CST Wed Jan 17 2018/

DISCUSSION...
Temperatures back above freezing and roads mostly open with a few
hold outs that were more heavily iced or with pooled water that
froze and these should thaw out this afternoon. Clear skies and
very dry airmass will set the stage for near optimal radiational
cooling. Have issued a Hard Freeze Warning again for the northeast
half of the region for tonight with temperatures falling to 25 to
15 degrees and concerned that the northeasternmost counties could
go 2 to 5 degrees colder. Clouds should expand northeast from
around Matagorda Bay area this afternoon throughout the night. If
the cloud deck develops and moves northeast faster then
temperatures along the southwest edge of the Hard Freeze Warning
may be too cold. If it erodes faster as it expands then the Hard
Freeze Warning may need to be expanded.

High pressure over OK will slide down into LA/MS Thursday
afternoon which should allow winds to turn to the east and warm up
to get underway. Moisture flowing back into the area Thursday
night/Friday as upper troughing deepens over Mexico and coastal
troughing enhances warm frontogenesis Friday. Possibility of
advection/sea fog developing over the coastal waters and
spreading inland Friday afternoon and night. Trajectories become
more southerly Saturday and lower the residency times and
introduce a little more dry air aloft which could lessen the fog
threat somewhat by late Saturday. Friday night as upper trough
shifts eastward rain chances should spike then fall Saturday as
the upper support departs. Warm both Saturday and Sunday with a
Pacific origin cold front sweeping through Sunday night/early
Monday. Sounds are capped Saturday-Sunday morning before capping
erodes in the afternoon and overnight. Profiles are loaded with
veering wind in the low levels but limited instability and may
have some tornado threat if these model predictions don`t change
much. This should bring a round of showers and thunderstorms to
the region followed by a rapid drying out and cooler weather not
anywhere nearly as cold as with the current airmass over the
region. The day 10 progs showing a deeper amplitude trough setting
up over the Rockies for Saturday the 27 which may be our next big
weather maker in the few days following.
45

MARINE...
Winds and seas continue their downward trend with only the 20-60 nm
offshore Gulf waters under Caution flags through this evening. The
eastward expansion of southeastern U.S. high pressure will maintain
easterlies through tomorrow. Although far northern bay and Ship
Channel waters may fall to around -1.0 ft below MLLW during
tomrrow`s low tides...will allow the Low Water Advisory to expire at
6 PM tonight. Easterly winds will keep many locations in the
southern bays and along the Gulf coast from only falling to within a
foot below MLLW during Thursday`s low tides. The late Thursday
development of a western Gulf trough of low pressure will increase
rain chances through Saturday while strengthening east-southeast
winds. As warmer more moist air draws in from the southwestern
offshore waters across very cool shelf waters...the threat for bay
and nearshore sea fog begins Friday. If/when fog does form...it would
linger on through the weekend.

The next cold front will come across the local waters either late
Sunday night or early Monday morning. Caution level onshore winds
Sunday just downstream of the front will transition to Caution (or
brief Advisory) level offshore winds in the wake of the front Monday
morning and afternoon.  Choppy bays with Gulf seas topping out in
the 3 to 4 foot nearshore...6 to 7 foot offshore...range early next
week.  31

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      22  42  31  52  49 /   0   0  10  30  30
Houston (IAH)              23  43  35  53  52 /   0   0  10  30  30
Galveston (GLS)            31  42  42  55  53 /   0   0  10  50  40

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Hard Freeze Warning until 9 AM CST Thursday for the following
     zones: Austin...Brazos...Burleson...Chambers...Colorado...
     Grimes...Harris...Houston...Liberty...Madison...
     Montgomery...Polk...San Jacinto...Trinity...Walker...
     Waller...Washington.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION until midnight CST tonight
     for the following zones: Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.

&&

$$

Discussion...43



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