Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 251620
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1020 AM CST Sat Feb 25 2017

.UPDATE...
A superb day across the region as southern plains high pressure
descends down into the state through the day. Clear skies with
mild cold air advection regulating afternoon temperatures to the
average middle 60s. The 1027 mb centered high over the Red River
Valley will weaken and advance east over the lower MS River Valley
early Sunday. The high`s movement will loosen the currently tight
post-frontal offshore pressure gradient and veer winds around to
east tonight...onshore by noon Sunday. This lower level flow pattern
will pull up a more moist air mass...pwats increasing from current
0.2 - 0.3 inch levels to over an inch Sunday afternoon. This all
translates to skies becoming overcast with increasing west-to-east
light rain chances (slight to low end probabilities). A very weak...
shallow shortwave trough moving out of the Rockies with associated
weak PVA may initiate a passing shower or two. Confidence is low
that there will be any significant rain due to the lack of lift
and slow return of column moisture. Any precipitation will likely be
periods of light rain or drizzle within a developing warm advection/
overrunning pattern. 31


&&

.PREV DISCUSSION... /ISSUED 524 AM CST Sat Feb 25 2017/

AVIATION...
VFR conditions expected with drier air funneling into Southeast
Texas behind the front. Northerly winds to begin the period will
be veering through Saturday, becoming easterly by the end of the
period, while generally diminishing from around 10 knots to 5
knots or less.

Beyond the forecast period, winds will continue to veer and become
southeasterly on Sunday and pick up a little in speed. Clouds
will also begin to increase. However, since these changes lie even
beyond the extended IAH forecast period (barely), will leave for
future forecast cycles to flesh out the timing on this. 25

&&

PREV DISCUSSION... /ISSUED 400 AM CST Sat Feb 25 2017/

DISCUSSION...
Cold front has pushed well into the Gulf of Mexico with high
pressure over the central Plains. Temperatures continue to fall
across the area with 40s mainly north of a Columbus to Conroe
line. Temperatures should continue to drop with 40s for much of
the area by sunrise. Airmass is quite dry with dewpoints in the
30s for most of the area. Cold air advection should continue today
with 850mb temperatures expected to drop to between 5C to 9C.
High temperatures should be in the 60s today which amazingly will
be a few degrees below normal.

Upper level analysis this morning shows an upper level low over
the Great Lakes and another strong shortwave off the West Coast.
This system should rotate within the mean trough over the Rockies
the next couple of days and move over the southern Plains Sunday
night into Monday.

After a cool dry day today, Sunday will be a recovery day in
terms of moisture. High pressure moves east quickly allowing
return flow to set up with moisture advection mainly over central
Texas. Hi-res WRF models show showers developing and spreading
over SE Texas Sunday afternoon so have 30 PoPs for western
portions of the area. Shower activity continues Sunday night into
Monday. A shortwave trough moving across the southern Rockies into
the Plains Sunday night may help enhance convection going into
Monday. Instability will be higher on Monday so will need to
monitor for at least a few storms with gusty winds, hail, and
lightning. Boundary layer flow in 925mb to 850mb may veer to the
SW with 850mb temperatures approaching the upper teens. This may
be enough for a cap to develop and limit overall thunderstorm
coverage.

Tuesday 850mb flow increases and temperatures approach 20C which
will again support temperatures back into the 80s. Another strong
shortwave moves across the Rockies into the Plains Tuesday night
into Wednesday. This system then moves quickly into the Midwest
with a cold front trailing it. The cold front pushes through SE
Texas during the day Wednesday. Moisture should be enough now to
support thunderstorm chances along the front. GFS/ECMWF/CMC area
all now showing rain chances Wednesday morning with the front.
Given the forcing and precipitable water values around 1.5 to 1.7
inches, think there could be a line of storms with the front. GFS
shows about 1000-1500 J/kg of CAPE so an isolated strong storm
maybe possible with some gusty winds.

High pressure builds into Texas behind the front so could see a
couple of days of below normal temperatures for the end of next
week. High temperatures in the 60s and lows in the 40s look
reasonable. Right now there are no real solid insights for the
extended forecast as the GFS and ECMWF have essentially flipped
from yesterday`s forecast. A coastal low may form next weekend
which the GFS no longer shows but the ECMWF now develops with a
strong trough moving across the Gulf coast. Paired back PoPs to
10/20s for the end of next week. 39

&&

MARINE...
Winds in excess of 20 knots with gusts up to 30 knots necessitated
the extension of the small craft advisory to include all Gulf waters
tonight. While those nearer to shore may be able to expire early
this morning, the 20-60 nm waters may be slower to come down, and
will extend the headlines to midday in all zones to play it safe.

These northerly post-frontal winds will veer to northeasterly today,
then northeasterly tonight while diminishing as high pressure slides
east of the area. Onshore flow will become established Sunday, and
increase in strength as the pressure gradient tightens between the
exiting high and developing low pressure in West Texas. Onshore flow
of 10 to 20 knots will persist until the next frontal passage. Tides
should run above normal due to these winds, but only in the range of
several inches up to a foot, so coastal flooding does not appear to
be a concern through early next week. The next cold front looks to
arrive Wednesday, likely Wednesday night. Strong winds in the wake
of this front may require a small craft advisory overnight. 25

&&


.FIRE WEATHER... One more day of low relative humidity is
expected as dry air funnels into Southeast Texas in the wake of
Friday`s frontal passage. Fortunately, though some breezy
conditions may be possible in the morning, passing high pressure
will relax the pressure gradient so that the lowest RH values in
the afternoon will coincide with weak northeasterly winds both at
the surface and through the transport layer.

Winds will continue to veer to southeasterly for the second half of
the weekend, and increasing moisture levels will keep relative
humidity high until after the next front passes for the second half
of next week. In the meantime, look for rain potential Sunday
afternoon through Monday morning, on Tuesday, and again on
Wednesday. The best potential for rain will be in the north towards
the Piney Woods, and decreasing towards the south and west down the
coast. Though the rain will be beneficial, all of the chances for
rain will also bring some potential for lightning as well. 25

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      61  44  72  60  80 /   0  10  30  40  30
Houston (IAH)              66  47  74  64  81 /   0  10  20  30  40
Galveston (GLS)            64  58  72  66  76 /   0  10  20  30  30

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION until noon CST today for the
     following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until noon CST today 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.

&&

$$

Discussion...31/43


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