Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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511
FXUS64 KHGX 211751
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1251 PM CDT Sat Oct 21 2017

.AVIATION...
Showers and an isolated storm will continue to pop up through the
afternoon, likely with less impact for the coastal LBX and GLS
sites. Lightning, in the cells the occur, seem to be limited in
activity and duration, so will likely keep showers as the dominant
type, and amend if sites look to be threatened by a more menacing
cell.

Otherwise, look for VFR conditions to prevail into tonight.
Overnight, MVFR ceilings should develop in advance of oncoming
front, and some warm sector showers are likely. Thunderstorms
should be expected in the morning along the front as it moves
across the area, with winds becoming gusty and turning
northwesterly in the front`s wake.

Luchs

&&

.PREV DISCUSSION... /ISSUED 1114 AM CDT Sat Oct 21 2017/

DISCUSSION...
Gradient tightening up today with southeasterly flow and scattered
CU. Closer to the coast this morning have had some showers and a
thunderstorm but these have begun to shift further inland.
Soundings clearly show drier air to the west - and to the east
much more moist and unstable conditions split right over the CWA.
Will likely continue to see scattered showers and thunderstorms
today as we warm up into the mid to upper 80s. Made some minor
tweaks to the forecast for today to line up with current radar
trends. Tonight and Sunday looks like active weather and rain
chances looking good for the area as deep convection along the
frontal boundary develops over NTX tonight and moves into and
through SETX 4 am to noon. Houston area should extensive rainfall
coverage between 7 am and noon. Guidance showing favorable
conditions to support a broad line of storms moving through and
will pose some low end wind threat probably 20-40mph with the
higher wind threat southeast of the Highway 59 corridor. Dry air
arrives in the wake of the front during the afternoon hours and a
second surge of cooler and drier air Tue morning/afternoon.
45

PREV DISCUSSION... /ISSUED 633 AM CDT Sat Oct 21 2017/

AVIATION...
A mix of MVFR down to VLIFR due to both ceilings and fog were
affecting the terminal sites at 1130Z. Expect these to lift to
VFR around 15Z. A few stubborn MVFR areas left over should lift to
VFR by 17Z. The high resolution models show the potential for
isolated coverage of showers to develop between 15Z and 17Z and
last through the afternoon. A prefrontal trough will be
approaching the KCLL area by 12Z Sunday morning with a cold front
advancing into that area later Sunday morning. Expect fairly
widespread coverage of rain and thunderstorms along and ahead of
the front. If the front speeds up, showers and thunderstorms
could arrive earlier than forecast during the early morning.

40

PREV DISCUSSION... /ISSUED 348 AM CDT Sat Oct 21 2017/

DISCUSSION...
At 3 AM, low pressure was developing in the lee of the Rockies
with a tightening pressure gradient over the state. Low level
moisture continues to deepen and dew points are in the lower 70`s
over most of the region. Low ceilings and some patchy fog prevail
over the region as well. Winds off the surface are around 20
knots so feel the predominant weather type this morning will
probably be the low ceilings. Radar is considerably less active
this morning but a few showers are still moving into the extreme
east. Temperatures remain unseasonably warm and are in the lower
70`s this morning. The warm start coupled with some breaks in the
cloud cover will allow high temps to warm into the mid/upper 80`s
today. Fcst soundings show convective temps between 83-84 degrees
so am expecting sct shra/tsra to develop with a bit of heating.
850 dew pts remain high with values between 12-15 C. At 00z. a
weak 700 mb trough was noted over East Texas and this feature
should act as a focus for precipitation later today. 300 mb winds
not as diffluent as yesterday but there is still a broad splitting
of the jet over East Texas. Will maintain the 30/40 PoPs for
today across the CWA but feel the best rain chances will be close
to the 700 mb trough...or over the western third of the CWA. Fcst
soundings show quite a bit of dry air in the 850-700 mb layer and
a bit of a cap at 850 mb. A bit worried that these features might
be enough of a player to limit rain chances this aftn.

Precipitation will end with the loss of heating so there should
be a break in the rainfall this evening but precipitation should
redevelop late tonight over the far NW zones as a cold front races
into the area. Overnight low temperatures will remain warm
tonight ahead of the front and lows will remain in the lower 70`s
yet again. The front will trigger a line of showers and
thunderstorms as it races to the SE. PW values will peak near
1.80 inches ahead of the front but the speed of the front should
limit the potential for heavy rain. That said, some of the Hi Res
guidance shows some 1-2 inch rainfall totals on Sunday morning.
Low level convergence not looking all that terrific but ascent on
the leading of an approaching upper trough and a departing 80 knot
speed max should aid in lift. Fcst soundings show potential for
thunder but the risk for strong/severe weather looks minimal at
best with modest lapse rates and low helicity values.

Skies should begin to clear rapidly from west to east during the
mid to late afternoon with clear skies areawide by Sunday night.
Temperatures will be much colder by Monday morning with low values
in the lower 50`s with a few spots cooling into the 40`s. A dry
NNW flow aloft in the wake of the upper trough and sfc high
pressure over West Texas should keep things quiet on Monday with
seasonal temperatures. A second surge of colder air will make its
way into the region late Monday night. Cooler temperatures will
prevail Tue/Wed as surface high pressure settles over South Texas.
The high moves east on Wednesday night and onshore winds return.
Moisture levels and temperatures will begin to increase and warmer
conditions are expected on Thursday. Moisture deepens enough by
Friday to introduce rain chances again. Global models again differ
on the timing, strength and evolution of a vigorous upper level
trough and surface cold front on Friday. At this time, will
compromise between the Can/EC/GFS and call for 20 PoPs and trend
toward cooler temperatures next Saturday. 43

MARINE...
Winds have slowly diminished early this morning across the Upper
Texas coastal waters. Caution conditions should lower before
sunrise. With the winds becoming more southeasterly as a whole, do
expect the tide situation to be lower than yesterday. Forecast tide
levels at Pleasure Pier from the Meteorological Development
Laboratory only shows tides reach to about 3.2 feet during high tide
later today. This could cause some problems at isolated locations
and will keep the Beach Hazards statement up through the day.

Otherwise, a cold front will be moving off of the coast on Sunday--
probably during the afternoon. Winds could reach caution conditions
once again this evening before falling off overnight tonight and
Sunday morning ahead of the front. Winds and seas will be higher in
and near thunderstorms. Offshore winds behind the front will pick up
to advisory conditions late Sunday afternoon and through the night.
These should gradually diminish on Monday. However, the models show
the potential for moderate to strong offshore winds through early
Wednesday and caution or advisory conditions are possible during
this period.

40

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      87  70  76  51  79 /  30  60  70  10   0
Houston (IAH)              86  74  83  56  78 /  40  40  90  10   0
Galveston (GLS)            81  77  83  63  77 /  30  30  80  10   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&

$$

Aviation/Marine...25



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