Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 181500

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1000 AM CDT Sun Mar 18 2018

Surface analysis shows a weak boundary extending from central LA
into east-central Texas. At 850 mb, a ribbon of deeper moisture
extends across extreme eastern TX and western LA with a weak 850
mb trough of low pressure over SE TX. AT 300 mb, upper level
ridging over the western Gulf was amplifying over the southern
plains and Texas. A 100 kt speed max over SW TX will move NE
today. 12z CRP soundings show a strong cap at 850 mb with 850
temps near 22 C and a very dry layer of air between 960 mb and 500
mb. The 12z LCH sounding shows a saturated layer from the surface
to 800 mb. A weak but breakable cap was noted at 700 mb and
convective temps are in the upper 70s. The 850 mb temp was only
14 C and PW values were a more robust 1.65 inches. Have modified
PoPs this afternoon and raised them over the extreme E-NE part of
the area and lowered them over the extreme W-SW where the capping
is stronger. The 12z ARW and NMM are aggressive this afternoon
with additional convection developing near the residual boundary
over East Texas and then bringing that precip through the eastern
zones late this afternoon. This is not unreasonable based on the
12z sounding data. Raised temps a few degrees over the southwest
zones and upper 80s not out of the question in that area. 43


.PREV DISCUSSION... /ISSUED 631 AM CDT Sun Mar 18 2018/

AVIATION [12Z TAF Issuance]...

Difficult ceiling forecast in the short term as cloud bases very
early this morning have been somewhat variable. But, in general,
looking for IFR/MVFR conditions to improve to VFR around noon.

Look for scattered showers and possibly some storms to develop
around mid-day or this afternoon. Models are indicating
instability available, but we may not be able to overcome a fairly
substantial cap. Take things a little conservatively and broad-
brush VCTS at all terminals but CLL, which future shifts may be
able to refine.

Finally, looking for clouds to briefly scatter out, but only
briefly as we should sneak in one last night of low clouds and
some patchy fog ahead of the cold front, which lies just beyond
the forecast period for all but IAH, which starts to sketch out
the associated wind shift.


PREV DISCUSSION /Issued 400 AM CDT Sun Mar 18 2018/...

NEAR TERM [Through Today]...

Sea fog is rearing its ugly head again this morning mainly in the
bays and along the Upper Texas Gulf coast. Visibility seem to
vary quite a bit based on available webcams but enough that we
will keep the marine dense fog advisory through the morning hours.

Surface analysis has a stalled frontal boundary basically across
north Texas and into Louisiana. Warm sector south of the boundary
remains moist and expect the airmass to become unstable again
today as it recovers from previous turn over from convection.
Based on last night`s convection, it appears that the cap is
strong enough over much of SE Texas to limit convection. Cap is
weaker over east Texas where convection is ongoing.

Water vapor imagery shows a jet streak coming across Mexico into
Big Bend region. The short wave with the jet streak is fairly
identifiable with nice cirrus leafing and then subsident drying
behind that back over Baja Peninsula. Jet streak is progged to
move over much of Texas later today and may provide enough lift
from divergence to weaken the cap. Most of the forecast models
erode the cap but hard to understand why when best lift is more
over north Texas. SPC has rightfully outlooked this area for
severe storms this evening with just a general thunderstorm
outlook for SE Texas. Overall thinking is that the cap will be
stronger than what the models suggest. Model still produce QPF
over the region so stands to reason there will be some showers and
a few storms later this afternoon, but cap will really limit
coverage and severity of the convection.

SHORT TERM [Tonight Through Tuesday]...

Tonight a Pacific front pushes through the region and off the
coast 15-18Z Monday based on latest WRF data. There may be a line
of showers with the front but overall expect capping to remain
over the region. Forecast will maintain some 20-30 percent PoPs
and hard to see there being much more than that.

A much drier airmass will move into the region Monday. Temps at
850mb will still be in the mid teens Celsius. With strong mixing
expected, temperatures behind the front with dry air could still
reach the mid 80s and possibly low 80s on Galveston Island.
Relative humidity will likely drop below 20 percent which raises
concerns over fire growth potential. Winds will be gusty and
likely monitored through the day for the need of a Red Flag
Warning. This decision will also hinge on fuel moisture and
coordination with Texas Fire Service. Fire weather section below
highlights the details but the bottom line is that conditions will
be close to Red Flag Warning.

Cold front finally pushes through Monday night into Tuesday which
will drop temperatures both at the surface up to 850mb. High
temperatures in the low 70s will not yield the low RH we expect on
Monday but still conditions worth monitoring for fire growth.

LONG TERM [Tuesday Night Through Sunday]...

High pressure builds into Texas Tuesday night into Wednesday so
we are expecting low temperatures Wednesday morning to reach the
40s for much of the area. Upper level ridging will be over the
Plains Wednesday through the end of the week when the flow
flattens more. A short wave trough in the flow on Friday will
support strong return flow off the Gulf and warming conditions.
Temperatures will be near normal for the middle of the week but
then back to above normal temperatures for the weekend.



Relatively light onshore winds will continue today, veering a little
more southwesterly and becoming a bit more southwesterly tonight.
With these winds and relatively cooler waters, periods of sea fog -
dense at times this morning - will continue. An early front will
turn winds to more offshore Monday afternoon, but will remain fairly
light. A reinforcing front late Monday into Tuesday will more
effectively shift the winds to just west of northerly, and to at
least caution levels.

High pressure moving into the region will slowly diminish winds and
turn them northeasterly in the mid-week. By late Wednesday, we
should see winds turn back to onshore.



A cold front Monday, reinforced on Tuesday, will usher in
considerably drier air for the first half of the week in Southeast
Texas. Without a big temperature drop on Monday (indeed, winds will
actually encourage a very warm day), RHs will plummet into the teens
in the afternoon hours as winds become northwesterly and push into
the 10-15 mph range, and uncomfortably close to the red flag
threshold. It would not take much of an increase in the forecast to
reach the criteria.

Tuesday will not see relative humidity drop as low thanks to cooler
temperatures - minimum RH numbers are currently forecast only for
the lower half of the 30s. But, the flip side of the reinforcing
cooler air is a tighter pressure gradient and stronger winds. We
should push into the middle teens from mid-morning to mid-afternoon
and given the speed of winds just aloft, gusts into the 20s. As a
result, we should see elevated fire weather conditions for a second

Unsure right now if any fire weather watches or warnings will be
needed early this week as the driest air and strongest winds are out
of phase by a day or so. Regardless, early this week will be a
period to keep a very close eye on weather conditions. This is
particularly true where fuels are lighter, as the dry air will push
fine fuel moistures down very effectively and even up to 100 hour
fuels are near their average moisture or drier than normal.
Additionally, we are not looking for much wetting rainfall with
these frontal passages and should not expect much help there.



College Station (CLL)      84  59  82  49  70 /  20  20   0   0   0
Houston (IAH)              84  68  85  51  72 /  30  30  10   0   0
Galveston (GLS)            78  69  82  58  71 /  20  20  10   0   0



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