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Area Forecast Discussion
National Weather Service Houston/Galveston TX
415 AM CST Wed Dec 26 2018
.DISCUSSION...
Confidence is increasing in the evolution of a thunderstorm event
today into Thursday morning. It appears that the potential for
severe weather this afternoon is decreasing, that is countered by
better confidence in a strong to severe line of storms that will
move in from Central Texas tonight, and cross the area through the
overnight hours before exiting to the east Thursday morning. This
rain could also bring high rainfall rates that could cause
flooding issues in sensitive locations. This potential is highest
in the north and northwest, where afternoon rain looks most likely
to prime the area, and the squall line will be strongest
overnight.
Once the front works through, much of the area will dry out...but
that front also will stall close enough to shore that slight
chances for rain will linger over the waters and near the coast
right into the weekend. From this weekend into early next week,
rain chances will pick up as the front`s remnants drift back
northward and develop into a coastal trough that then moves
northeastward.
.NEAR TERM [Through Today]...
A look at the midlevel chart shows that Southeast Texas is still
just barely under the influence of an upper ridge that has largely
moved off to the east. A subtle shortwave trough is evident in
water vapor imagery that may manage to spark some showers later
this morning, but the real action is still farther to the west.
Meanwhile, at the surface, a trough of low pressure in the desert
southwest is starting to coalesce into a center of low pressure in
New Mexico. That low will continue to develop and move into the
Panhandle. As it strengthens and moves closer, that will tighten
the pressure gradient and pick up the onshore breeze. Indeed, the
barrier islands and even portions inland to the southwest of
Houston may see winds start to push into the neighborhood of a
wind advisory. Shouldn`t quite get there, but the breeze will
likely be noticed.
As we push into the afternoon, in addition to the winds, we`ll see
rain chances increase, particularly north/northwest of Houston
around the Brazos Valley up towards the Pineywoods. Confidence is
increasing that we`ll be lacking the instability to really see
strong to severe convection get going in this early wave.
But...the environment beyond instability is still good and HREF
members still paint this northern area with some spots in the
Z > 40 dBZ paintball product, so a strong to severe storm can`t be
totally ruled out. But, for most, we should just be looking at
showers with perhaps some isolated lightning for the afternoon
portion of this event.
While the direct impact appears to be lessening, it is important
to note that any widespread areas that see rain in the afternoon
will be moistening the soils ahead of overnight convection. So,
this round need not be severe to make itself felt.
.SHORT TERM [Tonight Through Thursday]...
Confidence is also increasing in how things will play out tonight.
Unlike the afternoon, where it appears that the ceiling on
convection is lowering, the overnight period looks like it will be
better able to sustain some severe weather. It does appear that a
QLCS is expected to develop in Central Texas this afternoon, and
move through Southeast Texas overnight. Though it will likely
weaken slightly as its dynamic support ejects northeastward, there
is potential for severe weather across the entire area.
The primary threat will be for damaging straight-line winds in
the line, but there is sufficient low level shear and helicity
that there will be some tornado potential as well. The orientation
of the 0-3km shear vector is not ideal for QLCS tors, mitigating
this potential somewhat. But, the magnitude is relatively large,
so we`ll have to be on the lookout for smaller elements in the
line that may find locally better orientation for a brief spinup.
Larger hail may be possible in the northwest when the line is
strongest, but that threat should wane relatively quickly in the
event.
For timing, there is still some spread in the guidance on when
this line moves in from Central Texas. It looks to reach College
Station sometime between 11 pm and 3 am, but the consensus is
probably in the midnight to 1 am range. Houston looks to see the
leading edge of the line sometime after 3 am, with the current
consensus looking to be around 4 am. While the front will be the
main driver of this line, the forward speed of squall lines seem
to be influenced fairly strongly by their own internal cold pool
thermodynamics, and it may take until the line forms and gets
going to really get a firm handle on precise timing. Hopefully
these windows will provide a decent start on planning.
As the front pushes through on Thursday, rain should come to an
end fairly quickly for most. However, with the surface low heading
for La Crosse, Wisconsin and the upper low trailing after it, we
should expect the temperature gradient to weaken, and with it the
forward push of the front. Still expect it to get offshore and
through (or nearly so) our marine zones by the time we reach
Thursday night.
.LONG TERM [Thursday Night Through Wednesday]...
The front will continue to lose definition and stall. Look for it
to make it out of our marine zones, but it may not be push much
further into the open waters of the Gulf. Because of this, we may
see continued lift up the frontal surface, and lingering rain
chances over the waters and even to coastal spots on land. Only
keep in very slight chances over land, but at least wanted to
remain open to the possibility, particularly since the trend in
the guidance continues to be towards an upper trough and surface
low that eject sooner and more westward.
The remnants of the boundary are likely to be shoved back towards
the coast this weekend, and as a reinforcing shot of
northwesterlies makes its way into the picture, we should sharpen
that boundary back up into a coastal trough. As one might expect,
rain chances, particularly near the coast, will be on the way up
this weekend and for the early portion of next week. While there`s
some spread in the timing, there`s fair model consensus for the
trough to get kicked out to the northeast sometime in the first
half of next week as high pressure settles in from the Panhandle.
Model blended PoPs remain high into the New Year, but there`s
agreement with the surrounding offices to undercut that guidance
pretty significantly for the midweek.
&&
.AVIATION [06Z TAF Issuance]...
VFR conditions continue across all TAF sites. Some patchy fog may
still develop across some areas of the CWA overnight and early
morning hours resulting in periods of MVFR to possibly IFR
conditions. As a strong upper level low moves into N TX Wednesday
afternoon, rain activity will increase over the western counties
and expand eastward, resulting in MVFR/IFR cigs/vis through the
end of the forecast period. Scattered to widespread SHRA and
isolated to scattered TSRA expected from around 19Z Wed- 03Z Thu.
A stronger line of SHRA/TSRA is then expected to move across SE TX
from around 06Z-13Z Thu. Conditions expected to improve thereafter.
&&
.MARINE...
Increasing southeast winds and building seas can be expected through
Thursday morning as a west Texas area storm system strengthens and
gradually moves off to the northeast. Showers and thunderstorms are
possible today and are likely late tonight through Thursday morning.
Some of the overnight storms could become strong or severe. A cold
front will move across the waters Thursday afternoon with weak west
winds and lowering seas developing in its wake. Mainly moderate
northeast winds and an increase in seas can be expected at the end
of the week and on through the upcoming weekend.
The strengthening onshore flow could lead to some minor coastal
flooding late this afternoon through tomorrow morning, especially
in/around high tide times. 42
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 71 58 69 44 63 / 90 90 20 0 10
Houston (IAH) 70 63 73 48 68 / 90 90 70 10 10
Galveston (GLS) 68 63 69 54 65 / 70 80 80 10 20
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION through Thursday morning for
the following zones: Galveston Bay...Matagorda Bay.
Small Craft Advisory until noon CST Thursday for the following
zones: Coastal waters from Freeport to Matagorda Ship
Channel TX out 20 NM...Coastal waters from High Island to
Freeport TX out 20 NM...Waters from Freeport to Matagorda
Ship Channel TX from 20 to 60 NM...Waters from High Island
to Freeport TX from 20 to 60 NM.
&&
$$
