Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 182018

Area Forecast Discussion
National Weather Service Houston/Galveston TX
318 PM CDT Mon Sep 18 2017


A quintessential "Houston Summer" day today, complete with humidity
and scattered showers and storms. After a couple weeks to dry out,
there aren`t expectations for any new flooding issues. Given the
moisture present in the atmosphere, if one of the stronger cells
were to fall over a particularly vulnerable spot, there could be
some brief, highly localized area of water - in other words, what
you might expect from this area. With only slight variations in
coverage, expect more of the same over the next several days. In the
long term, the forecast is of relatively low confidence as the
guidance shows influence from a number of subtle, complex
interactions in the pattern, and a subtle difference in how these
interactions occur could nudge temperatures a handful of degrees or
result in different days shower better potential for afternoon
convective development.

.NEAR TERM [Through Tonight]...

We`re continuing to watch the radar as showers and some
thunderstorms march across the area this afternoon. Water Vapor
imagery shows a subtle shortwave trough moving through Southeast
Texas today. The GOES-16 TPW product shows precipitable water has
surged over 2 inches across the area, which is a notable bump for
inland portions of the area, which had been closer to 1.5 inches
when I came in this morning. Visible satellite shows a subtle
boundary from roughly Crockett to College Station to several miles
north of Victoria, and much of the early activity seems to be
initiating off of that boundary. A secondary line of showers and
storms seem to be preferentially firing off what may be a weak
seabreeze boundary near the coast. The vis also shows a cu field
coastward of this boundary as well, so there`s not a ton of capping
if this is indeed a seabreeze.

Regardless, activity so far is and is expected to be fairly well
behaved. CREST streamflow values show the strongest cells may create
some very highly localized areas of heavier rainfall, but so limited
that we`re not looking for much in the way of new flooding concerns.
As another potential threat, mesoanalysis shows some decent DCAPE
values and morning soundings near the area indicated a very shallow
inverted-V pattern, so there may be a bit of gustiness in the
strongest cells, so we`re looking to see a handful of strong
thunderstorms, but severe potential is still pretty limited.

These storms should continue through the afternoon and into the
evening hours before winding down. Depending on the timing of the
trough, we may end very rapidly around sunset as loss of heating and
subsidence on the back side of the trough shut down updrafts. A
slightly slower trough might allow things to linger a bit later, but
any difference is unlikely to make any real impacts.

.SHORT TERM [Tuesday Through Wednesday Night]...

As mentioned at the top of the discussion, don`t expect a ton of
change in the next several days. In the wake of today`s trough,
Tuesday would look to see a bit less coverage in showers and storms
today, with things bumping up a little bit Wednesday as the guidance
is pretty unanimous in showing another upper trough steam across.
But beyond that, don`t expect a lot of variation in temperatures or
cloud cover through the midweek.

.LONG TERM [Thursday Through Monday]...

As we approach the weekend, we look for a number of things to emerge
in the upper pattern. Jose and Maria may begin a complex dance in
the Northwest Atlantic underneath a growing Eastern ridge (both the
GFS and Euro indicate at least some interaction here). A trough over
the Pacific Coast looks to deepen significantly. And, perhaps most
important for our area, a shortwave trough looks to rotate around
the burgeoning ridge and could make its way back towards Southeast
Texas as a weak closed low.

The GFS is quite gung ho on this idea, while the Euro tends to hang
it back more towards the northern or northeastern Gulf Coast states.
Assuming this pulls off, the presence of the low would keep up the
potential for scattered showers and storms into the weekend. Either
way , this is even reminiscent of a Rex block, continuing to
amplify the longwave west trough/east ridge pattern. This would
keep the weather pattern over the area stagnant for several days.
The main difference in the two would be just how widespread
development would be. Now, for what it`s worth, the Canadian moves
Jose off to the northeast, leaving only Maria in the Atlantic.
The shortwave is much, much slower in being shunted around the
south side of the ridge, and ultimately gets drawn back up into
the northern stream and ejected. Though a bit more progressive,
this is still enough to amplify and stagnate the pattern, but
without lowering heights and cyclonic vorticity advection, the
result is a drier period over Southeast Texas.

This is all a very long-winded way of saying that the forecast for
late in the week and the weekend is of lower confidence. Ultimately,
the impact is relatively low, but these subtle differences may shift
which days in the longer term will have a better potential for
showers and storms. For now, keep the PoPs fairly splattered out to
account for this uncertainty.


.AVIATION [18Z TAF Issuance]...

Inland TAF sites will be dealing with SHRA/TSRA and associated MVFR
ceilings and visibilities this afternoon as activity continues to
develop and work its way generally off to the N and NE. Should see a
decrease in coverage later this afternoon through early this evening.
Some MVFR/IFR ceilings and visibilities are possible again overnight
at inland sites, and areas near the coast could see possible SHRA
toward morning. Activity should spread inland again tomorrow afternoon,
but current thinking is that we might see less coverage than what we
are seeing today.  42



Light to occasionally moderate southeast winds and low seas will
persist for the rest of the week and on into the upcoming weekend.
Slightly higher moisture levels favor additional periods of showers
and thunderstorms.  42



College Station (CLL)  75  91  74  93  75 /  10  20  10  30  10
Houston (IAH)          76  90  76  89  76 /  20  30  10  50  10
Galveston (GLS)        80  85  79  86  79 /  10  20  20  40  30






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