Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 211555

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1055 AM CDT Sun May 21 2017

Quick update to adjust rain chances based on latest radar trends.
The frontal boundary that pushed off the coast early this morning
is now beginning to lift back inland in response to daytime
heating, with scattered showers developing along it as it lifts
inland. The KHGX VAD wind profiler has shown low to mid level
winds increasing over the past hour and this, combined with the
boundary, should keep cells moving late morning through this
evening (unlike what has been observed with cells developing ahead
of the boundary over the Houston metro this morning). Despite some
cell movement anticipated, precipitable water values between 1.75
and 2.25 inches (per the Lake Charles and Corpus Christi morning
soundings) will mean even innocuous looking cells are capable of
producing very heavy downpours as they move over the area. Harris
County FWS gages have reported anywhere from 1-1.36 inches of rain
fell from showers earlier this morning near downtown Houston and
anything that develops later today will also be capable of totals
similar (or even higher) than what has already been observed.

Cannot rule out a thunderstorm or two being strong or possibly
severe and capable of hail/strong winds, but with less
instability present today (as compared to yesterday) expect
coverage to be much less than yesterday.

Farther offshore, a thunderstorm cluster (with one cell even
displaying supercellular characteristics) has been ongoing for
much of the morning and will be capable of producing brief, gusty
outflow winds along the coast between Freeport and Galveston
before it moves inland or dissipates later this morning.

Otherwise, minor updates made to temperatures and dew points based
on observations.



.PREV DISCUSSION... /ISSUED 459 AM CDT Sun May 21 2017/

Scattered showers and thunderstorms continue across the
area...aided by some impulses moving thru in the flow aloft. A few
cells continue to pulse up to severe levels with large hail and
heavy rain being the primary threat. Thankfully the cells are
moving which mitigates the observed high rain rates.

The frontal boundary/wind shift has sagged to near the coast and
is becoming more diffuse. With some daytime heating it might drift
further inland and possibly set up a convergent zone for additional
storms this afternoon. Vast majority of guidance indicates areas
along and south of I-10 will emerge as the favored area for the
best areal coverage. Though instability shouldn`t be quite as
high as Sat evening and overnight, there should still be enough
whereas a few pulse strong/severe cells can`t be ruled out. PW`s
around 2" and diminishing storm motions could also lead to some
localized heavy rain.

Main concern as far as the potential for widespread significant
wx looks to be late tonight thru the day Monday as a stronger
upper disturbance moves up from Mexico and across the area. It
should generate widespread precip that`ll begin moving in after
midnight and continue into at late Monday evening. Question is
where will the surface boundary be as this occurs? It`ll likely
serve as a focus for the higher rainfall amounts and potential for
any flash flooding. Guidance suggests it could be anywhere from
the Highway 59 corridor to just offshore. Due to the expected
prolonged rain, and the potential for heavy rain with pooling
1.9-2.2" PW`s, we`ve been placed in a moderate risk for flash
flooding. Holding off on issuing a watch attm as we still have
some time for the next shift or two to see how things evolve and
hopefully further narrow down a threat area should one exist.

Amplifying western ridge will send a few strong shortwaves from
the Rockies southeastward across east cntl Tx Monday night and
again Tue night. These will eventually carve out an eastern trof.
We may see some sct precip associated with one or both of these
shortwaves if there`s any leftover moisture from Sunday night and
Monday`s system. A cool front should be moving off the coast on
Tue. Improved (drier) conditions are expected for the remainder of
the work week. 47

No issues with the marine forecast these next few days with light
onshore winds prevailing. Scattered showers/thunderstorms will be
possible during this time as a weak frontal boundary lingers over
inland SE TX. Models remain on track with pushing a second strong-
er cold front into the coastal waters Tues. We could see SCEC/SCA
north winds by Weds morning, but will be very short-lived. Return
flow expected to be in place across the Gulf waters late Weds eve-
ning/early Thurs morning. 41

Forecast will remain messy and likely very dependent on mesoscale
factors through this next TAF package. Will keep with persistence
(generally) with a mix of MVFR CIGS/VCSH/VCTS. 41


College Station (CLL)      79  68  77  64  79 /  30  70  80  50  30
Houston (IAH)              84  72  78  66  82 /  50  60  80  50  30
Galveston (GLS)            83  77  81  71  81 /  50  70  80  70  30




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