Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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088
FXUS64 KHGX 201601
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1101 AM CDT Sat May 20 2017

.UPDATE...
The line of showers and thunderstorms that moved across the
northeastern counties earlier this morning along an outflow
boundary have cleared the region, but radar imagery shows isolated
cells developing across the northern half of the forecast area in
its wake. Visible satellite imagery shows an outflow boundary
associated with this line stretching from roughly Brenham to
Cleveland, with surface analysis revealing another outflow
boundary northwest of the region stretching from San Antonio to
Ft. Hood. Expect both of these features to serve as focusing
mechanisms for additional shower (and possibly thunderstorm
development) today as temperatures continue to rise into the 80s.

The environment across Southeast Texas today is capable of
producing thunderstorms today, but have low confidence in overall
coverage. Stabilization behind this morning`s thunderstorms may
limit development along the northwestern outflow boundary for the
next few hours as it sags across the Brazos Valley, with anvil
debris from ongoing convection near San Antonio`s area also
inhibiting better destabilization across the northern counties as
it advects to the northeast. Additional development is expected
along the southern outflow boundary, but aircraft soundings out of
Houston continue to show mid-level temperatures ranging from 10-12
degrees C and provide enough of a cap to also limit thunderstorm
development. However, morning upper air analysis showed strong
divergence across the region in response to the upper low lifting
across the Central Plains and this may be enough to overcome the
aforementioned factors. Cannot rule out a strong or severe
thunderstorm producing large hail or damaging winds given mid-
level lapse rates around 7 C/km per SPC mesoanalysis, but this is
all contingent on thunderstorm development. Brief heavy rain will
also be capable in stronger activity.

The HRRR/RAP show any activity developing today sweeping from
northwest to southeast as a cold front stretching from Presidio to
south of the D/FW Metroplex overtakes both outflow boundaries
this afternoon and evening. However, RAP VAD wind profilers show
mid level flow becoming increasingly weak/light during the
afternoon hours and consistent with the previous forecast am
concerned that this may result in the front slowing down or
stalling across the region. This would result in possibly another
round of rain this evening or tonight as subtle disturbances lift
out of Mexico.

Huffman

&&

.PREV DISCUSSION... /ISSUED 459 AM CDT Sat May 20 2017/

DISCUSSION...
Line of shra & embedded tstms is currently approaching northwest
portions of the region. SPC mesoanalysis indicates this is likely
along an outflow that has pushed ahead of the front. Few models
have started out as far se w/ the precip as it currently is.
Similar difficulties will likely be a common theme for the next
3-4 days, unfortunately, as the ongoing fcst will likely be driven
on small scale features that are tough to resolve well in advance.

Looks like the tail end of this precip will continue making s/se
progress into northern, and possibly central parts of the area
this morning. Aircraft soundings out the the Houston area still
indicate capping at play so suspect a gradual weakening trend can
be anticipated in the next several hours. But, we`ll need to keep
an eye where this boundary eventually ends up as could fire back
up w/ daytime heating. Healthy CAPES and low LI`s are depicted
later in the day, so can`t rule out a few strong cells either.

Tonight, an upper level disturbance is forecast to move ne out of
Mexico and toward se Tx. It appears this will travel parallel to
the frontal boundary which should be situated somewhere in the
area and serve to enhance overall lift. With PW`s pooling aoa
1.9" along it, slow moving/training & localized heavy rainfall
cannot be ruled out. NAM12 has been consistent in showing the
potential for several inches of rain tonight, generally in our NE
zones around Lake Livingston. The Tx Tech WRF and especially most
of the NCAR ensemble members support such a scenario. Whether the
sfc focusing boundary is actually in that region remains to be
seen, but signals like this do indicate at least the potential
somewhere in the vicinity.

Appears the front will make its way closer to the coast on Sunday
and stall. Synoptic models are in pretty good agreement showing a
the next strong shortwave embedded in the sw flow aloft to make
its way into the area Sunday night and Monday which should bring
widespread shra/tstms...again with highest accumulations near the
boundary which will probably be meandering across the southern
half of se Tx. We`re seeing more convective bullseyes nearby in
model QPF amounts which is also concerning in regards to a
possible evolving flash flood threat. Timing and placement are not
consistent, but these are oftentimes signals that need to be payed
attention to in these type of patterns.

Rain chances should persist into Tue night or Wed morning until
a stronger front pushes off the coast. Forecast confidence is
fairly high in regards to chances of rain. However, have very low
confidence in regards to the overall timing/placement of the
highest amounts that might cause impacts.

On the positive side, the 2nd half of the work week looks quite
nice in the wake of the front with lower humidities, mclr skies &
cool overnight temps. 47

MARINE...
Onshore winds remaining slightly elevated this morning and will go
ahead and keep the SCEC in place through mid morning. Winds are ex-
pected to decrease by this afternoon as the gradient weakens in re-
sponse to this line of storms moving into northern parts of SE TX.
Disturbances moving into the area will help keep an active weather
pattern in place the rest of the weekend...with generally light to
moderate onshore winds prevailing. Models are still indicating the
passage of a cold front into the Gulf late Tue/early Weds...with a
moderate/strong NNW flow developing Weds afternoon. This northerly
flow will begin to weaken Weds evening and remain in place through
early Thurs morning then slowly veering back to the SE Thurs after-
noon. Moderate onshore winds to prevail the rest of the week. 41

AVIATION...
Tricky forecast the next few days as a series of disturbances move
into the area from the west. The first being this morning as TSRAs
associated with the dryline finally make it into SE TX. Short-term
models indicating that another round is possible later this after-
noon. Thereafter the TAF forecast will be largely dependent on the
vagaries of the meso-scale features that linger in the wake of the
storms. Will likely keep with a mix of MVFR CIGS and VCSH/VCTS for
the overnight/early Sun morning hours. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      85  69  85  69  77 /  70  60  50  50  70
Houston (IAH)              88  73  86  72  80 /  50  50  40  40  70
Galveston (GLS)            86  77  85  77  81 /  20  40  50  50  70

&&

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

$$

Discussion...14



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