Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 201710
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1210 PM CDT Sun May 20 2018

.UPDATE...

Forecast made tricky by approaching line of thunderstorms, as well
as with streamer showers ahead of the line. Relatively certain
thunderstorms will survive to our northwestern zones, and high
moved PoPs up there, especially in the next couple of hours where
it`s already raining. Mesoanalysis shows most of our area has
1500-2000+ j/kg MLCAPE, and no real capping - thermodynamic
factors will not be the bottleneck in this situation. Indeed,
streamer showers in the area are putting down some isolated CGs of
their own this morning.

Rather, mesoanalysis is also showing 20 or so knots of bulk
shear...or worse. So, thinking that while some updrafts may pulse
up and become strong, they will not be maintaned nearly long
enough for a more organized threat of severe weather. However,
with those stronger updrafts and the potential for some drier low
level air to wedge in, storms could get a bit gusty as the
updrafts give out. DCAPE only looks to max out around 600 j/kg so
I`m not sure how much potential there is for severe winds, but it
seems fairly clearly the largest threat of the severe convective
hazards. Still, at least until heating wanes later this afternoon,
some hail is also possible as forecast soundings hint at more
instability in the hail growth region early on.

As we start to lose the sun late this afternoon into the evening,
the instability profile slims down, and with precipitable water up
in the 1.5-2.0 inch range, it does raise some worry about heavy
rainfall. Fortuantely, we again have the lack of shear to hamper
updraft organization, and we`ve also been quite dry lately.
Because of this, flooding concerns are largely blunted. Still, a
particularly strong downpour in the wrong spot (low spots, poor
drainage areas, feeder roads, the like) could manage to create
your typical localized issues with a fast couple inches coming
down too rapidly for said wrong spot to handle in such a short
time.

Since the decent instability is the primary supporter, we should
also see storms wind themselves down into the evening. It`s even
somewhat questionable how far south/southeast they will drive
towards the coast. Some models still bring a shot of rainfall to
the Houston metro tonight, while others fizzle things out to light
showers or even nothing before arriving. This will create a fairly
tight gradient in rain chances from northwest to southeast, and
unfortunately this gradient lands on the Houston area. Small
changes in the forecast then, could alter PoPs at a specific
location fairly significantly in this area.

Luchs

&&


.PREV DISCUSSION... /ISSUED 630 AM CDT Sun May 20 2018/

DISCUSSION...

12Z AVIATION...Isolated showers are occurring near the coast this
morning. Expect these to continue for the next couple of hours...
moving NW. The main issue for today is an area of thunderstorms
that is expected to form around mid-day NW of CLL on an outflow
boundary. This area or cluster of storms is expected to move E/SE
into southeast Texas near CLL around 20Z and then continue to move
E/SE likely affecting UTS and then possibly reaching CXO and/or
IAH in the 22Z-02Z window. Expect weakening of the storms by early
eve, so for now, will have VCTS at IAH and southward as storms
may dissipate before they make it that far to the S/SE. 33

PREV DISCUSSION... /ISSUED 405 AM CDT Sun May 20 2018/

DISCUSSION...
At 330 AM, radar is showing some action for a change and showers
are streaming in from Matagorda Bay into Jackson county.
Additional streamer showers will be possible this morning but more
robust activity is expected late this afternoon. A weak short
wave rotating around a long wave trough near CA will move into the
area this afternoon and produce scattered shra/tsra. A
southeastward moving outflow boundary over North Texas will move
toward the region this afternoon and also bring scattered
shra/tsra. The two individual areas of storms look like they will
phase into one complex over the Brazos Valley late this afternoon.
Jet dynamics remain favorable with a splitting 300 mb jet
structure. Forecast soundings show PW values reaching 1.90 inches
near KCLL with CAPE values above 3000 J/Kg, steep lapse rates and
inverted V signature. SPC has outlooked part of the area in
Marginal Risk for severe storms later today. Some of the stronger
storms will have the potential to produce large hail and gusty
winds. Will mention severe in the weather grids for the NW zones
for late this afternoon and this evening. The storms will weaken
by mid evening as they move E-SE and will likely diminish before
reaching rainfall starved coastal areas. Temperatures today will
trend cooler today primarily due to more cloud cover and cooler
850 temps as the upper ridge retreats slightly. Houston has had 8
consecutive days with 90 degree temps and will forecast a high of
90 degrees today. Precipitation should end around 03z with the
loss of heating and the exit of the short wave but clouds will
linger for a good part of the night.

The upper ridge tries to build back in on Monday but 500 mb
heights look low enough for daytime heating and the sea breeze to
generate isolated to scattered shra/tsra. PW values remain between
1.5-1.6 inches and convective temperatures are in the upper 80`s
which looks more than obtainable. Tuesday looks drier and the
sounding profiles reveal very dry air above 850 mb. Coverage
should be less but models show a weak short wave moving into the
area so will maintain a slight chance of precipitation.

500 mb heights climb back to 588 dm by Wednesday night as the
upper ridge amplifies and builds back over Texas. Heights begin to
fall on Thursday and Friday as a weakness over the Gulf deepens
and shifts slowly west. How close this features get will
eventually determine rain chances for the end of the week. This
time, the eastern half of the CWA is favored for rainfall as PW
values climb to 1.7-1.8 inches and heights fall. The ECMWF is
stronger with the upper trough over the Gulf and leaned toward
this solution. Ultimately the deepening trough will allow the
ridge over northern Mexico to expand over Texas bringing very warm
temperatures back to the area for the Memorial Day holiday. 43

MARINE...
Seeing SCEC conditions offshore tonight as depicted by buoy 19.
Expect winds to gradually diminish later today and will back to
the east on Monday as weak high pressure becomes positioned over
LA/Ark. Light winds will persist through the week with a weak
pressure gradient in place. Expect winds to become variable around
mid-week with a sfc ridge axis overhead, and then become S/SW late
in the week in response to deepening low pressure across west
Texas. 33

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      85  71  90  69  91 /  70  50  20  10  20
Houston (IAH)              90  73  91  72  92 /  50  20  20  10  20
Galveston (GLS)            85  78  86  77  86 /  20  20  20  10  20

&&

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

$$

Discussion...25



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