Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 292107
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
407 PM CDT Sat Apr 29 2017

.DISCUSSION...
The immediate forecast concern focuses on potential severe weather
threat for tonight. If capping can be defeated with the frontal
passage, potential exists for strong to severe storms. However,
the alternate scenario if the cap wins out is more a thin line of
showers and thunderstorms to move through the area. Skies should
clear quickly on Sunday in the wake of the front, with briefly
cooler temperatures. However, look for heat to come back Monday as
onshore flow resumes. Another chance for storms on Wednesday, and
many of the ingredients for severe weather look to be around there
as well. There is not enough consistency in the models bringing
all of these ingredients together in the same place at the same
time to speak very confidently on the severe threat at this time,
but it is a period that should be watched in the coming days.

Tonight through Sunday Morning...
Though the cap has been perhaps not quite the monster models
originally progged it to be (21 C vs 24 C at 850), and did allow
one rogue severe thunderstorm early this morning, it has still
been significant enough to otherwise shut down any warm sector
convection as expected. Radar does show that some streamer showers
are beginning to form beneath the cap. Though they will have to
be watched for any growth given the large instability present
above the cap, satellite gives no indications that this is a
likely scenario. At this time, satellite shows a thick cover of
stratocumulus clouds across the large majority of the area. Areas
southwest of Houston did see a brief window of sunnier skies
earlier today, which has allowed that area to warm more than the
rest, but even in cloudy areas temperatures have risen into the
80s. With dewpoints in the low to mid-70s, it`s been a muggy day.

In the far northwestern portion of the area (College Station to
Crockett or thereabouts), the stratocu has begun to break, and
some limited vertical development is apparent on satellite. Though
the data is still preliminary, a visible/infrared RGB composite
from GOES-16 does indicate some of these clouds are tall enough to
have an ice component. This indicates that as the front and upper
trough draw closer tonight with greater forcing, some convection
along the front can still be expected. If the forcing is strong
enough, there is ample instability and sufficient deep layer shear
to promote strong to severe thunderstorms. However, much of
Southeast Texas may simply be too far removed from the help of the
upper trough. As such, severe potential continues on a north to
south gradient, with the best potential likely in Houston County,
and decreasing as you head towards the coast.

There should be enough upward push on the front to expect at
least of line of showers and some isolated thunderstorms, with a
conditional chance of severe weather should any of them be able to
tap into the favorable environment above the cap. If this does
occur, would expect damaging winds to be the primary hazard give
the large amount of DCAPE shown in mesoanalysis. Hail would be a
secondary threat, but wet bulb zeroes are perhaps a bit high to
keep it from matching wind as a concern. A tornado or two can`t be
completely ruled out given what appears to be plentiful low level
shear, but SRH also appears fairly low, mitigating the concern
there.

By Sunday morning, the front should be approaching the coast, with
cooler and drier air pushing in and quickly bringing what rain
does manage to move through to an end pretty quickly.

Sunday through Tuesday...
Sunday looks to be quite pleasant in post-frontal flow, with skies
clearing and highs notably cooler - expected to be in the 70s.
Look for the high to move quickly off to the east, allowing for
onshore flow to re-establish on Monday. With that, warming will
return - with temperatures back into the 80s as quickly as Monday,
and pushing the mid to upper 80s on Tuesday. As moisture increases
on the onshore flow, skies should become cloudier from the
southwest Tuesday into Tuesday night.

Wednesday and beyond...
Late Tuesday an upper trough deepens again in the Central US, with
another front poised to move into Southeast Texas Wednesday,
likely Wednesday night. Like today, guidance shows strong
agreement in generating a large amount of instability above a low
level cap. However, both the Euro and GFS show a weaker cap
Wednesday than during today, which may indicate more potential for
severe weather. The midlevel trough looks very positively tilted
though, and neither the Euro nor GFS dig it deep enough into Texas
to play a significant role in assisting with convection. Instead,
we`d look for favorable phasing with shortwave troughs being
ejected across our area ahead of that positively tilted trough.
Given difficulty in timing these subtle features, there cannot be
much confidence that everything will line up just right for a
severe event. Given the favorable ingredients though, it is
worthwhile to keep an eye on the forecast for Wednesday in the
coming days.

After Wednesday, again look to temperatures to get knocked back
into the 70s followed by slow warming into the 80s into the
weekend on northwesterly winds and generally sunny skies.

Luchs

&&

.MARINE...
Strong onshore flow continues this afternoon. South to southeast
winds of 20-28G25-32kt with seas finally building quickly and up to
8-10 feet at the bouys. Gradient remains tight this evening but
should relax a little for just a few hours prior to FROPA. Seas
should build slowly and may reach 11-12 feet well offshore. Post
frontal winds abruptly turn to the NNW-NW and crank back up to 20+
knots which is going to make for very rough conditions offshore.
Winds and seas lowering after 10 am and SCA runs through 2 pm but
will probably need to extend it for the 20-60 waters for slower
dwindling seas.

High moves across the waters Sunday/Monday and return flow gets
underway Monday afternoon gradually increasing/persisting through
Wednesday evening. Next cold front swings through late Wednesday or
early Thursday morning. SCA likely in the wake of the Thursday front.

Longer period swells arriving and tide levels remain around 3 feet
but should drop in 2-4 hours as low tide nears. Although tide levels
will remain elevated by 1 to 2 feet the low tide and arrival of the
front should bring down tides prior to the arrival of the next high
tide. One thing to note though is the Bay side of the Island and
Bolivar will likely see a rise to near critical thresholds for an
hour or two as the cold front sloshes the water back south and it
backs up trying to get out through the ship channel entrance and
Rollover Pass.

45

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      58  73  50  84  60 /  70  10   0   0   0
Houston (IAH)              63  76  54  84  63 /  80  10   0   0   0
Galveston (GLS)            67  76  65  79  71 /  80  20   0   0   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Wind Advisory until 7 PM CDT this evening for the following
     zones: Austin...Brazoria...Chambers...Colorado...Fort
     Bend...Galveston...Harris...Jackson...Matagorda...Waller...
     Wharton.

GM...Small Craft Advisory until 2 PM CDT Sunday for the following
     zones: Coastal waters from Freeport to the Matagorda Ship
     Channel out 20 NM...Coastal waters from High Island to
     Freeport out 20 NM...Galveston Bay...Matagorda Bay...Waters
     from Freeport to the Matagorda Ship Channel from 20 to 60
     NM...Waters from High Island to Freeport from 20 to 60 NM.

&&

$$

Discussion...25
Aviation/Marine...45



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