Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 191124
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
624 AM CDT Tue Jun 19 2018

.AVIATION...
Models continue to be aggressive with rain chances today. PW
values are near 2.50 inches and fcst soundings show saturated
profiles. SE TX should lie on the east side of the 500 mb trough
axis today so would expect some increase in precipitation as the
day unfolds. Not enough confidence to pick one place or time over
another so will broad brush with VCSH this morning and VCTS this
afternoon. Will maintain VCSH wording for southern TAF sites over
night. Brief VFR ceilings will become MVFR later this morning with
a mix of VFR/MVF this afternoon. 43


&&

.PREV DISCUSSION... /ISSUED 458 AM CDT Tue Jun 19 2018/

SYNOPSIS...
A challenging forecast to continues to unfold for Southeast Texas
and the main emphasis has been placed on the next 48 hours for
the region. A Flash Flood Watch has been issued for roughly the
bottom two tiers of counties in the forecast area through 7 PM CDT
tonight. This watch is not to cover the potential for widespread
flash flooding, but for flash flooding that occurs as bands of
heavy rain rotating around a storm system along the Middle Texas
coast move inland through the day. Forecast rain amounts in the
watch area are only 1-3 inches today, which should , but it will
be the locally higher amounts of 4-6 inches that occur in smaller
streaks that create problems. Where those streaks occur is
anyone`s guess at this point and the Flash Flood Watch area is
there to highlight the most probable area in which rain bands may
develop. This is not another Memorial Day, Tax Day, or Harvey, but
this will be a day to check road conditions, use trusted sources
of weather information, and practice good judgment regarding
personal safety if encountering heavy rainfall.

Huffman

DISCUSSION...
Early morning surface analysis across the Southern Plains shows a
surface convergence zone/ weak trough axis stretching from near
Alice to Beaumont, with what appears to be some kind of weak or
transient MCV type feature churning across Bee and Goliad
counties. This has resulted in locally enhanced rain rates across
portions of South Central Texas early this morning with a storm
report from Rockport of 1.22 inches of rain in 15 minutes. Radar
imagery shows this feature translating northwards and it is not
expected to result in any hazards for Southeast Texas this
morning. Also evident on radar imagery this morning is a
developing band of rainfall approximately 30 nautical miles south
of Galveston that is lifting inland towards the Upper Texas coast.
GOES infrared satellite imagery has shown some cooling cloud tops
(now around -70 C as of this writing) over the past hour within
this band, indicating the potential for increased rainfall rates.

Evolution of this band over the next few hours is somewhat
uncertain as latest runs of the HRRR are struggling to resolve its
existence, but based on radar trends expect at least some of this
rainfall to push inland before sunrise. While high resolution
guidance has remained laughably inconsistent with its placement,
timing, and magnitude of rainfall over the past few days (believe
me, Southeast Texas, I feel your pain)... it is worth noting that
they do all show some kind of banding translating across either
South Central or Southeast Texas during the day today. This
banding is a result of energy rotating around a mid-level
circulation located near Corpus Christi early this morning on
water vapor imagery. Whether or not its the band that has
currently set up south of Galveston, or some other band that has
yet to develop, the main difference between yesterday and today in
terms of rainfall potential is the depth of the atmospheric
moisture. Aircraft soundings out of Houston yesterday morning only
showed column saturation up to around 800-750 MB, with soundings
this morning showing almost complete column saturation. This, in
addition to high freezing levels (above 16kft AGL) due to the
tropical air, only indicates a greater potential for
collision/coalescence processes and larger raindrop growth...
which means the potential for heavy rain within an environment
that has precipitable water values of 2.2-2.4 inches.
Southeasterly wind profiles this morning are also unidirectional
through a much deeper layer, indicating a high potential for
training. As was noted earlier, there is considerable uncertainty
on where these environmental parameters will be fully realized,
but because of the potential for banding to exist along the entire
Upper Texas coast, issued a Flash Flood Watch for the bottom two
rows of counties in the forecast area through 7 PM CDT. Diurnal
heating may enhance rain rates within these bands today, which
will make every band that is able to move inland something to
monitor. Rain bands may be able to spread farther inland by this
afternoon towards the northern counties, but regardless of their
inland progression, diurnal heating should contribute to scattered
shower and thunderstorm development towards the Brazos Valley and
Piney Woods by this afternoon.

A locally enhanced surface pressure gradient as a result of the
Coastal Plains surface trough is also resulting in southerly winds
in the 30-40 MPH range across the barrier islands this morning. As
this feature loses definition later this morning, expect these
gusts to relax somewhat but the coast will remain breezy through
the day. Rain should taper off this evening over the northern
counties as heating is lost and the mid-level low wobbles
southwest, but bands will continue to rotate into the coastal
areas this evening into tonight. A closed low now over Montana
swinging into the Central/Southern Plains on Wednesday will help
begin to nudge the Middle Texas coast low farther south and begin
to shear it out, which may result in lower banding coverage during
the day on Wednesday. While environmental conditions will still be
favorable for heavy rainfall, considerable uncertainty exists on
the coverage that the region will see tomorrow. The continued
shearing the Middle Texas low will result in dramatically lower
rain chances by Wednesday night with only scattered showers and a
few storms possible across Southeast Texas. The remnant shear axis
lingers over the region on Thursday, providing some enhancement in
coverage to diurnally driven convection roughly along the Highway
59 corridor.

For Friday and the weekend, broad and flat ridging spreading
across Texas will result in a return to a more typical summertime
pattern with low rain chances along the seabreeze with
temperatures warming above normal.

Huffman

MARINE...
The pressure gradient has tightened significantly this morning
with high pressure over the eastern Gulf and lower pressure off
the middle Texas coast. Sustained winds will exceed 25 kts early
this morning with gusts exceeding 35 knots at times. The gradient
will weaken slightly later this morning but winds will remain at
or above Small Craft Advisory criteria. Seas/Swells are above 8
feet offshore and will slowly drop later this afternoon but should
remain elevated through tonight. The gradient will weaken on
Thursday and onshore winds will begin to relax Thursday night into
Friday night. Wind speeds are expected to increase again next
weekend as low pressure develops in West Texas.

Tides will likely remain 1.0 to 1.5 feet above astronomical
levels today and minor coastal flooding will again be possible
along the Bolivar peninsula with high tide at the Galveston Bay
Entrance peaking a little over 3 feet above MLLW at high tide.
Given impacts to vulnerable locations, a coastal flood advisory
continues along the Bolivar Peninsula. Strong rip currents will
remain a threat on Gulf facing beaches again today. 43

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      83  74  86  74  90 /  70  40  60  40  40
Houston (IAH)              81  76  85  77  89 /  80  50  70  40  60
Galveston (GLS)            83  81  85  82  86 /  80  70  60  30  50

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...SMALL CRAFT SHOULD EXERCISE CAUTION from this evening through
     Wednesday morning for the following zones: Matagorda
     Islands.

     High Rip Current Risk through this evening for the following
     zones: Brazoria Islands...Matagorda Islands.

     Flash Flood Watch through this evening for the following zones:
     Brazoria Islands...Chambers...Coastal Brazoria...Coastal
     Galveston...Coastal Harris...Coastal Jackson...Coastal
     Matagorda...Fort Bend...Galveston Island and Bolivar
     Peninsula...Inland Brazoria...Inland Galveston...Inland
     Harris...Inland Jackson...Inland Matagorda...Matagorda
     Islands...Southern Liberty...Wharton.

     Wind Advisory until 10 AM CDT this morning for the following
     zones: Brazoria Islands...Galveston Island and Bolivar
     Peninsula...Matagorda Islands.

     High Rip Current Risk from 3 PM CDT this afternoon through this
     evening for the following zones: Galveston Island and
     Bolivar Peninsula.

     Coastal Flood Advisory until 3 PM CDT this afternoon for the
     following zones: Galveston Island and Bolivar Peninsula.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION through Wednesday morning
     for the following zones: Matagorda Bay.

     Small Craft Advisory until 7 PM CDT this evening for the
     following zones: Coastal waters from Freeport to Matagorda
     Ship Channel TX out 20 NM...Coastal waters from High Island
     to Freeport TX out 20 NM...Galveston Bay.

     SMALL CRAFT SHOULD EXERCISE CAUTION from this evening through
     Wednesday morning for the following zones: Coastal waters
     from Freeport to Matagorda Ship Channel TX out 20 NM...
     Coastal waters from High Island to Freeport TX out 20 NM...
     Galveston Bay.

     Small Craft Advisory until 10 AM CDT Wednesday for the following
     zones: Waters from Freeport to Matagorda Ship Channel TX
     from 20 to 60 NM...Waters from High Island to Freeport TX
     from 20 to 60 NM.

&&

$$

Discussion...14
Aviation/Marine...43


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