Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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227
FXUS64 KHGX 220455
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1155 PM CDT Sun May 21 2017

.AVIATION...
Tricky forecast this evening in terms of timing and precip
coverage, due to discrepancies in the model guidance and forecast
soundings. Currently, there is a small break from thunderstorms
across all of the terminals except for GLS. Showers and
thunderstorms over the bays and nearshore waters are continuing to
push northeastward interfering with GLS terminal and vicinity.
LBX may also see similar showers and possible thunderstorms within
its vicinity in the next hour. CLL and UTS also are experiencing
light rain across its terminals. Light scattered showers moving in
from the west across SGR, HOU, IAH, CXO should hold through early
morning when the next round of precipitation begins to move in.

Depending on how precipitation fires up to the west of our CWA,
expecting another round of showers and thunderstorms to move in
early morning around 09Z across TAF sites SGR, HOU, and terminals
northward. This round of precip associated with a shortwave
disturbance should continue through 12Z across much of these
terminals.

At the coast, shower and thunderstorms are expected to continue
through the late morning hours, between 07 to 15Z. The coast will
act as an area of convergence for both the shortwave disturbance
moving in from the west, the surface boundary currently over the
nearshore waters, and remnant outflow boundaries.

Hathaway

&&

.PREV DISCUSSION... /ISSUED 1009 PM CDT Sun May 21 2017/

DISCUSSION...
Bit of a lull in the activity for the most part across Southeast
Texas at 9:30 PM. An analysis of the 00Z upper air surfaces showed
a shortwave ridge over the area from 850 mb to 500 mb; although,
the 200 mb and 300 mb surfaces both had good divergent winds
developing overhead. Best moisture was over South Texas at 850 mb
and the central part of the state at 700 mb. An 850 mb trough and
surface trough was helping to generate showers and thunderstorms
over mainly the offshore Gulf waters. These may drift onshore
later this evening.

The splitting jet structure over the western part of the state
will be moving eastward with the upper level shortwave trough.
Think this feature will help set off another round of showers and
thunderstorms. However, still some uncertainty as to location and
amounts. Shorter range models were pointing toward some higher
QPF amounts in the southwestern counties from just before sunrise
through at least the midday period. These include the Texas Tech
WRF, HRRR, and NAM12. Older and global models were pointing toward
further north of the I-10 corridor with lessor amounts. Flash
flood guidance values are still high enough to not warrant a
watch tonight. Will reassess later tonight for the Monday morning
timeframe as more models come in and to see how things develop as
the shortwave trough moves through areas west of the forecast
area.

So, not much to change for this update with only a few minor
tweaks.

40

PREV DISCUSSION... /ISSUED 650 PM CDT Sun May 21 2017/

AVIATION...
Showers currently across much of the TAF sites this evening
besides currently UTS and CXO, but thunderstorms will be moving
in later tonight. Based off model guidance and forecast soundings,
expecting locations IAH, HOU, and SGR to see mostly shower and
thunderstorm development between 07-11Z, and CLL CXO and UTS to
see more of the thunderstorm development a little later between
09-12Z. The coastal TAF sites are also subject to shower and
thunderstorm development during much of the night and through
late morning tomorrow. Wind gusts can be expected associated with
some thunderstorm development. Confidence is not fairly high in
terms of timing, because this will be a multi-round precipitation
event due to outflow and surface boundaries, as well as a
shortwave moving in from the west. Therefore, there could be some
occasional breaks between these rounds of showers and
thunderstorms.

Hathaway

PREV DISCUSSION... /ISSUED 411 PM CDT Sun May 21 2017/

DISCUSSION...
A line of thunderstorms continues to lift north across Southeast
Texas this afternoon as an outflow boundary from the Gulf of
Mexico has overtaken a nearly stationary frontal boundary that was
draped along the Upper Texas Coast. Farther west, showers and
thunderstorms continue to develop along the Rio Grande ahead a
shortwave trough moving across the Big Bend. Expect the
frontal/outflow boundary to stall somewhere near or north of
Interstate 10 this evening with loss of daytime heating, with
convection along the line gradually diminishing as a result. After
a brief break in rain during the early evening hours, expect the
shortwave and associated thunderstorm complex near the Red River
to continue to translate east tonight and reach the western
portions of the forecast area between 12-6 AM.

Environmental conditions will be very supportive of thunderstorms
capable of locally heavy rainfall, both with the activity ongoing
this afternoon and the next round of rain later tonight. Some of
these conditions that indicate showers and thunderstorms will be
efficient rainfall producers include warm, moist inflow (15-20
knot low level jet) overriding the stationary boundary in advance
of the approaching thunderstorm complex, saturated sounding
profiles up to 450-500 MB, and CAPE values generally below 2000
j/KG. However, wind profiles are not unidirectional (which does
not support training storms) and mean winds in the cloud bearing
layer (850-300 MB) exceed 20 knots out of the west, which
indicates storms will be moving.

Essentially, while the environment is supportive of this locally
heavy rainfall... the thunderstorm attributes which allow these
rain totals to pile up are not supportive for that to occur. This
would mean flash flooding would be more of an isolated as opposed
to widespread occurrence. In addition, even after today`s
rainfall, one hour flash flood guidance values still range from
2.5-4 inches of rain. This has led to the decision to hold off on
issuing a flash flood watch at this time. For now, advertising
widespread 1-3 inch rainfall amounts but cannot rule out locally
higher amounts leading to isolated flash flooding especially
along any residual boundaries left across the region. Anticipate
the heaviest rainfall totals to occur along the frontal/outflow
boundary as it will serve as a good low level focusing mechanism,
but concerned that there may be a secondary swath of maxima near
or to the north of the boundary where RAP and NAM guidance have
advertised some 850 MB speed/directional convergence.

Drier air and subsidence behind the Big Bend shortwave may result
in rain chances tapering off Monday afternoon and evening. Another
round of showers and thunderstorms will be possible as a shortwave
now over Utah on water vapor imagery dives into the region Monday
night, sending a cold front across the region on Tuesday. Another (but
much lighter) round of rain is possible as a third shortwave near
Saskatchewan rotates around the upper low over Minnesota and
drags the upper trough axis across the region. This will end rain
chances for a few days as dry northwest flow quickly transitions
to shortwave ridging by the end of the work week. Gradually
strengthening onshore flow and returning moisture may bring low
rain chances back to the region by the end of the weekend/
beginning of the next week.

Huffman

MARINE...
Light to occasionally moderate onshore winds are expected to
persist through Monday night. Periods of showers and thunder-
storms are possible through Tuesday night. Some of the storms
could become strong or severe. Winds and seas will be higher in
and near the storms. A cold front will move through the area on
Tuesday with winds shifting to the north behind the boundary.
Caution flags might be needed Tuesday night through Wednesday due
to increasing north winds and building seas. Onshore winds come
back on Wednesday night and strengthen through the end of the
week. 42

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      67  77  65  80  59 /  70  80  40  30  20
Houston (IAH)              72  77  67  83  62 /  70  90  60  40  20
Galveston (GLS)            77  80  73  82  69 /  70  80  70  40  20

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 4 AM CDT Monday 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.

&&

$$

Discussion...40
Aviation/Marine...08



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