Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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672
FXUS64 KHGX 191458
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
958 AM CDT Thu Apr 19 2018

.DISCUSSION...
Just made a couple minor changes to the grids on the
morning update to match up with current observations
and satellite images. See discussions below for the
details.  42
&&

.PREV DISCUSSION... /ISSUED 552 AM CDT Thu Apr 19 2018/

AVIATION [12Z TAF Issuance]...
VFR conditions expected through the TAF period. Light to moderate
northeasterly winds will prevail through the afternoon hours
before decreasing in speed, as high pressure builds into the
region. Gusty conditions will be possible at HOU, SGR, and coastal
terminals.

PREV DISCUSSION /Issued 423 AM CDT Thu Apr 19 2018/...

SHORT TERM [Today Through Sunday Morning]...
A cold front working it`s way across SE TX is currently located just
south of I-10 this morning separating two airmasses with
dewpoints in the mid 40s to the north and upper 60s to the south.
This front should move off the coast near sunrise, with moderate
to strong northeasterly winds filling in behind the boundary as
high pressure builds into the region. 850mb Temperatures behind
this front will only drop 4-6 degrees, resulting in slightly
cooler temperatures for the remainder of the week. High
temperatures should top out in the mid 70s today, and the low 70s
on Friday. Dry conditions can be anticipated through the remainder
of the week with high pressure in play. As this ridge begins to
shift east Friday, moisture will again be on the rise.

Saturday will start the warming trend as onshore flow returns to
the forecast. The added advection of Gulf moisture, along with a
tightening moisture gradient out ahead of the approaching cold
front Saturday night, will result in muggy warm conditions. Mostly
cloudy skies Saturday should keep high temperatures in the upper
70s.

Global guidance such as the ECMWF, GFS, and NAM bring showers ahead
of the next cold front into the region as early as Saturday morning.
The most recent runs of the ECMWF and GFS have both backed off on
rainfall totals with the passage of this system, keeping the best
accumulation northeast of the forecast area over the Piney Winds.
The 00Z NAM shows a more saturated profile with showers lingering
into Saturday evening with PWs increasing to 1.8-1.9 inches
shortly after sunset, but low-level lapse rates only rise to near
5.5 C/km. The NAM also is a few hours faster with the front, and
slightly more aggressive with rainfall totals. The GFS which is
not so saturated, only increases PWs to 1.5-1.6 inches, but shows
a little more in terms of instability, with low level lapse rates
approaching 6.7 C/km Saturday afternoon. The best upper-level jet
dynamics moves over the region late Saturday afternoon into early
Sunday with decent diffluence ahead of the upper-level jet streak.
It still appears as though the best dynamic forcing will be
situated in a region from Colorado to San Jacinto county and
northward. Ample lift should also be available out ahead of the
upper-level trough axis which digs south into northern TX and
southern OK, late Saturday into Sunday. Therefore, the environment
will definitely have the necessary ingredients for the
development of thunderstorms Saturday afternoon and into the
evening hours. What remains a question is whether the best upper-
level forcing, moisture, lift and instability will all be in phase
to allow for the production of severe thunderstorms. SPC has just
released their Day 3 Outlook, which places most of SE TX under a
marginal risk of severe thunderstorms.

LONG TERM [Sunday Through Thursday]...
Behind the front, conditions will dry out by Sunday morning as
PWs fall to near a half inch. Cold air advection behind this
system will help to high temperatures just below 80 degrees to
start the week. Northwesterly winds will turn out of the northeast
by Tuesday, as a surface ridge builds into the NW Gulf of Mexico.
This area of high pressure will slide eastward late Wednesday,
and partly cloudy skies will move back into the forecast by mid-
week.

MARINE...
Moderate to strong northeasterly winds will prevail behind a cold
front expected to move across the Gulf waters this morning. A
Small Craft Advisory (SCA) is in effect for both Matagorda and
Galveston Bay, and the nearshore Gulf water through 11 AM. The
offshore waters will hold onto a Small Craft Advisory through 3 PM
this afternoon, when winds begin to lower in speed. Small craft
should still exercise caution (SCEC) into early this evening.

Moderate easterly winds will return as a region of high pressure
shifts east of SE TX early Friday, and SCEC criteria may be
necessary. Onshore flow will prevail Saturday morning out ahead
of our next cold front which will push through the region late
Saturday into Sunday morning. Showers and thunderstorms can be
anticipated with this front, which should reach the nearshore
waters shortly before sunrise on Sunday. Northwesterly winds will
fill in behind this system and slowly turn out of the northeast
through the beginning of next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      74  51  71  55  71 /   0   0  10  20  70
Houston (IAH)              76  53  72  59  74 /   0   0  10  20  60
Galveston (GLS)            74  61  69  66  74 /   0   0  10  20  40

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory until 1 PM CDT this afternoon 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.

     SMALL CRAFT SHOULD EXERCISE CAUTION from 1 PM to 3 PM CDT this
     afternoon 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.

     Small Craft Advisory until 3 PM CDT this afternoon for the
     following zones: Waters from Freeport to the Matagorda Ship
     Channel from 20 to 60 NM...Waters from High Island to
     Freeport from 20 to 60 NM.

&&

$$



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