Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 211735
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1135 AM CST Tue Nov 21 2017

.AVIATION...
A combination of an upper level shortwave trough and the left-
front quadrant of the upper level jet stream was helping to kick
off showers along the coastal areas at 11:30 AM. Expect these to
move eastward over the next couple of hours. An outside chance for
MVFR ceilings with any showers that occur, mainly at KGLS through
20Z.

Otherwise, VFR this afternoon. Chances for showers return from
KCXO southward after 00Z this evening with another upper level
shortwave trough moving overhead. Actual front will move through
KCLL after 06Z and then quickly through the coast the area
starting at 08Z at KCLL and KUTS and through to the coast sometime
toward 12Z. Timing has been tough to figure given that winds
should pick up but with ceilings probably lagging behind the
frontal passage. MVFR conditions should lift to VFR by or after
15Z. Breezy conditions continue after ceilings lift.

40

&&

.PREV DISCUSSION... /ISSUED 952 AM CST Tue Nov 21 2017/

DISCUSSION...
Just some extremely minor tweaks for the forecast update - mainly
to match obs/grids/trends. There`s currently a brief lull in the
precip, but suspect we`ll see additional development stretching
along an moisture axis situated across the se part of the CWA
south southwest into the offshore waters later this morning and
afternoon.  47

PREV DISCUSSION... /ISSUED 416 AM CST Tue Nov 21 2017/
Evening upper air analysis showed Southeast Texas located in
between two mid-level disturbances, one located over Louisiana and
a second located over the Texas Panhandle. Mid to high clouds
associated with this secondary disturbance are evident streaming
over the region on early morning GOES-16 infrared imagery with a
second, lower level of clouds building into the region as moisture
gradually returns from the Gulf. In addition to these lower
clouds, signs of this moisture return is also evident with the
isolated to scattered showers developing across the coastal waters
this morning along a coastal trough/ axis of surface convergence
stretching from Freeport southwest towards Brownsville.

As southwesterly low-level flow pushes this surface boundary up
the Texas coast through the day and nudges inland, shower
coverage is expected to expand across the coastal waters and move
into areas east of a Palacios to Huntsville line this morning.
Increasing instability as a result of mid-level cooling from the
approach of the Panhandle shortwave trough as well as a 60-80 knot
upper level jet nosing into the northwestern Gulf today should
provide favorable conditions for thunderstorms to also develop
along the boundary. 00Z Convection Allowing Models (CAMs) have a
fairly consistent signal for scattered showers and thunderstorms
to develop this morning with what appears to be a thunderstorm
complex developing somewhere across the eastern waters later this
afternoon. Have increased rain chances for the marine areas as a
result and pulled the isolated thunderstorm mention farther inland
east of a Freeport to Houston to Livingston line. 0-6 km bulk
shear values over the waters increase to 25-30 knots this
afternoon and have concerns that this may result in enough
organization for a few strong or severe thunderstorms over the
coastal waters today. Marine forecast soundings show a fairly
persistent dry layer of air between 700-500 MB with evaporative
cooling within this layer promoting lower freezing levels and at
least a small hail threat within stronger thunderstorms over the
waters this afternoon into this evening before the thunderstorms
push east towards coastal Louisiana. If the thunderstorm complex
the CAMs have shown does develop, rain chances farther inland
today may be much lower than the 20-40 percent currently
advertised as the coastal storms rob Southeast Texas of better
moisture inflow.

A cold front located across the Northern Plains early this morning
will continue to trudge south during the day today, reaching the
Brazos Valley this evening, Houston metro around midnight, and
pushes off the coast by sunrise Wednesday. Weak convergence along
the front may produce a few showers as it sweeps across the region
this evening and tonight with relatively drier and cooler air
being ushered back into the region behind it. Highs today in the
70s will be replaced by highs in the 60s tomorrow with dry
conditions again resuming across the region on Wednesday as north
flow aloft becomes established in the wake of today`s shortwave
trough.

The weather on Thanksgiving will certainly be something to give
thanks for as dry north flow aloft persists and results in sunny
skies and highs in the mid to upper 60s. North flow aloft becomes
more of a northwest flow aloft on Friday as a northern stream
shortwave trough dives across the Great Lakes towards the Mid-
Atlantic states and upper ridging builds in from the west.
Temperatures will gradually warm back into the mid 70s to near 80
by Saturday as ridging spreads eastward, but the northern stream
disturbance looks to send a weak cold front into the region late
Saturday into Sunday and moderate temperatures a few degrees by
the end of the weekend. With the region remaining closed off from
the Gulf through the end of the week, expect the frontal passage
to be dry. Upper ridging continues to build across Texas during
the late weekend and into early next week with temperatures again
warming back into the mid to upper 70s. Huffman

MARINE...
Light onshore flow will persist today with high pressure over the
eastern US and low pressure over the Southern Plains. A cold
front will cross the coastal waters this evening and usher in a
strong offshore flow. NNE winds will increase to 20-25 kts with
gusts approaching gale force over the Gulf waters. A Small Craft
Advisory is in effect for tonight into early Wednesday and a Gale
Watch/Warning may yet be required for the offshore waters as gusts
may frequently gust in excess of gale force. A moderate offshore
wind will persist Wednesday night and a SCEC will be needed. Winds
will decrease on Thursday as high pressure settles over S TX. The
high pressure will move east of the area on Friday and onshore
winds will return. Another cold front will cross the region
Saturday night with moderate offshore winds in the wake of the
front on Sunday. 43/22

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      51  61  36  66  42 /  20  10   0   0   0
Houston (IAH)              56  65  40  66  44 /  20  10   0   0   0
Galveston (GLS)            61  67  50  64  50 /  30  10   0   0   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory from 11 PM this evening to 11 AM CST
     Wednesday 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...47
Aviation/Marine...40


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