Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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525
FXUS64 KHGX 021202
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
602 AM CST Fri Dec 2 2016

.AVIATION...
Main aviation concerns for the 02/12Z TAFs center around
widespread SHRA at the terminals by this evening and easterly
winds increasing to around 15 knots during the day. VFR conditions
are expected to persist through the day, with scattered SHRA
spreading into the the terminals after 20Z. Forecast soundings
show a fairly dry subcloud layer persisting into the afternoon
hours, and expect initial returns that appear on radar to be virga
(which will be capable of producing stronger wind gusts in the
20-25 knot range as winds aloft are transported to the surface).
SHRA (with isolated, embedded TSRA) will become widespread this
evening as lift associated with both an upper level system west of
the region and a coastal trough increase near the terminals.
Expect conditions this evening to deteriorate to MVFR, but at
least temporary IFR and LIFR restrictions will be possible by late
in the TAF period due to heavy rain and lowered ceilings.

Huffman

&&

.PREV DISCUSSION... /ISSUED 522 AM CST Fri Dec 2 2016/

DISCUSSION...

Heavy rain threat this weekend.

This morning easterly flow with a warm front well south of the
area intersecting the Lower TX coast. A cool start to the morning
with temperatures in the lower 40s north to lower to mid 60s along
the coast. Cloud cover increasing as isentropic lift traverses the
region and by mid afternoon most of the region will be mostly
cloudy to overcast which should limit temperatures this afternoon
the mid 60s to around 70. With the isentropic lift through a
deepening layer expect to see some light radar returns develop in
the west (probably initially virga) and spread eastward. Rain
chances for measurable amounts should increase quickly this
afternoon from southwest to northeast.

Overnight the warm front nears the Matagorda Bay area and higher
rain chances expand across the area. Showers will probably
dominate the weather overnight becoming more stratiform with
embedded heavier showers and thunderstorms. Good potential for a
band to set up over the far north focusing heavier rain threat
generally north of a Columbus to Conroe to Groveton line in closer
proximity to the 850 front where impressive moisture advection
axis intersects. Stratiform rain will continue over the area
Saturday with the axis of heavier rain probably shifting south
with one of a series of upper disturbances traversing the area.
Heavy rainfall will be a threat across the entire region Saturday
given the extremely high precipitable water (99+ percentile for
the last 30 years!) in a saturated column and instability of
300-900 elevated CAPE with high speed shear (locally much high SRH
near the warm front). Storms should be moving fairly rapidly from
southwest to northeast but could easily train. Near the coast may
see a threat of short lived shallow topped supercells
transitioning to elevated inland. Waterspouts and possibly a weak
tornado but this will really depend on the warm front`s inland
penetration.

Saturday night the warm front remains near the coast and the upper
level jet comes more into play with increasing divergence aloft
but the 850 front weakens which may help to spread out the
rainfall and the attendant embedded bands of deeper convection.
Heavy rainfall threat continues with the PW still very high and
the approaching upper trough. Pressures falling in South TX
should in general begin to shift the focus for heavier rainfall
southward mainly along the Highway 59 corridor.

Sunday and Sunday night a much more favorable upper jet structure
(very slowly translating eastward) overlies the region and falling
pressures over the marine areas and Coastal Bend should continue
to focus heavy rainfall threat across the central and southern
areas. Storms will be slower moving and may cluster or become
anchored. Eventually cyclogenesis in the Western Gulf takes over
and kicks out Monday which should end the heavy rain threat...but
of course the track of the surface low will have to be monitored
closely (very uncertain on the track now) with the potential to
bring the warm sector inland near the coast.

Working through flash flood worksheet yields a very high flash
flood threat across the region this weekend. Rainfall
Saturday/Saturday night will likely see totals of 1-2" with
isolated 6-9" amounts...Sunday more widespread rainfall of 1-3"
and isolated amounts of 8-11". Although it has been dry recently
that is a lot of rain and flooding is likely to occur over the
weekend. With the rains likely saturating the area Saturday or
portions where the bands set up - the greater widespread flood
threat may be Sunday thanks to Saturday`s saturating rainfall. A
flash flood watch will likely be issued this afternoon for
Saturday and Sunday.

Last but not least the system kicks out Monday and a mild Pacific cold
front blows through drying out the area Tuesday. A much strong
Canadian cold front plunges south moving through Wednesday...a
second surge of colder Arctic air drops into East Texas and Friday
morning will likely see near freezing temperatures (per current
forecast areas from College Station to Conroe to Cleveland may
reach freezing) across the area.

45

MARINE...
Conditions will rapidly deteriorate across the marine
areas today, with elevated winds, waves, and periods of
thunderstorms expected across the coastal waters through the
upcoming weekend. Light to moderate easterly flow across the coastal
waters early this morning is expected to increase through the day in
response to a coastal trough developing along the Lower/Middle Texas
coast. This will result in caution to advisory flags needed for the
bays and coastal waters for elevated winds and waves from this
morning to Saturday morning. Seas in the 3 to 6 feet range this
morning are expected to build into the 6 to 9 feet range by this
evening and Small Craft Advisories will likely need to be extended
through the day Saturday as seas subside.

Increasing shower and thunderstorm coverage is also expected across
the coastal waters today, with rain continuing through Monday.
Conditions across the waters will be favorable for a few strong to
possibly severe thunderstorms, especially along a warm front
(associated with the coastal trough) that looks to set up across the
coastal waters today. Seeing a signal in some higher resolution
guidance (HRRR and TTU-WRF) for a supercell or two to develop along
this warm front, which would pose a damaging wind, hail, and
possibly waterspout threat for mariners this afternoon and evening.
The risk for strong to severe thunderstorms will exist for the
coastal waters into Saturday.

Movement of the coastal trough will result in easterly winds today,
southeast winds on Saturday, and northeast winds on Sunday. The
elevated easterly winds today will also result in tidal issues along
portions of the Upper Texas coast and have upgraded the Coastal
Flood Watch to a Coastal Flood Advisory for Chambers, Galveston,
Harris, and Brazoria. Tide levels early this morning are running
about 0.8 to 1.3 feet above normal and with winds increasing to 20
to 25 knots across the eastern waters this afternoon, tide levels
approaching 2-2.5 feet above normal are not out of the question.
Will need to monitor trends in Galveston County (particularly for
Bolivar Peninsula) as a Coastal Flood Warning may be needed for
possible tidal overwash of Highway 87. Expect tides to begin to
improve as winds veer on Saturday.

Huffman

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      64  52  53  48  55 /  40 100 100 100  80
Houston (IAH)              65  56  61  54  61 /  40  90 100 100 100
Galveston (GLS)            69  65  68  65  67 /  60  90 100  90 100

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Coastal Flood Advisory from noon today to 6 AM CST Saturday for
     the following zones: Brazoria...Chambers...Galveston...
     Harris.

GM...Small Craft Advisory from 9 AM this morning to 6 AM CST Saturday
     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...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.

     SMALL CRAFT SHOULD EXERCISE CAUTION until 9 AM CST this morning
     for the following zones: Matagorda Bay.

     Small Craft Advisory from 3 PM this afternoon to 6 AM CST
     Saturday for the following zones: Galveston Bay.

     SMALL CRAFT SHOULD EXERCISE CAUTION until 3 PM CST this
     afternoon for the following zones: Galveston Bay.

     SMALL CRAFT SHOULD EXERCISE CAUTION until 9 AM CST this morning
     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...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...45
Aviation/Marine...14



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