Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 041215
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
615 AM CST Sun Dec 4 2016

.AVIATION...
Southwest flow aloft ahead of an upper low near Baja California
will allow disturbances to move across Southeast Texas today,
resulting in periods of SHRA for most of the day. Breaks in rain
will be dictated by the passage of these disturbances and frequent
amendments are anticipated as these features are better resolved.
A few embedded TSRA are expected near Houston IAH and terminals
south where SHRA will be able to tap into some elevated
instability. Expect IFR to LIFR ceilings to persist for most of
the day, but ceilings may fluctuate between LIFR and MVFR in
heavier SHRA/TSRA. Northerly flow around 10 knots is expected to
veer to the northeast by the end of the TAF period, increasing to
between 10 to 15 knots. As the upper low lifts into Texas tonight
(after 06Z), widespread TSRA are expected to move across the
region from the southwest with visibility reductions from heavy
rain expected.

Huffman

&&

.PREV DISCUSSION... /ISSUED 528 AM CST Sun Dec 4 2016/

DISCUSSION...
Frontal boundary and outflow stalled off the coast about 80 miles
with SW flow aloft pulling moisture across the boundary and
lifting across the coastal waters and SETX. Water vapor clearly
showing the tap of mid/upper level Pacific moisture combining with
the low level moisture off the Gulf. This is producing widespread
showers and thunderstorm over the Gulf waters and a second band
from Deep South Texas through the Corpus area and into the Wharton
area (probably close to the 925mb front). As a speed max in the
flow aloft rides up into the Coastal Bend expect that deep
convection should blossom from the Coastal Bend and over
Upper Texas coastal waters spreading into SETX with storms
weakening and becoming more stratiform in nature over the northern
half of the CWA. Heavy rainfall should mainly be confined to
areas along and south of the I-10 corridor today. Have pared back
the Flash Flood watch to align with this thinking. After the
record setting rainfall along the coast (7.68" at Galveston
Scholes Field) and amounts of 10-12 inches in Texas City obviously
any additional heavy rains will rapidly runoff becoming flooding.
Over the course of the day expect to see rainfall of 1-2 inches
mainly south with some isolated amounts of 3 to 5 inches. The
greater amounts should probably be focused closer to the coast.
May even get a report or two of pea hail in stronger storms. A
break in the precip inland will probably take place late this
afternoon or early this evening before the next big shot of rain.
Early Monday-Monday afternoon the upper low will be coming out
across SETX and should bring a large swath of showers and
thunderstorms wrapping around across the northern CWA. Impressive
jet aloft with a glancing blow of the LLJ should drag back in PW
of 1.5-1.85" across much of the southern CWA. Cool surface
temperatures and cloud cover should limit the instability and
hence limit severe threat but enough remains for thunderstorms to
fire off inland...over the Gulf stronger storms and possibly some
severe will be possible. Breezy conditions possible south of the
I-10 corridor and along and east of the I-45 corridor as surface
low tracks through the nearshore waters (possibly coming ashore in
Chambers county). Pacific cold front sweeps out the moisture and
by Monday evening precip should depart. Winds go from westerly
Monday night to east/southeast Wednesday morning. Much colder
airmass plunges down the Front Range Wednesday and should swing
through SETX Wednesday night. This should wring out the last of
the moisture possibly bring scattered showers to the area and
Thursday strong CAA dominates with drying and clearing of the low
levels though aloft Pacific moisture should continue and lead to a
belt of high clouds over the region. Tail end of the shear axis
comes through Friday morning and clouds should be diminishing.
This will make temperature forecasts very iffy...what should be
the coldest day of the season with freezing temperatures down to
around a Columbus-Conroe-Cleveland line problematic. If the clouds
are thicker and hang on then freeze line may be further north...if
the clouds clear earlier then the freeze line will be further
south. With some confidence the northern counties (Burleson-
Trinity) should see the first freeze. The cold airmass races on
eastward Friday and by Saturday afternoon return flow underway
with a warm up and increased rain chances. 45

MARINE...
Unsettled weather continues across the marine areas early this
morning. Periods of showers and thunderstorms are expected to sweep
across the coastal waters from the west through Monday as a compact
upper level system lifts into Texas from near the tip of Baja
California. Expect most showers and storms today to be capable of
heavy rainfall limiting visibilities to 1 NM or less at times.
However, there will be an increasing potential for strong to severe
thunderstorms tonight into early Monday morning as a frontal
boundary over the north central Gulf retreats westward in response
to a surface low lifting up the Texas coast and cannot rule out a
few strong wind gusts or an isolated waterspout as this happens.

Additionally, a strengthening surface pressure gradient as this low
lifts up the coast will result in east to southeast winds increasing
to near 20 knots and Small Craft Advisories are in effect for the 0-
60 NM waters this afternoon and tonight. Expect winds and waves to
remain elevated overnight, but decrease by Monday afternoon as the
surface low clears the western Gulf.

Tide levels this morning are running about 2-2.5 feet above MLLW and
have cancelled the Coastal Flood Advisory early as levels have
fallen below those producing impacts along the coast. Will need to
keep an eye on tide levels for the overnight period as easterly
winds ahead of the surface low may again result in tide levels
approaching 3 feet above MLLW, and minor wave run-up on coastal
highways will be possible.

Otherwise, dry conditions are expected to return to the marine areas
by mid-week as the early week system exits the area. A strong cold
front looks to move across the waters late Wednesday, and Small
Craft Advisories to possibly Gale Warnings may be needed behind the
front.

Huffman

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      55  49  58  45  69 / 100  80  80  10  10
Houston (IAH)              57  52  60  49  70 /  60 100 100  10   0
Galveston (GLS)            62  59  66  58  68 /  90 100 100  10   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Flash Flood Watch through this evening for the following zones:
     Austin...Brazoria...Chambers...Colorado...Fort Bend...
     Galveston...Harris...Jackson...Liberty...Matagorda...
     Waller...Wharton.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION from 4 PM CST this afternoon
     through late tonight for the following zones: Galveston
     Bay...Matagorda Bay.

     Small Craft Advisory from 4 PM this afternoon to 4 AM CST 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...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 4 PM CST 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...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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