Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 121756
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1156 AM CST Thu Jan 12 2017

.DISCUSSION...
Surface analysis this morning shows a cold front pushing into
north central Texas with temperatures dropping sharply behind it.
Across SE Texas, warm and humid conditions continue. Visible
satellite imagery shows typical flow off the Gulf with cloud
streets and stratocumulus decks streaming northward. There may be
a few showers across the area but rather isolated if any.
Satellite imagery also shows clearing from Livingston to Palacios.
Further clearing may occur northward during the afternoon.
Temperatures were adjusted for latest trends and forecast high
temperatures were bumped up a couple of degrees since there could
be more heating. Otherwise the forecast looks on track. Early
glances at model guidance suggest the potential for heavy rain
Monday night into Tuesday. This will be evaluated in more detail
with the afternoon forecast update.

Overpeck

&&

.AVIATION.../18Z TAF/
Not a lot of changes with these latest TAFS. While winds are not as
strong today, there does appear to be enough mixing for VFR condit-
ions across SE TX at this time. We should see a bit more fog versus
low clouds tonight if the forecasts of much lighter/calm winds come
to pass. Did go for improving VIS/CIG by mid Friday morning for all
sites but GLS. Sea fog issues will likely persist on the Island for
much of the day/into the weekend. 41

&&

.PREV DISCUSSION... /ISSUED 613 AM CST Thu Jan 12 2017/

AVIATION...
Satellite imagery shows lower ceilings have pushed in over the
northern half of the forecast area this morning, leaving most of
the airports north of the I-10 in MVFR criteria. Winds were quite
gusty in the northern zones earlier this morning, but have since
lowered to around 10kts. Currently, VFR conditions across the
Houston metro, with high cirrus clouds and winds out of the south
at 10kts.

Late in the morning and into early afternoon, low level moisture and
onshore flow will aid in the development of sea fog across the
coast. Warm moist flow across cool ocean sea surface temperatures
will likely result in lower visibilities near Galveston and
possibly Angleton tonight as sea fog pushes onshore.

Tomorrow morning around 09Z, short term guidance is hinting
at the development of some streamer showers. These will move from
south to north, and push into the College Station airport
vicinity, and possibly impact CXO/IAH.

08

PREV DISCUSSION... /ISSUED 414 AM CST Thu Jan 12 2017/

DISCUSSION...
Surface analysis this morning shows a strong Arctic front pushing
south into Oklahoma and the Texas Panhandle this morning, with
temperatures behind the front in the low 20s. A tight surface
pressure gradient was noted across Southeast Texas early this
morning, due to the combination of the front`s approach and
surface riding stretching along the Gulf coast states. This
gradient is also resulting in elevated southerly flow (10-15 MPH)
this morning which is keeping temperatures in the upper 60s to low
70s as of 4 AM CST, which is 20 to almost 30 degrees above normal
for overnight temperatures this time of year.

Webcams around Galveston Bay show patchy sea fog has developed
and expect sea fog to remain a threat through at least the
upcoming weekend for the bays and nearshore waters. Southerly
winds will begin to back to the southeast and east later today as
the surface ridge builds farther west, possibly resulting in
increasing sea fog coverage as these winds push cooler waters
towards the Upper Texas coast. Have continued the mention of sea
fog in the forecast through Monday morning with onshore flow
expected to persist over the next few days.

The Arctic front will continue to ooze into North and Northwest
Texas today, with persistent warm air advection ahead of the front
resulting in another warm day with near record highs again in the
mid 70s to near 80 a few light showers possible across the region.
Forecast soundings show moisture remaining trapped beneath a
capping inversion located between 800-700 MB today and this will
make the temperature forecast (as well as any records achieved) a
bit tricky today as parts of the region may experience enough
clouds to limit the heating needed for record-setting
temperatures. The most likely areas affected by these clouds would
be along and north of a Columbus to Cleveland line, as drier air
evident moving towards the region across the northwest Gulf should
erode cloud cover farther south later today.

Regarding the Arctic front, have some concerns that the frontal
boundary may make a farther south run than most model guidance is
currently indicating. This front is very shallow with VAD wind
profilers from Omaha, NE and Topeka, KS showing the post-frontal
layer only as deep as about 3-4 kft above the surface. This means
that the front is too shallow to be driven by most mid/upper level
support and actually propagates as a result of hydrostatic
pressure differences (or really... the density of the airmass
behind the front). What this all means is that model guidance may
be stalling the front too far north of the region and it may
actually make a farther run south towards our northern counties.
The NAM12 is showing some signs of this (although not quite as
aggressive as dropping it to the forecast area), but will have to
watch how the front progresses through the remainder of tonight
and again Friday night. Expect the front to decelerate or possibly
stall near the Red River during the day today as daytime heating
warms the boundary layer ahead of the front, allowing for
additional flow opposing the front to mix to the surface. However,
radiational cooling overnight will help increase density
differences along the front (cold, dry air is more dense than
warm, moist air) and actually allow the boundary to begin moving
farther south Friday night. All this depends on how much the air
behind the front modifies during the day today though. If the
front were to make it farther south than currently indicated in
guidance, rain chances for this weekend would shift farther south
with it as southerly flow overrides the boundary and temperatures
may be a bit cooler than currently forecast.

Assuming the front doesn`t make farther south progress, little
overall change in sensible weather is forecast tonight through
Friday night with lows in the mid 60s and highs in the mid 70s.
May see a few warm air advection showers develop at times
underneath the persistent cap in place across the region, but
overall rain totals are expected to be light and generally under
one-quarter inch.

An upper low now over Oregon on early morning water vapor imagery
will continue to drop southward along the West Coast this weekend,
reaching southern New Mexico by Sunday. Increasingly southwest
flow aloft ahead of this feature will result in rain chances
spreading from west to east across the region as lift ahead of the
disturbance moves across Texas. Height falls associated with the
disturbance are expected to induce surface cyclogenesis somewhere
across eastern New Mexico or western Texas, with this system
lifting across Texas Sunday night and Monday as the upper low
ejects towards the northeast. An associated frontal boundary and
line of thunderstorms will push into the region and stall along
the coast by Tuesday as it loses support from its parent system,
with overrunning along the front resulting in widespread rain
chances Tuesday into Wednesday. The front will also result in
cooler temperatures with highs in the 60s and lows in the 40s.

Regarding severe weather and heavy rain potential for the
beginning of next week (Monday-Wednesday), model guidance
continues to delay the passage of this frontal system and
confidence with any timing on development of strong to possibly
severe storms is low. However, environmental conditions for
whenever this system does reach Southeast Texas still appear to
remain favorable for at least a low-end severe weather threat.
Ahead of the approaching frontal boundary, bulk shear values
increase 45-55 knots across the region as a split in the upper jet
moves across the region on Monday. An associated 120+ knot jet
streak is then forecast to move across the region Tuesday and
Wednesday. Flow aloft (southwest) oriented parallel to the
approaching frontal boundary should result in the development of a
line of thunderstorms as the front moves into the region on
Tuesday, with available shear encouraging at least some
thunderstorm organization. Similar to 00Z guidance from yesterday
however, instability during this time remains meager (MUCAPE
around 500 J/kg) and may be a big limiting factor for the overall
threat. However, with the stalled boundary remaining in the region
on Wednesday as the right entrance region of the upper jet moves
across the region means that the first half of the upcoming work
week will continue to need to be monitored for possibly hazardous
weather. Precipitable water values are still forecast to rise to
near 1.4 inches by Tuesday and these above normal values, combined
with dormant vegetation and uniform flow promoting training
storms, continues concerns for at least a localized flooding
threat during the first half of the work week.

A passing upper trough by mid-week should looks to bring a
reinforcing Pacific front across the region by Thursday, ending
rain chances through the end of the work week.

Huffman

MARINE...
Onshore winds remain rather strong over the offshore waters, with
sea heights ranging between 5 to 7 feet. Expecting these
conditions to persist through the morning, and therefore extended
the Small Craft Advisory through noon today. Winds will begin to
back out of the southeast this afternoon, which is more favorable
for sea fog to develop. These conditions are likely to persist
throughout the weekend, with visibilities dropping down to a mile
or less at times. 08

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      80  68  77  64  75 /  20  20  30  20  20
Houston (IAH)              81  66  77  64  75 /  20  20  20  20  20
Galveston (GLS)            76  64  72  63  69 /  10  10  10  10  20

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory until noon CST today 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.

&&

$$

Discussion...39
Aviation/Marine...41



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