Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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173
FXUS64 KHGX 170218
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
818 PM CST Sat Dec 16 2017

.UPDATE...
As of 8 PM local time, looks like the center of the low is inland
over Matagorda County with storms right near the center starting
to get their act together. Dewpoints right out ahead of this
feature at Bay City have climbed to 60 degrees, with 68 degree
dewpoints just offshore. Strong inflow is also occurring, with the KHGX
VAD wind profile showing 50 knot SE winds at 3,000 feet. Could
still see some isolated tornadoes along the coastal counties with
this system over the next few hours. Gusty winds are also
possible, and we`ve already had some 20-30kt gusts measured across
the coastal counties in the inflow region and right on the
backside of the surface low. These storms are moving pretty
quickly, so the severe threat should be over with for our area by
around midnight.

11


&&

.PREV DISCUSSION... /ISSUED 547 PM CST Sat Dec 16 2017/

AVIATION [00Z TAF Issuance]...

Yesterday we knew the next 6 to 9 hours will be a messy aviation
forecast and it looks like the forecast is on track with that
scenario. Surface analysis has a coastal low pressure forming off
the middle Texas coast with a warm front extending east just off
the Upper Texas Coast. Upper level trough remains west of the area
with water vapor imagery showing a strong vorticity max moving
out of Mexico in south Texas with the main upper low centered over
west central Texas. Over the next 6 hours we expect ceilings to
continue to decrease into MVFR and then IFR categories from
02-05Z. This also corresponds to when showers and thunderstorms
will be most numerous. Thunderstorm activity may be confined to
KLBX and KGLS but cannot rule out elevated thunderstorms for the
Houston metro terminals. KCLL/KUTS/KCXO can expect a broad area of
moderate rain for much of the next 6 hours. System should move
east out of the area 06-07Z but low ceilings and visibility may
remain. Model guidance keeps low ceilings in place for much of
Sunday morning with improvement by afternoon. With wet grounds,
light winds and a cool airmass, this seems reasonable. Look for
TAF amendments based on convection through the next 6 hours to
convey the latest timing.

Overpeck

PREV DISCUSSION /Issued 401 PM CST Sat Dec 16 2017/...

A complicated forecast continues to unfold for Southeast Texas as
a series of upper level disturbances cross Texas over the next
week. The first disturbance is noted on afternoon water vapor
imagery lifting across Southwest Texas from Mexico, with a coastal
low having developed in response near the Texas Coastal Bend.
Little has changed in the forecast thinking through tonight
regarding the evolution of this feature, with the surface low
lifting up the coast towards Matagorda Bay and then into the
Sabine Pass tonight. Light to moderate rain across the northern
counties this afternoon will spread south towards the coast in
response to enhanced convergence from the low this evening, with
a line of showers and thunderstorms sweeping across the region
from west to east as the parent storm system ejects towards the
Midwest.

VAD wind profilers from Corpus Christi show southeast to south
winds above 2000 feet and expect more of a northerly movement to
the surface low within the next few hours. This would begin to
shunt some of the moisture near the low into the Upper Texas coast
(dew points in the 64-68 degree range) and increase the potential
for a few surface-based thunderstorms. SPC Mesoanalysis shows 0-1
km shear values in the vicinity of this surface low have already
increased into the 20-30 knot range (more than sufficient low
level rotation to support tornado or waterspout development) and
expect an isolated tornado/waterspout potential to increase for
the coastal waters and immediate coastal areas within the next few
hours. The speed of the overall system should limit any
widespread flooding threat with this rainfall, but widespread rain
amounts of 1-2 inches will be possible. Expect the line of
thunderstorms to clear the region 1-2 AM with the potential for
some patchy fog development across the western counties behind the
line as the surface pressure gradient slackens.

The next system to impact the region is currently in the process
of closing off near Baja California this afternoon and will ensure
that the region maintains southwest flow aloft in the wake of
tonight`s system. Moisture behind tonight`s storm system is never
fully scoured out of the northwest Gulf and east to southeast
winds becoming reestablished on Sunday will draw this moisture
back into the region. This will mean that smaller disturbances
moving across the region in the flow aloft will result in periods
of showers and thunderstorms spreading into the region from the
Gulf beginning Sunday night. Rain chances continue through late
Tuesday into Wednesday as the Baja closed low translates eastward
and crosses Texas. This looks to drag another cold front across
the region and end rain chances for a brief period of time during
the mid-week period.

A second, stronger cold front is expected to enter the state late
Thursday into Friday. Warm air advection ahead of this front may
result in light rain spreading inland from the Gulf on Thursday,
with a combination of isentropic upglide and mid/upper level
forcing from another disturbance west of the region resulting in
periods of rain continuing into the upcoming weekend. Much colder
air is expected to move into the region behind this cold front,
with high temperatures on Friday almost 20 degrees cooler than
Thursday.

Huffman

Long Term Discussion...

Speculation continues about the forecast for the weekend before
Christmas. CIPS Extended Analogs have shown a consistent signal
for sub-freezing temperatures to reach at least parts of Southeast
Texas sometime during the December 24 through December 26
timeframe... and this increases confidence that the region will
see a spell of much below normal temperatures around Christmas.
What remains more uncertain though is when this cold air will
actually reach the region. This airmass looks to originate over
western/northwestern Canada before being dislodged southward
towards the Great Plains as a disturbance now near the Bering
Strait also dives south. These polar airmasses tend to be very
shallow by the time they reach Texas, which means that model
guidance can struggle with when the coldest temperatures arrive
behind the initial cold front. This is because the airmass behind
the front moves moreso from propagation due to density differences
at the surface than forcing from higher up in the atmosphere. The
medium range guidance that was inspected today continues to shows
signs of that struggle with the deterministic GFS/European not
bringing in sub-freezing surface temperatures until Christmas and
the Canadian, well, doesn`t bring them into the region at all.

This is further compounded by the signal that at least a weak
over-running regime sets up across the region behind the front.
This over-running, where more warm, moist air is lifted over the
colder, shallow post-frontal air, looks to produce periods of
light to moderate precipitation across the region over the weekend
before Christmas and into Christmas week. Not only is the timing
for arrival of colder temperatures in question, but whether or not
the atmosphere will be precipitating when they arrive is too.

So what do we know?
- It will be colder than normal the weekend before Christmas into
  the beginning of next week.
- Winter weather could certainly be possible... but several
  things have to come together for that to be the case.
- It is too early to determine impacts, if any.
- Model guidance is still in poor agreement on what will actually
  happen in that portion of the forecast. This makes forecast
  confidence low.
- Forecast confidence probably won`t begin to increase until the
  middle of next week when the disturbance near Alaska begins to
  dislodge our next round of colder air.

Huffman

MARINE...
A Small Craft Advisory is currently in effect across the coastal
waters and Matagorda Bay, with easterly winds sitting around 20 kts.
A SCEC is in effect for Galveston Bay until 00Z this evening, as
winds hover between 15 to 20 kts. As the coastal trough continues to
slide up the northern Texas coastline, winds in Galveston Bay will
also increase in speed, and a Small Craft Advisory will be in effect
beginning at 00Z Sunday. Wave heights will also rise and 4 to 7 feet
with this system, as it moves through the coastal waters during the
overnight hours.

Scattered to numerous showers and thunderstorms will fill in across
the waters later this afternoon into the early evening hours as the
coastal low slides north/northeast. This activity should continue
until shortly after midnight when this system shifts eastward.
Embedded in these showers and thunderstorms will be the potential
for gusty winds and isolated waterspouts through early Sunday
morning.

Wave heights will begin to lower Sunday afternoon to between 2 to 4
feet, and onshore flow will return. With winds out of the
east/southeast, warm moist air will flow over cooler shelf waters
and periods of sea fog will be possible out ahead of the next cold
front expected to push through the region early Wednesday. Light to
moderate offshore winds will prevail behind this front. This
offshore flow will not last for long though, as moderate onshore
winds will return Thursday. Towards the end of the week, the next
frontal passage is expected to reach our bays and coastal waters on
Friday afternoon. Moderate to strong offshore flow is anticipated
behind this front, resulting in elevated wave heights between 3 to 5
feet.

Hathaway

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      45  63  52  64  60 /  90  10  10  50  70
Houston (IAH)              48  67  57  69  63 /  90  10  30  60  60
Galveston (GLS)            56  63  60  67  64 /  80  20  40  70  40

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory until midnight CST tonight 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 Advisory until 10 PM CST this evening for the
     following zones: Galveston Bay.

&&

$$

Discussion...11



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