Area Forecast Discussion
Issued by NWS Brownsville, TX

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000
FXUS64 KBRO 031803 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1203 PM CST Sat Dec 3 2016

.DISCUSSION...In process of making wholesale changes to the short
term forecast based on many factors, first of which is the current
and expected position of the "wavy" front which appears to be
shifting from the Friday/Friday night warm front back to a cold
front this morning. This is just the first "pushback" of cooler
air, with more to follow as upper level trough plows eastward and
brings the surface low with it across northeast Mexico overnight
into Sunday.

The 12Z GFS has finally jumped on the cooling train...and is in
fact now the fastest of all models in oozing the first front
across locations well inland primarly from the Upper Valley
through the ranchlands - areas mainly west of US 281/IH 69C.
Already, Hebbronville and Zapata have dipped into the lower 60s
with generally north winds and very low ceilings - and given time
of year and nature of shallow fronts such as these the cold tends
to win the battle in time. The trick will be how to handle the
populated mid Valley this evening and overnight. With sunset and
reduction of strong low level southerly flow to no longer shove
the front back to the north - not to mention the persistence of
the ECMWF which was first to the gate on the overnight/early
Sunday timing of the cooldown for all but the coast - will be
leaning into the cooling and retaining the summer-like warmth
mainly along/east of US 77 south of Harlingen/Raymondville.

As for the marginal threat for severe storms? This is also very
tricky. Plenty of potential in warm sector later tonight and into
Sunday morning, but how much warm sector will be left by then? The
surface boundary would effective kill off chance for mix-down of
convective winds unless some type of gravity wave can overcome,
but hail potential continues and "cool weather hail" is not
uncommon in Texas during this time of year. Because the threat is
marginal will likely leave out "severe" wording in the forecast
but may provide some independent partner decision support on the
reasonable worse case scenario. More details in the upcoming short
term discussion by 3 PM.
52/BSG

&&

.AVIATION...As mentioned above, front timing is very difficult for
all but Brownsville/SPI Airport which should hold off until pre-
dawn or daybreak on Sunday. Decided to blend GFS/ECMWF/NAM trends
for McAllen and bring the wind shift a couple hours later than the
raw data (02Z); same for Harlingen, broad brushing a shift at 06Z.
Behind the front, ceilings will fall to either low MVFR or IFR as
is typical in the shallow cool zone and already occuring in the
Upper Valley and northwest ranchlands.

Upper level jet divergence and support arriving with the front
(Brownsville) and behind the front elsewhere may be enough for
rumbles, so added TSRA to all areas, beginning late tonight near
McAllen and around daybreak at Brownsville/Harlingen. Confidence
in exact timing of any thunder is low to medium at this point.
52/BSG

&&

.MARINE...Updated to add a Small Craft Advisory for winds for
Laguna Madre based on near 40 knots off the deck at Brownsville
and gusts near 30 mph at 9-10 AM at Cameron County Airport just a
few miles west of the Bay. Elected to include all zones, but it is
likely that north of Mansfield with the wind shift coming sooner
(already to Baffin Bay) surface winds will lay down and advisory
area may need to be trimmed to just the southern legs.
52/BSG

.PREV DISCUSSION... /issued 607 AM CST Sat Dec 3 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Following some -SHRA which moved over the RGV
terminals over the past couple of hours, CIG`s have risen to VFR
for the near-term. Effect should be short-lived, however, per
satellite trends and model guidance, with MVFR ceilings expected
to overspread the area again by around 15Z. Southerly winds will
also be on the increase this morning, as we are situated in the
area south of a warm front and to the east of a surface low. Winds
likely to gust to 30KT at BRO and HRL but should stay shy of AWW
criterion. CIG`s expected to lift back to MVFR in the afternoon
with daytime heating and mixing.

Overnight, the warm front begins moving back to the
south/southeast as a cold front. It will not reach the terminals
until well beyond the current TAF period, but MVFR (possibly
IFR...but low confidence) ceilings and visibility in mist are
likely tonight ahead of the front. Precip is possible at the
terminals after 06Z...with MFE the first to be affected with
thunder also possible. Still some degree of uncertainty with how
rapidly the front will approach the RGV tonight, so expect future
refinements, especially with respect to precip/thunder.

PREV DISCUSSION... /issued 432 AM CST Sat Dec 3 2016/

SHORT TERM (Today through Sunday):
Current water-vapor satellite imagery indicates a good tap of
tropical moisture moving up over much of TX ahead of an upper-
level low over the northern Gulf of California at this time. 00Z
BRO raob measured 1.77" of precipitable water. Meanwhile, at the
surface, a warm front is draped across the northern tier of the
CWA. This boundary is expected to retreat northward a bit further
during the day today, as robust southerly winds develop ahead of a
surface low located along the same boundary, but further west
over the Sierra Madre. The bulk of the precip is expected to be
along and north of the warm front, so the more populated areas of
the RGV will see little to no rain. A warm, humid day is in store
with highs in the 80`s most all locales.

As tonight goes on, the upstream upper low moves slowly toward the
SE.  A secondary low along the Middle Texas coast, along with high
pressure bulging into the Plains, will begin to nudge the boundary
back toward the south as a cold front, albeit slowly at first.
Shower and thunder chances increase through the night...with highest
chances near the boundary.  There is potential for some of the
storms to become strong to severe with wind gusts and marginally
severe hail the main threats.  SPC has continues its Marginal Risk
area over the entire CWA...though it seems the northern tier
counties would be favored.

The front continues pressing southward during the day on Sunday,
though some timing/alignment differences still evident in the NAM
and GFS.  NAM is faster and features a more bent-back, "S-shape" to
the boundary as it develops a surface low inland.  GFS is a few
hours slower.  Attempted to split the difference in temp and wind
grids.  At any rate, it should be through the NW counties early in
the morning, then slow down during the day...not reaching the Lower
RGV until around sunset.  Biggest question mark on timing is for
areas such as MFE.  Numerous showers and thunderstorms will
accompany the front as it moves into a moist airmass.  Temps will
fall into the 50`s over the NW Ranchlands by sunset (unless the NAM
is more correct, in which case temps may never rise above that level
during the day)...but Brownsville still likely to reach the upper
70s.

LONG TERM (Sunday Night through Friday):
First forecast issue is the departure of the cyclogenesis low and
the ending of precipitation. Sunday night, models are consistent
in the surface low beginning to wrap up just offshore. This will
provide widespread rainfall to the region Sunday night into Monday
morning. The speed of the departure of the low varies, with the
GFS lagging well behind just off SPI, the NAM racing ahead just
off Matagorda Bay, and the Canadian and EC in between. With the
length behind the GFS is from the remainder of the models, have
decided on a blend of the remaining, with the surface low just off
Galveston around noon. This should bring the drier air into the
region by that time, with rain chances ending, and cloud cover
decreasing quickly through the afternoon. Tuesday will be a nice
recovery day with light winds and highs around 80 as the trailing
high pressure moves offshore.

The second forecast issue is the arrival of the next intrusion of
cold air into Deep South Texas. Model spread is an issue again
with arrival timing of the front between sunset Wednesday and just
before dawn Thursday. With the front not arriving until around
sunset, Wednesday should see a full day of heating, so have leaned
toward a blend of the warmer model runs, with highs in the lower
80s. If any prefrontal trough tries to set up during the
afternoon, we could see another incident of temperatures soaring
into the upper 80s and lower 90s. For the arrival of the cold
front, as with any significant cold pool, a faster progression is
nearly always the case, so have kept closer to Canadian and GFS
solutions and away from the slower EC. Lows Wednesday night will
plunge into the low 50s and upper 40s, accompanied by a brisk
north wind. Good cold air advection will be ongoing through the
day Thursday as the cold pool advects into the region, so highs
Thursday will remain steady in the upper 40s across the ranchlands
to the mid 50s in the lower valley. The north winds and clearing
skies will bring a chilly night Thursday night, with lows in the
middle 30s north to lower 40s in the lower valley.

MARINE:
Now through Sunday: Fresh southerly winds have returned to Buoy
020 with combined seas of 8 ft. as of last report at 350am CST. A
east swell has been persistent, though it should be trending down
today. However, stronger southerly winds will keep SCA conditions
in effect for at least the outer Gulf waters into early evening.
Additional SCA`s may be needed for the Laguna Madre during the
daytime today...and possibly the inner Gulf waters. SCEC likely to
continue overnight for seas. Front arrives in the northern marine
zones Sunday afternoon with N-NE winds returning SCEC or possibly
low-end SCA conditions. Showers and thunderstorms will also be
increasing from north-to-south on Sunday.

Sunday night through Wednesday: Sunday night into monday will see
the development of the surface low pressure right along the
coast, which will move off to the northeast during the day Monday.
This will provide northwest winds, agitating the onshore swells
into smaller periods of swells at 5 to 7 feet. High pressure
Tuesday into early Wednesday will lighten winds and allow seas to
relax down to around 3 feet. Strong cold front forecast for late
Wednesday night into Thursday with strong north winds and seas
jumping above 10 feet across the Gulf.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 9 PM CST this evening for GMZ170-175.

     Small Craft Advisory for winds until 6 PM CST this evening for
     GMZ130-132-135.

&&

$$

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