Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 100954
AFDBRO

Area Forecast Discussion
National Weather Service BROWNSVILLE TX
354 AM CST Sat Dec 10 2016

...The Roller Coaster Continues: Winter`s Chill Hangs on Saturday
and Balmy Breezes Return Spring Temperatures Sunday and Beyond...

.SHORT TERM (Now through Sunday):  The cold air mass with polar
origins drained itself out Friday morning, and despite continued
northwest flow over land due to nose of surface high centered over
the South Texas Brush Country into the lee of the Sierra Madre with
general low pressure over the southwest Gulf, temperatures have
remained steady and even risen slightly at most locations through 3
AM Saturday morning.  Credit also the insulating effect of the low
clouds; this situation is quite familiar with events in the past
where overrunning within the inversion is enough to lock in the
overcast but forcing is weak at best.

For today (Saturday), models continue to trend in their own
directions, but the big picture strongly suggests one more day of
slate gray overcast and northwest flow for all but the immediate
coast which should veer around to the northeast at some point during
the afternoon.  The surface high will weaken through the day and
with the polar-source airmass exhausted, temperatures will slooowly
moderate but not until noon or so. Overall, still a good nip in the
air for holiday activities - not necessarily a bad thing for those
who like the chilly air for the holiday season. Current forecast was
on track but undercut a few degrees especially west of US 77/IH 69E
where northwest surface winds will only allow moderation from the
weakening high, not from non-existent warm advection.   Bottom line?
50 across the northern ranchlands, 50s elsewhere, squeaking over 60
at the beaches where the northeast winds arrive late.

As for precipitation, not getting a warm and fuzzy feeling for
organized precipitation with moisture trapped below 700 mb and very
dry air above it as well as minimal vertical motion or upper level
support. That said, low level convergence zone near the coast may be
enough to kick off patchy light rain/drizzle mainly over the Lower
RGV later today; any precipitation should be light enough to cause
minimal impact on outdoor activities - but, it will keep a nuisance
"bite" to the air where it falls.

Last but not least, continued 8 to 10 second swell period with a
still brisk east to northeast onshore flow will continue to
maintain tidal run-up to the dune line at high tide especially at
thinner or more heavily eroded beaches. High tide cycle is by
early to mid afternoon and with inshore tides running about 1.2
feet above predicted see no reason to let the coastal flood
statement drop especially on a Saturday - even if it won`t be a
great beach day. Will update to extend through sunset.

For tonight, the forecast gets a little tricky. The easy part is
steady temperatures, perhaps a couple degree rise in the Lower
Valley as winds lay down with perhaps a few puffs of south/
southwest wind reaching the surface. Any time cold temperatures
rise with the dewpoints the potential for "compressional" (sinking
ceilings toward ground level) fog increases, and have added
patchy to areas beginning at midnight and more focused from 3 AM
to 9 AM Sunday. Dense fog is certainly a possibility - favoring
the ranchlands but could be just about anywhere. By daybreak
Sunday, temperatures will continue to range through the 50s into
the lower 60s on the beach.

Whatever chilly air remains early Sunday is soon wiped out by
increasing southerly flow and subsidence allowing very dry air to
sink to 850 mb and perhaps all the way toward the surface,
particularly from the mid Valley to the ranchlands. Strength of
southerly flow remains somewhat uncertain, but will lean into the
higher speeds after noon (20 to 25 mph). By 3 PM, all areas will
be some 20 to 25 degrees warmer than today, completing the
transition back to springlike weather. Valley locations should
reach into the lower 80s while the ranchlands peak in the mid to
upper 70s.

LONG TERM (Sunday night through Friday): The mid level pattern will
remain relatively zonal through much of the long term. Both the GFS
and the ECMWF agree in principle with that scenario. Only late in
the game, roughly Thurs and beyond, will a sharp short wave trough
dive south through the Pacific Northwest and rotate or lift across
the Great Basin. That will support backing in the mid level pattern
to southwest over the fcst area, with a late next weekend surface
front in the offing, but that is already past the boundaries of this
cycle.

While the trough is descending into the CONUS, cold air from
Canada will move down into the Northern Plains on Tuesday. The
outbreak will initially move south along the western side of an
upper polar low over Central Canada, but will be on its own after
that, moving south into Central texas by Wednesday in both long
range models, the GFS and the ECMWF. The ECMWF guidance numbers
reflect a stronger push of colder air into the CWA on Thursday,
whereas the GFS colder air gets shunted to the east. These
differences are not as pronounced as previously. The blend seemed
to reflect the cooler ECMWF a bit more from Thursday through
Friday so added two or three degrees to give the GFS a bit more
representation. Rain chances will remain low through the forecast
with only a slight chance Thursday and Friday.

Monday through Wednesday will be warm with a few clouds. Temps
will be mainly in the 60s at night and will reach into the 80s
during the day for most areas. Slight rain chances will develop
from Wednesday night through Thursday night with increasing cloud
cover as a cold front pushes close to the area. The ECMWF will
push a wind shift through on Thurs, also pulling the values of the
temperature blend down. Temperatures will recover Friday and
Saturday as both models advertise southeast winds, and will push
warm Gulf air back over the area.

&&

.MARINE:
Now through Sunday...After more than 36 hours with Beaufort Force
7 or 8 seas...not fit for small craft and difficult for ships...
sea heights have begun to slowly subside. Still, as of 3 AM values
remained above 8 feet in the 20 to 60 nautical mile leg and with
orthogonal flow these heights likely extend close to the coast.
Small craft advisory remains in effect until 9 AM for the Gulf and
will likely need to be extended through the afternoon for the
still dangerous seas.

By nightfall seas should finally drop below 7 feet but will still
be somewhat dangerous for small craft into the night and not
finally improve a little more until Sunday when surface winds turn
southeast and the gradient slackens. Still moderate values to
close the weekend. Better conditions Sunday on Laguna Madre for
morning fishing, but fog could become an issue and it will still
be a bit nippy. Stronger winds will arrive during the afternoon
and could reach 15 to 20 knots in some locations.

Sunday night through Wednesday night...Moderate south winds and
moderate seas Sunday night will settle down to light to moderate
southeast to south winds and low to moderate seas the rest of the
long term. Broad high pressure will cover the north and east Gulf
during this period, supporting the onshore flow.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  61  61  77  69 /  40  20  10  10
BROWNSVILLE          57  57  80  68 /  40  20  10  10
HARLINGEN            56  56  81  67 /  30  10  10  10
MCALLEN              55  55  81  65 /  20  10   0   0
RIO GRANDE CITY      53  53  81  62 /  10  10   0   0
SOUTH PADRE ISLAND   65  64  73  70 /  40  20  10  10

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 10 AM CST this morning for
GMZ150-155-170-175.

&&

$$

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