NWS Brownsville, TX wrote its last area forecast discussion before the first event at Dec 21, 11:03 PM CST, 57 minutes earlier.
Area forecast discussion, Dec 21, 11:03 PM CST, 57 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1103 PM CST Mon Dec 21 2020
.DISCUSSION...Updated for latest aviation discussion below.
&&
.AVIATION...IR satl imagery shows areas of mid level cld cover
moving over the RGV from west to east. Otherwise, VFR conditions
will prevail over the RGV airports. The lower level SE flow will
allow the moisture levels to steadily increase over the upcoming
24 hours which will allow the ceilings to lower down a bit more by
tomorrow afternoon. These will likely still remain above VFR
levels over the next 24 hours.
&&
.PREV DISCUSSION... /issued 558 PM CST Mon Dec 21 2020/
DISCUSSION...Updated for latest aviation discussion below.
AVIATION...Vsbl satl imagery and surface obs indicate that mid
and high level cld cover is moving over the RGV from west to east.
Meanwhile, the low level flow is starting to increase the moisture
levels over the region tonight into Tues which will allow the
ceilings to drop down some more Tues afternoon. However, although
the ceilings will be lowering through tomorrow afternoon, expect
VFR conditions to persist for all 3 RGV airports over the upcoming
24 hours. The SE surface flow will likely be strong enough tonight
to limit any serious fog formation over the RGV airports.
PREV DISCUSSION... /issued 251 PM CST Mon Dec 21 2020/
SHORT TERM (Now through Tuesday Night): A beautiful day has
transpired across the Rio Grande Valley and Deep South Texas today.
Anticipate similar conditions in the short term period. A slight
warming trend will continue across the district as low-level WAA
increases. Flat and weak riding aloft will persist over much of
Texas through the period. A very weak shortwave trough will move
across Northern Texas tomorrow, but will have little to no effect on
Deep South Texas. Surface ridging over the northwest Gulf will slide
northeastward over the central Appalachian Mtns by tomorrow
afternoon. This will establish a more southeasterly wind flow at the
surface and help develop a few more lower level clouds tomorrow.
Lows tonight will be about 5-10 degrees warmer than last night,
dropping into the mid-upper 50s to near 60 degrees. Afternoon highs
tomorrow should only be a couple of degrees warmer than today,
ranging in the upper 70s to lower 80s. By Tuesday night, lows will
be back into the upper 50s and lower-mid 60s.
LONG TERM (Wednesday through Monday): The main highlight in the
long-term continues to be the mid-week passage of a strong cold
front, which will impact our weather for the Christmas
Eve/Christmas Day period. Ahead of the front, Wednesday the 23rd
still appears to be a very warm late December day, with afternoon
high temperatures approximately 15 degrees F above normal (though
no records will be threatened), which equates to lower-mid 80s
everywhere except right along the coast.
Latest deterministic model suite has sped up arrival of the cold front
by a few hours. Looks like a pre-frontal trough (or more
accurately, remnants of a dryline) will begin shifting winds to
the NW as Wednesday afternoon progresses. The actual thermal
gradient, associated with more northerly winds, is now progged to
arrive in the northern tier of counties around 03Z Thu, give or
take a couple of hours. (12Z GFS is the quickest, with the NAM not
far behind.) Breezy winds will develop overnight, advecting in
much drier air with source regions in the Mexican Plateau and
Desert Southwest of the U. S. Unusually, relative humidity values
will be decreasing through the night, as dewpoints crater into
the the teens and 20s F west of I-69E/Hwy. 77. See Fire Weather
discussion below for more details.
So, Christmas Eve Day (24th) remains on the cool and very dry
side, with temps managing to rise into the low-mid 60s, though it
is likely to feel a bit blustery much of the day. As surface high
pressure settles overhead by early Christmas morning, favorable
radiational cooling conditions set up, for a festively cool/cold
start to Christmas Day: Most all areas should see temps dip into
the 30s for at least a couple of hours. A light freeze is possible
across portions of the Ranchlands, mainly for Brooks, inland
Kenedy, and northern Hidalgo counties where the coldest spots have
been noted thus far this season. Still some uncertainty with
exactly how far these temps will dip, though, with NBM 10-90th
percentile min temps for Falfurrias ranging from 28-36F, for
example.
Christmas Day itself looks fantastic, following the cold start.
Light onshore develops by later in the day as the surface high
scoots east. Plenty of sunshine should allow daytime temps to
rebound into the lower 70s for much of the RGV. Warming and
gradual re-moistening trend continues on Friday, as a short-wave
ridge aloft de-amplifies overhead and southerly flow develops at
850mb.
Latest models now a little more bullish on precip with a
short-wave trough cutting across Texas Friday night and Saturday,
with the trough now a little sharper and further south than
previous solutions indicated. Have raised PoP's a touch to account
for this, with areas east of Hwy. 77 again favored. This wave
will move out quickly, with flat ridging aloft on Sunday allowing
temps to again touch 80F ahead of the next potential front. Models
still not too sure what to make of this next front, though
consensus is for it to stall north of our CWA (which makes sense,
given that zonal flow aloft is progged) or perhaps "ooze" through
with a NE wind shift heading into Monday. Stuck very close to
blended guidance out that far.
MARINE (Now through Tuesday Night): Favorable marine
conditions are expected to persist through the day on Tuesday.
Surface high over the northwestern Gulf this afternoon will continue
to slide northeast through this evening and into Tuesday as mid-
level high pressure builds over Texas. Low pressure will develop
across the central and northern Plains tomorrow afternoon,
increasing the pressure gradient along the lower Texas coast. As a
result, southeasterly winds will increase by late afternoon and
evening, with the breeziest winds occurring over the offshore Gulf
waters. Winds may be strong enough to warrant small craft exercise
caution (SCEC) wording on the Gulf by Tuesday evening. Winds on the
bay should remain below SCEC levels.
Wednesday through Sunday: Moderate southerly winds should be on a
decreasing trend early Wednesday as the pressure gradient relaxes
ahead of an approaching cold front. Seas should remain in the 3-4
ft. range through the day, though a few showers and thunderstorms
may occur farther offshore. A strong cold front is expected to
arrive in the coastal waters Wednesday evening, ushering in very
strong northerly winds. Frequent gusts to 34 knots or greater
appear likely for the Gulf waters, and perhaps on the Laguna Madre
as well, beginning around 06Z Thursday and lasting to around mid-
day. Gale Watches and Warnings may be needed. Seas will also build
to dangerous levels of 10-12 ft., with Small Craft Advisories
(SCA) likely to replace any gale headlines on the Gulf waters as
seas gradually subside into Thursday evening (Christmas Eve).
Conditions look much more favorable for Christmas Day, with light
winds and seas around 3 ft. More moderate S-SE winds kick in for
the weekend, perhaps building seas to SCEC levels on the outer
Gulf waters by Saturday night.
FIRE WEATHER (Thursday)...Very dry air will move in behind
Wednesday night's cold front. Relatively humidity values should
begin falling even during the pre-dawn hours of Thursday, falling
as low as 10-15% across much of the area west of I-69E/Hwy. 77.
Combined with cured grasses and drying brush, elevated to even
critical fire weather conditions may develop. One slight
mitigating factor is that the front now appears to arrive a few
hours earlier Wednesday evening, so 20-ft. winds may begin to
subside to 8-12 mph across Zapata, Jim Hogg, Brooks, and Hidalgo
counties by early Thursday afternoon. Will have to watch along the
Hwy. 77 corridor where winds stay up a little longer and may
coincide with the lowest RH of the day. A Fire Danger Statement is
likely for Thursday, and Red Flag Watches or Warnings may even be
needed over portions of the area.
&&
.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL 60 79 65 82 / 0 0 10 10
BROWNSVILLE 60 80 65 84 / 0 0 10 10
HARLINGEN 57 80 62 85 / 0 0 0 10
MCALLEN 60 81 65 85 / 0 0 0 10
RIO GRANDE CITY 56 83 61 85 / 0 0 0 10
SOUTH PADRE ISLAND 68 73 70 76 / 0 0 10 10
&&
.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&
This product is also available on the web at:
HTTP://WEATHER.GOV/RGV
Short Term/Aviation...60-Speece
Long Term/Upper Air...56-Hallman