Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Brownsville Texas
455 am CST Wednesday Dec 11 2013
Discussion...updated for latest aviation discussion below.
Aviation...wedge of colder low level air under capping southwest
flow aloft remains in place...locking in a deep inversion and overcast
skies today. Light north northeast winds will continue and be
reinforced later today and tonight as fresh high pressure moves
into the northwest Gulf and East Texas. Ceilings may lift to VFR
during day at times...but would primarily still expect MVFR for
the majority of the time.
Previous discussion... /issued 229 am CST Wednesday Dec 11 2013/
Short term /today through Thursday/...broad middle level troughing
remains over the eastern United States...with 1030 mb high pressure
over the lower Mississippi Valley. Thick inversion to 850 or 800 mb
will continue with overcast skies and high temperatures about ten degrees below
normal...middle 50s to middle 60s. Light north northeast winds with token
mention of probability of precipitation.
Reinforcing shot of primarily Canadian pressure will rush from the
northern to Southern Plains today and will tighten the gradient from
the lower Mississippi Valley to East Texas. This will also sharpen a
coastal trough along the Lower Texas coast...maintaining the threat
of low probability light rain drizzle for the Lower Valley. Winds tonight
will be light north northeast and temperatures will be close to normal near
40 to near 50.
High pressure will spread out and shift east on Thursday...toward
the southeast United States...while the Baja California cut off low begins to
eject into the southwest United States. Conditions will remain much
the same for the Rio Grande Valley...with high temperatures still cooler
than normal due to the influx of new high pressure...middle 50s to middle
60s under overcast skies...and with winds lighter than today out of the
north to northeast at 5 to 10 miles per hour.
Long term /Thursday night through Wednesday/...
Thursday night into Friday a positively tilted longer wave trough
will be digging south just inside the West Coast with a leading
shortwave moving in zonal flow into the panhandles region of TX/OK.
These features will help get a low level cyclone going over the
Front Range of The Rockies that will generate southeasterly return
flow and strengthen our southwesterly flow aloft. This will provide
an increase in low level moisture and humidity...and provide a
subsidence inversion capping convective potential. Friday should be
a pretty pleasant day with much more seasonable temperatures than the last
few days. Middle and lower level drying should also scour cloud cover
down to afternoon cumulus.
Saturday the Pacific low flattens out into a more distinct open wave
with the forerunning shortwave ejected into the Great Lakes region.
The upper convergence behind the trough increases plains high
pressure and that sends a frontal boundary southward into deep south
Texas. Looking at the central Canadian source airmass and the
projected 1029mb ridging over Kansas expect that the initial frontal
surge will again outrun the global model consensus and sped up the
frontal timing to around 12-18z on Saturday and lowered Saturday
maxes accordingly. Also introduced a chance of thunder with the area
in a right exit region of the leading jet ahead of the upper trough
fairly close to frontal passage. Forecast soundings show moderate instability
that may be released by the frontal boundary or just behind it.
Precipitable water values are also projected to be around two
Standard deviations above normal so those trends will need to be
The upper level trough axis is a little more reluctant to clear in
the 00z guidance suite showing a passage around 00z Monday.
European model (ecmwf)/GFS both show decent middle and upper level ascent over a dry
Post frontal passage boundary layer. The possibility for some middle level
overrunning or elevated instability producing showers seems to be
higher and included a 20 percent pop through Sunday evening to account
for this. Went from more of a sunny cool day to a mostly cloudy
spotty shower day. But a cold drizzly day does not seem too likely
given the frontal slope and advancing upper trough.
The trough axis does however clear by Monday and another pleasant
and seasonable day looks to be on the horizon with a few low clouds.
Northeasterly low level flow continues on Tuesday before shifting
more east to southeasterly on Wednesday. Went a little above
seasonal norms in the extended with rising thicknesses and low cloud
today through Thursday...moderate north northeast winds and moderate
wave heights will prevail today...as broad high pressure to the north
spreads out over the north Gulf Coast and northwest Gulf of Mexico.
Reinforcing high pressure moving southeast out of the Central Plains
will tighten up the gradient and increase winds to fresh this afternoon
through Thursday morning...with wave heights building to seven feet
out over the open Gulf tonight. Will Post Small Craft Advisory
conditions beginning this afternoon with winds hitting 20 knots. Seas
will follow up to seven feet tonight.
Thursday night through Sunday...
light to moderate southeast winds and seas are expected Thursday
night through Friday and Friday night with low to moderate seas.
Another cold front is expected on Saturday with a sharp wind shift
to the north and winds of 20 to 25 knots with higher gusts lasting
through Sunday morning and possibly Sunday afternoon. Seas will
quickly become rough behind the front. Small craft advisories will
be needed for this period with at least a possibility of frequent
gale force gusts. Conditions are currently expected to be
improving by Sunday night.
GM...Small Craft Advisory from 3 PM this afternoon to 3 am CST
Thursday for gmz150-155-170-175.
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