Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 141148 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
548 AM CST Thu Dec 14 2017

.DISCUSSION...Updated for latest aviation discussion below.


.AVIATION...VFR to prevail through the daytime hours today with
ceilings gradually lowering (but remaining VFR) and light
southerly winds. An approaching cold front will cause winds to
back this evening with areas of rain showers developing. The front
proper should reach the RGV airports beginning around 09Z Friday
with a sharp increase in northerly winds and widespread light
rain. Ceilings fall to MVFR levels and could fall below 020 at the
very end of the current TAF period. Rain, low ceilings, and mist
will persist through at least mid-day on Friday.


.PREV DISCUSSION... /issued 428 AM CST Thu Dec 14 2017/

SHORT TERM (Now through Friday): A strong cold front will combine
with a subtropical moisture tap aloft to bring widespread rain to
Deep South Texas later Thursday night through much of Friday.  After
finally warming several degrees above normal (for a change!) today,
much colder and brisk conditions will occur on Friday, in addition
to the rain.

Current setup includes our old friend the Baja low "reloading" as
additional shortwave energy drops into it from the U. S. Desert
Southwest.  High clouds continue to stream overhead, and these
layers will eventually thicken down to mid-levels as today goes on.
Meanwhile, at the surface, the aforementioned cold front is analyzed
across North Texas.  Zonal flow aloft and southerly winds ahead of
the front should allow for a decent warmup into the low-mid 70s F
for most locales today, despite the thickening cloud cover.
Dewpoints in the Lower Valley also rise into the lower 60s, for a
bit more muggy feel than we have seen recently.  Latest model
solutions in good agreement in slowing down the front just a bit, so
have pulled mentionable PoP`s from the forecast through the daylight
hours today.

Things change quickly overnight.  Winds begin to back ahead of the
front by this evening, but main "blast" doesn`t arrive until late
this evening, progressing from NW to SE.  Some showery precip may
occur along and immediately ahead of the front.  Can`t totally rule
out a rumble of thunder right along the front with good dynamics
moving in, but perusal of forecast soundings shows very limited
instability.  Precip should quickly change over to a steady rain as
isentropic upglide over the colder aimass commences, aided by lift
aloft courtesy of the right-entrance region of a 150-knot jet streak
over NE Texas.  Mid-level shortwaves may also move through,
enhancing precip at times.  Column quickly becomes saturated, with
the best omega progged for Friday morning. High temps for the
calendar day Friday will occur at midnight, with temps falling into
the upper 40s-low 50s areawide for the afternoon, so it will be a
cold rain, especially in conjunction with the breezy winds.  Winds
may touch advisory criteria in coastal zones, but confidence on the
duration not yet high enough to issue.

Model/WPC total QPF`s generally in the 1.0-1.5" range through Friday
afternoon, so pretty much everybody should see a good, soaking rain.
Started trending PoP`s down a bit (to 40-60%) for Friday afternoon,
as some low-level subsidence appears to kick in (per model time-
heights) with the jet-streak pulling away.

LONG TERM (Friday Night through Wednesday): The majority of the
period will be active due to the progression of a pair of closed
500 mb low pressure systems through Mexico and the United States.
The first system, originating just northeast of the tip of the
Baja Peninsula, will rapidly fill and eject northeast into the
central United States after the passage of a cold front through
Deep South Texas and the Rio Grande Valley. Widespread coverage of
light rain, drizzle, and showers is expected Friday night through
Saturday night for the BRO CWFA before dry weather and greatly
reduced cloud cover kicks in late in the weekend. The second mid-
level low will transition gradually and generally east from the
Desert Southwest across the Lone Star State, resulting in more
isolated to scattered shower coverage Sunday night through Tuesday
for the BRO CWFA with a cold front passage at the tail end of this
period ushering in dry weather afterwards.

Given the passage of the two cold fronts and progression of the
mid-level lows, temperatures understandably are a bit tricky
surrounding those events. Went nearly isothermal Friday night
through Saturday night given the cold air advection behind the
cold front with significant rainfall and cloud cover in place.
Temperatures with the second low and front were a little easier,
as it appears an onshore flow will dominate for most of that
rainy period.

MARINE (Now through Friday): Rather pleasant boating conditions
today will give way to adverse conditions starting around 1am Friday
as a strong cold front plunges southward through the coastal waters.
 Small craft advisories have been posted with this package as north
winds of 20-30 knots are expected behind the front.  A few to even
occasional gale-force gusts are likely to occur, but after
collaboration with other Texas coastal WFO`s, confidence on
frequency/duration not enough to warrant gale headlines at this
time.  Seas build fairly rapidly 6-9 feet nearshore and 8-11 feet
offshore.  Conditions will remain hazardous through Friday daytime
as coastal troughing develops, maintaining the pressure gradient.
Adverse conditions are likely to persist on the Gulf waters into the
long-term period.

Friday Night through Wednesday: Light to moderate winds and low to
moderate seas are expected through a majority of the period with
weak and broad low pressure over the western Gulf of Mexico. The
two period of concern, however, will be Friday night through
Saturday morning and on Tuesday, with stronger winds and higher
seas anticipated due to the passage of cold fronts. Small Craft
Advisories are highly likely during both of these periods, with
even Gale Warning conditions not completely able to be ruled out
Friday night.


GM...Small Craft Advisory for winds from 1 AM to 6 PM CST Friday for

     Small Craft Advisory from 1 AM to 6 PM CST Friday for GMZ150-155-



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