Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 200116 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
816 PM CDT Mon Mar 19 2018

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


.AVIATION...Surface obs and KBRO/KCRP radar both indicate the pcpn
free cold front has pushed through the western half of the RGV
but apparently has stalled along a line roughly from just east of
CRP to just east of La Joya. Radar imagery indicates that this
boundary is starting to drift back west very slowly. To the west
of the boundary dewpoints have crashed into the 20s while to the
east dewpoints remain in the upper 60s to near 70. Since the drier
air to the west is taking its time to push into the eastern
counties of Deep South TX, concerns are rising on the potential
for some fog formation near the HRL and BRO airports later tonight
through Tues morning. Surface winds have dropped off some already
this evening over the region as the temp/dewpoint spreads narrow.
So will include some mention of MVFR/IFR fog for the HRL and BRO
TAFs tonight, but will leave it out of the MFE TAF for the moment.
The drier air will finally start moving into all of Deep South TX
and the RGV early tomorrow as the old cold front finally clears
the region. This will maintain VFR conditions for the RGV airports
for the latter half of the current TAF period. Overall
condfidence in this TAF issuance is not the greatest in light of
the fog potential associated with the meandering cold front.


.PREV DISCUSSION... /issued 240 PM CDT Mon Mar 19 2018/

SHORT TERM (Now through Tuesday night): 500 mb low over Missouri
this afternoon will continue to progress eastward towards the
Mid- Atlantic with dry, northwesterly flow over South Texas
through the short term. A surface low over the Mississippi Valley
is currently bringing a very dry cold front through Deep South
Texas and will clear the region by this evening. A couple of very
isolated showers have tried to develop along the boundary, but
generally rain-free conditions will continue. Gusty winds and dry
air are already quickly surging in from the northwest with dew
points dropping to near 30F from Zapata to Hebbronville. With
afternoon temperatures in the upper 80s, RH values will range from
10 to 15% and push into Brooks and Hidalgo counties by mid
afternoon. A Red Flag Warning is in effect through 7 PM CDT along
and west of Interstate 69-C with critical fire weather conditions

Conditions will be very pleasant tonight with cooler temperatures
and very low humidity, especially in the Mid to Upper RGV. Mostly
sunny skies are expected tomorrow with highs in the mid 80s and dew
points in the 40s for all except the immediate coastline. The threat
of rapid fire spread will be a concern again tomorrow, but winds
will likely not reach criterion for Red Flag Warnings. However,
burning is still discouraged as the ground remains very dry.

LONG TERM (Wednesday through Monday): Early in the period, the
combination of surface ridging easing into east Texas while the
RGV is on the front end of the subtropical ridge centered over
central Mexico will keep Chamber of Commerce conditions going with
a refreshing Tuesday night and Wednesday morning followed by
plenty of sunshine and a strengthening easterly breeze and
comfortably low relative humidity.

Both ridges will gradually shift east Thursday, with surface high
pressure building as it moves into the Mississippi Valley, while
the 500 mb subtropical ridge expands north to along the Rio Grande
through the Big Bend region. May see fair weather cumulus during
the day as winds begin to ramp up, but still plenty of sunshine.
Temps rise a few degrees above average but it should still be a
nice day overall.

That will begin to change Friday and through the weekend. The 500 mb
ridge will shift east and flatten a bit as short waves racing
through the fast westerly flow knock down the northern extent of
the ridge across the central Plains. At the surface, a spring
cyclone will develop in the foothills of the Rockies, wrapping up
across Kansas and getting a kick from the speedy 500 mb flow. At
the same time, surface ridging will slide to the eastern Gulf and
the gradient flow across much of Texas will really crank. Winds
could reach advisory levels (30 to 40 mph including gusts)
underneath a persistent southerly 850-925 mb jet favoring the US
77 corridor. The atmosphere will warm appreciably as well, with
1000-500 mb thicknesses blowing past 576 dam, and 850 mb
temperatures surging above 20 C. The current forecast ranges
temperatures from around 90 degrees along US 77 to the mid 90s in
the mid Valley and near 100 degrees along the Rio Grande near
Falcon. These temperatures are a few degrees above guidance

By Sunday, the now flat 500 mb ridge will shift into the Gulf of
Mexico and the beginnings of west-southwest flow aloft will arrive.
This will help strengthen the more typical lee of Sierra Madre
trough, which will maintain a breezy to windy Sunday afternoon with
temperatures a bit lower than on Saturday. The southerly flow Friday
through Sunday will raise dew points, but drying soils and a dry to
very dry atmosphere above the overnight inversions will allow some
drop off in RH each afternoon.

Expect hazy skies to return for Saturday and Sunday, courtesy of the
higher humidity, but also possibly from a long fetch of southerly
low level flow that may bring pollutants from agricultural burning
in eastern Mexico. This could cause some breathing difficulties for
sensitive groups. Keeps tabs on air quality at and
by clicking on the south Texas region.

Now through Tuesday night: A weak cold front will move through
the coastal waters this afternoon with weak flow and low seas of 2
feet or less persisting through late tonight. Higher pressure,
however, will move southward and increase winds from the NNE to 20
knots just after sunrise Tuesday morning. A Small Craft Advisory
will go into effect through early afternoon with seas also coming
up to 5 to 7 feet briefly. Conditions will improve in the evening
as winds diminish and shift to the east.

Wednesday through Saturday night: Light to moderate onshore winds
and low to moderate seas will prevail Wednesday through Thursday
night. Winds will begin to strengthen from the southeast to south
Thursday night, becoming moderate to fresh with elevated seas.
Small craft should exercise caution to low end small craft
advisory conditions will be the norm during that period. The
reason for the stronger winds will be a tighter local pressure
gradient between Gulf high pressure and Plains low pressure.

FIRE WEATHER: Slow improvement in RH values will occur tonight.
Winds will subside very quickly around or just after sunset. RH
values will once again drop into the 15-25 percent range west of
I- 69E on Tuesday, however, 20 ft winds will be less than 15 mph
for much of the region, which should limit the rapid spread of

Wednesday may be a fire danger day if southeast winds pick up
sufficiently and carry across the entire region. Areas of concern
would be mainly along and west of US 281/I-69C where afternoon RH
values in the 20s could combine with 15 to 20 mph 20 ft winds to
result in a threat for rapid wildfire growth and spread.

For Thursday through Saturday...the current forecast holds
interior RH values in the 30 to 40 percent range Thursday and in
the 30 to 50 percent range Friday and Saturday in areas of
strongest winds (20+ mph from the south/southeast). That said,
fine fuels (grasses) and medium fuels (brush) will likely still be
dry to critically dry, and erratic wildfire behavior will be
possible if RH values dip a little lower than forecast. Bottom
line? There really won`t be any totally safe days to conduct
burning this week, particularly west of US 77/I-69E and more so
west of US 281/I-69C.


GM...Small Craft Advisory from 6 AM to 1 PM CDT Tuesday for GMZ150-



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