Area Forecast Discussion
Issued by NWS Brownsville, TX

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370
FXUS64 KBRO 191201 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
701 AM CDT Wed Sep 19 2018

.DISCUSSION...Updated for latest /12z/ aviation discussion below.

&&

.AVIATION...Virtually no change to the current forecast, just an
extension of the current forecast. Cirrus has thickened a bit
since the forecast issued at 1 AM which fits nicely with the high
ceiling (VFR by far) already forecast. No change in wind, and
still looks like precipitation will be difficult to find at any
terminal, including McAllen where VCSH was left for the late
afternoon. For what it`s worth, the 06Z GFS hints at more mid
level clouds right at the end of the forecast period...just before
12Z Thursday...but can let future shifts evaluate the potential
toward daybreak with updates this afternoon and evening.
52/BSG

&&

.PREV DISCUSSION... /issued 421 AM CDT Wed Sep 19 2018/

SHORT TERM (Today through Thursday): After a respite from the wet
week that was September 9 through 16, deeper moisture will gradually
creep back into Deep S. Texas and the Rio Grande Valley beginning at
the end of the short term period (Thursday) and continue beyond,
initially courtesy of pieces of a short wave trough rotating into
Texas from northern Mexico as the 500 mb ridge whose backside was
over the state through today eases into the southeast U.S.,
helping to dry out the Carolinas who desperately need it. More on
the end of the week and weekend in the long term section to
follow.

For the sensible weather, time-height sections show quite a bit of
thick high level moisture with 90+ percent mean relative humidity
between 500 and 300 mb by late morning through afternoon. IR/WV
satellite loops show this underway with slowly advancing cirrus from
the Sierra Madre associated with the aforementioned trough.  Expect
the cirrus to turn the sun milky or blot it out altogether at
times...but not enough to affect temperatures which should still
reach the low to mid 90s in most areas.  Lower level moisture
increases a bit toward afternoon, and given the isolated nature of
showers/storms that was a tad surprising Tuesday afternoon, have
added back 20 percent chances to the upper Valley late this
afternoon.

Tonight should be fairly copacetic but the fairly thick cirrus
should continue.  The high cloud canopy may impact low
temperatures...and guidance has risen a degree or two over values
seen this time Tuesday so have nudged up the minimum temperature
forecast accordingly.

By Thursday, the moisture thickens in more layers, particularly
filling the previously dry layer between 850 and 500 mb by
afternoon.  The additional moisture pushes precipitable water levels
in the column up over 2.1 inches, and combined with deepening
southerly flow (up to 500 mb) ahead of the 500 mb trough nudging
east will favor increasing rain chances.  For now went in the middle
of the model blends which is fairly close to inherited forecast
(mainly 30 percent probability), though did time out rain chances to
favor mainly the afternoon on land with daytime heating percolating
the moistening atmosphere. Worded as "chance" rather than
"scattered" for now given some uncertainty in how the moisture will
translate to rainfall.

High temperatures will likely be affected by the increasing cloud
cover and potential rainfall late, but difficult to pinpoint when
showers/storms may knock down the values.  For now, nudged down a
degree or two from previous forecast.

LONG TERM (Thursday Night through Tuesday): 500mb trough across
New Mexico and west Texas Thursday will move slowly eastward into
the southern plains Friday as the 500mb ridge across the eastern
United States moves eastward across the U.S. mid-Atlantic coast.
Global models are in good agreement with low to mid level moisture
increasing across Texas from west to east Thurs night into Friday.
This will result in an increasing chance of showers and
thunderstorms across south Texas Friday and rain chances will
continue through the weekend as a broad shear axis develops across
the state between the 500mb ridge across northern Mexico and the
500mb ridge across the southeast United States. Both the GFS and
ECMWF prog a broad 500mb low/inverted trough across the eastern
Gulf of Mexico Saturday moving slowly westward into the central or
western Gulf Sunday into Monday. This feature should provide some
subsidence across south Texas early next week even as low to mid
level moisture remains high across the area. Will continue to
mention a slight chance of showers and thunderstorms through the
rest of the forecast period even as the global models diverge on
whether the upper level ridge across northern Mexico builds
eastward across south Texas or the upper ridge across the
southeast U.S. builds westward into south TX.

MARINE (Now through Thursday): Light winds and slight seas will
continue today into this evening.  As surface ridge that has covered
the northern and western Gulf for the past few days slides east,
expect southeast flow to pick up modestly overnight and probably
hold on a bit into Thursday despite diurnal trends downward. This
will slowly increase seas but 2 feet should do it for most of the
period.  Some increase in fetch by late Thursday may provide areas
of 3 feet, and isolated to scattered showers and a few thunderstorms
will add a little nuisance to boating/fishing excursions.

Thursday night through Sunday...Light to moderate southeast winds
will prevail across the coastal waters Thurs night into Friday
with low pressure across west Texas and high pressure across the
eastern Gulf of Mexico. A weak cold front is expected to move into
northwest Texas Friday and stall. Light to moderate southeast
winds Saturday should diminish and back slightly to the east and
southeast Sunday as a surface trough lingers across north Texas.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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