Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 201909 AAC

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
209 PM CDT Tue Mar 20 2018

.DISCUSSION...Updated for latest marine discussion below.

.MARINE...Seas were near 8 feet with strong north to northeast
winds at buoy020 late this morning so have extended the small
craft advisory through 4 pm for the far offshore waters.

.PREV DISCUSSION... /issued 1234 PM CDT Tue Mar 20 2018/
DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Satellite images and surface observations indicate
clear skies across the CWA early this afternoon. Expect VFR
conditions to prevail across the Rio Grande valley and the
northern ranchlands the rest of the afternoon through tonight as
surface high pressure continues to build into the area providing
subsidence across deep south Texas.

PREV DISCUSSION... /issued 634 AM CDT Tue Mar 20 2018/
DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Lingering low ceilings and visibilities will quickly
improve around dawn as winds increase. North to northeast winds
will increase as drier air starts to filter into deep south Texas
later this morning courtesy of a surge of high pressure. VFR
conditions expected for the next 24 hours.

PREV DISCUSSION... /issued 452 AM CDT Tue Mar 20 2018/

SHORT TERM (Today through Wednesday): A stalled frontal boundary
extends from Baffin Bay to near Falcon Lake early this morning.
Dewpoints west of this boundary are in the 30s and 40s, while to the
east dewpoints remain in the upper 50s to the mid 60s. The
combination of higher dewpoints and very light winds has allowed
areas of dense fog to develop across portions the Rio Grande Valley
this morning. The fog is expected to dissipate around dawn as winds
begin to increase across the area. A Dense Fog Advisory is in effect
for Cameron, Willacy and Hidalgo counties through 6 am.

A surge of high pressure will arrive later this morning into the
afternoon bringing drier air and gusty winds into the region today.
Mostly sunny skies are expected today with high temperatures in the
low to mid 80s and dew points in the 30 and 40s for all except the
immediate coastline. The threat of rapid fire spread will be a
concern again today, but winds will likely not reach Red Flag
Warning criteria. However, burning is still discouraged as the
ground remains very dry. Low temperatures will fall into the 50s
across much of deep south Texas tonight under mostly clear skies.
High temperatures Wednesday are expected to climb into the mid to
upper 80s, except near the coast, under sunny skies. Rain-free
conditions will prevail through the forecast period as dry northwest
flow aloft and surface high pressure remains in place through

LONG TERM (Wednesday Night through Monday): Little change to the
gist of the forecast through the period, as upper level pattern
and forecast is stable among the various global models, which
keeps confidence high in the forecast. Upper ridge builds in
amplitude along the Rio Grande through the Texas/New Mexico border
into Thursday and Friday, initially centered over central Mexico
and easing into eastern Mexico by early Saturday. The ridge will
flatten but hold forth as it slips east into the western Gulf by
early Sunday. By the end of the period, the ridge moves back into
what has been a fairly dominant position since February - the
eastern Gulf - which will allow a deepening southwesterly flow at
mid levels to arrive and could be the portent of some hope for
rainfall in the form of thunderstorms at the very tail end of the
forecast. At that point, GFS/ECMWF differ in the treatment of the
Gulf ridge, with the ECMWF stronger and a bit farther west,
holding off precipitation until next Tuesday. Time will tell and
trends *possibly* favor some rainfall relief in the form of
thunderstorms, but until then it`s a dry and increasingly hot

For the sensible weather, the takeaways include mainly partly to
mostly sunny days through the period with all days seeing the
"Valley Wind Machine" courtesy of nearly stationary surface
pressure systems...strong high pressure in the eastern Gulf and a
stationary west Texas through Sierra Madre trough. The "Machine"
begins moderately on Thursday but kicks into high gear Friday
through Sunday. For the US 77/IH 69E corridor, 20 to 30 mph winds
with some gusts near or above 40 are quite likely each day, and
with limited daytime clouds, sufficient mixing should realize
these speeds. Wind may drop a bit Monday per latest GFS run but
confidence is low at this point and prefer to lean toward the
higher winds hanging on between the upper level systems shown by
the ECMWF. May well need repeated wind advisories for the higher
end wind favoring the Cameron/Willacy/Eastern Hidalgo area.

Regarding the clouds...while low level moisture recovers
overnight, not sold (and neither is blended guidance) on overcast
late nights and early mornings beginning Thursday and continuing
through Saturday. Plenty of dry air above the surface and could
see some mix in from time to time leaving scooting cumulus fractus
especially Friday and Saturday night, mainly after midnight. Those
clouds will lift into fair weather clouds (and fewer as the day
wears on) with sunshine the dominant memory. As for
temperatures...virtually no change to current forecast. Kept
entire package above guidance and consensus based on dry to very
dry ground, plenty of sunshine, and May-like atmospheric "heat"
(100-500 thickness nearing 580 in the Rio Grande Plains Friday
through Sunday afternoon and 850 mb temperature 20 to 22C+ as
well) Kept the populated mid-Valley in the mid 90s or so, with
near 100 along the Rio Grande in Starr and Zapata each day. A few
degrees lower on Monday but still well above late March averages.

MARINE: Buoy 42020 reported north-northwest winds around 4 knots
with seas slightly under 1.6 feet with a period of 5 seconds at
03 CDT/08 UTC. A surge of higher pressure will arrive this morning
with north to northeast winds increasing to around 20 knots just
after sunrise this morning. A Small Craft Advisory is in effect
through early this afternoon with seas building to 5 to 7 feet
briefly. Marine conditions will improve in the evening as winds

Wednesday night through Saturday: While marine layer is much less
robust now that it was through winter and early this month, it
will be able to temper winds a bit early on (Wednesday night
through Thursday night) with Caution levels (15 to 20 knots) over
the Gulf, though Laguna Madre could flirt with 20+ knots and
advisory conditions Thursday afternoon.

That changes Friday through Saturday. The dryness of the
atmosphere and expected near full sunshine over the Gulf, along
with 850 mb winds holding tough in the 30 to 35 knot range for
much of the day may be the exception to the typical marine layer
day, with less of a difference (10-15 knots) and 20 knots quite
possible day and night (slightly higher at night), even near
shore. Laguna Madre should reach 20 to 25 knots each afternoon and
could see a low level jet pop 20 knot wind from time to time at
night Friday and Saturday night. Long and persistent fetch will
build seas from moderate levels Thursday to high levels Friday
night through Saturday night. Bottom line? Fish and boat early to
mid week, as conditions go downhill in time for the weekend.

FIRE WEATHER...(Today through Wednesday): The threat of rapid
fire spread will be a concern again today, but winds will likely
not reach Red Flag Warning criteria. However, burning is still
discouraged as the ground remains very dry. Wednesday is fraught
with potential danger for burning, too. While dewpoints will rise
somewhat in easterly flow (vs. north/northeast flow today), winds
will rise as well; often this type of setup with `dry` east flow
can become surprising difficult for controlled burns. So...may be
another day for inland areas to see a Fire Danger Statement and
hopefully nothing more assuming dew points "behave" as forecast
and don`t slide into the 20s to lower 30s west of US 281/IH 69C.

Friday and Saturday: Unlike more `traditional` Valley Wind Machine
events that bring higher dewpoints and a mix of clouds and
sunshine, very dry air develops and sinks lower in the
atmosphere...all the way to ~950 mb Saturday. This combines with
the distinct possibility of 25+ mph sustained winds and the
likelihood of dry to critically dry grasses/brush which would meet
Red Flag Criteria for all or part of the area. Just something to
keep in mind on a hot south Texas weekend.

GM...Small Craft Advisory until 4 PM CDT this afternoon for GMZ170-


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