Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 191122 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
622 AM CDT Mon Mar 19 2018

...12z Aviation Update...

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


.AVIATION...Southerly flow overnight kept our terminals from
dropping as initially thought with visibilities, however,
overnight we`ve been flirting with MVFR ceilings and in some
cases brief IFR. Expect that to continue this morning out ahead of
a cold frontal boundary due into Deep South Texas by later this
morning. Behind the front, skies should begin to clear allowing
for eventual VFR conditions, which should then persist through the
remainder of the period. Winds behind the front will range from
10-20kts from the northwest eventually transitioning to more of a
northeast with time.


.PREV DISCUSSION... /issued 357 AM CDT Mon Mar 19 2018/

SHORT TERM (Today through Tuesday):  A mild, mostly cloudy and hazy
start the new workweek this morning. Southerly flow of 5-10kts is
helping keep low level moisture in place. There has been some
reports of hazy/foggy conditions out there, especially where the
wind has dropped to around or slightly less than 5kts (HBV). That
said, no widespread dense fog is anticipated for the morning

A weak cold frontal boundary will slide through the region later
this morning. Mostly a dry passage is expected, however, short term
model guidance still show some very weak QPF signal, so kept slights
in, especially across our northern marine zones. The front will help
shift winds to a more northeasterly direction and help clear skies
of clouds and haze shortly after fropa. Cold air will lag behind the
front, however, dry conditions will filter southward (also downward
due to a well mixed boundary layer) by early afternoon. As
temperatures climb and dewpoints decrease, critical fire weather
concerns will begin to increase (see more in Fire Weather section

Winds will begin to drop off tonight around or shortly after sunset
as we decouple. The cooler air finally arrives later tonight and
drops much of the area off into the 50s or lower 60s with the warmer
values residing in the lower valley and along the coast.

On Tuesday temperatures will be right around average or just a bit
above with temperatures climbing into the 80s for much of the
region. Northeast flow will continue as pressure gradient tightens
just a bit Tuesday morning between a high pressure dropping
southward from the high plains and a strengthening low pressure
along the east coast. As the high pressure settles into the area by
the end of the period, surface winds will relax and tranquil
conditions are expected.

LONG TERM (Tuesday Night through Sunday): Accelerated
spring...April in March...will be the main story for the balance
of the work week and especially the weekend. Early in the period,
the last of very pleasant weather continues as combination of
surface ridge easing into east Texas while the RGV is on the front
end of the subtropical ridge centered over central Mexico keeps
Chamber of Commerce conditions with a refreshing windows-open
Tuesday night and Wednesday morning followed by plenty sunshine
and a freshening easterly breeze with plenty of sunshine over
comfortably low humidity.

Both ridges gradually shift east Thursday, with the surface high
strengthening as it moves into the Mississippi Valley Thursday
while 500 mb subtropical ridge expands northward to along the Rio
Grande through the Big Bend region. May see fair weather cumulus
during the day as the Valley Wind Machine begins to crank but
still plenty of "clean" sunshine. Temperatures rise a few degrees
above average but still a nice day overall.

That begins to change Friday and through the weekend. The 500 mb
ridge shifts east and gets a bit flatter 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 develops in the foothills of the Rockies and wraps up
across Kansas but gets a nice "kick" from the speedy 500 mb flow.
At the same time, surface ridge slides to the eastern Gulf and
the gradient flow across much of Texas really cranks. The Valley
Wind Machine will do its thing, and winds could reach advisory
levels (30 to 40 mph including gusts) underneath persistent
southerly 850-925 mb jet favoring the US 77 corridor. The
atmosphere warms appreciably as well, with 1000-500 mb thicknesses
blowing past 576 dm and 850 mb temperatures surging above 20C.
Current forecast ranges temperatures from around 90 along US 77 to
the mid 90s in the mid Valley and near 100 along the Rio Grande
near Falcon. These temperatures are a few degrees above guidance
consensus but are on par with the warmer ECMWF, as well as recent
trends including positive feedback from drying conditions where
moderate to potentially severe drought is expanding.

By Sunday, the now flat 500 mb ridge now shifts into the Gulf of
Mexico and the beginnings of west-southwest flow aloft arrives.
This helps strengthen the more typical lee of Sierra Madre trough
which will maintain a breezy to windy Sunday afternoon with
temperatures just a smidge lower than on Saturday. The southerly
flow Friday through Sunday will bring dewpoints back, but
aforementioned drying soils and generally dry to very dry
atmosphere above the overnight inversions will allow some drop off
each afternoon. ECMWF showing afternoon 850-925 mb wind shifts to
northwest which could bring significantly lower humidity than
currently forecast once the bigger heat arrives, similar to the
heat and several hour very low humidity (<15%) observed this
weekend in a pocket from Roma to Hebbronville and points west.

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

MARINE: (Now through Tuesday): Very tranquil marine conditions
expected to start the short term, however, that will be short lived
as the weather begin to deteriorate on Tuesday. A strong pressure
gradient is expected to develop between a high dropping southward
from the high plains and a strengthening low over the mid Atlantic.
This will increase our northeasterly fetch across the northwest
Gulf. Winds and perhaps seas will likely reach SCA thresholds at
some point Tuesday morning/early afternoon. The high pressure drops
fairly rapidly into central Texas by the end of the period allowing
for pressure gradient to relax quickly and overall improving marine
conditions to round out the short term period.

Tuesday Night through Friday: A quick drop in winds behind the
Tuesday morning surge will return slight/light seas Tuesday night
into Wednesday, even as moderate east winds return. Moderate winds
and seas will continue through Thursday as flow veers southeast
with the shifting surface ridge, and marine layer holds things
down a bit compared to land - though the western Gulf is beginning
to warm (low to mid 70s sea surface temperatures) and winds won`t
be held back as much in southerly flow as we move later into the
year. By Friday, increasing southerly flow will be able to
overcome the marine layer and bring caution winds (15 to 20 knots)
in the Gulf and 20 knots and gusty to parts of Laguna Madre,
which is likely to continue into the evening as low level jet is
established. Long period swell and wind waves will likely build to
7 feet or higher in spots Friday and Friday night...and weekend
boating/fishing for small craft looks difficult to dangerous at
this point.

FIRE WEATHER...(Today through Tuesday): A weak cold frontal boundary
will slide through the region later this morning. Although the winds
will shift, any cooler air will be delayed from reaching deep South
Texas and the RGV until later in the period. This will allow
temperatures to warm into the 90s west of I69E this afternoon. Dew
points will crash as dry air advects into the region and mixing
begins as surface heating ramps up this afternoon. Winds are
somewhat marginal with 20ft winds around 20kts or so give or take
for the upper Ranchlands. To offset the slightly lower end wind
issue, fuels in this part of the CWFA are very dry in some cases
so with the low RH values and elevated winds, fires should have no
issue spreading. Closer to the coast, winds are below RFW
criteria, but even with RH values 10-20 percent fires could easily
start, especially across Kenedy and western Willacy counties.

Slow improvement with RH values tonight. Winds will subside very
quickly around or just after sunset. RH values will once again drop
into the 15-25 percent range generally west of I69E on Tuesday,
however, 20ft winds will generally be less than 15 mph for much of
the region, which should limit at least the rapid spread of fires.

(Wednesday through Saturday):
Wednesday...continues to look like a fire danger day at minimum
and perhaps a Red Flag day if temperatures can recover and east
winds pick up sufficiently and carry across the entire region.
Areas of concern would be mainly along and west of US 281 where
teens afternoon humidity could combine with 15 to 20 mph 20 foot
winds to result in a significant threat for rapid wildfire spread.

For Thursday through Saturday...current forecast holds interior
RH in the 30 to 40 percent range Thursday and 35 to ~50 percent
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 be "set up" as dry to critically dry by
conditions through Wednesday, it erratic wildfire behavior is
possible if humidity can dip a little lower than forecast. Bottom
line? There really won`t be any totally safe days to conduct
burning this week, particularly between US 77 and US 281 and


TX...Red Flag Warning from noon today to 7 PM CDT this evening for



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