Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 251152 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
552 AM CST Sun Feb 25 2018

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

&&

.AVIATION...IFR to LIRF cigs this morning becoming MVFR to VFR mid
to late morning as drier air mixes through the moist surface
inversion. Cumulus clouds to increase later this afternoon
and lower to MVFR at times as moisture increases off the Gulf in
advance of a slow moving cold front. This front moves into the Mid
and Lower Rio Grande Valley tonight and Monday with fog and
ceilings dropping to IFR levels along with an increase in showers
and possible thunderstorms. South winds to increase in the KHRL
and KBRO terminals with gusts to 20 knot this morning before
lowering this afternoon.

&&

.PREV DISCUSSION... /issued 411 AM CST Sun Feb 25 2018/

SHORT TERM (Today through Monday): A weak cold front has temporarily
stalled over the far western counties of Deep South Texas extending
north through South central Texas. The front is expected to move
very little today getting a slight boost eastward later this
afternoon and tonight as a mid level shortwave trough moves east
through the Southern/Central Plains. As this shortwave moves well
east of the area Monday the front is likely to pull up stationary
once again just offshore. Moisture deepens and pools along and
behind the frontal boundary and as the front approaches the RGV
and stalls offshore rain chances increase significantly. Most
model guidance increases the chance of rain later tonight with
categorical or widespread rainfall in the RGV Monday as weak low
pressure forms along the stalled front increasing the isentropic
lift. Weak disturbances in the deep southwest flow aloft and some
elevated instability will allow for isolated thunder at times.
Rainfall amounts of up to one half inch or greater are likely from
Hidalgo to Willacy to Cameron counties where the best lift sets
up and the close proximity to the frontal boundary. Fog over SPI
this morning has become dense (prompting a Dense Fog Advisory for
the Island) and may spread into coastal Willacy and Cameron
counties for a few hours this morning. With the front nearing the
coast tonight and stalling tomorrow fog may be an issue at times
over the coastal waters as well as portions of the RGV.

Temperatures to warm up ahead of the front and may be exceed the
forecast highs if some sun peaks through especially in the Lower/Mid
Valley. Slightly cooler tonight and 10-15 degrees cooler Monday with
the front passing to the east. Temperatures could even end up lower
in the Mid/Lower valley if rain becomes more persistent.

LONG TERM (Monday Night through Saturday): Upper level southwest
flow will continue as we move into the long term period. A ~590dam
high will be centered over the Gulf/Bay of Campeche allowing for
continued active upper levels. Back down at the surface the
frontal boundary that pushes through and gets stuck right along or
just south of the lower portion of the Rio Grande will begin to be
forced northward as a warm front by Tuesday morning. This should
veer surface winds back to southerly allowing for more moist air
to advect into the region (will need to watch fog potential during
the overnight early next week).

Still think as the front lifts north and away from our location,
we`ll see a bit more sun on Tuesday. Decided to bump temps a bit
and thin clouds, especially closer to the Rio. Also, can`t ignore
the mid level high trying to slide westward during this time,
which is more evidence of warmth. Low level flow also increases as
pressure gradient tightens with leeside troughing across N
Mexico. Went ahead and slightly bumped winds a tad for Tuesday as
well.

Going into Wednesday, pressure gradient will continue to gradually
tighten out ahead of our next cold frontal boundary due into Deep
South Texas by late Wednesday night. Wednesday looks quite breezy,
especially across the lower valley with some gusts nearing Wind
Advisory criteria once again. Outside of increasing pressure
gradient, there will also be mechanical mixing taking place
during peak heating adding to the gusty conditions. Speaking of
heating, temperatures are expected to soar into the 90s,
especially the southwest portion of the CWA where middle 90s may
be realized.

Models are showing very little in the way of rainfall with the
midweek front. There is a very subtle QPF signal which is enough
to continue 20 POPs for some/most of the region, but uncertainty
in regards to rainfall with the front is high.

The front slides all the way through by Thursday allowing for
clouds to decrease and a return to mostly sunny skies across the
board. Although it will be cooler behind the front, temperatures
will still be able to warm into the lower 80s with insolation
alone, even despite the cold air advection.

Surface winds lighten finally by early next weekend and perhaps
even a signal of a slight pattern change as the mid level high
continues to try to work west. If this does indeed occur, the
rather active southwesterly flow will veer to a calmer and much
drier northwesterly flow by next weekend.

MARINE (Today through Monday): Dense sea fog has formed with
advisories posted for the Laguna Madre and Nearshore Gulf waters
through the morning. Some brief thinning of the fog is possible this
afternoon before becoming dense once again tonight and lingering
well into Monday with a frontal boundary stalling near or over the
coastal waters. Light southeast winds today become northeast Monday
as high pressure builds over the northern Gulf. Moderate seas
expected through Monday with exercise caution levels mainly for the
far offshore waters.Dense sea fog has formed with
advisories posted for the Laguna Madre and Nearshore Gulf waters
through the morning. Some brief thinning of the fog is possible this
afternoon before becoming dense once again tonight and lingering
well into Monday with a frontal boundary stalling near or over the
coastal waters. Light southeast winds today become northeast Monday
as high pressure builds over the northern Gulf. Moderate seas
expected through Monday with exercise caution levels mainly for the
far offshore waters.

(Monday Night through Saturday): Moderate winds and seas will
continue into the long term. Northerly flow will eventually
transition back to southeasterly flow on Tuesday. This will help
bring in warmer/humid air over relatively cool waters, which will
likely cause additional sea fog Monday night and Tuesday morning.
Fog should mix out as winds slowly ramp up Tuesday and especially
into Wednesday where low-end Small Craft Advisories may be needed
for the Laguna Madre and the nearshore waters.

The frontal boundary should pass through late Wednesday bringing
some scattered showers and perhaps a rumble of thunder offshore.
Winds will then veer back to the north and northeast as high
pressure moves southward into the midwest. This should allow for
some slight improvement as pressure gradient weakens, however,
surface flow has a pretty decent fetch setting up across the
northern gulf, which may lead to increasing swells by early next
weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  81  66  70  67 /  20  40  70  20
BROWNSVILLE          84  66  71  67 /  20  40  70  20
HARLINGEN            84  65  69  67 /  20  40  70  30
MCALLEN              82  64  70  67 /  20  40  60  40
RIO GRANDE CITY      79  62  72  65 /  20  30  40  40
SOUTH PADRE ISLAND   75  65  68  67 /  20  40  70  20

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Dense Fog Advisory until noon CST today for TXZ256-257.

GM...Dense Fog Advisory until noon CST today for GMZ130-132-135-150-
     155.

&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

59...Aviation/short term
55...Long term
62...PSU/graphics



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