Area Forecast Discussion
Issued by NWS Brownsville, TX

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525
FXUS64 KBRO 211725 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1225 PM CDT Fri Apr 21 2017

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

.AVIATION...Cloud decks might slip into MVFR tonight, otherwise
VFR will prevail outside of that time frame. Moderate to breezy
surface winds will prevail due to high pressure and low pressure
interaction.
&&

.PREV DISCUSSION... /issued 620 AM CDT Fri Apr 21 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Satellite images and surface observations indicate
some low clouds across western portions of the CWA this morning.
Ceilings were near 1100ft at KHBV to near 2400ft at KAPY.
Visibilities were near 4SM with fog at KBKS. Expect VFR conditions
to prevail today as 500mb ridge across northern Mexico and
southwest Texas continues to provide subsidence across the CWA.
MVFR conditions will develop late tonight into early Sat morning
as low level moisture allows low clouds/patchy fog to develop
ahead of a cold front moving into central TX tonight.

PREV DISCUSSION... /issued 423 AM CDT Fri Apr 21 2017/
.HOT TO COOL THAN BACK TO HOT: COOLEST WEATHER IN SIX WEEKS ON
SUNDAY THEN HEAT RETURNS IN FORCE NEXT WEEK AS APRIL NEARS END...

SHORT TERM (Today through Saturday): 500mb ridge across northern
Mexico into south Texas will continue to provide subsidence across
the CWA today through Saturday. Low to mid level moisture will be
limited across the area as a result. A cold front will move into
central TX tonight and approach the CWA Sat afternoon. Some moisture
will pool along and ahead of the front and isolated convection will
develop across deep south TX Saturday as the front moves into the
area Sat afternoon. The main impacts will be temperatures today with
highs at least 5 degrees above normal across most of the CWA this
afternoon. Will likely see similar conditions across the southern
portions of the CWA Sat afternoon before the front brings drier and
slightly cooler air across the northern portions of the CWA.

LONG TERM (Saturday Night through Thursday):  A front will bring a
refreshing surge of cooler air, the coolest since mid March(!)
Saturday night with a very noticeable difference in the weather
compared to the early May-like warmth we`ve experienced so far this
month. By Sunday afternoon, sunshine will take an edge off the
relative chill but clear and calm conditions Sunday night will set
the stage for the coolest morning since March 14th. Why does this
matter?  Because the heat returns in spades by the middle of next
week.

For Saturday night, aside from the front - whose timing looks pretty
much on track for just after sunset across the ranchlands and
through the Valley a little before midnight - the question remains:
Is there enough lifting available to push through the very dry air
above 700 mb to build thunderstorms?  Latest GFS and ECMWF suggest
the best vertical motions are held below 700 mb *post* front, when
atmospheric parameters for convection crash - hard. That said, there
is a window ahead of the actual front when surface winds will veer
east/east-northeast and potential for convection remains as lifted
index/CAPE/etc. are sufficient for low-topped cells.  Though 500 mb
ridging holds forth, northwest flow on the front side of said ridge
combined with the lift provided by the front is a bit worrisome -
stranger things have occurred in such flow including gusty wind and
hail in convection that can initiate especially this time of year.

For these reasons, will keep the mention of thunder through the
evening hours along/ahead of the front, but fade back to plain
showers overnight and behind the front. Temperatures by daybreak are
a bit of a tricky call; NAM-12 shows 2 meter readings dipping into
the mid to upper 50s for the ranchlands and perhaps reaching the
upper Valley despite the overcast skies; given the shallow nature of
the air mass this would not surprise totally. Current numbers are in
range though may lower slightly in a couple of spots.

For Sunday, morning overcast across the Valley will thin in strong
April sunshine and dry air mixing in at all levels, and by early
afternoon expect just patchy stratus with increasing sunshine, and
full clearing by 3 PM across the Lower Valley.  Ranchland locations
will clear sooner. Warmest temperatures may be in the ranchlands
with the longer sunshine but even there readings will likely fall
just shy of 80. Chamber of commerce overnight weather dominates
Sunday night with light winds allowing temperatures to fall into the
low to mid 50s everywhere as the surface high briefly settles in.

Briefly is the word here, and Monday will be a true "Chamber of
Commerce" day with light veering winds back to the east, low
humidity, but rapidly recovering temperatures as the atmosphere
warms quickly and highs surge back into the mid to upper 80s.  A
great time to extend the weekend if you can.

The heat returns quickly thereafter.  Increasing zonal westerly flow
aloft - despite 500 mb height *falls* - brings lower surface
pressures into northern Mexico and west Texas which slide eastward
through Thursday.  The core of the low sets up in west Texas
Wednesday and Thursday ahead of developing/digging 500 mb trough
into the US Four Corners region.  The lower pressures bring very
high thicknesses for late April to Deep South Texas especially
Wednesday through Friday; the position of the 500 mb trough well
northwest of the region brings an additional warming/drying from
downslope/compressional heating off the Sierra Madre by Wednesday
and Thursday. Sultry surface southerly flow will bring some of the
highest dewpoints since last October especially Thursday, and heat
index may well reach to or even slightly above 105 at some point.

Any precipitation will remain well north of the Valley in this warm
sector; this pattern is similar to the one where we waved to the
action in central and north Texas during the latter half of March
while we said "hello" to a top ten warmest (all-time) month when all
was said and done.  During that period, daily temperatures were 5 to
10 degrees above the average.  Such a trend late next week will push
the Upper Valley/Rio Grande Plains to the century mark, with much of
the Valley in the mid to upper 90s. In fact, we are outlooked for a
moderate threat of "much above normal" temperatures beginning next
Thursday/Friday.

MARINE: Seas were near 3 feet with southeast winds near 14 knots
at buoy020 early this morning. Surface high pressure across the
northeast Gulf of Mexico and low pressure across the Texas
panhandle will provide light to moderate southeast to south winds
across the coastal water today. The pressure gradient will
increase across the lower TX coast tonight allowing SCEC
conditions to develop offshore the lower TX coast. A cold front
moving into central TX tonight will approach the middle and upper
TX coast Sat morning and reach the lower TX coastal waters late
Sat afternoon into early Sat evening. The pressure gradient will
weaken across the lower TX coast Sat morning and winds will
diminish and back to the east Sat afternoon.

Saturday Night through Monday Night:  A mixed bag with a rough
end to the weekend, then stellar conditions before the winds
return after the forecast period. Enough cool advection over above
normal waters (upper 70s) combined with the gradient associated
with this `norther Saturday night through Sunday will bring the
possibility of gale force gusts into Sunday morning, along with
hazardous seas which will pretty much make for difficult to
dangerous boating/fishing conditions in the Gulf all day, and over
the Laguna Madre early Sunday. Not expecting to hoist Gale
Warnings, but not too far away from that.

As quickly as the winds/seas rise up they come drop off rapidly
Sunday night into Monday as the high settles in and winds drop to
nearly calm before flipping to the south/southeast by Monday night,
but still at light speeds.  Bottom line? Best to postpone the
fishing trips Sunday, but take full advantage of Monday into early
Tuesday before southerly flow picks up a bit toward mid week.
&&

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

$$

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