Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 272348

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
648 PM CDT Mon Mar 27 2017

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


.AVIATION...VFR cloud decks to gradually dissipate through the
evening hours as daytime mixing wanes. As the surface inversion
forms and a low level jet strengthens MVFR stratus develop with
the three regional airports seeing increasing cloud cover after
06Z. MVFR conditions to persist through sunrise breaking up late
morning with VFR conditions Tuesday afternoon.  Sufficient winds
through the evening and overnight should limit any fog formation.
Southeast winds overnight to remain near 10 knots with a few
higher gust. Stronger winds likely Tuesday with gusts 25-30 knots
maybe nearing 35 knots at times.


.PREV DISCUSSION... /issued 332 PM CDT Mon Mar 27 2017/

SHORT TERM (Tonight through Tuesday Night): Water vapor imagery this
afternoon shows a 500mb trough moving over southern California with
a nearly 500mb zonal flow over Deep South Texas. The mid-level flow
will become more southwesterly over the region Tuesday and Tuesday
night as the 500mb low moves over northern New Mexico. The pressure
gradient will strengthen Tuesday and Tuesday night resulting in
breezy to windy onshore flow along the Lower Texas coast. Very warm
and rain-free conditions will prevail through the short term period
with very limited moisture in place. Well above normal temperatures
with continue tonight and Tuesday night with low temperatures in the
70s and high temperatures Tuesday ranging from the 80s near the
coast to the mid 90s across the west.

LONG TERM (Wednesday through Monday): A large and very strong 500
mb closed low will move east across the south central Plains
states on Wed ushering a weak and dry cold front through the area
late Wed/early Thurs. The 500 mb dynamics associated with this
closed low will be very substantial for south TX throughout the
day. However...the limiting factor for the RGV will be the
lacking deep layer moisture values associated with this closed
low. The GFS and ECMWF models both keep the better deep layer
moisture values pooled much further north concentrated mainly
over northern TX/OK and NE with only marginal values advecting
over the RGV from the west. So this will likely keep the conv
potential pretty low around the region. However isolated pockets
of conv cannot be ruled out on Wed. Also due to the combination of
the path of the center of the closed 500 mb low and the overall
concentration of the better moisture and instability values...SPC
is maintaining the best chcs for svr conv over the lower Miss
River Valley and the surrounding areas. SPC places the
Marginal/Slgt risk values down into the eastern third of TX for
Wed but leave out the RGV due to the limiting factors mentioned

CAA on Thurs will be pretty limited with a fairly light northerly
low level flow prevailing into Thurs. With drier air moving into
the region expect the daytime heating after the front to warm
temps up pretty quickly. Then the WAA will return pretty quickly
to the region on Fri as the surface winds swing around from the
south to southeast. This southerly flow will prevail on into Fri
and Sat as 500 mb ridging moves back in place over the RGV.
Another large 500 mb closed low then moves into the south central
Plains states Fri night into Sat/Sun which once again maintains
good PVA over the region for this weekend. However deep layer
moisture will again be limited over the RGV on Sat. The longer
range guidance then tries to pool deeper layer moisture just to
the north of the RGV over central TX with this moisture advecting
east quickly late Sun as the 500 mb trough axis lifts out. For
now will leave only slgt chc pops in place for the weekend and
will see how future model runs try to resolve this pattern for the

The model temps for the longer range period have shown more
disagreement starting on the weekend with better agreement evident
through Fri. The GFS is maintaining a cooler bias for the weekend
temps versus the ECMWf which is apparently due to the GFS digging
the closed 500 mb low much further south this weekend in
comparison to the GFS. So will opt for a general blend of the
GFS/ECMWF temps through Day 7.

The pre-frontal wind regime on Wed ahead of the first 500 mb
feature looks to set the stage for a potential heat spike for the
RGV. The latest ECMWF run keeps most temps across the RGF pushing
well into the 90s with guidance for MFE threatening the century
mark. The GFS MEX MOS for MFE is also pushing maxes for MFE on Wed
at 99 degrees. So 100 degrees is not out of the question for the
RGV for Wed.

Overall forecast confidence is a little lower today versus
yesterday due to the stronger model differences for this coming
weekend. Run to run comparisons of the GFS MEX MOS also show
forecast temps for this weekend swinging more wildly.

MARINE (Now through Tuesday Night): Buoy 42020 reported south-
southeast winds around 8 knots gusting to around 10 knots with seas
slightly under 4.9 feet with a period of 7 seconds at 14 CDT/19 UTC.
High pressure and low pressure interaction will produce breezy winds
and building seas along the Lower Texas Coast through the period.
Small Craft Exercise Caution conditions expected to continue across
the Gulf waters tonight with Small Craft Advisories likely on the
Laguna Madre on Tuesday. A Small Craft Advisory maybe be needed on
the Gulf waters by late Tuesday afternoon due to building seas and
stronger winds.

Wednesday through Saturday Night: The PGF ahead of the 1st 500 mb
closed low will strengthen the S-SE surface flow along the lower
TX coastline up near SCA levels. The passage of this system will
then weaken the PGF for Thurs and Fri allowing the seas and winds
to decrease somewhat. The approach of the next 500 mb closed low
will restrengthen the PGF once again for Sat possibly pushing the
marine conditions back up to SCEC/borderline SCA levels.




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