Area Forecast Discussion
Issued by NWS Brownsville, TX

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244
FXUS64 KBRO 271146 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
646 AM CDT Tue Jun 27 2017

.DISCUSSION...Updated for latest /12z/ aviation discussion below.

&&

.AVIATION...Little change to the current forecast as day breaks
across the Rio Grande Valley. Forecast for Vicinity showers (VCSH)
is on track for the first few hours of the morning, and despite
struggles for the rain to survive its trek onshore for the hours
between 3 and 6 AM, latest radar shows enough of a band set to
move into Brownsville`s airspace by around 7-730 AM and with
enough reflectivity (up to 45-50 dbz in small pockets) to add low
MVFR visibility in tropical-type showers. Same for
Harlingen/Valley, just starting an hour later. Slow movement and
situation favoring the coast will save McAllen from all but a
possible two hour window for TEMPO MVFR ceilings soon after
sunrise, similar to what occurred Monday morning.

The somewhat difficult part of the forecast follows for late
morning through mid afternoon. Confidence remains low for
forecasting direct restrictions at terminals from thunder based on
reasoning provided in discussion below and in earlier discussions
about deeper moisture and forcing being best offshore. With this
in mind, did not include additional TEMPO groups for Brownsville
and Harlingen and let the prior forecast ride, which includes a
clear-out this evening with light to near calm winds, extended
through daybreak Wednesday.

For McAllen/Miller, did add a PROB30 group for any late developing
thunder around 4 or 5 PM; did not want to go with TEMPO based on
40 percent rain chances and uncertainty on a cell ending up at the
field, but this may need to be updated on the fly in the window
between 19Z and 22Z with the next full forecast package. At
McAllen, too, the weather will quickly clear out at or before
sunset followed by a steady state VFR forecast with light winds
thereafter.
52/BSG

&&

.PREV DISCUSSION... /issued 424 AM CDT Tue Jun 27 2017/

SHORT TERM (Now through Wednesday): After some good fanfare for the
potential for more widespread rainfall than even Sunday produced,
Monday was a bit of a letdown, as precipitation coverage was largely
scattered (30-40 percent coverage) and area-wide average totals were
actually lower than those on Sunday, with just a few small pockets
of radar estimated 1-2 inches. Thunder was also minimal, a sign of
the lack of organized convection across Deep South Texas and the Rio
Grande Valley. One of the big picture reasons for the failure to
launch may have been as simple as the position of the mid level
trough, which remained over the western Gulf rather than sliding
inland from east to west, which would have brought the more
favorable east side of the mid level trough axis onshore. Instead,
atmospheric flow up to 500 mb was more *northerly* rather than
southerly; unless associated with a vigorous westward moving wave or
tropical cyclone, such flow is unfavorable here for big summer
convection.

For today, there is little change to the position of the trough
axis. The GFS is clearest on this situation, but even the recently
arrived ECMWF which has a vorticity center rotating northwest to
southeast moving through southeast Texas favors the deeper moisture
associated with the better forcing over the Gulf waters mainly east
of the Coastal Bend through southeast Texas today. By tonight and
especially Wednesday, all models hold or shift the deeper moisture
into the west central Gulf and edge deeper drying into parts, if not
all, of Deep South Texas.

With all this in mind, the sensible weather forecast is still a bit
unsettled today with some improvements on Wednesday - with diurnal
quieting once again tonight. By unsettled, we`re talking about
taking the umbrella along but not canceling outdoor activities as
the duration of rain will be short in most cases even as
precipitation tends to move slowly - there`s just not enough
forcing to maintain anything more than pop and and rain-out type
showers. As for thunder, we can`t rule out any but lightning
storms will be more isolated than scattered. Temperatures today
will be very close to those on Monday, though did not go as high
as 98 in McAllen as boundaries and cloud cover should be enough to
keep more in line with mid 90s.

For tonight, could see leftovers this evening across the
ranchlands/Rio Grande Plains; thereafter, quiet conditions except
for another round of scattered showers across most of the Gulf
waters that will struggle to edge inland - so removed mention for
land areas and kept isolated-scattered over the water.

Reduced rain chances a hair on Wednesday, holding onto mentionable
over the lower-mid Valley to the Gulf.  GFS time-height sections
show quite a bit of drying above 700 mb for the upper Valley (RH
below 10 percent on average) which would kill of convection with
northerly flow all the way down to 850 mb.  That dry air slides east
toward the coast through the day and would not surprise to see any
convection end over all but the Gulf waters, including the beach -
but still enough moisture to make case for isolated to scattered
showers and a few storms through part of the afternoon.  With more
of a partly sunny vs. mostly cloudy sky the GFS temperature numbers
look best, which was close to the inherited forecast so just a
slight bump in temperatures.

LONG TERM (Wednesday Night through Monday): The forecast reasoning
for the long term portion of the forecast remains persistent and
stable. An elongated mid-level weakness over the northwest Gulf of
Mexico will gradually be replaced by 500 mb high pressure centered
over both the Bahamas and the Gulf of California that will build
west and east, respectively, and come to dominate the weather
pattern over the entire Gulf of Mexico, Deep South Texas, and the
Rio Grande Valley. Isolated to scattered showers and thunderstorms
over inland portions of the BRO CWFA and the adjacent coastal
waters Wednesday night and Thursday will transition to dry weather
for the remainder of the forecast period as the aforementioned
synoptic pattern comes to fruition. With diminishing cloud cover
and precipitation chances ending, near normal temperatures early
on will become well above normal.

MARINE (Now through Wednesday): The action is only in the
weather part of the forecast, where today could still see numerous
showers and a few thunderstorms and kept this idea over the
waters beyond 20 nautical miles this morning where activity
underway and forecast to continue based on deeper moisture
arguments above. Elsewhere, scattered showers will prevail most of
the time, with some dips near the coastline this afternoon
through midnight or so before redevelopment of scattered coverage
which will hold into most of Wednesday. Stronger cells could
produce gusts and brief choppy waters. Otherwise, virtually no
pressure gradient through the period will lead to light east to
northeast flow and seas, which have fallen to 2-2.5 feet, will
continue in that ballpark through at least tonight. Could see
slightly higher seas Wednesday but still well below any
highlights.

Wednesday Night through Monday: Small Craft Exercise Caution
and/or Small Craft Advisory winds and/or seas continue to be
expected through most of the long term portion of the marine
forecast as high pressure over the Gulf of Mexico interacts with a
low pressure system that will drop into the Texas Panhandle. More
favorable marine conditions are likely during the final 48 hours
of the forecast period as the pressure gradient weakens.

&&

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

$$

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