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Area Forecast Discussion (AFD)

Issued by Brownsville/Rio Grande Valley, TX (BRO)

                            
372 
FXUS64 KBRO 262343
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
643 PM CDT Sat May 26 2018

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

&&

.AVIATION...VFR expected to predominate at the RGV terminals for
the next 24 hours. High-pressure ridging upstream will continue to
shift closer, further drying out the atmospheric column. RAP 
BUFKIT soundings suggest that an MVFR-level cloud deck will form 
tonight, but other guidance leaning away from it. Will retain FEW-
SCT 015 in TAF's but not go with a prevailing CIG due to lower 
confidence. Winds will generally be light SE-S, becoming more ESE 
Sunday afternoon as the sea-breeze front progresses inland.


&&

.PREV DISCUSSION... /issued 321 PM CDT Sat May 26 2018/ 

..The HEAT of the moment...

SHORT TERM (Tonight through Sunday Night): Upstream ridging aloft 
remains in place over west Texas and will shift slowly east toward 
central Texas through the next day or two, further solidifying local 
stability. A deep upper low will hover over the Southwest United 
States during the short term, while in the GOMEX Subtropical Storm 
Alberto moves north from the Yucatan Channel and across the Gulf 
toward the north central to northeast Gulf coast during the next two 
to three days. Across the west Gulf and the CWA, high pressure will 
remain in control at the surface, with light to moderate southeast 
to south winds and mostly clear skies. Temperatures will continue to 
be above normal, particularly during the day, and the forecast will 
feature high temperatures a couple of degrees above guidance. The 
forecast values will dance around record numbers, with the 98 degs 
forecast at Brownsville, the 100 degs forecast at Harlingen, and the 
103 degs forecast at McAllen for Sunday comparing to 99, 100 and 107 
degree records, respectively. These values will combine with ambient 
humidity to produce mid afternoon heat index values of 103 to 108 
degrees.

LONG TERM (Monday through Saturday):  The heat that is already in 
place builds further through the week, peaking on Wednesday and 
Thursday as position and strength of the early onset of the 500 mb 
"La Canicula" ridge centered from Coahuila through the Texas Big 
Bend combines with lower surface pressures over west Texas...forced 
a little bit by a slow moving 500 mb trough easing across the 
intermountain west and into the U.S. Four Corners region by early 
Friday.  In my 10+ years tracking such overview parameters as 1000-
500 mb thickness and raw 850 mb temperatures, never have I seen a 
587+ decameter value over McAllen and nearly 590 decameter (590!) 
over the Rio Grande Plains (GFS).  The ECMWF is just a hair behind, 
and 850 mb temperatures *on the Rio Grande Plains* approach 28 or
29C (Wednesday). Combine this with deep dry air, near solstice 
sun angle, and rapidly drying top soils and super-adiabatic 
possibilities and it would not surprise to see 110+ for some areas
in southern Starr/Zapata Wednesday, perhaps beginning Tuesday and
continuing Thursday and perhaps beyond. The ridge shifts slowly 
northward by next Friday/Saturday which raises surface pressures a
bit over west Texas and thereby slightly lowers the 1000-500 mb 
thickness and 850 temperature, but not appreciably enough to put a
dent into this impressive late May and early June heat wave.

So, what does this all mean? First, given the parameters above, have 
raised temperatures in all areas 2 to 5 degrees over previous 
forecast, with 104 to 107 along and west of US 281 Tuesday through 
Friday, only slightly lower on Memorial Day and next Saturday. The 
good news (if you can call it that) is abundant dry air above the 
few cumulus bases will mix down given expected mixing heights at or 
above 8000 feet, so afternoon humidity in the 20s to lower 30s will 
keep "feels like" temperatures close to the actual temperatures, 
with perhaps an exception late next week/weekend across the more 
populated RGV where near 110 apparent temperatures may start to 
appear.

The aforementioned lowering surface pressures over west Texas will 
help enhance the afternoon sea breeze, which will gradually progress 
from near the coast into the mid/upper Valley by late afternoon and 
bring slight cooling after the highs are reached.  Until then, 
however, 10 mph or lower winds will keep ventilation low.  Bottom 
line?  A pretty remarkable, searing heat wave more similar to July 
2009 and 2015-2017 than the end of May and start of June.

One final but important item:  The impact of Alberto on Lower Texas 
shoreline.  This afternoon's "jump" reformation of the center and 
latest model trends suggest a far eastern Gulf of Mexico track in 
the short term, before some "curling" into the upper low that 
captures the system on Memorial Day.  With the most strengthening 
expected after the center passes our latitude, and landfall 
somewhere closer to the western Florida panhandle than Louisiana, 
swell impacts may be a bit more limited.  While there should be some 
tidal run up peaking at high tide Monday morning, it may only be 
half to two thirds to the dune line at narrow beaches, more a 
curiosity than anything else.  Rip currents, however, which are 
already at moderate levels may amp up a tad and due to the huge 
crowds on a hot Memorial Day weekend a Rip Current Statement may be 
needed this evening which could extend into Memorial Day.

MARINE (Tonight through Sunday Night)...Light to moderate 
southeast to south winds and low to moderate seas will continue 
along the lower Texas coast due to a persistent weak local 
pressure gradient over the northwest Gulf. Subtropical Storm 
Alberto will move north from the Yucatan Channel toward the north 
central to northeast Gulf coast over the next couple of days, 
posing no threat to the lower Texas coast.

(Monday through Thursday Night)...Based on the eastward nudging 
of the new center for Alberto, landfall Monday will likely be a 
bit farther east as well (and sooner) hence there may not be ample
time for significant swell to generate seas any higher than 4 
feet. Blended close to TAFB in the offshore waters on Monday 
before whatever enhancements come from Alberto recede.

Thereafter, gradient picks up a bit by Tuesday and especially 
Wednesday and Thursday, but mainly the typical diurnal marine layer 
arguments for the Gulf.  Could edge near Caution (15 to 20 knot) 
territory Tuesday night through Thursday night bur for now kept just 
below. Laguna Madre may well see caution afternoons with the 
gradient enhanced by the sea breeze Tuesday through Thursday. 
Otherwise, moderate seas over the Gulf with some increase in wind 
waves, perhaps to 3 feet, are expected especially Wednesday night 
on. 

CLIMATE...Been flirting with records in recent days and the 
expected heat of the atmosphere upcoming will challenge more. The 
following are records for McAllen/Miller and Brownsville May 28 
through 31:

McAllen/Miller
May 27   107 in 1973
May 28   101 in 2017 
May 29   102 in 1975 
May 30   103 in 2004 
May 31   106 in 1967

Brownsville
May 27   99 in 1973
May 28   98 in 1919
May 29   98 in 1922
May 30   100 in 1919
May 31   100 in 2005

&&

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

$$

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