Area Forecast Discussion
Issued by NWS Brownsville, TX
FXUS64 KBRO 220702 AAA
Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
202 AM CDT MON AUG 22 2016
.DISCUSSION...Updated for latest aviation discussion below.
.AVIATION...The 00z BRO sounding showed a pretty elevated PWAT of
2.11 inches. This deeper layer moisture is interacting with some
500 mb PVA moving north along the west side of the ridge axis
centered over the Gulf of Mex to generate some scattered conv over
the region. This convection has moved north of the RGV airports
which should prevent much of an impact to aviation operations. As
the 500 mb ridging over the Gulf of Mex builds further west the
atms over the RGV should stabilize and dry out gradually allowing
for VFR conditions to prevail. Some brief MVFR ceilings may be
possible over the next few hours due to the current deep layer
moisture in place over the region. Will handle this with some
tempo wording through 12z.
.PREV DISCUSSION... /issued 644 PM CDT SUN AUG 21 2016/
DISCUSSION...Updated for latest aviation discussion below.
AVIATION...Satellite and radar images as well as surface
observations indicate convection across portions of Willacy and
Kenedy counties as well as across portions of Zapata county.
Ceilings were near 3500ft at KAPY to near 4000ft at KBKS. Expect
VFR conditions in general to prevail across most of the CWA this
evening into early Monday morning even as low to mid level
moisture remains high across the extreme western portions of the
CWA with a weak 500mb inverted trough/low across northern Mexico.
Expect brief MVFR conditions with showers/thunderstorms through
PREV DISCUSSION... /issued 305 PM CDT SUN AUG 21 2016/
.Flooding Ranchland Rainfall Fading into Monday then Dry and Hot
for the Workweek...
SHORT TERM (Tonight through Monday Night): Models continue on track with
building 500mb ridge westward. This will ushering much drier mid and
upper level dry air cutting of the chance of rain for Monday.
Until than moisture transport continues south to north across South
Texas on the eastern side of a mid level low pressure area over
Nuevo Leon Mexico. A cold front extends along a Houston-Cotulla to
Eagle Pass line with a surface trough axis extending south splitting
our CWA. This trough is acting as a convergence line with most of
the afternoon convection line up along and either side it. With the
atmosphere worked over from last nights MCS, over the Rio Grande
Plains, and a veil of high clouds limiting the max instability
convection remains scattered at best. The southerly low level flow
remains in a moderate range keeping the showers and storms moving
north at a decent clip reducing the threat of flooding. However with
PWATS remaining in the 2 inch range any continuing convection will be
efficient rain makers with rainfall rates of 1 to 2 inches per hour.
for tonight...as the mid level ridge begins to build westward all
synoptic features, front, mid level low, and surface trough should
begin to weaken as they get shunted north or west. Higher moisture
theta-e air should get pushed westward placing the highest pops once
again over the Upper Valley and the Ranchlands. Weakening upper
level divergence and the worked over atmosphere should limit
widespread heavy rainfall but can not rule out localized heavy
downpours. The lower probabilities of widespread torrential rainfall
and the overall waning chances of rain should lower the flash flood
threat so will go ahead and cancel the flash flood watch for Zapata
and Jim Hogg counties.
Monday and Monday night...the aforementioned 500mb, Bermuda high,
works it way into the Western Gulf with a gradual lowering of mid
level drier air. Low level moisture remains rather high so expect
the day to start off mostly cloudy before sufficient breaks develop
in the afternoon. temperatures will remain hot and with humidities
remaining high heat indices to approach 110 degrees mainly along and
east of 69C. Western areas will be a little slow in recovering back
to the century mark due to the wet soils.
LONG TERM (Tuesday through Sunday): As the climatological daily
rainfall begins to climb the late August ladder, the atmospheric
pattern won`t follow...at least not immediately. 500 mb ridge will
stretch back from Florida and the southeast U.S. coast all the way
to South Texas Tuesday and remain in place through Friday with
perhaps a little scouring away of the higher 500 mb heights on
Friday. By later Friday and especially into the weekend, the ridge
axis shifts north into the mid Atlantic/Southeast U.S. and stretches
west to north Texas, which brings a broader easterly flow and
ultimately *some* increase in mean relative humidity. The 12Z GFS
and just arrived ECMWF each show some type of weak easterly wave
action by the end of the period, which will likely be the best
opportunity for the next notable rainfall - though exactly how much
and when/where remain to be seen.
For the sensible weather, then - if you`ve got plans for the beach
or the pool, Tuesday through Thursday, and probably Friday, are your
days. Plenty of sunshine with slowly diminishing inland winds each
day with mainly clear nights. Despite lowering 1000-500 mb
thickness values, persistence and full sunshine argue for high
temperatures in line with recent trends each day, with triple digits
along/west of Highway 281/IH 69C and upper 90s elsewhere each
afternoon except the usual near 90 at South Padre. For Friday, the
GFS and to some degree the ECMWF hold onto the dry air one last day,
particularly in a stripe from the Lower Valley through Upper Valley.
Trimmed mentionable rain to the ranchlands, closer to the possible
arrival of the deeper moisture.
Thereafter, gradually increased rain chances each day through next
weekend while easing down maximum temperatures - though still a
degree or two above consensus, as for now no all-day rain and cloud
situation appears on tap. The best chances will be next Sunday which
could see isolated to scattered rains right through the day.
MARINE: Surface pressure gradient across the
Western Gulf of Mexico to weaken slowly Monday as surface high
pressure moves west and a cold front over south Texas begins to
retreat northward and weakens. A light to moderate southeast
southeast flow is expected Monday with a slow subsiding sea.
(Tuesday through Friday Night)...After a fairly long
period of nuisance wind chop and moderate seas and winds conditions
will be on the up and up for the period. Initial remnant moderate
winds and wind chop Tuesday and early Wednesday will ease down
Wednesday night through early Friday as winds slowly back to the
east/southeast and dip to 10 knots or less beginning late Wednesday.
With no rain in the forecast the only concern will be the wind chop
early; Wednesday will be an improvement on Tuesday and Thursday and
Friday mornings look optimal for now.
HYDROLOGY: Another night and morning of widespread heavy rains
over parts of the Rio Grande Basin will continue to increase water
flows into the river itself between Amistad and Falcon reservoir.
The great news in all of this is Falcon may now see even more of a
bump than expected yesterday, with Texas percentage of the lake
levels rising for the next few days, from the 32.6 percent value
somewhere into the upper 30s or so during the upcoming week. The
heavy rainfall over the Rio San Juan east of Monterrey, Mexico,
should be taken in by both El CUCHILLO and Marte Gomez but the
level of Marte Gomez may rise to a level that requires some
release toward the Rio Grande bordering Starr County. As of this
writing, can`t envision a case where whatever is released couldn`t
be absorbed by the Rio Grande since Falcon won`t likely change
their distribution, but should additional unforecast heavy rains
fall along the Sierra Madre tonight, that might be worth watching.
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