Area Forecast Discussion
Issued by NWS Brownsville, TX
FXUS64 KBRO 281735 AAB
Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1235 PM CDT SUN AUG 28 2016
.DISCUSSION...Updated for latest aviation discussion below.
.AVIATION...Light generally easterly flow continues across deep
south Texas. The seabreeze is just starting to drift inland, with
modest northeast winds expected once the seabreeze passes. The
seabreeze may not reach KMFE this evening, so winds will remain
lighter there all afternoon. Low clouds are not expected
overnight, but some high cirrus may begin to move into the region
from storms to the north. There is an increased chance for showers
and thunderstorms during the day tomorrow as moisture from the
north begins to return to the region.
.PREV DISCUSSION... /issued 631 AM CDT SUN AUG 28 2016/
DISCUSSION...Updated for latest aviation discussion below.
AVIATION...No changes were needed to the previous suite of TAFs.
Light and variable winds with a few clouds this morning will
increase to moderate levels from roughly the east this afternoon
as the seabreeze boundary moves from east to west within Deep
South Texas and the Rio Grande Valley. A return to light and
variable winds is anticipated tonight. VFR remains likely through
the next 24 hours.
PREV DISCUSSION... /issued 429 AM CDT SUN AUG 28 2016/
SHORT TERM (Today through Monday): Closed 500 mb low pressure is
evident on water vapor satellite imagery spinning over extreme
southeast Texas. Over the course of the short term forecast period,
this feature will meander slowly south over the Upper and Middle
Texas Coasts. Aided by daytime heating and an increasing amount of
deep tropical moisture, isolated showers and thunderstorms today and
tonight will become more scattered tomorrow across Deep South Texas,
the Rio Grande Valley, and the adjacent coastal waters. With
additional tropical moisture arriving on Monday, a few cells may
produce locally heavy rainfall - but with uncertainty on coverage
and model/official QPF values still somewhat paltry have gone with
moderate wording for now. Daytime high and overnight low
temperatures will be at above normal levels today and tonight, but
should be tempered to more near normal levels for Monday due to
increased cloudiness and the opportunity for rainfall.
LONG TERM (Monday Night through Saturday): Somewhat complicated
forecast for Monday Night through Wednesday Night in terms of
specific, but the big picture looks fairly stable. That big
picture continues in the form of generally welcome rainfall
(hopefully) for areas that need it most, including most of the
populated Rio Grande Valley who are in moderate drought levels
after missing both torrential events that soaked the ranches of
Deep South Texas on the 13th and again early on the 21st.
Unfortunately, confidence on higher average rainfall is best
across the King Ranch who was one of the areas soaked on the 21st.
Beyond Wednesday night, the summer heat returns once again.
There are some subtle differences among the GFS/ECMWF this
morning for the potentially wet period. The ECMWF moves the core
of the upper disturbance slowly onshore along the coastal bend by
late Tuesday, allowing the deepest of the tropical moisture to
ease onto the entire south Texas coast Tuesday night through at
least midday Wednesday. The GFS hangs the disturbance out just
east of the Coastal Bend through Wednesday before dissipating it.
Should the GFS win out, deep northerly flow would still favor
spokes of rainfall but amount, intensity, and timing would be in
question - and the latest deterministic guidance has followed suit
with lowered rain chances and QPF for Monday night through
Wednesday night. Should the ECMWF solution rule the period, its
improved vertical motion fields and associated higher mean RH
through the column would produce better coverage and higher rain
totals in all areas. Due to the high uncertainty, elected to due a
rare model blend and lean into higher rain chances (coverage)
Tuesday night and Wednesday. The one exception is the King Ranch
and Brooks County hunting ground, where proximity to the energy,
which has been well advertised over several days, should bring the
best opportunity for coverage.
For now, rolled a broad brush ~1 inch average QPF for these areas,
with a fairly sharp drop toward the Lower/Mid Valley, where
three day averages are closering to 0.25 to 0.5". One thing to be
aware of are the showers and a few storms rolling from *northeast*
to southwest Monday night through Tuesday, and perhaps Tuesday
night and Wednesday. With the Gulf at peak temperature and
assuming atmosphere is as moist as it will get during the week,
rain could continue off and on day and night (i.e. no diurnal sea
breeze considerations) from Monday night through Wednesday,
perhaps into Wednesday night. This will break the streak of
perfect beach weather, with early week visitors playing "dodge
`em" with brief downpours and plenty of leftover mid and high
clouds between cells.
As far as temperatures...it`s always a trick when shower/storm
coverage increases through day and night, and/or residual cloud
cover can hold values down. For Tuesday, when the moisture isn`t
as deep, there should be enough sunshine before things really
percolate to bring highs into the low to mid 90s - below average
and below the triple-digits that have dominated inland areas the
past two months. Same goes for Wednesday, though afternoon
temperatures may struggle to reach 90 from Falfurrias to Sarita
and points north. Calm winds and some stabilization of the
atmosphere especially inland will allow minimum tempeartures to
bottom out near average (low to mid 70s except upper 70s to 80
near the coast and in urban centers).
By Wednesday night and especially Thursday, the upper ridge takes
control as whatever is left of disturbance 99L finds the weakness
between the western (Canicula) ridge and a ridge strengthening
over the Bahamas to exit stage right across the northeastern Gulf.
Drier air will gradually drop rain chances, which are likely gone
altogether by the weekend as southerly flow increases solidly and
the atmosphere heats up into early August territory. The southerly
flow is enhanced by fairly strong southern Plains surface low,
while at the same time the 500 mb ridge "pancakes" across south
Texas through north central Mexico and could cut off the Monsoon
for the U.S. Southwest. For this reason, jacked up the afternoon
temperatures back into triple digits for the mid/upper Valley
beginning Friday and surging Saturday and Sunday.
MARINE: (Now through Monday)...Buoy 42020 reported Southeast
winds around 4 knots with seas of 2 feet with a period of 7
seconds at 03 CDT/08 UTC. A weak pressure gradient over the
western Gulf of Mexico will produce generally light winds and low
seas for the Lower Texas coastal waters during the forecast
period. Small Craft Exercise Caution and Small Craft Advisory are
not likely to be needed. The approach of a disturbance in the
middle layers of the atmosphere will lead to a gradual increase in
showers and thunderstorms along the Lower Texas Coast.
Monday Night through Thursday: Other than the nuisance showers and
storms that will become somewhat numerous especially along/north
of Mansfield but still decent coverage elsewhere Monday night
through Wednesday night, light winds and slight seas will
dominate early. Wednesday through Thursday could see a swell up to
3 feet or perhaps a shade higher pending any strengthening of the
eastern Gulf low, but with uncertainty so high this far out capped
the seas at 3 feet for now, with a later decrease.
CLIMATE...July and August 2016 are amazingly paralleling the same
months in 2009, with most Valley locations ranking among the top 5
hottest thus far, with today (Sunday) keeping the ball rolling.
By month`s end, even with the downturn in temperatures, most
locations will settle into a top 5 to top 10 warmest all time
August. Details coming in about a week. Perhaps more interesting
is the *timing* of the end of month rain relief. After the 2009
swelter, the final three days brought 0.57 inches, 1.44 inches,
and 0.25 inches to Brownsville, Harlingen, and McAllen
respectively. Last year, another top 5 hottest August (for several
sites) ended with 1.65 inches in Brownsville and 1.49 inches in
Harlingen. Monday, August 29th 2016 is the beginning of - you got
it - another three-day stretch of unsettled weather where rainfall
numbers may well be similar at Valley Airports. Stay tuned for the
final numbers when the dry weather returns by the first of
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