Scientific Forecaster Discussion
fxus64 kbro 160540 aab
Area forecast discussion...updated
National Weather Service Brownsville Texas
1240 am CDT Mon Sep 16 2019
Discussion...updated for latest aviation discussion below.
Aviation...VFR conditions expected to prevail through most of the
taf period again with very similar conditions to yesterday. An
increase in showers and thunderstorms is expected by mid to late
this morning and through this afternoon from east to west. Brief
MVFR to potentially IFR conditions are possible during any
thunderstorms with locally heavy to torrential rainfall. Light
onshore northeast and easterly winds continue, with higher gusts
possible near any convective activity.
Previous discussion... /issued 608 PM CDT sun Sep 15 2019/
Discussion...updated for latest aviation discussion below.
Aviation...convection is waning across deep south Texas and the
Rio Grande Valley with the loss of the heating of the day. Bro and
hrl may encounter additional convection for a few hours around
sunrise Monday as a low pressure system in the middle layers of
the atmosphere continues to March west. All of the aerodromes
stand an equal chance of convection on Monday beginning mid-
morning as the low continues to approach and daytime heating kicks
in. Overall, VFR with a tempo MVFR is a sure bet through the next
Previous discussion... /issued 316 PM CDT sun Sep 15 2019/
Short term (now through Monday night): radar trends so far this
afternoon have mainly featured scattered shower activity coming
off the Gulf of Mexico waters. Thunderstorm activity has been
limited so far but MLCAPE values of 1000 to 1500 j/kg will support
possibility for isolated thunderstorms for the rest of this
afternoon and into this evening.
As the upper level low continues to slowly move westward across the
western Gulf of Mexico, expect for the moisture to remain elevated
across the atmospheric column through the short term period with
precipitable water values ranging from 2.00 inches to 2.50 inches
along the coastal sections to 1.75 inches to 2.00 inches further
west. Showers and thunderstorms remain a mainstay in the forecast
with the best coverage expected along the coast. Rain chances do
look to decline a tad overnight, especially across the western
counties. The 12z European model (ecmwf) run has come in a tad drier but general
model consensus supports the idea that the greatest rainfall will
concentrate along the coast. The latest quantitative precipitation forecast from the wpc shows
around 1 to 1.5" for the coast through 00z Wednesday with lower
totals further inland. Localized higher rainfall amounts of up to
around 3 inches will be possible with any heavier showers or
thunderstorm activity. Minor flooding could be possible in these
locations that do receive any higher localized rainfall totals.
Latest wpc excessive rainfall outlook has included the coastal
regions within a marginal risk from 12z Monday through 12z
Wednesday for rainfall to exceed flash flood guidance.
Continued to trend the temperatures near or slightly below average
levels through the short term as a result of the areal cloud and
rainfall coverage. Despite that, I did decide to increase the
afternoon highs on Monday by a degree or two as the 12z European model (ecmwf)
trended warmer due to its slightly drier output on its latest run.
Long term (tuesday through sunday): forecast model solutions (i.
E., 12z GFS/12z NAM/00z ecmwf) have come into pretty good
agreement for the Tuesday-Wednesday period. Still looks like this
will be a continuation of our current wetter period, under the
influence of a mid-level low. A drying trend then appears to take
hold for Thursday through Saturday, before uncertainty returns by
later next weekend regarding another inverted trough or low in the
Gulf of Mexico.
Left inherited pop's pretty much alone for Tuesday, though do have
some concerns about heavy rainfall/localized flooding. Per
forecast soundings, deep tropical moisture will still reside over
the area. Mid/lower-level troughing or low should be in the
process of filling, but still nearly overhead. Low-level feature
should provide some convergence, whereas mid-level low adds weak
steering flow and instability. This could make for some slow
storm motions and torrential rainfall rates. Of course, flooding
potential depends on how much rain falls between now and then.
For Wednesday, what is now a strong model consensus drifts the
500-mb low, along with deeper tropical moisture, northward toward
the coastal Bend. Drier air at lower to mid-levels is progged to
filter in from south-to-north (somewhat counter-intuitively), but
should be slow enough to retain chance of showers and storms for
Heading into Thursday and Friday, some discrepancies in the
details between GFS and European model (ecmwf) solutions, but upshot for US is
largely the same. GFS continues to gradually fill the low as it
meanders nearby, whereas European model (ecmwf) keeps it more intact but moves it
into North Texas...with a ridge axis over our neck of the Woods.
Both models show lesser moisture quality with precipitable water back near climo
(1.7-1.9"), so will have to rely on diurnal effects to generate
slight chances for rain, mainly east of Hwy. 281/I-69c, daily.
By next weekend, GFS places the subtropical high back over
northern Mexico and Texas, with moisture largely limited to the
boundary layer. Hung onto slight chance pop's for coastal areas,
but even these may disappear in a few more forecast cycles.
European model (ecmwf) brings another upper-level trough or low
(again with mid-latitude origins, as with the current system)
into the western Gulf, but still highly uncertain of this with no
sign in the GFS.
Temperature-wise, should be a slow increase in afternoon temps
back to a few degrees above normal by mid-week. Blends want to
drift temps back a couple degrees lower into next weekend, but
will fight this trend, in line with previous forecaster, as there
doesn't seem to be a good reason for this, particularly with the
ridging aloft and relatively dry atmosphere.
Marine (now through Monday night): light to moderate breezes from
out of the east-northeast and moderate seas of 3 to 5 feet will
continue through tonight and much of Monday. The pressure gradient
will weaken into Monday night as a weak surface low approaches the
region from the east. This will help for the wind flow to become
southerly and lighter. Seas will decline slightly as well as a
result. Expect scattered to numerous rain showers and isolated to
scattered thunderstorms to remain in the forecast throughout the
short term thanks to elevated moisture levels ahead of a 500 mb
low approaching the area. Some of the stronger thunderstorms
could produce locally strong winds and elevated seas.
Tuesday through saturday: light-to-moderate southerly flow takes
hold for Tuesday and Wednesday, possibly reaching low-end small
craft exercise caution (scec) levels Wednesday afternoon on the
Laguna Madre and near-shore waters. Flow backs slightly to the southeast
for Thursday through Saturday but remains at 10-15 knots. With the
modest flow and no tropical cyclones expected in the Gulf,
combined seas should stay in the 2-4 ft. Range through the period.