Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 021718 AAB

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1118 AM CST Fri Dec 2 2016

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


.AVIATION...A synoptically driven southeast flow will sustain low
level moisture advection into the CWA...saturating a portion of
the column. Southwest flow aloft into a weak ridge will tend to
provide subsidence and capping. A cut off low is well to the west.
Under this scenario, MVFR ceilings now in place will continue,
with a period of IFR possible this evening and into tonight as
winds decrease slightly.


.PREV DISCUSSION... /issued 600 AM CST Fri Dec 2 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Several challenges in the aviation forecast today.
Light showers have already developed in the vicinity of the Rio
Grande Valley airports, per current radar imagery. Ceilings have
been variable the past few hours but generally are being pulled
down to MVFR in these showers but around 5kft. otherwise.
Shower/rain coverage should taper off some after noontime for the
mid-Lower RGV as a warm front lifts northward away from the area.
Thunderstorms are not out of the question this afternoon but
areal coverage expected to be too sparse to include in TAF`s. SE
winds will pick up to breezy levels with MVFR ceilings persisting
through the afternoon at BRO...but possibly lifting to VFR for a
few hours later this afternoon at HRL and MFE. Continued moist SE
flow beneath a low-level inversion should return MVFR ceilings to
the terminals this evening through the overnight, possibly
dipping to IFR late tonight at MFE.

PREV DISCUSSION... /issued 424 AM CST Fri Dec 2 2016/

SHORT TERM (Today through Saturday):
A warm front will lift northward through the CWA this morning,
then become more-or-less stationary just north of the area through
Saturday. Southwesterly flow aloft will be maintained as an
upper-level trough, currently dropping into the Desert Southwest,
dips further southward and cuts off over Sonora state in Mexico.
Southeasterly winds at the surface will continue to pump in Gulf
moisture at low levels with dewpoints in the mid-Lower RGV rising
into the low 70s today. So the atmosphere will be moisture-rich,
especially today, with precipitable water values running in the
1.6-2.0" range, or in the upper 10% of climatology.

Upshot is that scattered to numerous showers and thunderstorms are
on tap for today, with activity becoming more isolated to scattered
(especially south) on Saturday.  In spite of all this, generally
modest rainfall amounts are expected, with WPC progs indicating a
few tenths of an inch accumulation for most locations, though more
is certainly possible where heavier downpours occur. Limiting factor
seems to be lift/dynamics.  Once the warm front progresses northward
today, best isentropic lift will be north of the CWA.  Although the
entire area will remain in the warm sector, the core of the upper
low will still be far to the west, and no areas of enhanced jet
dynamics are evident.

Storm Prediction Center has placed much of the area...except for
Cameron, SE Hidalgo, and most of Willacy counties...into a Marginal
Risk for severe thunderstorms today.  All things considered, this
seems reasonable.  Fast mid-level flow will yield 0-6km bulk shear
values of 35-45 knots this afternoon and evening as CAPE values
reach 1000-1500 J/kg per BUFKIT forecast soundings.  Low-level warm
advection will form a cap, however, so widespread severe storms are
not expected.  Primary threats from any strong/severe storms would
seem to be wind and marginally severe hail.  Although low
probability, a tornado cannot be ruled out due to good directional
shear with helicity values in the 150-200 m^2/s^2 range.  May need
to monitor this if any storms moving off the higher terrain in
Mexico can become surface-based over northwestern part of the CWA
this evening. Storm motion will be fairly rapid at 30-40 knots from
southwest to northeast.

Not a great deal of change for Saturday, though PoP`s are lower,
especially south, as the warm frontal dynamics nudge even a bit
further north.  Warm and humid conditions, with highs in the low-mid
80s and scattered showers and thunderstorms, will be the rule.

LONG TERM (Saturday Night through Thursday):
With the models still remaining consistent in the development and
cutting off of H5 low into northwest Mexico, same forecast puzzle
remains for Deep South Texas in the long term forecast: when will
the H5 low eject, and what path will it take, when will the
surface cyclogenesis commence, which path will it take when it
ejects, and when will the wraparound cold front sweep through the
region? Models have come together considerably in the past 36
hours, focusing the precipitation on the Saturday Evening through
Sunday time periods. With deep moisture in the parcel and strong
jet dynamics, have no trouble with likely pops with embedded
thunder all of Sunday into Sunday evening. Once the surface low
departs to the northeast Monday morning, the northwest flow sfc
and aloft will draw drier air quickly into the region. Any
leftover rainfall should be complete by noon, with quickly
decreasing cloud cover through the remainder of the day. The high
pressure behind the front moves to the east quickly, so that by
Tuesday, weak onshore flow is already returning. This will be
short-lived, as the next rush of cold air is noted in all models.
Unfortunately, the models are again differing from timing issues,
with the final arrival of this front ranging from Wednesday
afternoon to Thursday Morning. Have gone with a middle ground
between models, depicting a frontal passage Wednesday evening.
This will mean cold lows for dawn Thursday, with minimal diurnal
temperature increases for the day Thursday struggling to reach 60

Now through Saturday: Latest conditions at Buoy 020 as of 250am
CST show winds ESE at 16G20 knots with seas about 6 ft. Small
Craft Exercise Caution Conditions have been maintained for the
Gulf Waters 20-60nm out. These moderate to fresh SE winds will
continue today and strengthen a bit more on Saturday, as the
pressure gradient tightens in response to a surface low
approaching from the west. Seas are expected to build to near 7
ft. over the Gulf waters by Saturday morning, so Small Craft
Advisories may be required. Winds may also increase to SCA levels
over the Laguna Madre in the afternoon.

Saturday night through Tuesday: Agitated marine conditions are
forecast the latter half of the weekend, with low pressure
developing along the immediate coast. Saturday night into Sunday,
the surface trough will be lurking inland, keeping marine winds
from the southeast around 15 knots, with seas generally running 6
to 7 feet. Around midnight Monday, the surface low will be
deepening just off the coastline, and sweeping off to the
northeast. This will bring brisk northwest winds of 15 to 20 knots
by sunrise Monday, continuing through the afternoon. This will
continue to produce swells around 6 feet for the Gulf waters, with
a likely shortening of the period as the northwest Gulf
experiences some significant churn. High pressure quickly moves
into the northwest gulf Monday night, with much lighter winds and
seas relaxing to 2 to 3 feet during the day Tuesday.




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