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Area Forecast Discussion (AFD)

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

FXUS64 KBRO 150009 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
609 PM CST Mon Jan 14 2019

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


.AVIATION...Lower ceilings and visibility will persist the next 
24 hours with a low level inversion in place, and overrunning and
isentropic lift squeezing light rain/drizzle out of the moist 
boundary layer. Ceilings should be more limiting than visibility 
tonight, hovering between low MVFR and high IFR. Light rain 
chances will increase tomorrow, especially near the coast, as a 
coastal trough enhances low level convergence. Ceilings will trend
down to a more certain IFR by Tuesday, with lower visibility in 
light rain and fog adding to the IFR certainty later in the day.


.PREV DISCUSSION... /issued 310 PM CST Mon Jan 14 2019/ 

SHORT TERM (Now through Tuesday Night):  The chill has arrived 
courtesy of a minor but notable surge of northerly flow as the 1030+ 
mb ridge continued to nose south/southwest into the Rio Grande 
Valley and the lee of the Sierra Madre this afternoon. Somewhat mild 
morning temperatures along/east of US 281 at sunrise fell a few 
degrees, then rose back to their morning lows by mid afternoon - 
while the northern ranchlands have budged little only moving to 
around 50 or so after lows in the mid to upper 40s.

The big picture for the next two days remains the same: Isentropic 
upglide begins in the 850-700 mb layer overnight, which will help 
patchy light rain break out toward the Rio Grande mainly west of 
McAllen before daybreak, with some nuisance drizzle more likely 
elsewhere except mainly dry across the northern ranchlands which 
will take longest to saturate. Approach of dampeniNg but still 
holding together 500 mb short wave Tuesday will enhance the lift 
and fill atmosphere with moisture, enough for widespread 
occasional light rain for most of the day. The wave scoots into 
southeast Texas and the northwest Gulf Tuesday night, but leaves a
tail of shear vorticity behind over the border region which will 
be enough to continue the yucky light rain regime for the first 
half of the night across the entire Valley, fading to the Lower 
Valley by daybreak Wednesday. QPF will be on the modest side but 
still welcome...when all is said and done roughly 0.10 to 0.30 
inches of liquid will fall.

Forecast challenges are two-fold, though rather minor in the grand 
scheme of things. First, temperatures.  The NAM-12 has been lights 
out in chilly rain/drizzle events this winter.  With weak coastal 
troughing expected to develop, and combined with shallow cool air 
locking in under the inversion, expect a north to even north-
northwest flow to dominate mainly along/west of US 77/IH 69E.  We're 
already seeing typical cold air damming low clouds to the spine of 
the Sierra Madre, and the surface ridge is nosing south-southwest as 
the NAM has hinted at into this area.  That said, the surface ridge 
source and movement is not in the true "classic" fashion...rather, 
it is sliding south from east central Texas and the southern Plains. 
With cold air advection more of an ooze than a slam, this should 
allow low level easterly flow off the deck to nudge milder air 
toward US 77 especially Tuesday and Tuesday evening. 

Therefore, expect a fairly significant range of afternoon 
temperatures across the Valley, from the lower 50s in Starr and 
western Hidalgo to the mid to upper 50s in western Cameron, and 
likely low or even mid 60s from Brownsville-Bayview-South Padre. On 
Tuesday night, the coastal trough edges northward then dissipates; 
weak backside north/northwest flow will continue but with no 
addition advection expect surface temperatures to dip only slightly 
from Tuesday afternoon highs.  With dewpoints sliding up to meet 
these temperatures, and winds going slack (below 5 knots), patchy 
fog is a good bet sometime after midnight Tuesday night, if not 
sooner in locations where the "magic" value of mid to upper 50s 
temperatures are locked in. 

Would not surprise if dense fog covers many areas - a result of the 
inversion near the ground, mild temperatures especially along/east 
of US 281, and wet top soil. Will cross that bridge tonight and 
Tuesday, but will mention in the Hazardous Weather Outlook. Finally, 
the NAM does develop a fairly notable coastal trough/front which 
could enhance low level lift and push QPF over 0.5 inches in banded 
moisture along it.  Not going with that just yet, but something to 
watch.  Favored areas would be from Brownsville to Harlingen and 
points northeast from there.

LONG TERM (Wednesday through Monday): After some lingering
effects of the coastal trough for at least the first part of 
Wednesday, well above-normal temperatures are expected for 
Thursday and Friday. A strong cold front is forecast to move 
through the area sometime on Saturday, ushering in colder 
temperatures than we've seen in several weeks. For Wednesday, have
kept previous forecast trend of gradual drying of the atmospheric
column and decreasing PoP's from west-to-east as the day goes on.
NAM is quicker to dry things out, with GFS/ECM hanging on to more
moisture along the coast. Have compromised and ended PoP's more 
quickly once we reach Wednesday afternoon. Also some signal in the
guidance for patchy fog early (as mentioned in short-term), so 
will keep that going into mid-morning in weather grids.

Quasi-zonal flow aloft, with SSE flow at the surface, takes hold
for Thursday. This should allow afternoon temps to jump to the
upper 70s-near 80 in most areas. Probably a couple of degrees
warmer yet on Friday as upper-level flow gradually backs in
response to a trough digging into the Rockies. This trough will
develop a lee cyclone at the surface, increasing the southerly
breezes for us.

The "main event" then arrives sometime on Saturday, with still
some subtle differences in handling of the front(s) between the 
GFS and ECMWF. What is pretty clear is that the airmass behind the
front will be of more Canadian/Arctic origin, with GFS 5-wave 
progs showing deep H5 low over Hudson Bay coupled with ridging 
over the Yukon/Alaska region allowing the cold air to spill 
southward. 00Z run of ECMWF suggesting somewhat of a split in the
trough, which allows more WSW flow at mid-levels to be maintained
at H5 through part of the day on Saturday. At the surface, this
would seem to translate to a Pacific frontal passage early on
Saturday. This followed by the "real" cold blast Saturday 
evening. 12Z GFS solution has the main Arctic front overtaking the
Pacific front and blasting on through the RGV by mid-day. Both 
have similar timing on the initial wind-shift to NW, just the 
difference in the when the much colder airmass arrives, with some 
implications for the temp forecast that day. At any rate, strong 
cold air advection continues on through Saturday night. By 12Z 
Sunday, raw model 2-m temps are 40-45F in the GFS and a couple of 
degrees colder in the ECM. Though there is a some hint that the 
core of the cold air will pass further easter (nearer the 
ArkLaTex), will still need to keep an eye on forecast wind chill 
temps which, at this point, look to be at least flirting with 
advisory criteria. Much drier air will also come in behind the 
front, so may have to keep an eye on fire weather parameters as 
dewpoints crash into the 30s and 40s during the afternoon on 

After a chilly day on Sunday, with highs generally in the 50s, it
looks like Sunday night/early Monday morning will be the coldest
time, as the surface ridge axis is forecast to lie nearby. Light
winds and clear skies suggest good radiational cooling conditions.
Have gone along with trend of lower temps in the blended guidance
for Monday morning. Northern tier of counties could be flirting 
with a freeze, though of course this is still almost a full week 

Some uncertainty on what will happen next Monday, as GFS has a
transitory ridge aloft passing over and surface winds veering
onshore, whereas ECM pinches off a mid-level low over NE Mexico.

MARINE (Now through Tuesday Night):  Moderate winds and seas will
be the rule as stiff but not altogether tough northeast flow this
afternoon veers east overnight and holds from the east on 
Tuesday...before edging southeast and south mainly across the 
offshore waters beyond 20 nm Tuesday night. The bigger nuisance 
may be on and off showers Tuesday and especially Tuesday night. 
Laguna Madre conditions will feature a fresh north/northeast flow 
tonight, veering east/northeast Tuesday at lighter speeds and 
likely going slack overnight Tuesday night. With light rain and 
fog in the mix for the early morning fishers on Wednesday, not the
best situation for clear boating.

Wednesday through Sunday: Generally favorable winds and seas should
hold from Wednesday into Friday morning, though the aforementioned
light rain and possible fog for much of Wednesday may make things 
otherwise unpleasant. Light-to-moderate southerly winds and 
moderate Gulf seas in the 3-4 ft. range will be the rule for this 
period. Southerly winds increase from Friday into Friday night as 
a surface low deepens in the lee of the Rockies. Small Craft 
Exercise Caution-level winds and seas are likely on the Gulf by 
later Friday night, with Small Craft Advisories possible.

A strong cold front is expected to reach the Lower Texas Coastal
waters on Saturday, with strong north winds creating very
hazardous conditions. Confidence in gale conditions (gusts, at a
minimum) is beginning to increase for later Saturday into early
Sunday. Seas could build as high as 10-12 ft. in the outer Gulf




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