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Area Forecast Discussion (AFD)

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

FXUS64 KBRO 172027

Area Forecast Discussion
National Weather Service Brownsville TX
327 PM CDT Sun Jun 17 2018

...Increasing rain coverage and intensity through the week but how
much remains uncertain...

.SHORT TERM (Tonight through Monday Night): A few scattered showers 
and isolated thunderstorms have developed along the sea breeze this 
afternoon. Coverage is a little better and may continue to increase 
this afternoon as PWATs increase to 2.0-2.2 locally. Convection has 
released a few outflow boundaries, which may spark additional 
development further west along the I69C corridor later this 
afternoon. CAMs indicates this will probably occur by 23z, though 
nothing more than spotty activity is being indicated at this time. 

Aloft, a broad upper level low is currently just southeast of our 
CWFA. At the surface, a broad trough associated with a tropical wave 
will continue to move northwestward toward the TX Coastal Bend and 
points northeast later tonight. This may be enough to fire off some 
additional convection overnight, especially closer to the 
coast...gradually shifting west with time as tropical moisture 
continues to deepen and move westward. 

Models bring the elongated upper level low/inverted trough into our 
region tomorrow afternoon. POPs inherited from the overnight look 
reasonable with the best chances residing along and east of the I69C 
corridor through tomorrow afternoon. Guidance varies a lot toward 
the end of the short term period in terms of a QPF signal. NAM/GFS 
are both drier than CMC/ECMWF/UKMET...with the proximity of the 
elongated upper low confidence is there for at least scattered 
activity overnight Monday night so maintained POPs for this time as 
well with again the highest potential residing right along the 
coast. As for temperatures, not much change is expected, though 
clouds and precipitation will likely be the ultimate deciding 
factor. Given the better precipitation chances closer to the coast, 
think interior will be above average where the rest of the RGV/Deep 
South TX will be around average for mid June.

.LONG TERM (Tuesday through Sunday): Uncertainty reigns (rains?) 
with the first couple of days in the long term period.  Not whether 
it will or won't rain (it will) but how much as well as diurnal 
trends. The somewhat crazy solutions shown by some recent ECMWF 
runs, including the deterministic 00Z run where >20 inches of 
rainfall were forecast over parts of the Valley, have gotten in line 
with the more logical GFS. The GFS, which has been staunchly 
insistent on a slowly west moving shear axis from the western Gulf 
onto the Texas coast and eventually over interior south Texas and 
northern Mexico, was followed more closely on the 12Z runs by both 
the NAM and the ECMWF.  The ECMWF *still* closes off a circulation 
aloft over the Big Bend/Sierra Madre region at the close of the 
week, but this too seems to make little sense given the big picture 
of things and is well beyond the critical concern period which 
remains Tuesday and Wednesday, possibly edging into Thursday. 

The 12Z GFS and ECMWF are much more similar in periodic 0.25 to 0.5 
inch average six hour rainfall Tuesday through early morning 
Thursday...when removing the convective feedback Wednesday afternoon 
across the ranchlands the values are in very good agreeement. 
Interestingly, the NAM now hugs the coast with the deepest moisture 
and QPF through Wednesday morning before pivoting a cyclonic 
vorticity center southwest from north to south across the ranchlands 
and RGV by afternoon. Such a solution would keep all but the coastal 
counties "wet" for a time...which would be far from the welcome and 
locally soaking rains we've been advertising. 

So, for the big picture:  The oozing west nature of the 500 mb shear 
axis and vorticity in an elongated sense looks to be the best trend. 
 By Tuesday, the leading edge should reach the coast and produce 
the expected coverage of rainfall, but the highest totals look to 
favor the upper Texas coast.  By Wednesday, the axis of the deepest 
moisture finally reaches the Lower Texas coast, and this looks to be 
the best day for more widespread QPF stretching farther inland per 
GFS/ECMWF/Canadian model consensus. By Thursday, there are some 
subtle differences with the ECMWF holding the shear axis over the 
region with a stronger bulge of vorticity across the Upper Texas 
coasts with a stronger rainfall signal there, while the GFS weakens 
the forcing while maintaining a decent plume of moisture that lifts 
north and west through the afternoon.

For the sensible weather...and per collaboration with the WPC QPF 
forecasters from Day 2 to 4/5...have generally maintained the idea 
of morning activity underway or quickly developing Tuesday near the 
coast and edging inland through the day.  With a bit more 
uncertainty leaning in from the GFS/NAM, elected to raise 
temperatures a couple degrees...still below guidance consensus. 
Wednesday now offers the best chance to kill off any early morning 
temperature rises and keep maximum values in the upper 80s by early 
afternoon toward the coast and kept this idea with plenty of 
leftover clouds.

For Thursday...still enough moisture for chance to likely rain 
probabilities but expect in and out sunshine to allow temperatures 
to sneak up a couple degrees over prior days.  Did not spend much 
time for the Friday to Sunday period, but trends are running hotter 
with partly cloudy skies and low rain chances...mainly isolated to 
scattered at this point.  How hot may be determined by how much rain 
falls...along and west of US 281/IH 69C may remain in low enough QPF 
to have little influence on deep surface moisture to stop 
temperatures from jumping back to normal.


.MARINE: Marine conditions continue to decline across the western
Gulf this afternoon. Buoy 42002 has indicated swells up to around
8 feet at times with the constant southeasterly fetch in place. 
Buoy 42020 has been a bit slower to respond, however, over the 
last hour there has been a slight uptick in swell there as well. 
Seas will continue to increase through the evening and overnight. 
Will hoist a Small Craft Advisory for rough seas for 0-60 NM 
through Monday Midday. This of course may be extended as 
conditions shouldn't improve through the period. We'll also have 
to consider SCA for both seas and winds by Monday evening as 
pressure gradient increases allowing winds to pick up to SCA 
thresholds beyond 20 NM. Showers and thunderstorm chances will 
continue to increase through the period. Stronger convection will 
likely lead to increased winds and seas locally.

Tuesday through Friday Night...Leading in with propagating high 
and rough seas (due to convection) continues from Monday night 
right through Wednesday as persistent southeast fetch near 20 
knots holds offshore. While the highest seas are likely to 
propagate north/northeast there's enough movement to bring 6 to 8 
foot seas into the outer waters through at least Wednesday morning
before a very slow drop off through the remainder of the period. 
Small Craft Advisory conditions will continue at these times.

The bad news?  Terrible boating/fishing conditions continue right 
into Thursday morning but some slow improvement by Friday 
particularly on Laguna Madre.  Cross fingers for a drier and 
slightly quieter weekend beyond this forecast period.


BAYVIEW/PORT ISABEL  78  88  78  86 /  50  70  70  70 
BROWNSVILLE          78  90  78  89 /  50  70  70  70 
HARLINGEN            77  91  77  89 /  40  60  60  70 
MCALLEN              79  93  77  91 /  30  60  50  60 
RIO GRANDE CITY      78  96  76  95 /  30  40  50  60 
SOUTH PADRE ISLAND   79  84  79  84 /  60  70  80  90 


GM...Small Craft Advisory for hazardous seas from 7 PM this evening 
     to 1 PM CDT Monday for GMZ150-155-170-175.



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