Central Gulf Coast
~~~~~~~~~~~~~~~~~~
...High Risk of Excessive Rainfall Once Again Across Portions of
the Central Gulf Coast...
Abundant moisture remains in place across the Southeast, with the
exceptionally anomalous 2.5-2.75+ inch PW values along the central
Gulf Coast, including an observed 2.71" per the KLIX 00Z 08/12
RAOB. These values are between 3-4 standard deviations above the
norm (approaching record values) per the SREF and GEFS. The best
description of this system is that of a sheared inland tropical
depression, as the upper high over the system has moseyed off to
the TX/LA border, advecting north-northeast flow over the top of
the system. The best analog is Tropical Depression Five (2010)
during its second trip into the Gulf Coast. Given the deep high
theta-e/moist profile for optimal warm rain processes (wet bulb
zero heights a whopping 16KFT per the 00Z KLIX sounding), the
threat for heavy tropical banding rains across this area will
continue given the slow westward drift of the TUTT.
WPC maintained a fairly large swath of areal-average QPF between
3-6" across eastern LA and southern MS. However, all of the
higher-resolution CAMs indicate pockets of much higher amounts,
7-10+ inches between 12Z Fri-12Z Sat. Given the additional heavy
rains over already saturated soils (low FFG values), WPC will
maintain a fairly broad MODERATE and HIGH risk area in the day 1
Excessive Rainfall Outlook (ERO) over the central Gulf region --
similar to the day 2 ERO on Thursday.
...SOUTHERN PLAINS INTO THE LOWER MS VALLEY...
THE DISTURBANCE OVER THE NORTHERN GULF WILL CONTINUE TO SLOWLY
PUSH WEST...AND EVENTUALLY BEGIN TO BE PICKED UP BY THE TROUGH
MOVING ACROSS THE PLAINS. A HEAVY RAIN THREAT LIKELY CONTINUES
INTO AT LEAST SATURDAY MORNING ACROSS LA. BY LATER SATURDAY THE
MID LEVEL VORTICITY CENTER IS GENERALLY FORECAST TO BECOME LESS
FOCUSED AND MORE STRUNG OUT AS IT BEGINS TO FEEL THE EFFECT OF THE
TROUGH LIFTING TO THE NORTH. SOME DIFFERENCES WITH REGARDS TO HOW
FAR WEST THE SYSTEM GETS AND THEN HOW QUICKLY IT BEGINS TO MOVE
NORTH...WHICH END UP HAVING A PRETTY SIGNIFICANT IMPACT ON QPF.
OVERALL THE NAM AND GEM HANDLE THE MID/UPPER LEVEL ENERGY
DIFFERENT THAN THE BEST CLUSTERING AND ARE THUS NOT PREFERRED. THE
0Z GFS IS THE FURTHEST WEST SOLUTION...AND THE SLOWEST TO BRING
THE SYSTEM NORTH...RESULTING IN SIGNIFICANT RAINFALL CROSS
SOUTHERN LA. THE 0Z ECMWF AND UKMET ARE A BIT EAST AND
QUICKER...AND WHILE STILL SIGNIFICANT...BOTH HAVE QPF MAGNITUDES
ABOUT HALF THE MAGNITUDE OF THE GFS AND SHIFTED NORTH ACROSS
CENTRAL AND NORTHERN LA. OVERALL THE TREND APPEARS TO BE FOR A
SLIGHTLY EAST AND QUICKER SOLUTION...WITH THE 0Z ECMWF PROBABLY
CLOSEST TO THE CURRENT MODEL CONSENSUS. GIVEN SOME LINGERING
UNCERTAINTY...WPC QPF STAYED CLOSE TO A BLEND OF THE 0Z
GFS/ECMWF/UKMET...WHICH RESULTS IN A PRETTY BROAD 2-4" ACROSS MUCH
OF CENTRAL AND WESTERN LA. BASED ON RECENT TRENDS THINK THE GFS
MAY VERY WELL BE A BIT TOO WET AND TOO FAR SOUTH. CURRENTLY WOULD
EXPECT THE BEST THREAT OF ISOLATED TO SCATTERED 4-7" AMOUNTS ON
DAY 2 (12Z SAT-12Z SUN) TO FOCUS ACROSS CENTRAL LA. OPTED TO
INTRODUCE A MODERATE RISK ON DAY 2 FOR PORTIONS OF CENTRAL
LA...WHERE THIS RAIN ON TOP OF WHAT FALLS TODAY WILL LIKELY CAUSE
CONTINUED AREAS OF FLOOD CONCERN.
HEAVY RAINFALL IS ALSO LIKELY ACROSS PORTIONS OF TEXAS THIS
WEEKEND. EXPECT AN AXIS OF STRONG LOW LEVEL CONVERGENCE TO
DEVELOP...WITH NORTHERLY FLOW BEHIND A COLD FRONT PUSHING THROUGH
THE PLAINS...AND SOUTHERLY FLOW OUT OF THE GULF ASSOCIATED WITH
THE TROPICAL DISTURBANCE. EXPECTING NEAR RECORD PRECIPITABLE WATER
VALUES TO POOL ALONG THIS CONVERGENCE AXIS OVER CENTRAL AND
EASTERN TEXAS. THIS AXIS OF VERY HIGH MOISTURE IS FORECAST TO BE
NEARLY STATIONARY. MEANWHILE EXPECT A FEW SUBTLE SHORTWAVES TO
TRAVERSE THE AREA ON THE SOUTHERN EDGE OF THE PLAINS TROUGH...WITH
THE AREA ALSO SEEING PERIODS OF INCREASED DIFFLUENCE ALOFT. THUS
WOULD EXPECT PERIODS OF CONVECTION WITHIN THIS MOISTURE AXIS.
CORFIDI VECTORS BECOME VERY LIGHT...SUGGESTING THAT NEARLY
STATIONARY CONVECTION WILL BE POSSIBLE...WITH ONLY A SLOW
SOUTHWARD PUSH OF THE FRONT HELPING PROGRESS THINGS. THUS SEEMS
LIKE A GOOD SETUP FOR HEAVY RAINFALL.
OVERALL THE TREND HAS BEEN FOR SOUTHEASTWARD SHIFT IN THE QPF AXIS
ACROSS TEXAS...WITH THIS TREND SEEN IN ALL THE DETERMINISTIC
GUIDANCE. THIS IS THE RESULT OF A QUICKER FRONTAL PROGRESSION AND
A FURTHER EAST POSITION OF THE TROPICAL DISTURBANCE. GIVEN THE
GOOD MODEL AGREEMENT AND CONSISTENT TREND...WPC ALSO SHIFTED QPF
SOUTHEASTWARD. STILL SOME DIFFERENCES IN THE AXIS OF HIGHEST
QPF...AND WPC GENERALLY FOLLOWED A CONSENSUS OF THE
ECMWF/GFS/UKMET. AMOUNTS WERE KEPT ON THE CONSERVATIVE SIDE GIVEN
THE LINGERING UNCERTAINTY. A SLIGHT RISK WAS MAINTAINED FOR DAY 2
AND ISSUED FOR DAY 3 ACROSS PORTIONS OF CENTRAL AND EAST TEXAS.
THE SETUP DOES SEEM TO HAVE THE POTENTIAL FOR AN EVENTUAL UPGRADE
TO A MODERATE RISK WHEN CONFIDENCE IN THE EXACT LOCATION OF
HIGHEST AMOUNTS INCREASES...SO WILL HAVE TO CONTINUE TO MONITOR
