MESOSCALE PRECIPITATION DISCUSSION 0014
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EST SUN FEB 11 2018

AREAS AFFECTED...FL PANHANDLE...SRN AL...SW GA...SE MS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 110759Z - 111159Z

SUMMARY...A SQUALL LINE HAD FIRMED UP OVER FL/AL AS OF 0745Z.
SATURATED GROUND BEHIND THE SQUALL WILL CONTINUE TO SEE SHOWERS
WHICH MAY AGGRAVATE ANY SURFACE RUNOFF CONDITIONS. THE LEADING
EDGE OF THE SQUALL LINE WILL MOVE SLOWLY ENOUGH TO PRODUCE LOCAL 3
TO 4 INCH RAINFALL OVER A SHORT TIME...POSSIBLY CAUSING FLASH
FLOODING.

DISCUSSION...WARM ADVECTION AND CONFLUENCE AROUND THE PERIPHERY OF
A LOW LEVEL HIGH PRESSURE CIRCULATION...HAD PRODUCED A SWATH OF 1
TO 2 INCHES OF RAIN / LOCALLY HIGHER WEST OF ELGIN AIR FORCE BASE
UP TO ANDALUSIA ALABAMA. A SHORTWAVE TROUGH UPSTREAM WAS HOPED TO
PUSH THE RAINFALL FORWARD...BUT HAD INITIALLY HAD THE EFFECT OF
SHARPENING A SQUALL LINE THAT WAS LOCATED FROM MIRAMAR BEACH TO
OZARK ALABAMA AT 0745Z. GOES-16 MULTI-LAYER WATER VAPOR IMAGERY
SUGGESTED THE SHORTWAVE WILL NOT INDUCE ANY SHARP LARGE SCALE
VEERING OF THE FLOW...AS IT HAD THE APPEARANCE OF A WAVE SHEARING
AND BECOMING EMBEDDED IN THE DEEP LAYER SOUTHWESTERLY FLOW. INDEED
THE RAP INDICATES A VERY SLOW VEERING. THE INFLUENCE OF DRIER AIR
ASSOCIATED WITH THE SHORTWAVE MAY ALSO BE OFFSET BY VERY MOIST LOW
LEVELS IN THE PRE-SQUALL-LINE AIR...RESULTING IN LITTLE COLD POOL
PUSH...AND ULTIMATELY A VERY SLOW FORWARD PROGRESSION. IN
FACT...THERE IS LITTLE DIFFERENCE BETWEEN THE 0-6KM MEAN WIND AND
CORFIDI VECTORS FOR FORWARD PROPAGATION.

THE MODELS WERE NOT DOING A VERY GOOD JOB OF LOCATION AND
EVOLUTION OF THIS EVENT NEAR THE GULF COAST. THE 06Z HRRR WAS AT
LEAST IN THE NEIGHBORHOOD. BASED ON IT...AND DUAL-POL RADAR
ESTIMATES OF ABOUT 1.5 INCH PER HOUR RATES IN THE SQUALL
LINE...SOME LOCATIONS WILL LIKELY RECEIVE 3 TO 4 INCHES OVER THE
COURSE OF 3 HOURS...WHICH WOULD APPROACH OR MINIMALLY EXCEED FLASH
FLOOD GUIDANCE...NEAR AND EAST OF THE ONGOING SQUALL LINE. BACK
WEST...UPSTREAM SHOWERS MAY PERSIST AND MOVE OVER RECENTLY
SATURATED SOILS IN PARTS OF MS/AL/FL UNTIL AFTER 12Z WHEN LARGER
SCALE DEEP LAYER FORCING MAY LIFT NORTHWARD AND BECOME DETACHED
FROM THE GULF-BASED SOURCE OF INSTABILITY. ALTHOUGH THIS WOULD
LEAD TO A DECLINE OF DEPTH/INTENSITY...SOME FORM OF MODERATE
RAINFALL WOULD BE EXPECTED TO PERSIST INTO THE DAYTIME HOURS AS
WELL.

BURKE

ATTN...WFO...BMX...FFC...LIX...MOB...TAE...

ATTN...RFC...LMRFC...SERFC...

LAT...LON   32208497 31888446 30418508 29818577 30048679
            29848823 30238885 31138831 32068617