MESOSCALE PRECIPITATION DISCUSSION 0025
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
856 PM EST WED FEB 21 2018

AREAS AFFECTED...NRN/CTRL LA...FAR ERN TX...NRN MS...SERN AR...WRN
TN

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 220155Z - 220745Z

SUMMARY...ONGOING FLASH FLOODING IS LIKELY TO CONTINUE IN PARTS OF
NORTHERN LOUISIANA...AND MAY WORSEN AS WAVES OF CONVECTION
CONTINUE TO PUSH OVER THAT AREA. ADDITIONAL FLASH FLOODING MAY
DEVELOP FURTHER NORTHEAST WITH RAIN RATES POSSIBLY APPROACHING 1.0
TO 1.5 IN/HR.

DISCUSSION...BASED ON THE CURRENT DISTRIBUTION OF FLASH FLOOD
WARNINGS...RECENT REPORTS...AND THE CREST HYDROLOGIC MODEL...FLASH
FLOODING IS LIKELY ONCOING IN THE NORTHERN HALF OF LOUISIANA THIS
EVENING. TRENDS IN THE CREST UNIT STREAMFLOW ANALYSIS SUGGEST THAT
THE FLOODING MAY ACTUALLY BE GETTING WORSE IN THE 0030-0200Z TIME
FRAME...PARTICULARLY WITH THE PRIMARY CONVECTIVE BAND THAT
STRETCHED FROM RUSTON LA TO JASPER TX AS OF 0145Z. THIS CONVECTIVE
BAND WAS SITUATED NEAR A WEAK SURFACE LOW AND THE NORTHWEST
PERIPHERY OF A LOW-LEVEL JET...AND HAD RELATIVELY UNDISTURBED
BROAD SOUTHERLY INFLOW WITH SURFACE DEWPOINTS IN THE 70S.
ISOTHERMAL REFLECTIVITY ANALYSIS ALOFT REVEALED LIMITED STORM
DEPTH ABOVE -15C. A LACK OF SIGNIFICANT ICE IN THE CLOUD LAYER
COMBINED WITH PRECIPITABLE WATER VALUES AROUND 1.7 INCHES (RAP
ANALYSIS AND GPS-PW MEASUREMENTS) SUGGEST THAT ANY CONVECTION IN
LOUISIANA SHOULD BE VERY EFFICIENT RAINFALL PRODUCERS. EXPECT
ONGOING FLASH FLOODING IN NRN/CTRL LOUISIANA TO CONTINUE AND
POSSIBLY WORSEN IN SOME AREAS PROVIDED TRAINING CONVECTIVE BANDS.
RADAR ESTIMATES HAVE SUGGESTED RATES AS HIGH AS 1.5 IN/HR OVER THE
PAST HOUR.

MODELS ARE THEN IN GOOD AGREEMENT THAT THE WEAK LOW OR FRONTAL
WAVE WILL LIFT QUICKLY NORTHEAST OVERNIGHT...ALONG WITH THE LLJ
SPEED MAX...AND THIS SHOULD INCREASINGLY FOCUS HEAVIER RAIN AND
PERHAPS SOME ELEVATED THUNDERSTORMS FURTHER NORTHEAST WITH TIME.
THESE AREAS HAVE RECEIVED LESS RAINFALL AS COMPARED TO NORTHERN
LOUISIANA...BUT THE POTENTIAL FOR TRAINING CONVECTION WITH RAIN
RATES UP TO 1.0 TO 1.5 IN/HR SHOULD PERSIST WITH AT LEAST SEVERAL
HUNDRED J/KG OF MUCAPE LIKELY. THE RAP PREDICTION OF 1.5 INCH
PWATS EXTENDING AS FAR NORTH AS THE KY-TN BORDER IS SIGNIFICANT AS
THIS IS OUTSIDE THE HISTORICAL OBSERVATIONS FOR NASHVILLE FOR THE
MONTH OF FEBRUARY. THE ANOMALOUSLY HIGH MOISTURE LEVELS MAY
COMPENSATE FOR RATHER MARGINAL INSTABILITY TO PRODUCE LOCALIZED
BANDS OF HEAVY RAINFALL CAPABLE OF CAUSING FLASH FLOODING.

LAMERS

ATTN...WFO...JAN...LCH...LZK...MEG...SHV...

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

LAT...LON   36368877 36138803 35408848 34378881 32988926
            32288996 32309107 31549176 31129266 30529350
            30159451 30939444 32239383 33429293 34649151
            35829001