Flash Flood Guidance
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000
AWUS01 KWNH 220424
FFGMPD
TNZ000-KYZ000-INZ000-ALZ000-MSZ000-221015-

MESOSCALE PRECIPITATION DISCUSSION 0026
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1123 PM EST WED FEB 21 2018

AREAS AFFECTED...WRN/CTRL TN...WRN/CTRL KY...FAR NE MS

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

VALID 220422Z - 221015Z

SUMMARY...FLASH FLOODING WILL BE POSSIBLE AS A SURGE OF DEEP
MOISTURE PUSHES INTO PORTIONS OF TENNESSEE AND KENTUCKY OVERNIGHT.
RAINFALL RATES OF UP TO 1.5 IN/HR WILL BE POSSIBLE WITH THE
HEAVIER CONVECTIVE RAIN BANDS.

DISCUSSION...REGIONAL RADARS SHOWED SCATTERED CONVECTIVE ELEMENTS
EMBEDDED WITHIN A BROADER PLUME OF MODERATE RAIN BEGINNING TO
CONSOLIDATE AND BECOME MORE FOCUSED AS THEY LIFT N/NE INTO
PORTIONS OF TENNESSEE AND KENTUCKY. CONVECTIVE BANDS WERE
PRODUCING THE HEAVIEST RAINFALL WHERE THE GREATEST OVERLAP OF
INSTABILITY CURRENTLY EXISTS...ACROSS MIDDLE TENNESSEE. A RAIN
BAND THAT MOVED THROUGH NASHVILLE METRO BY 04Z WAS ESTIMATED BY
KOHX DUAL POL TO HAVE RAIN RATES APPROACHING 1.5 IN/HR AND SIMILAR
RAIN RATES WERE MEASURED ON SOME MESONET SITES AND PERSONAL
WEATHER STATIONS IN THE AREA. ONE SUCH SITE MEASURED 0.59 INCHES
OF RAIN IN JUST 15 MINUTES AND 1.39 INCHES OF RAIN IN AN HOUR. THE
MARGINAL INSTABILITY MAY BE OFFSET SOMEWHAT BY THE DEEP MOISTURE
AVAILABLE ACROSS THE REGION...AND THE STRONG NORTHWARD MOISTURE
TRANSPORT IN THE LOW-LEVEL JET CURRENTLY LIFTING NORTHEAST IN
ASSOCIATION WITH A FRONTAL WAVE. THE NOSE OF THE LLJ SHOULD HELP
FOCUS SOME OF THE HEAVIER RAIN RATES THROUGH THE OVERNIGHT...AND
THE RAP FORECASTS IT TO MOVE FROM WRN TENNESSEE AROUND 04-05Z...TO
CTRL KENTUCKY BY 09Z.

ANY CONVECTIVE BANDS EMBEDDED IN THE MARGINALLY UNSTABLE INFLOW
REGION (ELEVATED ABOVE A COOLER SURFACE LAYER) MAY CONTINUE TO
PRODUCE RAIN RATES PEAKING IN THE 1.0 TO 1.5 IN/HR RANGE PROVIDED
FAVORABLE TRAINING CAN OCCUR. LOCALIZED RAINFALL TOTALS BY 10Z
COULD EXCEED 3 INCHES IN SOME SPOTS. THESE RATES/TOTALS MAY BE
SUFFICIENT TO PRODUCE SOME FLOODING INTO PORTIONS OF TENNESSEE AND
KENTUCKY DURING THE REST OF THE NIGHT...WITH FLASH FLOODING MOST
LIKELY IN URBANIZED AREAS. ONE THING TO MONITOR IS THAT SHALLOW
CONVECTIVE BANDS MAY BE DOMINATED BY WARM RAIN PROCESSES...WHICH
COULD ALLOW THEM TO CONTINUE PRODUCING HEAVY RAINFALL RATES EVEN
IF OVERALL INSTABILITY OR MAXIMUM REFLECTIVITY VALUES SEEM TO WANE
SLIGHTLY.

LAMERS

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...HUN...JKL...LMK...MEG...OHX...PAH...

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

LAT...LON   38428530 38218430 37768383 37118458 35998624
            34928754 34468857 34858919 35718945 36558914
            37168834 37688742 38198623



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