MESOSCALE PRECIPITATION DISCUSSION 0875
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
531 AM EDT SUN OCT 08 2017

AREAS AFFECTED...KY...WRN WV...CTRL AND ERN TN...FAR NW GA...NE AL

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

VALID 080930Z - 081530Z

SUMMARY...RENEWED DEVELOPMENT OF HEAVY RAIN BANDS EXPECTED 09-12Z
OVER THE TENNESSEE VALLEY REGION...WHICH MAY LEAD TO AREAS OF
FLASH FLOODING BY 15Z.

DISCUSSION...THE HEAVY RAIN BAND THAT HAS PRODUCED TOTALS IN
EXCESS OF 3 INCHES ALREADY IN PARTS OF WRN/NRN KENTUCKY HAS NOT
MOVED APPRECIABLY IN THE PAST 6 HOURS. IT WAS STILL LOCATED ALONG
A STALLING FRONTAL BOUNDARY IN THE VICINITY OF THE OHIO RIVER.
REFLECTIVITY DIMINISHED IN THE 07-09Z TIME FRAME...LIKELY DUE TO A
GRADUAL WEAKENING OF LOW-LEVEL CONVERGENCE ALONG THE
FRONT...COUPLED WITH DECREASING INSTABILITY. THESE TRENDS SUGGEST
IT WILL LIKELY TAKE ADDITIONAL DEVELOPMENT OF SHALLOW CONVECTIVE
RAIN BANDS TO PRODUCE FLASH FLOODING ACROSS THE REGION. THE
OVERALL ENVIRONMENT CONTINUES TO BE SUPPORTIVE OF HEAVY RAIN RATES
WITH PWATS IN EXCESS OF 2 INCHES AND A NARROW CAPE PROFILE. MRMS
30 DBZ ECHO TOPS HAVE GENERALLY BEEN BELOW 20 KFT OVER THE PAST
FEW HOURS (BELOW THE -10C LEVEL) SUGGESTIVE OF A LACK OF ICE IN
THE CLOUDS AND DOMINANT WARM RAIN PROCESSES. THIS WAS LIKELY
LEADING TO UNDERESTIMATION OF RAIN RATES BY RADARS. AN AVERAGE OF
SIX KENTUCKY MESONET SITES FROM GRAYSON COUNTY TO NEAR LEXINGTON
SHOWED THAT RADAR ESTIMATES FROM KLMK WERE ABOUT 63 PERCENT OF
OBSERVED VALUES. THIS TREND SHOULD BE MONITORED THROUGH THE
DAY...ESPECIALLY AS PWATS ARE EXPECTED TO INCREASE WITH A SURGE OF
TROPICAL MOISTURE FROM CLOSER TO NATE.

THE RENEWED DEVELOPMENT APPEARS LIKELY GIVEN SIGNALS ON BOTH
HI-RES AND GLOBAL MODELS. THE PRIMARY AXIS OF LOW-LEVEL
CONVERGENCE APPEARED TO BE REFOCUSING A BIT FURTHER SOUTH (KHZD TO
KCKV TO KGLW TO KRGA; OR NEAR THE KY-TN BORDER) AND REGIONAL
RADARS SHOWED GROWTH OF SHOWERS IN THIS CORRIDOR. THE 08Z RAP
SHOWED AN INCREASE IN UPPER LEVEL DIVERGENCE IN THE RIGHT ENTRANCE
REGION OF A JET STREAK...ALONG WITH A SUBSTANTIAL INCREASE IN
SFC-850MB MOISTURE FLUX CONVERGENCE AS STRONGER LOW LEVEL FLOW ON
THE E/NE SIDE OF NATE SURGES INTO TENNESSEE. THE COMBINATION OF
THESE FACTORS SUGGESTS NEW RAIN BAND(S) SHOULD BECOME ESTABLISHED
OVER THE NEXT FEW HOURS WITH SOME PERSISTENCE OR A SLOW NORTHWARD
DRIFT POSSIBLE THEREAFTER. IF HEAVY RAIN CAN MOVE INTO AREAS OF
SATURATED SOILS...FLASH FLOODING MAY BE MORE LIKELY...ALTHOUGH IT
WOULD BE POSSIBLE WITH ANY SLOW-MOVING RAIN BAND WITH A COUPLE
HOURS OF PERSISTENCE. RAIN RATES OF 1-2 IN/HR SHOULD BE FAIRLY
COMMON WITH HEAVIER RAIN BANDS...AND ISOLATED EXTREMES UP TO 3
IN/HR CANNOT BE RULED OUT...PARTICULARLY AFTER 12Z AS HIGHER PWATS
PUSH INTO THE REGION.

ONE OTHER AREA TO MONITOR IS A REGION OF RELATIVELY NEW CONVECTIVE
DEVELOPMENT ALONG THE NORTHERN PORTION OF THE AL-GA BORDER WHICH
HAS BEEN SLOWLY MOVING NORTH. SOME HI-RES MODELS CONTINUE THIS
SLOW TRAINING MOTION AND EXPAND THE HEAVY RAIN INTO EASTERN
TENNESSEE AND THE CUMBERLAND PLATEAU REGION THIS MORNING.
HOWEVER...THERE IS LESS OF A DISTINCT FOCUSING MECHANISM IN THIS
AREA THROUGH 15Z...SO THE FLASH FLOOD THREAT MAY BE MORE LOCALIZED
IN THESE AREAS.

LAMERS

ATTN...WFO...BMX...FFC...HUN...ILN...JKL...LMK...MEG...MRX...
OHX...PAH...RLX...

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

LAT...LON   38938217 38898138 38418097 37558178 36948263
            36568319 36128405 35238464 34768480 34388481
            33928501 33988584 34308637 34968664 35568664
            35838684 35898757 36198827 36338879 36488930
            36958950 37428847 37778749 38278570 38708392