MESOSCALE PRECIPITATION DISCUSSION 0929
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1116 PM EST MON NOV 06 2017

AREAS AFFECTED...MIDDLE AND EASTERN TENNESSEE

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

VALID 070415Z - 070945Z

SUMMARY...ONE OR TWO BANDS OF HEAVY RAIN AND THUNDERSTORMS
ORIENTED IN A WEST-TO-EAST FASHION MAY MOVE IN THE SAME
DIRECTION...REPEATEDLY AFFECTING SIMILAR AREAS AND POTENTIALLY
LEADING TO LOCALIZED HEAVY RAINFALL AND FLASH FLOODING. THIS SEEMS
MOST LIKELY ACROSS PORTIONS OF MIDDLE TENNESSEE WITH RAIN RATES
PEAKING AS HIGH AS 1-2 IN/HR.

DISCUSSION...REGIONAL RADARS AND IR SATELLITE IMAGERY SHOW A BAND
OF CONVECTION VERY NEAR THE I-40 CORRIDOR IN WEST-CENTRAL
TENNESSEE AS OF 04Z. THIS CONVECTION WAS FORMING IN A REGION OF
ENHANCED CONVERGENCE IN THE LOWEST 1KM AGL...VERY NEAR A SUBTLE
SURFACE TROUGH. THE CONVECTIVE BAND WAS ORIENTED ROUGHLY PARALLEL
TO THE MEAN FLOW...OR APPROXIMATELY WEST-TO-EAST...AND THUS WAS
ALREADY IN A FAVORABLE CONFIGURATION FOR CELL TRAINING. THE
MESOSCALE ENVIRONMENT ALSO FAVORS SOME BACKBUILDING...WITH MOIST
AND UNSTABLE INFLOW UPSTREAM (IN W TN AND AR) VEERING WITH TIME
AND BECOMING INCREASINGLY ALIGNED WITH THE MEAN WIND. THE VEERING
WINDS ARE CONFIRMED BY TRENDS ON THE TMEM AND KNQA RADAR VERTICAL
WIND PROFILES. NEW CELL GROWTH WOULD BE FAVORED ON THE UPSHEAR
FLANK OF THE CONVECTIVE BAND TOWARD THE MORE UNSTABLE AIR...AND
THE ALIGNMENT OF THE LOW-LEVEL FLOW WITH THE DEEPER LAYER MEAN
FLOW SHOULD ALLOW THE NEW CELL GROWTH TO OCCUR IN FAVORABLE
POSITIONS FOR CONTINUED TRAINING.

EXTRAPOLATION OF RADAR TRENDS OF THE CONVECTIVE BAND AS OF 04Z
(STRETCHING FROM THE SOUTHWEST SIDE OF NASHVILLE METRO TO WAVERLY
TN) SUGGESTS THAT HEAVY RAIN WITH THIS BAND COULD LAST BETWEEN 60
AND 90 MINUTES AT SOME LOCATIONS. AND RECENT IR SATELLITE TRENDS
SUGGEST BACKBUILDING MAY BE OCCURRING WITH A SEPARATE CLUSTER OF
COOLING CLOUD TOPS AND A FEW LIGHTNING STRIKES JUST TO THE WEST.
MRMS RAINFALL ESTIMATES THUS FAR HAVE PEAKED AROUND 1.5 IN/HR. IF
THESE WERE TO LAST FOR A COUPLE HOURS IN CERTAIN LOCATIONS...FLASH
FLOODING WOULD BE POSSIBLE. AND EVEN HOURLY RATES AROUND 1.5 IN/HR
COULD POTENTIALLY PRODUCE FLASH FLOODING IN LARGER URBAN AREAS
SUCH AS NASHVILLE. HI-RES MODEL GUIDANCE DOES NOT SEEM TO HAVE A
GOOD HANDLE ON THE PLACEMENT OF HEAVIEST RAINFALL OVERNIGHT...WITH
MOST MODELS DISPLACED NORTH INTO KENTUCKY. HOWEVER...SOME OF THE
HI-RES RUNS HAVE SHOWN ISOLATED RAIN TOTALS CLOSE TO 3
INCHES...WHICH WOULD BE CLOSE TO 6HR FFG VALUES ACROSS MOST OF THE
REGION. THESE SEEM TO BE REASONABLE VALUES...BUT THE HEAVIEST RAIN
BAND IN THE HI-RES MODELS SHOULD BE SHIFTED TO THE SOUTH (AND
PERHAPS A BIT FURTHER WEST CLOSER TO THE BETTER
INSTABILITY)...ROUGHLY INTO THE I-40 CORRIDOR IN MIDDLE TENNESSEE.

LAMERS

ATTN...WFO...HUN...MEG...MRX...OHX...

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

LAT...LON   36608629 36568509 36408430 35958401 35468453
            35248546 35128659 35208781 35518863 36068877
            36398829 36588743