MESOSCALE PRECIPITATION DISCUSSION 0406
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1253 PM EDT THU JUL 05 2018

AREAS AFFECTED...OH...NWRN WV...WRN/CTRL PA...SWRN TIER OF NY

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

VALID 051652Z - 052245Z

SUMMARY...THUNDERSTORMS WILL BECOME MORE NUMEROUS ACROSS THE UPPER
OHIO RIVER VALLEY THIS AFTERNOON. ANY STORMS THAT FORM IN THE
MOIST AND UNSTABLE ENVIRONMENT WOULD BE CAPABLE OF PRODUCING VERY
HEAVY RAIN RATES AS HIGH AS 2-3 IN/HR. THIS WILL LIKELY LEAD TO
SOME AREAS OF FLASH FLOODING. THE HIGHEST CHANCES OF FLASH
FLOODING ARE CURRENTLY ANTICIPATED FROM SERN OHIO INTO THE NRN WV
PANHANDLE AND INTO SWRN PENNSYLVANIA.

DISCUSSION...RADAR AND SATELLITE TRENDS ACROSS THE UPPER OHIO
RIVER VALLEY REGION INDICATED AN INCREASE IN THE COVERAGE AND
INTENSITY OF CONVECTION IN THE PAST HOUR OR SO (1500-1630Z).
GOES-16 DAY CONVECTION RGB AND CLOUD TOP HEIGHT PRODUCTS SHOW THIS
WELL... WITH THE MOST SIGNIFICANT TRENDS ALONG A WEAK SURFACE
CONVERGENCE AXIS (AND RIBBON OF ENHANCED CUMULUS DEVELOPMENT) FROM
NEAR ERI... TO NEAR CLE... TO NEAR MIE. THIS ALSO CORRESPONDS WELL
TO AN EXPECTED INCREASE IN CONVECTIVE COVERAGE ADVERTISED BY MOST
HI-RES MODELS AROUND MIDDAY...A TREND THAT THESE MODELS CONTINUE
THROUGH 00Z. THE FLASH FLOOD THREAT WILL BE ELEVATED VIA A
COMBINATION OF FAVORABLE ENVIRONMENT AND ANTECEDENT CONDITIONS.

THE ATMOSPHERE ACROSS THE REGION IS VERY MOIST...AND ANALYSIS OF
MODEL INITIALIZATION FIELDS SHOWS PWATS ABOVE THE 99TH PERCENTILE
OVER MUCH OF THE OUTLINED DISCUSSION AREA. THIS SHOULD DRIVE THE
POTENTIAL FOR VERY HEAVY RAINFALL. TWO THINGS WILL TEND TO
EXACERBATE THAT POTENTIAL: RELATIVELY STRONG INSTABILITY AND WEAK
WIND FIELDS. LATEST RAP ANALYSIS SHOWS WIDESPREAD MLCAPE AROUND
1500-2000 J/KG...WITH SLIGHTLY LOWER VALUES ONLY FOUND IN AREAS
WITH LINGERING CLOUD COVER FROM MORNING CONVECTION (NRN WV INTO
CTRL PA). THIS AMOUNT OF INSTABILITY COMBINED WITH LOW-LEVEL
(925-850MB) WESTERLY INFLOW AROUND 15 KNOTS SHOULD BE SUFFICIENT
TO ALLOW FOR SOME STORM ORGANIZATION (LIKELY INTO MULTICELL
CLUSTERS) AND POSSIBLY BACKBUILDING. FURTHER ALOFT...WIND FIELDS
ARE RATHER WEAK...AND THIS SHOULD ALLOW FOR SLOW STORM MOTIONS.
THUS...ANY ORGANIZED STORM CLUSTERS PRODUCING VERY HEAVY RAINFALL
SHOULD SEE INCREASED RESIDENCE TIME OVER INDIVIDUAL LOCATIONS...
INCREASING THE CHANCES OF FLASH FLOODING.

GOES-16 WATER VAPOR CHANNELS SHOW A DISTINCT ANTICYCLONIC CURL
CENTERED ON THE NRN WEST VIRGINIA PANHANDLE...LIKELY AN INDICATION
OF A RELATIVE MINIMUM IN UPPER LEVEL FLOW AND A REGION WHERE STORM
MOTIONS MAY BE MORE CHAOTIC AND/OR LOWER. THIS FEATURE IS
CO-LOCATED WITH THE STRONGEST HI-RES MODEL SIGNAL FOR HEAVY
RAINFALL...PER THE 12Z HREF AND THE 15Z HRRR (SLIGHTLY OFFSET TO
THE NORTH). THIS REGION ALSO DID SEE SOME CONVECTION DURING THE
MORNING HOURS...AND THUS MAY (AT LEAST TEMPORARILY) SEE REDUCED
INSTABILITY EARLY THIS AFTERNOON. THEREFORE...IT IS POSSIBLE THAT
THE GREATEST CHANCES OF FLASH FLOODING MAY EVENTUALLY BE DISPLACED
SLIGHTLY TO THE NORTH FROM WHERE 12Z HI-RES MODELS ARE
CONCENTRATED.

MUCH OF THIS REGION ALSO HAS REDUCED FLASH FLOOD GUIDANCE GIVEN
RECENT RAINFALL. THE NATIONAL WATER MODEL ANALYSIS ALSO SHOWS MUCH
OF WESTERN PENNSYLVANIA WITH ABOVE NORMAL STREAMFLOW...AND THUS
THE HYDROLOGIC CONDITIONS MAY ALSO ENHANCE THE OVERALL CHANCES OF
FLASH FLOODING IN THIS REGION.

LAMERS

ATTN...WFO...BGM...BUF...CLE...CTP...ILN...PBZ...RLX...

ATTN...RFC...MARFC...NERFC...OHRFC...

LAT...LON   42697861 42307705 41497688 40207815 39238016
            38628187 38758359 39998390 41458220 42138058