MESOSCALE PRECIPITATION DISCUSSION 0192
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
459 PM EDT FRI MAY 05 2017

AREAS AFFECTED...NORTHERN WV...SOUTHWEST PA

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

VALID 052058Z - 060158Z

SUMMARY...ISOLATED FLASH FLOOD ISSUES MAY ARISE WITH LOCALIZED 1"
IN AN HOUR AMOUNTS OVER AN AREA WITH RELATIVELY LOW FLASH FLOOD
GUIDANCE.

DISCUSSION...CONVECTION IS EXPANDING IN COVERAGE ALONG A LOW LEVEL
CONVERGENCE AXIS STRETCHING FROM CENTRAL WV INTO SOUTHWEST PA.
DAYTIME HEATING HAS RESULTED IN ROUGHLY 500 J/KG OF ML CAPE ALONG
THIS BOUNDARY. THE APPROACH OF A SHORTWAVE FEATURE CURRENTLY SEEN
ENTERING SOUTHERN WV ON WATER VAPOR IMAGERY IS HELPING INCREASE
LARGE SCALE ASCENT TO RESULT IN THE INCREASED CONVECTIVE COVERAGE
ALONG THE CONVERGENCE BOUNDARY. PRECIPITABLE WATER VALUES BELOW 1"
ARE NOT THAT IMPRESSIVE AND SHOULD GENERALLY HELP LIMIT RAINFALL
RATES AND AMOUNTS. HOWEVER WITH THE BOUNDARY ONLY SLOWLY MOVING
EAST WITH TIME...AND THE MEAN STORM LAYER WINDS HAVING A LARGE
COMPONENT PARALLEL TO THE BOUNDARY...SOME SOUTH TO NORTH REPEAT
CONVECTION IS POSSIBLE.

THE 12Z EXPERIMENTAL HREF VERSION 2 SHOWS A HIGH PROBABILITY OF
SCATTERED RAINFALL TOTALS EXCEEDING 0.5" IN AN HOUR...WITH
MODERATE PROBABILITIES OF ISOLATED 1" IN AN HOUR AMOUNTS. THESE
PROBABILITIES ARE ALSO BACKED UP BY RECENT HRRR RUNS. IN GENERAL
LOOKING AT ISOLATED TO SCATTERED 1-2" RAINFALL AMOUNTS OVER THE
MPD AREA. AMOUNTS OF THIS MAGNITUDE SHOULD NOT CAUSE SIGNIFICANT
ISSUES...HOWEVER WITH FLASH FLOOD GUIDANCE AS LOW AS 0.5" IN AN
HOUR OVER THE MPD AREA...THESE RAINFALL RATES COULD CAUSE SOME
RUNOFF AND FLOOD ISSUES. THE CONSENSUS IS FOR THIS THREAT TO
CONTINUE TILL AROUND 02Z...THEREAFTER DECREASED INSTABILITY AND AN
EASTWARD PUSH OF THE CONVERGENCE AXIS SHOULD END THE THREAT.

ATTN...WFO...CTP...PBZ...RLX...

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

LAT...LON   41267874 40757861 39727936 38897999 38778027
            38918095 39548074 40138019 40447986 40707982
            41197953