MESOSCALE PRECIPITATION DISCUSSION 0500
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 PM EDT THU JUL 28 2016
 
AREAS AFFECTED...CENTRAL MID ATLANTIC 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 281855Z - 290055Z
 
SUMMARY...EXPANDING CONVECTION SHOULD RESULT IN AN AXIS OF HEAVY
RAINFALL AND AT LEAST AN ISOLATED FLASH FLOOD RISK...

DISCUSSION...CONVECTION CONTINUES TO INCREASE IN COVERAGE EARLY
THIS AFTERNOON ALONG AHEAD OF A MID LEAVE VORTICITY AXIS...AND IN
THE RIGHT ENTRANCE REGION OF A 110KT 250MB JET MOVING ACROSS NEW
YORK STATE. UNDERNEATH THIS FAVORABLE SYNOPTIC ENVIRONMENT IS A
PLUME OF WELL ABOVE NORMAL PRECIPITABLE WATER VALUES...AT OR ABOVE
2" OUTSIDE OF THE HIGHER TERRAIN. THIS ANOMALOUS AND DEEP MOISTURE
COMBINED WITH HIGH WET BULB ZERO HEIGHTS SHOULD RESULT IN
EFFICIENT WARM RAIN PROCESSES AND THE POTENTIAL OF HEAVY RAINFALL
RATES.

A COUPLE OF INDIVIDUAL SHORTWAVE IMPULSES WITHIN THE VORTICITY
AXIS MENTIONED ABOVE APPEAR TO BE HELPING FOCUS CONVECTION...WITH
ONE CLUSTER DEVELOPING ACROSS NORTHERN VA AND WESTERN MD...AND
ANOTHER ACROSS EASTERN KY INTO WV. LOWEST FLASH FLOOD GUIDANCE IS
ACROSS WV...AND WE ARE SEEING INCREASED CONVECTIVE DEVELOPMENT.
HOWEVER CONVECTION HS NOT BEEN THAT VIGOROUS...LIKELY DUE TO WEAK
LAPSE RATES RESULTING IN LIMITED INSTABILITY. RECENT RUNS OF THE
HRRR AND PARALLEL HRRR DO SUGGEST INCREASE CONVECTIVE
COVERAGE...BUT ARE NOT ALL THAT IMPRESSIVE WITH QPF AMOUNTS. THINK
THE LIMITED INSTABILITY MAY END UP KEEPING THE FLASH FLOOD THREAT
MARGINAL ACROSS MOST OF WV. ALTHOUGH WITH THE LOW FLASH FLOOD
GUIDANCE...HIGH PWATS AND SOME SHORT TERM TRAINING...AN ISOLATED
FLASH FLOOD RISK LIKELY STILL EXISTS.

THE GREATER INSTABILITY IS LOCATED FURTHER EAST...ACROSS VA/MD/DE.
THE CONVECTION ACROSS NORTHEAST WV/NORTHERN VA/WESTERN MD AT 18Z
IS BEGINNING TO PUSH SOUTH ALONG AN OUTFLOW. ALSO THE CONVECTION
ACROSS WV SHOULD INTENSIFY AS IT MOVES EASTWARD INTO VA AND THE
GREATER INSTABILITY POOL. THUS SHOULD SEE EXPANDING CONVECTIVE
COVERAGE ACROSS EASTERN WV INTO NORTHERN VA AS WE HEAD THROUGH THE
AFTERNOON. LOOKS LIKE WE MIGHT SEE AN AXIS OF PERIODIC TRAINING
CONVECTION SET UP FROM EASTERN WV INTO NORTHERN VA AND MD. THIS
ACTIVITY WILL ALSO LIKELY EVENTUALLY MERGE WITH THE DISCRETE
STORMS THAT HAVE FORMED AHEAD OF AN INITIAL SUBTLE SHORTWAVE...IN
A LOW LEVEL CONVERGENCE AXIS OVER THE EASTERN MID ATLANTIC.

WOULD EXPECT RAINFALL RATES GENERALLY TO PEAK AROUND 1-2" IN AN
HOUR...WITH AMOUNTS AROUND 3-4" IN A 3 TO 6 HOUR PERIOD POSSIBLE.
THE BEST CHANCE FOR THESE HIGHER AMOUNTS APPEARS TO BE ON THE
SOUTHERN PERIPHERY OF THE CURRENT PRECIPITATION SHIELD WHERE
TRAINING SHOULD BE MAXIMIZED. THE HIGHER FLASH FLOOD GUIDANCE
ACROSS THIS AREA MAY LIMIT THE OVERALL FLOOD THREAT...BUT RAINFALL
RATES AND TOTALS OF THE EXPECTED MAGNITUDE SHOULD AT LEAST POSE AN
ISOLATED FLASH FLOOD RISK. LIMITED THE MPD AREA TO REGIONS OF LOW
FFG...AND ACROSS AREAS WHERE THERE IS A GREATER TRAINING POTENTIAL
ACROSS NORTHERN VA...INTO PORTIONS OF MD AND DE. THE RECENT RUNS
OF THE HRRR AND PARALLEL HRRR HAVE A DECENT HANDLE ON
THINGS...JUST PROBABLY AN HOUR OR SO TOO SLOW.

CHENARD

ATTN...WFO...AKQ...CTP...JKL...LWX...PBZ...PHI...RLX...RNK...

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

LAT...LON   39997652 39917535 38557476 38137525 37987563 
            38227578 38307756 38047949 37948042 37528316 
            38898232 39597928 39687791