MESOSCALE PRECIPITATION DISCUSSION 0527
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
650 PM EDT MON JUL 23 2018

AREAS AFFECTED...NORTHERN VA...D.C...MD...CENTRAL/EASTERN
PA...NORTHERN DE...NORTHERN NJ...CENTRAL/SOUTHERN NY

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

VALID 232250Z - 240450Z

SUMMARY...SHOWERS AND THUNDERSTORMS WILL CONTINUE INTO THE
OVERNIGHT HOURS. GIVEN THE WET ANTECEDENT CONDITIONS AND VERY
HEAVY RAINFALL RATES...FLOODING AND/OR FLASH FLOODING WILL REMAIN
LIKELY.

DISCUSSION...THE LATE-DAY VISIBLE SATELLITE IMAGERY CONTINUES TO
SHOW BROKEN AREAS OF CONVECTION IMPACTING A LARGE AREA OF THE
NORTHERN MID-ATLANTIC REGION AS PLENTY OF BOUNDARY LAYER
INSTABILITY REMAINS IN PLACE WITHIN A VERY MOIST ENVIRONMENT. THE
REGION AGAIN REMAINS EMBEDDED WITHIN A DEEP LAYER SOUTH TO
SOUTHEAST FLOW PATTERN ALOFT IN RELATION TO AN UPPER TROUGH OVER
THE SOUTHEAST U.S. AND A STRONG SUBTROPICAL RIDGE OFFSHORE OFF NEW
ENGLAND. A DEEP LAYER CHANNEL OF TROPICAL MOISTURE IS SEEN IN THE
BLENDED-TPW IMAGERY FOCUSED ALONG AND THE EAST COAST...AND THERE
HAS BEEN SOME GRADUAL WESTWARD ADVANCE OF THIS AXIS TODAY GIVEN
THE RETROGRESSIVE FLOW SEEN OVER THE WESTERN ATLANTIC. PWATS ARE
VERY HIGH AND AS HIGH AS 2.25 INCHES OVER THE MID-ATLANTIC REGION.

GOING INTO THE OVERNIGHT HOURS...THE THREAT OF VERY HEAVY SHOWERS
AND THUNDERSTORMS WILL CONTINUE AS THERE WILL ONLY BE A GRADUAL
DIMINISHING OF SURFACE-BASED INSTABILITY WHILE THE FLOW ALOFT
REMAINS BROADLY DIFFLUENT. MEANWHILE...THE PERSISTENT DEEP LAYER
SOUTH-SOUTHEAST FLOW ACROSS THE REGION INTO THE BLUE RIDGE AND
ADJACENT AREAS OF THE CENTRAL APPALACHIAN PLATEAU WILL FAVOR
SUFFICIENT OROGRAPHIC FORCING FOR ADDITIONAL CONVECTIVE
ENHANCEMENT. THE NEAR UNIDIRECTIONAL FLOW THROUGH THE COLUMN WILL
ALSO FAVOR SOME NW/SE OR N/S ORIENTED CONVECTIVE BANDS THAT WILL
BE CAPABLE OF TRAINING OVER THE SAME AREA AND ESPECIALLY WITH THE
AID OF SOME RELATIVELY ENHANCED SURFACE CONVERGENCE WHICH THE
LATEST SUITE OF HRRR AND 12Z/18Z CAM GUIDANCE HAS BEEN SUPPORTING
FOR OVERNIGHT.

RAINFALL RATES WILL BE VERY HIGH GIVEN THE EXTREMELY EFFICIENT
RAINFALL PROCESSES THAT WILL BE IN PLACE...AND THERE WILL AGAIN BE
THE POTENTIAL FOR THESE RATES TO REACH 2 TO 3 INCHES/HR WITHIN THE
MORE ORGANIZED CONVECTIVE BANDS AND WHERE OROGRAPHIC FORCING IS
NOTED. ADDITIONAL RAINFALL TOTALS OF AS MUCH AS 3 TO 4+ INCHES
WILL BE POSSIBLE FOR THIS VALID TIME PERIOD. THESE AMOUNTS WILL
LIKELY CAUSE MORE AREAS OF FLOODING AND/OR FLASH FLOODING GIVEN
THE VERY MOIST IF NOT SATURATED SOIL CONDITIONS.

ORRISON

ATTN...WFO...AKQ...ALY...BGM...BUF...CTP...LWX...OKX...PHI...

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

LAT...LON   44097529 43877472 42957410 42157400 41487413
            40617458 39707547 38477623 38167661 37977737
            38627798 40147828 41227851 42167847 42997804
            43147729 43267677 43577624 44027583