MESOSCALE PRECIPITATION DISCUSSION 0523
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1257 PM EDT MON JUL 23 2018

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

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

VALID 231655Z - 232255Z

SUMMARY...SHOWERS AND THUNDERSTORMS WILL BECOME MORE NUMEROUS THIS
AFTERNOON. HEAVY RAINFALL RATES AND LOCALLY SATURATED SOIL
CONDITIONS WILL LIKELY PROMOTE SOME FLASH FLOODING.

DISCUSSION...THE LATEST VISIBLE SATELLITE IMAGERY IS SHOWING AN
INCREASE IN CU/TCU DEVELOPMENT ACROSS MUCH OF THE NORTHERN
MID-ATLANTIC REGION AS DIURNAL HEATING CONTINUES TO DESTABILIZE
THE BOUNDARY LAYER WITHIN A VERY MOIST ENVIRONMENT. THE REGION IS
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 THE EAST COAST. A DEEP LAYER
CHANNEL OF TROPICAL MOISTURE IS SEEN IN CIRA-LPW DATA SETS AND
BLENDED-TPW IMAGERY...WITH A HIGHLY CONCENTRATED DEGREE OF
MOISTURE IN THE 500/300 MB LAYER. THIS IS CONTRIBUTING TO PWATS
THAT ARE NEAR AND LOCALLY EXCEEDING 2.25 INCHES.

OVER THE NEXT SEVERAL HOURS...SHOWERS AND THUNDERSTORMS WILL
BECOME MORE NUMEROUS AS ADDITIONAL BOUNDARY LAYER HEATING ENSUES
AND COUPLES WITH A RATHER BROADLY DIFFLUENT FLOW PATTERN ALOFT.
DEEP LAYER UNIDIRECTIONAL FLOW ACROSS THE REGION WILL PROMOTE SOME
NW/SE OR N/S ORIENTED CONVECTIVE BANDS THAT WILL BE CAPABLE OF
TRAINING OVER THE SAME AREA. THIS COUPLED WITH THE AID OF
OROGRAPHIC ASCENT OVER THE SOME OF THE HIGHER TERRAIN OF THE
CENTRAL APPALACHIANS AND BLUE RIDGE WILL FAVOR ENHANCED RAINFALL
AMOUNTS THIS AFTERNOON...WITH RAINFALL RATES AS HIGH AS 2 TO 3
INCHES/HR WITHIN SOME OF THE MORE VIGOROUS CONVECTIVE CELLS.

THE LATEST HRRR GUIDANCE AND SUITE OF 12Z CAM GUIDANCE SUGGESTS
MULTIPLE BANDS OF AS MUCH AS 2 TO 3 INCHES OF RAIN THROUGH
21Z...BUT THESE TOTALS COULD BE A LITTLE TOO CONSERVATIVE GIVEN
THE VERY HIGH PWAT ENVIRONMENT AND POTENTIAL FOR TRAINING BANDS.
WILL EXPECT TO SEE LOCALLY 3 TO 4+ INCHES AS A RESULT BASED ON THE
SET-UP...AND THESE AMOUNTS WILL LIKELY CAUSE SOME FLASH FLOODING
GIVEN THAT MUCH OF THE REGION IS SEEING VERY MOIST IF NOT
SATURATED SOIL CONDITIONS.

ORRISON

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

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

LAT...LON   42877693 42867616 42567539 42417450 41887401
            41447383 40937403 40267473 39697534 38817594
            38487671 38627744 38967797 39337835 40197874
            41307887 42437817