MESOSCALE PRECIPITATION DISCUSSION 0883
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1157 PM EDT TUE OCT 10 2017

AREAS AFFECTED...VA...SRN MD AND EASTERN SHORE...FAR ERN WV

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

VALID 110356Z - 110956Z

SUMMARY...SLOW-MOVING CONVECTION SHOULD CONTINUE TO BUILD THROUGH
THE EARLY MORNING HOURS AND WILL POSE A RISK FOR LOCALLY VERY
HEAVY RAINFALL RATES UP TO 3 IN/HR AND FLASH FLOODING.

DISCUSSION...REGIONAL RADARS ARE INDICATING SHALLOW CONVECTION
GRADUALLY BUILDING IN AN ARC NEAR A QUASI-STATIONARY FRONT IN
VIRGINIA. CROSS SECTIONS THROUGH INDIVIDUAL CELLS AND
CELL-CLUSTERS (WITH LOW-ECHO CENTROID APPEARANCE) AND A LACK OF
COLD CLOUD TOPS IN IR SATELLITE IS SUGGESTIVE OF FAIRLY DOMINANT
WARM RAIN PROCESSES. THIS COULD INCREASE PRECIPITATION
EFFICIENCY...AND IS CONSISTENT WITH AN ENVIRONMENT CHARACTERIZED
BY NARROW INSTABILITY PROFILES AND HIGH PRECIPITABLE WATER VALUES.
IN FACT...PWATS ARE ANOMALOUSLY HIGH (AROUND THE 99TH PERCENTILE)
AND WERE SAMPLED TO BE AROUND 2.4 TO 2.5 INCHES BY GPS SENSORS
NEAR THE VIRGINIA TIDEWATER REGION.

THE GRADUAL UPTICK IN COVERAGE OF CONVECTION OVER THE PAST HOUR OR
SO IS SUGGESTIVE OF AN INCREASE IN FORCING...AND SIGNALS IN THE
RAP MODEL INDICATE THAT WILL LIKELY CONTINUE OVER THE NEXT FEW
HOURS. LOW-LEVEL SOUTHWESTERLY FLOW FROM THE PIEDMONT OF NORTH
CAROLINA INTO FAR SOUTHERN VIRGINIA IS PROJECTED TO INCREASE
THROUGH 09Z...WITH THE NOSE OF 850MB MOISTURE TRANSPORT EVENTUALLY
LINING UP NEAR THE ANALYZED SURFACE FRONT IN THE I-64 CORRIDOR OF
CENTRAL VIRGINIA. FURTHER ALOFT...DIFFLUENT FLOW WAS NOTED AROUND
THE NORTHERN PERIPHERY OF THE SUBTROPICAL RIDGE...AND SHOULD
CONTINUE TO MAKE THE LARGE SCALE ENVIRONMENT FAVORABLE FOR ASCENT.

DESPITE THE VERY DRY CONDITIONS ACROSS THE REGION OVER THE PAST
SEVERAL WEEKS...THE COMBINATION OF SLOW STORM MOTIONS AND
POTENTIAL FOR VERY HIGH RAIN RATES SUPPORT A LOCALIZED FLASH
FLOODING THREAT. STORM PROPAGATION VECTORS ARE FORECAST TO WEAKEN
OVER THE NEXT SEVERAL HOURS...AND OBSERVED CELL MOTIONS HAVE
ALREADY BEEN QUITE SLOW (THE SHALLOW NATURE OF THE CONVECTION IS
LIKELY CONTRIBUTING AS WELL). IF SLOWLY DRIFTING STORMS CAN ALIGN
OVER A GIVEN LOCATION FOR AN HOUR OR TWO...RAIN RATES COULD BE
QUITE SUBSTANTIAL AND PERHAPS APPROACH 3 IN/HR. KAKQ RADAR DUAL
POL ESTIMATES HAVE ALREADY SHOWN THIS ONCE WITH A CELL STALLED
OVER NW HANOVER COUNTY VIRGINIA...AND A CWOP OBSERVING SITE ALONG
THE PERIPHERY OF THE CELL IS...IF ANYTHING...SLIGHTLY LOWER THAN
THE RADAR RAINFALL ESTIMATES. RAIN RATES SUCH AS THESE...ALIGNING
WITH SMALLER/FLASHIER BASINS OR URBAN AREAS...MAY LEAD TO FLOODING
REGARDLESS OF THE RECENT DRY CONDITIONS.

LAMERS

ATTN...WFO...AKQ...LWX...RLX...RNK...

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

LAT...LON   38617888 38447640 38137514 37577540 37027568
            36687572 36627641 36607798 36617891 36617996
            36688079 37368091 38008039