Graphic for MPD #0521

MESOSCALE PRECIPITATION DISCUSSION 0521
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1007 PM EDT THU JUL 20 2017

AREAS AFFECTED...CENTRAL/EASTERN IA...WESTERN/CENTRAL IL

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

VALID 210205Z - 210805Z

SUMMARY...VERY SLOW-MOVING AND BACKBUILDING THUNDERSTORMS MAY
ORGANIZE FURTHER AND EXPAND IN COVERAGE OFF TO THE EAST OVER THE
NEXT FEW HOURS. FLASH FLOODING WILL BE POSSIBLE.

DISCUSSION...VERY COLD-TOPPED CONVECTION HAS FORMED ALONG AND JUST
NORTH OF A QUASI-STATIONARY FRONT STRETCHING W/E FROM EASTERN NE
THROUGH SOUTHERN IA AND INTO CENTRAL IL. THE FRONT ALSO REPRESENTS
A SHARP INSTABILITY GRADIENT WITH A VERY UNSTABLE AIRMASS POOLED
ALONG THE BOUNDARY. MEANWHILE...PLENTY OF MOISTURE IS IN PLACE
WITH PWATS NEAR 2 INCHES AND SUPPORTED BY VERY HIGH SURFACE
DEWPOINTS RANGING FROM THE MID 70S TO THE LOW 80S.

THERE IS RATHER SIGNIFICANT DISAGREEMENT OUT OF THE HIRES MODELS
WITH HOW THIS CONVECTION WILL EVOLVE OVERNIGHT AS THE 12Z ARW
SHOWS VERY LITTLE ACTIVITY ACROSS THE MPD THREAT AREA THROUGH
06Z...AND THE 12Z NMMB SUGGESTS ANY CONVECTION SHOULD ALREADY BE
FOCUSING FARTHER EAST ACROSS PARTS OF WESTERN/CENTRAL IL. THE
BETTER PERFORMING GUIDANCE APPEARS OVERALL TO BE THE MOST RECENT
HRRR GUIDANCE AND THE 12Z NSSL-WRF...BUT THEY APPEAR TO BE
UNDERDONE WITH THEIR QPF AMOUNTS AND DOWNPLAYING THE POTENTIAL
OVERNIGHT ACROSS PARTS OF WESTERN/CENTRAL IL DOWNSTREAM OF THE
ACTIVITY IN CENTRAL/EASTERN IA. ALREADY SOME OF THE CONVECTION HAS
PRODUCED LOCALLY SEVERAL INCHES OF RAIN AS PER DUAL-POL ESTIMATES.

THE FORCING FOR THE CURRENT ACTIVITY APPEARS TO BE TIED INTO A
VERY SUBTLE/COMPACT MID LEVEL VORT THAT EJECTED EAST OUT OF
CENTRAL/EASTERN NE OVER THE LAST FEW HOURS AND IS INTERACTING WITH
THE SURFACE CONVERGENCE AND VERY STRONG INSTABILITY AXIS/GRADIENT
ALONG THE FRONTAL ZONE.

THIS ENERGY SHOULD MOVE EAST AND ALLOW FOR AT LEAST SOME INCREASED
CONVECTIVE POTENTIAL FOR MORE AREAS OF EASTERN IA AND GRADUALLY
INTO WESTERN/CENTRAL IL. VERY INTENSE RAINFALL RATES OF 2 TO 3
INCHES/HR WILL BE POSSIBLE GIVEN THE VERY HIGH LEVEL OF
INSTABILITY AND DEEP MOISTURE PROFILES. SEVERAL OVERSHOOTING TOPS
ALREADY ONGOING WITH THE CURRENT ACTIVITY WILL ONLY ENHANCE RATES
FURTHER. THE POTENTIAL IS IN PLACE FOR A MORE ORGANIZED CONVECTIVE
EVOLUTION WITH LOCALIZED 5+ INCH AMOUNTS WHICH WILL BE CAPABLE OF
PRODUCING FLASH FLOODING. WILL CONTINUE TO MONITOR THESE TRENDS
OVERNIGHT.

ORRISON

ATTN...WFO...DMX...DVN...ILX...LOT...

ATTN...RFC...MBRFC...NCRFC...OHRFC...

LAT...LON   42489341 42369258 41849088 41538907 41098773
            40488770 40108826 40269034 40569148 40809228
            41149317 41269421 41599486 42139491 42439431
           


Last Updated: 1007 PM EDT THU JUL 20 2017