MESOSCALE PRECIPITATION DISCUSSION 0273
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1022 PM EDT FRI JUN 08 2018

AREAS AFFECTED...SERN IA...NRN MO...NWRN IL

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

VALID 090221Z - 090730Z

SUMMARY...SLOW-MOVING THUNDERSTORMS WERE BEGINNING TO DEVELOP
ACROSS THE REGION AS OF 9 PM CDT...AND MAY EXPAND IN COVERAGE INTO
THE OVERNIGHT HOURS. RAIN RATES TO AROUND 2 IN/HR WILL BE POSSIBLE
WITH THE LARGER AND MORE ORGANIZED STORMS...AND LOCALIZED RAINFALL
TOTALS COULD REACH 4 INCHES. THIS MAY LEAD TO FLASH FLOODING.

DISCUSSION...CONVECTIVE INITIATION OCCURRED QUICKLY IN NORTHERN
MISSOURI BETWEEN 0030Z AND 0130Z AND HAVE SINCE ORGANIZED INTO
SLOW-MOVING HP SUPERCELLS THAT ARE DRIFTING SLOWLY SSE. THE WIND
PROFILES ACROSS THE ENTIRE REGION ARE VERY WEAK...AND STORM
MOTIONS ARE GENERALLY AROUND 10 KNOTS. THUS FAR THE OBSERVED
MOTION OF THE SUPERCELLS IN NRN MISSOURI HAS CLOSELY MIRRORED THE
BUNKERS RIGHT-MOVING SUPERCELL VECTORS. SEVERAL OTHER ISOLATED
THUNDERSTORMS HAVE DEVELOPED FURTHER NORTH...IN PORTIONS OF
EASTERN IOWA AND NORTHWEST ILLINOIS...BUT THESE HAVE BEEN
DISORGANIZED THUS FAR.

MANY MODELS (AND OBSERVATIONAL DATA TRENDS) SUGGEST A GRADUAL
SHARPENING OF THE LOW-LEVEL WIND FIELD THIS EVENING ALONG A
SURFACE FRONT DRAPED ACROSS THE REGION. THIS MAY BE ENHANCED BY
COOLER AND MORE STABLE AIR BEING ADVECTED INTO NE/NC ILLINOIS FROM
LAKE MICHIGAN. TEMPERATURES TO THE EAST OF I-39 AND NORTH OF I-80
WERE GENERALLY LESS THAN 70 DEGREES...WITH DEWPOINTS LESS THAN
60-65 DEGREES. MEANWHILE...THE AIR MASS FURTHER TO THE SOUTHWEST
REMAINS VERY MOIST AND UNSTABLE...WITH MLCAPE VALUES AROUND
1500-2500 J/KG AND DEEPER MOISTURE WITH GPS-PW OBSERVATIONS AROUND
1.7 INCHES IN EASTERN IOWA...ABOVE THE 90TH PERCENTILE FOR
EARLY-MID JUNE. IN ADDITION TO THE SHARPENING SURFACE FRONT AND
DEEP MOISTURE IN PLACE...MODELS ALSO FORECAST AN INCREASE IN SWLY
LOW-LEVEL MOISTURE TRANSPORT FOCUSED INTO THE IA-IL BORDER REGION
BY 04-06Z. THE COMBINATION OF A MOIST UNSTABLE AIR MASS AND
INCREASING LOW-LEVEL FORCING SHOULD BE SUFFICIENT FOR CONVECTIVE
COVERAGE TO INCREASE IN THE NEXT SEVERAL HOURS.

GIVEN THAT STORMS MAY MOVE ONLY AROUND 10 KNOTS...RAIN RATES
REACHING 2 IN/HR SEEM ACHIEVABLE WITH THE BACKGROUND ENVIRONMENT
IN PLACE. HI-RES MODELS SEEM TO BE UNDERESTIMATING THIS
POTENTIAL...WITH MRMS AND KEAX/KDMX DUAL POL RATES ALREADY
REACHING THOSE VALUES WITH THE SUPERCELLS IN MISSOURI. 1-2 IN/HR
RATES COULD CONCEIVABLY BE SUSTAINED FOR A COUPLE HOURS IN ANY
GIVEN LOCATION DESPITE THE LACK OF STRONG FORCING (WITH THE SLOW
STORM MOTION) AND THUS LOCALIZED TOTALS OF 4 INCHES ALSO SEEM
POSSIBLE. THESE MAY LEAD TO FLASH FLOODING...PARTICULARLY INTO
PORTIONS OF NORTHERN ILLINOIS WHERE FLASH FLOOD GUIDANCE IS LOWER.

LAMERS

ATTN...WFO...DMX...DVN...EAX...ILX...LOT...LSX...

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

LAT...LON   42469086 42358937 41138965 40059125 39749336
            40409352 41639215