MESOSCALE PRECIPITATION DISCUSSION 0855
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
720 PM EDT MON OCT 02 2017

AREAS AFFECTED...NW IOWA...SE SOUTH DAKOTA...CTRL AND ERN NEBRASKA

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

VALID 022318Z - 030500Z

SUMMARY...TRAINING THUNDERSTORMS SHOULD DEVELOP THIS EVENING ALONG
AND NORTHWEST OF A QUASI-STATIONARY FRONT...AND MAY LEAD TO AREAS
OF FLASH FLOODING WITH RAIN RATES UP TO 2 IN/HR.

DISCUSSION...IR SATELLITE IMAGERY AT 23Z SHOWED NEW UPDRAFT GROWTH
TO THE NORTHWEST OF A QUASI-STATIONARY SURFACE FRONT OVER PORTIONS
OF SOUTH-CENTRAL AND EASTERN NEBRASKA. THESE NEW UPDRAFTS WERE
FORMING IN A CORRIDOR THAT EXTENDED SOUTHWEST FROM ONGOING
CONVECTION IN MINNESOTA...AND ROUGHLY ALIGNED WITH A MAXIMUM IN
FRONTOGENESIS IN THE 925MB LAYER. HI-RES MODELS HAVE BEEN FAIRLY
CONSISTENT IN DEVELOPING A PROMINENT BAND OF CONVECTION THIS
EVENING ALONG THE 925MB FGEN MAX...BUT RECENT HAVE BEEN GRADUALLY
SHIFTING THE LOCATION OF THE LOW-LEVEL FORCING FURTHER SOUTHEAST.
SOME OF THE NEW CELL GROWTH NOTED ON IR SATELLITE HAS BEEN ON THE
SOUTHEASTERN PERIPHERY OF MEASURABLE RAINFALL IN THE HREF ENSEMBLE
BLENDED TOTALS. THEREFORE...THE OUTLINED DISCUSSION AREA HAS BEEN
DRAWN TO EXTEND SOUTHEAST OF THE PRIMARY ACCUMULATIONS ON MOST OF
THE HI-RES MODELS TO ACCOUNT FOR POSITIONAL UNCERTAINTY IN WHERE
THE CONVECTIVE BAND WILL SET UP.

OVERALL THE INGREDIENTS ACROSS THE REGION ARE FAVORABLE FOR
TRAINING CONVECTION...FOCUSED BANDS OF HEAVY RAINFALL...AND
POSSIBLY FLASH FLOODING IF THEY CAN PERSIST LONG ENOUGH. THE
LOW-LEVEL FORCING (AS APPROXIMATED BY FGEN AND MOISTURE
CONVERGENCE) IS PROJECTED TO INCREASE SUBSTANTIALLY IN THE 00-03Z
TIMEFRAME BY MOST MODELS...COINCIDING WITH THE NOCTURNAL INCREASE
IN THE LLJ AND A STRONGER SHORTWAVE ALOFT IN COLORADO EJECTING
INTO THE PLAINS. IN RESPONSE TO THE EJECTING WAVE...THE LOW-LEVEL
FRONTS MAY SHIFT BACK A BIT TOWARD THE NORTHWEST THIS
EVENING...ALTHOUGH THIS MAY COMPETE WITH RAIN-COOLED AIR FROM
ONGOING CONVECTION TENDING TO ANCHOR THE SURFACE FRONT IN PLACE.

THE BOTTOM LINE IS THAT WE ARE LIKELY TO SEE A WELL-DEVELOPED
CONVECTIVE BAND BY LATE THIS EVENING...ROUGHLY ALIGNED WITH THE
MEAN FLOW VECTORS...AND THUS CAPABLE OF PRODUCING FLASH FLOODING.
HI-RES MODELS SHOW AREAS OF UP TO 2 IN/HR RAIN RATES AND THIS
SEEMS REASONABLE GIVEN THE POTENTIAL FOR TRAINING...STRONG
MOISTURE FLUX...AND PRECIPITABLE WATER VALUES ABOVE 1.5 INCHES.

LAMERS

ATTN...WFO...DMX...FSD...GID...LBF...OAX...TOP...

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

LAT...LON   43889644 43599598 43459517 43479451 43389413
            42839445 42229531 41709577 41099623 40459670
            40019734 39989842 40139915 40389975 40609994
            41149978 41889909 42819820 43489745 43829687