Graphic for MPD #0813

MESOSCALE PRECIPITATION DISCUSSION 0813...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
408 PM EDT TUE SEP 19 2017

CORRECTED FOR TYPOS IN THE FIRST PARAGRAPH

AREAS AFFECTED...EASTERN DAKOTAS & WESTERNMOST MN

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

VALID 192002Z - 200202Z

SUMMARY...ORGANIZED CONVECTION WITH INCREASING COVERAGE IS
EXPECTED TO LEAD TO HEAVY RAIN.  HOURLY RAIN TOTALS TO 1.75" WITH
LOCAL AMOUNTS TO 4" ARE EXPECTED.

DISCUSSION...THE SURFACE ANALYSIS REVEALS AN OCCLUDED FRONT MOVING
INTO THE CENTRAL DAKOTAS WHICH IS ACTING AS THE BACK EDGE OF THE
ML CAPE GRADIENT SEEN ON RAP ANALYSES/FORECASTS AND THE SPC
MESOANALYSIS.  INFLOW AT 850 HPA AHEAD OF THIS FEATURE IS 25-50
KTS PER VAD WIND PROFILES.  EFFECTIVE BULK SHEAR OF 30-50 KTS IS
SEEN ACROSS THE REGION, WHICH WHEN COMBINED WITH FAIRLY
UNIDIRECTIONAL SOUTHERLY FLOW UP TO 600 HPA IS LEADING TO
CONVECTIVE LINE SEGMENTS.  THE CONVECTION/OCCLUDED FRONT ARE BEING
FORCED EASTWARD BY A SLOWLY PROGRESSIVE UPPER TROUGH WHICH IS
BECOMING NEGATIVELY TILTED.  PRECIPITABLE WATER VALUES OF 1-1.5"
INHABIT THE REGION PER RAP ANALYSES AND SUPPLEMENTAL 18Z SOUNDINGS
FROM SD.

WITH INCREASING LOW-LEVEL INFLOW OVER THE NEXT SEVERAL HOURS,
BACKBUILDING CONVECTION IS POSSIBLE, WHICH WOULD ENHANCE RAIN
RATES/TOTALS.  AS CIN CONTINUES TO ERODE, CONVECTION SHOULD BECOME
INCREASINGLY SURFACE BASED.  IN SUCH A DYNAMIC ENVIRONMENT,
EAST-SOUTHEASTWARD PROPAGATION AT 30-35 KTS (PER FORWARD
PROPAGATING VECTORS/THE ML CAPE ORIENTATION/THE 1000-500 HPA
THICKNESS ORIENTATION) IS NORMALLY EXPECTED.  HOWEVER, WITH THE
LOW-LEVEL FLOW OUT OF THE SOUTH-SOUTHEAST, SUCH PROPAGATION COULD
BE HELD UP FROM TIME TO TIME.  THE AVAILABLE BULK SHEAR SUPPORTS
MESOCYCLONE FORMATION, WHICH WOULD ALSO ENHANCE RAIN TOTALS/RATES.
 RECENT RADAR IMAGERY GIVES AN INDICATION THAT MULTIPLE ROUNDS OF
THUNDERSTORMS ARE POSSIBLE WITHIN THE DISCUSSION AREA.

THE MESOSCALE GUIDANCE SHOWS A STRONG SIGNAL FOR LOCAL AMOUNTS IN
THE 3-4" THROUGH 02Z.  THE 12Z SSEO NEIGHBORHOOD PROBABILITIES
SHOW A BROAD AREA OF 50%+ EXCEEDANCE OF THE THREE HOUR FLASH FLOOD
GUIDANCE VALUES STARTING IN THE 22-01Z PERIOD, REACHING 70%+
EXCEEDANCE IN THE 23-03Z TIME FRAME.  THUNDERSTORMS APPEAR TO BE
FORMING A COUPLE HOURS EARLIER THAN THE GUIDANCE SUGGESTED, SO
THESE TIME FRAMES MIGHT BE TOO DELAYED.  USED THE 9C ISOTHERM AT
700 HPA AND BOUNDARY LAYER MOISTURE CONVERGENCE RAP FORECASTS TO
HELP DEFINE THE SOUTHERN BOUND OF THE DISCUSSION AREA AS IT
APPEARS TO BE ACTING AS THE NORTHERN EDGE OF THE MID-LEVEL CAPPING
INVERSION -- NORTH OF THERE CONVECTION HAS BEEN SHOWING INCREASING
COVERAGE AS OF LATE.

ROTH

ATTN...WFO...ABR...BIS...FGF...

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

LAT...LON   49079927 48889683 46859615 45539629 44989729
            44769900 44929962 45260004 46430047 48390054
            48940021


Last Updated: 408 PM EDT TUE SEP 19 2017