MESOSCALE PRECIPITATION DISCUSSION 0422
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
551 PM EDT SUN JUL 08 2018

AREAS AFFECTED...FAR ERN DAKOTAS...NRN MINNESOTA

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

VALID 082149Z - 090330Z

SUMMARY...THUNDERSTORMS ARE EXPECTED TO DEVELOP LATE THIS
AFTERNOON ACROSS NORTHERN MINNESOTA AND PERHAPS BACK INTO THE FAR
EASTERN DAKOTAS. THESE STORMS WILL GRADUALLY SHIFT SOUTH WITH
TIME...BUT MAY REPEAT OVER SIMILAR AREAS ENOUGH TO PRODUCE HEAVY
RAINFALL. RAIN RATES OF 1-2 IN/HR WILL BE POSSIBLE WITH LOCALIZED
TOTALS OVER 3 INCHES...AND THIS MAY LEAD TO SOME FLASH FLOODING.

DISCUSSION...HI-RES MODELS ARE IN EXCELLENT AGREEMENT THAT
THUNDERSTORMS WILL DEVELOP IN THE NEXT FEW HOURS OVER NRN MN...AS
LOW-LEVEL SOUTHERLY INFLOW INCREASES AND A SURFACE COLD FRONT
SHIFTS SOUTH OF THE INTERNATIONAL BORDER. THE 12Z HREF SHOWS
NEIGHBORHOOD PROBABILITIES OF 1 INCH OF RAIN BETWEEN 21-00Z AS
HIGH AS 70-90 PERCENT IN PARTS OF NORTH-CENTRAL MINNESOTA AND THE
HRRR TIME-LAGGED ENSEMBLE IS SLIGHTLY LOWER ON THE VALUES BUT IN
GOOD AGREEMENT ON THE GENERAL LOCATION (LAKES COUNTRY NEAR
WADENA...ENE TO THE IRON RANGE NEAR EVELETH). AS THE FRONT
CONTINUES TO GRADUALLY PUSH SOUTH DURING THE EVENING...THE
CONVECTION IS EXPECTED TO SIMILARLY SHIFT SOUTH FROM THE INITIAL
DEVELOPMENT LOCATION...ALTHOUGH THIS TREND SHOULD BE RELATIVELY
GRADUAL. AS OF 21Z...THERE WAS ONLY A WEAK BAND OF CONVECTION FROM
NEAR KBAZ TO NEAR KFOZ...AND GOES-16 VISIBLE SATELLITE SHOWED
LIMITED CLOUD DEVELOPMENT BEYOND 150KM SOUTH OF THAT DESPITE
CONSIDERABLE CONDITIONAL INSTABILITY. THIS MAY BE DUE TO A WARM
LAYER AROUND 850-700MB NOSING INTO SOUTH DAKOTA AND CENTRAL
MINNESOTA. THE EDGE OF THIS CAP MAY GRADUALLY ERODE THROUGH
CONVECTIVE DEVELOPMENT AND SUSTAINED VERTICAL MOTION...BUT FOR
SEVERAL HOURS THE CAP SHOULD SOMEWHAT LIMIT THE SOUTHWARD EXTENT
OF CONVECTION.

AS THE LLJ INCREASES AROUND 00Z...IT SHOULD VEER TO A MORE
WESTERLY DIRECTION AS IT APPROACHES THE FRONT...AND PRODUCE MORE
UNI-DIRECTIONAL WIND PROFILES IN THE CORRIDOR OF EXPECTED
CONVECTIVE DEVELOPMENT. AS THE FRONT CONTINUES TO PUSH SOUTH AT
THE SAME TIME...THIS SHOULD RESULT IN AN INCREASINGLY FOCUSED
RIBBON OF RAIN AND CONVECTION AND A NARROWING OF THE PLUME OF
HIGHER PWS. THE PRECIPITABLE WATER VALUES ARE ANALYZED BY THE RAP
MODEL TO BE AROUND 2 INCHES...WHICH IS ABOVE THE 95TH PERCENTILE
PER CFSR CLIMATOLOGY. THUS...A FOCUSED BAND OF HEAVY RAINFALL
SEEMS SOMEWHAT LIKELY THIS EVENING. GIVEN THE ELEVATED PWS...RAIN
RATES APPROACHING 2 IN/HR WILL BE POSSIBLE WHICH COULD LEAD TO
FLASH FLOODING.

THE PRIMARY UNCERTAINTY RELATES TO THE EROSION OF THE CAP AND JUST
HOW QUICKLY THE FRONT WILL BE ABLE TO ADD A MORE SUBSTANTIAL
NORTH-SOUTH COMPONENT TO THE MOTION OF THE CONVECTIVE BAND. SOME
HI-RES MODELS ARE MORE AGGRESSIVE IN SHOWING THIS (SUCH AS THE
NSSL WRF)...BUT THEY MAY BE TOO FAST. IF THE MORE FOCUSED CHANNEL
OF HIGH PW AIR CAN BE MAINTAINED AT A SIMILAR LATITUDE FOR SEVERAL
HOURS...FLASH FLOODING WOULD BE MORE LIKELY. ANOTHER UNCERTAINTY
RELATES TO POCKETS OF HIGHER FFG IN THE BRAINERD LAKES REGION. IF
THE CONVECTIVE BAND HAS A STEADY DRIFT...IT MAY BE SUFFICIENT TO
PREVENT EXTREMELY HIGH RAINFALL TOTALS AT ANY ONE LOCATION.

OVERALL THOUGH...IT DOES APPEAR THERE WILL BE A CHANCE OF FLASH
FLOODING THIS EVENING. THE BEST CHANCES LOOK TO BE NEAR THE NOSE
OF THE INCREASING LOW-LEVEL FLOW WHERE IT ALIGNS WITH THE EDGE OF
THE CAP (AROUND +10 TO +11C AT 700MB) WHICH WOULD LIKELY BE IN
PORTIONS OF NORTH-CENTRAL MINNESOTA.

LAMERS

ATTN...WFO...ABR...BIS...DLH...FGF...MPX...

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

LAT...LON   48109389 47959006 47009090 46319235 45749409
            45319620 45559780 46649804 47619681