Mesoscale Precipitation Discussion 0922
NWS Weather Prediction Center College Park MD
334 PM EDT Wed Oct 03 2018

Areas affected...northern WI into the U.P. of Michigan

Concerning...Heavy rainfall...Flash flooding possible

Valid 031933Z - 040130Z

Summary...Increasing coverage and intensity of thunderstorms
across northern Wisconsin into the U.P. of Michigan are expected
through 00Z. 1-3 inches of rain, locally higher, can be expected
for these locations which may cause flash flooding through 02Z.

Discussion...19Z surface observations showed a warm front lifting
north across eastern MN into WI, coincident with a line of
thunderstorms over northwestern WI which were rapidly moving off
toward the northeast with the 850-300 mb mean flow of 45-55 kt.
The 18Z SPC mesoanalysis showed that MLCAPE had increased to
500-1000 J/kg along and south of the warm front in central WI with
area dewpoints in the upper 60s to low 70s. Needless to say that
moisture was rather anomalous in the region for early October,
both MPX and GRB 12Z soundings showed precipitable water values in
the 95th percentile.

Instability is forecast to continue increasing through this
evening through eastern WI into the U.P. of MI into the 1000+ J/kg
range. Deep layered southwesterly unidirectional flow is
supportive of repeating segments of thunderstorms but their
forward speed will initially temper flash flood potential. As a
shortwave currently over western ND continues to advance east into
northern MN by 03Z, per recent model consensus, 850 mb winds are
expected to increase into the 50-65 kt range by 00Z across WI (via
18Z RAP) which should help to allow periods of repeating/training
heavy rain to develop as low level wind speeds approach the
850-300 mb mean wind.

Initial warm advection forced convection with repeating segments
of heavy rain will be possible across northern WI and the U.P. of
MI with an increasing coverage and intensity as CAPE values rise
into the evening. This will be followed by a second round of
thunderstorms associated with the cold front itself. While the
cold front will be rather progressive, a couple of rounds of heavy
rain through 02Z with rainfall rates peaking in the 1-2 in/hr
range may allow for local exceedance of 3-hr Flash Flood Guidance
values which were as low as 1.5 inches in several locations.

Otto

ATTN...WFO...APX...ARX...DLH...GRB...MPX...MQT...

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

LAT...LON   47728815 47478627 46998502 46498467 46008471
            45568587 44898764 44748972 44919194 46309270
            47138973