WPC Met Watch |
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Mesoscale Precipitation Discussion: #0613 (2020) |
(Issued at 615 PM EDT Sun Aug 09 2020
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MPD Selection |
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Mesoscale Precipitation Discussion 0613
NWS Weather Prediction Center College Park MD
615 PM EDT Sun Aug 09 2020
Areas affected...northern WI into the western U.P. of MI
Concerning...Heavy rainfall...Flash flooding possible
Valid 092208Z - 100345Z
SUMMARY...Scattered thunderstorms with brief training may produce
flash flooding across portions of northern WI into the western
U.P. of Michigan. Rainfall rates of 1-2 inches in 30 to 60 minutes
with storm totals of 2-4 inches are possible through 03Z.
DISCUSSION...Regional radar imagery from 22Z showed a small
cluster of convection over far northern WI, just ahead of an MCV
near the northern WI/MN border. In addition, scattered storms were
observed near the western WI/MI border, out ahead of the small
convective cluster. Moisture was anomalously high across the
region with PWATs over 1.5 inches, but visible satellite imagery
has shown several failed attempts to develop convection so far in
much of northern WI, except for immediately ahead of the MCV
track. This is likely due to a warm capping layer noted on RAP
analysis soundings between 850-700 mb that weakens with northward
extent. Cloud cover over central WI may continue to limit
convective development over much of central WI this evening.
Vertical shear supports a mixture of storm modes but storm motions
have so far averaged near LFC-EL mean winds of ~30 kt from the
southwest. While the MCV track will likely remain progressive
toward the ENE, a corridor of stronger 850 mb winds, in excess of
30 kt, was noted via VAD wind plots and RAP forecasts to the south
of the MCV along the MN/WI border. The northern fringes of this
region of stronger low level flow will help to support training of
convection, but the time scale should be limited by the fairly
progressive nature of the MCV track. The other potential for flash
flooding rainfall will come from mergers and repeating of cells
that develop out ahead of the main axis of thunderstorms located
just ahead of the MCV.
Earlier rainfall in the vicinity of the Minneapolis-St. Paul
metropolitan area was noted to be rather efficient with rates of
about 1 inch in 20-30 minutes. Similar intense rainfall rates will
be possible in northern WI and the U.P. of Michigan where MLCAPE
is estimated to be 1000-2000 J/kg (SPC mesoanalysis). While the
coverage of flash flooding is expected to remain small, low flash
flood guidance values across portions of northern WI and the U.P.
of MI warrant a flash flood risk over the next few hours.
Otto
ATTN...WFO...ARX...DLH...GRB...MPX...MQT...
ATTN...RFC...NCRFC...NWC...
LAT...LON 47618841 47518762 47158740 46818738 46268796
45778877 45478987 45269089 45339152 45699189
46349167 47059092 47488930
Last Updated: 615 PM EDT Sun Aug 09 2020
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