SPC MD 1578

MD 1578 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PARTS OF NORTHEASTERN MINNESOTA...NORTHERN WISCONSIN AND THE WESTERN UPPER PENINSULA OF MICHIGAN

Mesoscale Discussion 1578
NWS Storm Prediction Center Norman OK
1140 PM CDT Tue Aug 25 2020

Areas affected...Parts of northeastern Minnesota...northern
Wisconsin and the western Upper Peninsula of Michigan

Concerning...Severe potential...Watch unlikely 

Valid 260440Z - 260715Z

Probability of Watch Issuance...20 percent

SUMMARY...Intensifying thunderstorm development is possible
overnight, particularly near the western Wisconsin/Upper Michigan
border area by 2-3 AM CDT.  This may include a couple hour period
where storms may become capable of producing severe hail.  It is not
anticipated that a severe weather watch will be needed, but trends
will be monitored.

DISCUSSION...Nocturnal southwesterly low-level jet strengthening
(30-40 kt at 850 mb) has occurred across the mid Missouri Valley
toward the western Lake Superior vicinity.  A gradual veering of
this jet to a greater west-southwesterly component is likely across
the Upper Midwest through 06-08z, while short wave perturbations
also begin to flatten mid-level ridging near the central
Canadian/U.S. border area.

Isentropic ascent near and north/east of the nose of the low-level
jet is already contributing to the initiation of thunderstorm
activity (west-southwest of Hibbing MN as of 0430Z).  Activity may
still be suppressed by inhibition associated with a plume of warm
elevated mixed-layer air, and this may remain the case another
couple of hours.  However, models suggest a more substantive
intensification of thunderstorm activity is possible closer to
07-08Z, as this forcing spreads east-southeastward toward the
Wisconsin/Upper Michigan state border vicinity.  This appears to be
closer to the northeastern edge of the strong capping, and may
coincide with a rapid increase in low-level moisture associated with
the 850 mb jet.

CAPE for moistening parcels rooted above a stable boundary-layer may
increase in excess of 2000 J/kg, in the presence of sufficient shear
for mid-level updraft rotation, beneath moderate to strong
northwesterly mid/upper flow.  Severe hail may be the primary severe
hazard in initial stronger development, before activity increases in
coverage/grows upscale.  The boundary layer may remain too cool to
support an appreciable risk for strong surface gusts associated with
downbursts.  However, trends will continue to be monitored for the
possibility of convective organization which could support
increasing risk for strong surface gusts overnight.

..Kerr/Grams.. 08/26/2020

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...MQT...GRB...DLH...

LAT...LON   48039281 46598835 45948697 45158765 46049069 46949303
            48039281 

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