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Date : 04 Jun 20 07:45:17
From : COD Weather Processor 1:2320/105
To : wx-storm@lists.illinois.edu
Subj : DAY3SVR: Nws Storm Prediction Center Norman Ok
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ACUS03 KWNS 040745
SPC AC 040744

Day 3 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0244 AM CDT Thu Jun 04 2020

Valid 061200Z - 071200Z



Severe thunderstorms capable of producing large hail, damaging wind
gusts and a tornado threat will be possible in parts of the northern
Plains Saturday afternoon and evening.

...Northern Plains...
An upper-level ridge will amplify and move quickly northeastward
across the northern Plains on Saturday. At the surface, a low will
deepen in the northern Montana as a warm front moves northward into
the northern Plains. Along and south of the warm front, a moist
airmass will be in place across much of South Dakota. A narrow
corridor of maximized low-level moisture will exist across eastern
and northern Montana. Strong instability is forecast to develop to
the south of the warm front Saturday afternoon across parts of
southern and western South Dakota with moderate instability further
northwestward along the moist axis. Large-scale ascent will increase
across the northern Plains during the day as a shortwave trough
approaches from the southwest. Although a capping inversion is
forecast to be in place across much of the instability corridor, the
cap should weaken by late afternoon. This combined with increasing
large-scale ascent and low-level convergence along the warm front
should result in surface-based convective initiation across the
instability corridor Saturday afternoon from southern and western
South Dakota into central and eastern Montana. Other thunderstorms
may develop further to the northeast in the cooler airmass across
northeastern South Dakota and southern North Dakota. Thunderstorms
are expected to grow upscale and move northeastward across the
northern Plains from late Saturday afternoon through the evening.

Forecast soundings along the instability axis by 22Z on Saturday to
the east of Rapid City, South Dakota show moderate to strong
instability with MLCAPE in the 2500 to 4500 J/kg range, lower or
higher depending upon which model solution you look at. The wind
profile shows directional shear from the surface to 700 mb with
strong speed shear in the mid-levels. The wind profile will likely
support supercell development. 700-500 mb lapse rates of 7.5 C/km
will be favorable for large hail with the more intense cores. The
large hail threat along with a possibility of hailstones greater
than 2 inches in diameter, is forecast to be greatest across western
and central South Dakota along the corridor with the strongest
instability. Large hail will also be possible in central and
northern Montana but weaker instability there may keep the threat a
bit more isolated. A few tornadoes may occur as supercells mature
and move northeastward into strengthening low-level flow during the
late afternoon and early evening. The wind damage threat will be
maximized with the stronger supercells. This wind damage threat
should become greater during the early to mid evening, especially if
a transition to linear mode occurs.

..Broyles.. 06/04/2020


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