SPC 0600Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1248 AM CDT Wed Sep 15 2021

Valid 161200Z - 171200Z


Isolated to scattered severe storms may occur Thursday into Thursday
night across parts of the northern Plains and Upper Midwest, with
damaging winds and large hail the main threats.

...Northern Plains into the Upper Midwest...
A large-scale upper trough/low will advance slowly eastward across
central Canada and the north-central CONUS on Thursday. At the
surface, a cold front should move southeastward across parts of the
northern Plains and Upper Midwest through the period. Some guidance
suggests that a secondary surface low will form along the front over
the eastern Dakotas during the day, and subsequently develop
northeastward into adjacent parts of Ontario by Thursday evening.
Low-level moisture, with surface dewpoints generally in the low to
mid 60s, should advect northward across much of MN ahead of the
front. Diurnal heating of this moist low-level airmass, along with
steepening mid-level lapse rates from an EML emanating from the
Rockies and High Plains, will likely result in moderate to strong
instability developing in a narrow corridor ahead of the front by
peak afternoon heating.

This heating should also act to erode the convective inhibition
associated with the EML, and current expectations are for isolated
to scattered storms to develop along the cold front by late Thursday
afternoon or early evening. Rather strong wind fields at both low
and mid levels attendant to the upper trough appear likely to
overspread the warm sector by the time convective initiation occurs.
The strongest effective bulk shear (50+ kt) should be located across
northern MN, closer to the upper trough in Canada. Around 30-40 kt
of deep-layer should be present farther south into central/southern
MN and the northern Plains.

While there are still some concerns about overall storm coverage,
any convection that forms along or ahead of the cold front appears
likely to become severe given the favorable thermodynamic and
kinematic environment. Initial supercells may pose an isolated large
hail threat, but storms will probably grow upscale fairly quickly
along the front by Thursday evening and have more of a
severe/damaging wind threat as they move east-southeastward. Given
the strength of the low-level flow and related shear, a tornado or
two also appears possible, both with any initially discrete
supercells or low-level circulations embedded within the line. The
eastward extent of the severe threat Thursday evening/night will
likely be constrained by rapidly increasing convective inhibition
across southeastern MN/western WI. Finally, an isolated hail/wind
threat may also exist along the length of the front across parts of
southern SD into northern NE, where a strengthening low-level jet
should aid storm development by late Thursday evening.

The remnants of tropical cyclone Nicholas should be located over LA
at the start of the period Thursday morning per latest NHC advisory.
The surface low is expected to be rather weak by this time, and the
low-level wind field to the east of the low should gradually
diminish throughout the day. The potential for even isolated severe
storms along/near the central Gulf Coast still appears too low to
include any severe probabilities.

..Gleason.. 09/15/2021

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