SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1237 AM CDT Tue Jul 27 2021

Valid 271200Z - 281200Z


Isolated thunderstorms are possible late Tuesday across the upper
Mississippi Valley.  A conditional risk for large hail and damaging
gusts will accompany the stronger storms.  Strong to locally severe
gusts will be possible mainly this afternoon and evening for parts
of the Northeast.


Upper trough is expected to shift east across the lower Great Lakes
region into eastern NY by 28/00z. Currently this feature is located
over the UP of MI/WI along with an associated MCS. This activity
should propagate across northern lower MI very early in the period
and likely weaken considerably after sunrise. Remnants of this MCS
should move over southern ON into far western NY by early afternoon.
Latest model guidance suggests strong boundary-layer heating will
occur ahead of the short-wave trough and renewed convective
development is anticipated by 17-18z. 500mb flow will also increase
across the lower Great Lakes, and forecast soundings suggest
adequate surface-6km bulk shear for potential storm organization.
Deep westerly flow should encourage line segments with potentially
damaging wind as this activity spreads east-southeast. 

...Upper MS Valley...

In the wake of the early-day short-wave trough that ejects toward
the lower Great Lakes, weak mid-level height rises are forecast
across the upper MS Valley. Latest thinking is a surface boundary
will orient itself across southern WI (aided by the early
MCS)-southern MN-SD. Models generate very strong instability along
this frontal zone by afternoon, as surface temperatures warm through
the 90s to near 100F as far east as the southwestern corner of MN.
Inhibition will weaken along the western portion of the boundary,
and there is some indication that a weak disturbance will top the
ridge over western ND early in the period, then track southeast
toward southern MN by late afternoon. Timing of this feature
suggests isolated supercells should develop near the frontal zone
then propagate southeast. Large hail is the primary risk along with
some threat for damaging winds. It's not entirely clear how far this
activity may spread after dark, but sustained low-level warm
advection across the eastern Dakotas may contribute to additional
isolated elevated convection overnight.

..Darrow/Bentley.. 07/27/2021

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