025
ACUS01 KWNS 200558
SWODY1
SPC AC 200557

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1257 AM CDT Mon May 20 2024

Valid 201200Z - 211200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
NORTHEAST COLORADO...SOUTHWEST NEBRASKA...AND FAR NORTHWEST
KANSAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
NORTHEAST ILLINOIS INTO SOUTHEAST WISCONSIN AND NORTHWEST INDIANA...

...SUMMARY...
Severe thunderstorms are expected across parts of the central Plains
this evening into the overnight hours. Other severe storms are
possible from the Middle Mississippi Valley to Lower Michigan
through early evening.

...Synopsis...
The large-scale pattern on Monday will feature a positively tilted
mid-level ridge across the eastern CONUS with a trough in the west.
A shortwave trough currently across northeast Kansas early this
morning will progress through the Midwest and into the Great Lakes
by tonight. In addition, a closed upper low across the eastern
Pacific will phase with the upper-level trough across the western
CONUS and accelerate toward the Plains by the end of the period.

A lee cyclone will deepen across the southern High Plains through
the period as mid-level troughing amplifies across the western CONUS
and mid-level flow strengthens across the Southwest. To the north of
this surface low, a stationary front will become better defined from
near the Palmer Divide to northwest Iowa with a dryline extending
southward from this front.

...Front Range into the central Plains and Upper Midwest...
A significant void in low-level moisture is present across western
Kansas and eastern Colorado early this morning in the wake of the
MCS. By late morning, moisture should start to increase across
Kansas in response to low-level moisture advection. This moisture
advection will intensify during the afternoon as the primary lee
cyclone develops across southeast Colorado and low-level flow
strengthens. By late afternoon, low-level upslope flow will develop
across northeast Colorado with dewpoints into the 50s. Within this
region of upslope flow, supercells are expected to develop by late
afternoon to early evening. Long hodographs will support the threat
for large to very large hail initially. Any storms which are surface
based, along or south of the frontal boundary, will pose some
tornado threat, particularly after 00Z as the low-level hodograph
enlarges and low-level moisture increases.

Multiple rounds of supercells are possible within the Enhanced risk,
as the initial supercells progress east, before additional
supercells likely develop in a similar region as the primary trough
ejects into the Plains. Eventually, supercells are expected to
congeal into one or more bowing MCSs with an increasing severe-wind
threat into south-central Nebraska. Some CAMs suggest the greater
coverage severe-wind threat may persist into eastern Nebraska during
the overnight period. Strong low-level moisture advection will
maintain moderate to strong instability south of the frontal zone
during the overnight period, and a strengthening low-level jet will
also aid in MCS maintenance and intensity. However, the stable
boundary layer during the overnight period casts doubt on how
widespread the severe-wind threat will be after midnight. Therefore,
the Enhanced risk is focused during the initial stages of
supercell-to-bow transition where low-level thermodynamics will be
more favorable during the evening hours.

...Northern Illinois vicinity into Wisconsin and Michigan...
A compact mid-level shortwave trough over northeastern Kansas this
morning will traverse the Midwest with ascent overspreading northern
Illinois by mid-afternoon. Once the cap erodes, expect scattered
thunderstorms to develop across central Illinois and move into
northern Illinois, southeast Wisconsin and northwest Indiana.
NAM/RAP forecast soundings show around 2000 to 2500 J/kg MLCAPE and
around 30 knots of effective shear (in a confined region near the
upper shortwave trough). This combination of instability, shear, and
ascent with the passing wave should be sufficient for severe
thunderstorms capable of primarily large hail and damaging wind
gusts. These storms are expected to weaken as they move over the
colder water of Lake Michigan and the boundary layer cools near
sunset.

...South Florida...
Scattered thunderstorms are expected across south Florida during the
afternoon and evening hours amid 1500 to 2000 J/kg MLCAPE. Shear
will be relatively weak (20 to 25 knots) and lapse rates will be
quite meager. Therefore, multicells with isolated large hail and
damaging wind gusts will be the primary hazards.

..Bentley/Grams.. 05/20/2024

$$