940
ACUS01 KWNS 201248
SWODY1
SPC AC 201247

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Mon May 20 2024

Valid 201300Z - 211200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF NORTHEASTERN COLORADO...CENTRAL/SOUTHWESTERN NEBRASKA...AND
NORTHWESTERN KANSAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK...AND ALSO OVER PARTS OF THE SOUTHWESTERN LAKE
MICHIGAN VICINITY...

...SUMMARY...
The greatest severe-thunderstorm threat today appears to be over
parts of the central Plains, with large to very large hail, severe
gusts (some 75 mph or greater), and a few tornadoes all possible.

...Synopsis...
Mid/upper-level troughing over the western CONUS is expected to
shift slowly eastward through the period, as an associated, strong
shortwave trough pivots southeastward from the interior Northwest to
WY/UT.  A southern-stream trough -- apparent in moisture-channel
imagery over the Pacific west of southern CA/northern Baja -- should
eject northeastward and reach AZ by 00Z, then the central High
Plains by 12Z tomorrow.  A broad fetch of difluent flow aloft will
extend downstream across the Great Plains States, with westerlies
over OK and southwesterlies from CO to the Upper Midwest.
Meanwhile, a positively tilted trough will drift southeastward over
the northern/central FL Peninsula.

The 11Z surface map showed a low in the FSD/SUX region, with
quasistationary front southwestward to another low over southwestern
KS, then arching westward into southeastern CO.  Cyclogenesis should
occur along the western boundary today, over eastern CO, as well as
tonight along a dryline over the northern TX/western OK Panhandles.
The lows should combine along the front, over western KS, by the end
of the period.  A warm front was drawn from the Siouxland low across
southeastern IA, and should move northeastward over northern IL
today.

...Central Plains...
Thunderstorm development is expected by late afternoon, initially
over higher terrain and in post-frontal convergence/upslope-lift
zones near the Palmer Divide and/or Cheyenne Ridge.  Some of this
activity should move east-northeastward over northeastern CO and
adjoining portions of NE, with both supercells and upscale-
aggregating storm cluster(s) expected.  Multiple rounds of
convection may cross the region in and near the "Enhanced" area
through this evening.  Large to very large hail and a few tornadoes
are possible from any sustained supercells.  Severe gusts may occur
from supercell downdrafts, but should be more common once cells
evolve to clusters and cold pools combine.  Some significant (65+
kt) gusts are possible.  Activity should move/grow into a corridor
of favorable instability across and south of the front in NE, with
more convection developing near the front tonight.  Isolated
development also is possible near the dryline over eastern
CO/western KS, but should be shorter-lived.

Large-scale ascent aloft (ahead of the approaching Intermountain
West trough) and low-level lift each should increase across the
outlook area throughout the afternoon and into the evening.  The
boundary layer also should destabilize (preferentially more at
higher altitudes) diurnally -- though the influence of moisture will
contribute to increasing CAPE eastward across CO to the southwestern
NE/northwestern KS area.  The airmass-recovery process over that
region will continue for most of the day, and the threat may develop
and peak later in the day/evening than usual for this region.
Relatively moisture-rich boundary-layer trajectories (still bearing
the imprint of some upstream convective modification) may not advect
into the area until late afternoon or evening.  Still, with steep
midlevel lapse rates attributable to the EML, MLCAPE values commonly
around 2000 J/kg should develop, amidst 40-50 kt effective-shear
magnitudes.  Long hodographs will support the hail threat.
Nocturnal boundary-layer stabilization and delayed airmass recovery
with eastward extent make probabilities more conditional into
eastern NE, IA and southern MN.

...Upper Great Lakes region...
A prominent MCV -- produced by last evening`s destructive
thunderstorm complex over KS -- is apparent in composite radar
imagery over IA.  This feature should move northeastward across the
DBQ ares and southern WI today, then over central Lake Michigan and
the northern Lower Peninsula through tonight.  As the associated
field of strengthening low/middle-level ascent and mesoscale shear
enhancement spreads over a destabilizing boundary layer in northern
IL and northwestern IN, one or more episodes of thunderstorms should
develop and move across the area from later this morning through
late afternoon.  Pockets of heating in cloud breaks, along with
surface dewpoints in the low/mid 60s F, should support MLCAPE in the
1500-2500 J/kg range.  Ambient deep shear is progged to be modest,
though this may be underrepresented by synoptic models as often seen
east through southeast of a well-formed MCV.  Organized multicells/
clusters, and perhaps a few supercells, are possible.  Damaging
gusts and isolated severe hail are the main concerns.

Some convection-allowing progs depict either:
1.  A rejuvenated version of the IL convection later over Lower MI,
past the impediment of relatively cold lake waters, or
2.  Additional development in the Lower Peninsula as the MCV
approaches Lake Michigan and the associated ascent field spreads
overhead.
While either scenario cannot be ruled out, the timing would place
the activity into marginally surface-based effective-inflow parcels,
amid decreasing instability eastward and with time.  Therefore, the
unconditional severe probabilities will be kept marginal over Lower
MI for now, though even slightly more favorable mesoscale trends or
more-consistent progs may compel an upgrade to wind probabilities
for parts of the area.

...Southeastern FL...
Scattered multicell thunderstorms are expected to form over mainly
eastern parts of the region today, especially south of a weak cold
front, along the Atlantic sea breeze and on outflow boundaries from
initial-stage convection.  Isolated severe gusts and hail are
possible.  Surface dewpoints will remain in the 70s F, acting in
concert with strong heating to weaken MLCINH, and offset modest
midlevel lapse rates enough for around 1500-2500 J/kg MLCAPE.  The
main limiting factor will be modest shear.  Mid/upper-level flow
will not be as strong as previous days, limiting deep shear
(effective-shear magnitudes less than 30 kt), while weak winds in
the lowest km keep hodographs fairly small.

..Edwards/Goss.. 05/20/2024

$$