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

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