SPC AC 270548

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1248 AM CDT Mon May 27 2019

   Valid 271200Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO TONIGHT ACROSS PARTS OF EASTERN IOWA AND NORTHERN
   ILLINOIS INTO PARTS OF SOUTHWESTERN MICHIGAN AND NORTHWEST OHIO...AS
   WELL AS ACROSS PARTS OF SOUTHEASTERN WYOMING AND NORTHEAST COLORADO
   THROUGH MUCH OF WESTERN INTO CENTRAL NEBRASKA...

   ...SUMMARY...
   Severe thunderstorms, including a risk for tornadoes, severe hail
   and damaging wind gusts, are expected in a corridor across southern
   portions of the Upper Midwest and Great Lakes region this afternoon
   into tonight.  Additional severe storms are expected near the
   Cheyenne Ridge of the central High Plains, and perhaps southward
   across the central Plains through the Texas South Plains.

   ...Synopsis...
   Models indicate that the significant mid-level closed low, now
   digging into the base of larger-scale troughing encompassing much of
   the West, will remain progressive through this period.  After
   reaching the lower Colorado Valley by 12Z this morning, it appears
   likely to turn northeastward toward the Colorado Rockies later
   today, through tonight.  As it does, a belt of 60-70+ kt cyclonic
   500 mb flow is forecast to propagate around its southern and eastern
   periphery, across the southern Rockies through the central Plains by
   12Z Tuesday.  This will be accompanied by the continued advection of
   warm elevated mixed-layer air northeast of the southern Plains
   toward the Upper Midwest and Great Lakes region, in the wake of an
   initial perturbation emanating from the Southwest.  The lead
   impulse, is forecast to progress around the northern periphery of a
   persistent, prominent subtropical high (centered over the eastern
   Gulf of Mexico), northeast of the middle Missouri Valley, then
   eastward and southeastward across southern portions of the Great
   Lakes region by late tonight.

   ...Southern portions of the Upper Midwest and Great Lakes region...
   Model output indicates that the lead impulse will continue to be
   accompanied by a belt of southwesterly to westerly, 40-50+ kt flow
   in the 850-500 mb layer, enhancing vertical shear and forcing for
   ascent along a strengthening warm front across parts of eastern Iowa
   through northern Illinois by mid to late afternoon.  This appears
   likely to coincide with the northward return of seasonably moist
   boundary layer air along and south of the front, which is forecast
   to contribute to moderate to large mixed-layer CAPE on the order of
   2000-3000 J/kg.

   As the boundary layer destabilizes, remnants of overnight convection
   spreading northeastward out of the central Plains may re-intensify
   across eastern Iowa, with new storm development also possible along
   the warm front into Illinois.  The eventual evolution of another
   upscale growing convective system seems probable, but there may be a
   period with sustained discrete storm development, including
   supercells.  Either mode may support potential for tornadoes, in the
   presence of large clockwise curved low-level hodographs along the
   front.  Discrete activity would seem to pose a more substantive risk
   for a strong tornado or two.  Otherwise, damaging wind gusts
   probably will tend to become the more prominent risk, as a cold pool
   consolidates and strengthens with upscale growing convection. This
   is expected to tend to propagate east-southeastward along the warm
   frontal zone, across southern portions of the Great Lakes region
   into parts of the mid/upper Ohio Valley by daybreak Tuesday.

   ...North central High Plains...
   With a moist easterly flow to the north of a stalled frontal zone
   across the Plains, at least modest boundary layer destabilization
   appears possible by late this afternoon.  Focused near the Cheyenne
   Ridge, storm development may be aided by forcing for ascent in the
   left exit region of the strong mid-level jet crossing the southern
   Rockies.  Storms may initiate near the Laramie Mountains, before
   tending to propagate east southeastward across portions of western
   into central Nebraska.  This may include a few supercells capable of
   producing large to very large hail and an isolated tornado or two,
   before perhaps growing upscale into a small mesoscale convective
   system with potential to producing strong surface gusts by this
   evening.

   ...Central Plains through the Texas South Plains...
   Large-scale forcing for ascent may remain relatively weak to the
   south of the cyclonic mid-level jet, but various model output does
   indicate some cooling or developing weakness in the inhibition in
   lower/mid troposphere, which may allow for isolated to widely
   scattered thunderstorm development near/east of the dryline.  This
   appears most notable in a corridor across parts of western/central
   Kansas, southward toward the eastern Texas Panhandle, by late this
   afternoon or early evening.  If sustained convection forms, the
   environment will likely be conducive to supercells with a risk for
   severe hail/wind and tornadoes.

   ..Kerr/Squitieri.. 05/27/2019

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