SPC AC 151948

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0248 PM CDT Mon May 15 2017

   Valid 152000Z - 161200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHEASTERN MN...NORTHERN IA...SOUTHERN WI...AND ADJACENT PORTIONS
   OF NORTHERN IL...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
   AREAS OF THE UPPER MIDWEST...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE TX AND OK PANHANDLES INTO WESTERN KS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE BLACK
   HILLS VICINITY AND ADJACENT AREAS OF THE NORTHERN HIGH PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
   THE REMAINDER OF THE HIGH PLAINS INTO THE MID MISSOURI VALLEY AND
   PORTIONS OF THE UPPER MISSISSIPPI VALLEY/GREAT LAKES REGION...

   ...SUMMARY...
   Thunderstorms will impact parts of the Upper Midwest and high Plains
   this afternoon through tonight, accompanied by a risk for severe
   weather.  This may include at least some potential for a couple of
   tornadoes, but severe hail and wind appear the primary severe
   threats.

   ...20Z Outlook Discussion...
   The only notable change to the ongoing forecast has been a westward
   adjustment of the Marginal delineation across parts of southwest NE,
   eastward CO, and vicinity. Ascent accompanying a weak midlevel
   impulse glancing the western extent of Gulf moisture over the Great
   Plains may support isolated storms capable of severe hail/wind
   within areas added to Marginal designation. Otherwise, no changes
   have been made to the ongoing forecast.

   ..Cohen.. 05/15/2017

   .PREV DISCUSSION... /ISSUED 1127 AM CDT Mon May 15 2017/

   ...Synopsis...
   Mid/upper troughing, with an embedded closed low, is forecast to
   pivot inland across southern California, and take on an increasing
   negative tilt orientation across the southern Great Basin/Colorado
   Valley region by late tonight, as another vigorous upper
   impulse/closed low dig into the Pacific Northwest.  Downstream,
   large-scale mid/upper ridging appears likely to remain prominent
   east of the Rockies, through the lower Great Lakes region and
   Appalachians.  

   Weak to modest mid-level height falls may occur across much of the
   Rockies through the northern half of the Plains by late tonight in
   response to progression of the upstream perturbations, but models
   suggest that mid/upper flow, while at least modestly strong, will
   remain broadly anticyclonic.  Still, beneath increasingly divergent
   upper flow between a couple of mid/upper jet streaks (one rounding
   the crest of the ridging and the other within the base of
   Southwestern troughing), a consolidating and deepening surface low
   seems likely to emerge within lee surface troughing across parts of
   western into central South Dakota and Nebraska by late tonight.  In
   response to this latter development, considerable strengthening of a
   southerly 850 mb jet (to 50+ kt) is forecast across the central
   Plains, and within a branch across the mid Missouri Valley into the
   Upper Midwest, late this evening into tonight.

   Beneath steep mid-level lapse rates, associated with a plume of warm
   elevated mixed layer air advecting northward to the west of the
   mid-level ridge axis, across the southern through central Plains,
   toward the Upper Midwest, continued low-level moistening is
   beginning to contribute to areas of moderate to strong potential
   instability.  This may remain mostly focused beneath stronger
   mid-level inhibition across the mid Missouri Valley through this
   period, but areas of focused strong/severe storm potential appear to
   exist near the surface trough across the high Plains, and on the
   nose of the stronger capping elevated mixed layer air across the
   Upper Midwest.

   ...Upper Midwest...
   Forcing for ascent supporting an ongoing area of thunderstorms
   spreading northeast of the mid Missouri Valley appears largely in
   mid-levels, associated with a weak perturbation rounding the crest
   of the larger-scale ridging, which may build further northward
   across much of the region through the day.  Given the large
   potential instability now evolving across the mid Missouri Valley,
   supported by a combination of modest low-level moisture and very
   steep mid-level lapse rates, there does appear at least considerable
   conditional severe weather potential in the wake of this activity. 
   However, in the presence of larger-scale mid-level height rises and
   generally weak low-level forcing for storm development, convective
   potential remains unclear through at least the daytime hours.  And
   it is not clear that convection allowing model guidance is handling
   this well.

   A northward advancing warm frontal zone could provide a focus for an
   evolving cluster or two of thunderstorm development during this
   period.  It is possible that this could initiate near or in the wake
   of a mid-level cyclonic vorticity center progressing across southern
   Minnesota into Wisconsin this afternoon and evening.  However, it
   seems more likely that strengthening low-level warm advection
   associated with the 850 mb jet will provide better opportunity for
   organized convective development later this evening into the
   overnight hours.

   ...Central and southern high Plains...
   Model output continues to indicate the initiation of considerable
   thunderstorm activity in the peak late afternoon surface heating
   near/east of the lee surface trough, particularly from portions of
   the Texas Panhandle into western Kansas.  In the presence of
   thermodynamic profiles characterized by a modestly moist and deeply
   mixed boundary layer and steep lower/mid tropospheric lapse rates,
   coupled with 30-40 kt southwesterly lower/mid tropospheric flow, the
   environment appears conducive to storms capable of producing severe
   hail and wind gusts.

   ...Northern high Plains...
   Strengthening vertical shear and destabilization within
   southeasterly low-level flow across the Black Hills region into
   northeastern Wyoming and southeastern Montana probably will become
   sufficient to support a few severe storms, including isolated
   supercells, late this afternoon and evening.

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