SPC AC 291958

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0258 PM CDT Thu Jun 29 2017

   Valid 292000Z - 301200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF FAR EASTERN CO INTO WESTERN KS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE SOUTHERN/CENTRAL PLAINS TO THE MO VALLEY AND MIDWEST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE CENTRAL HIGH PLAINS/ROCKIES TO THE NORTHEAST...

   ...SUMMARY...
   Severe thunderstorms are expected this afternoon into tonight,
   especially across portions of the central Plains and middle Missouri
   River Valley into parts of the Midwest. Large hail and damaging
   winds will be the main threats.

   ...20Z Update...
   15% hail probabilities have been expanded northward/westward to
   include more of southeastern SD, central/northern NE, northern KS,
   and northern IA based on latest surface observations and visible
   satellite imagery showing re-destabilization occurring ahead of a
   weak front.

   Very large hail may occur within an upslope flow regime across
   eastern CO into far western KS with initial supercell development,
   and a small significant hail area (2+ inch diameter) has been added
   to this region. Upscale growth of these supercells into a small
   bowing cluster seems probable by early evening across far eastern CO
   into parts of western KS. Confidence has increased in the damaging
   wind potential through at least mid evening with this small bow
   given steep low and mid-level lapse rates present across this
   region, strong instability with a moist low-level airmass, and 40-50
   kt of effective bulk shear. Have included 30% wind probabilities to
   account for this scenario, resulting in a categorical Enhanced Risk.

   Very isolated instances of marginally severe hail may continue to be
   possible across parts of ND through the remainder of peak heating
   with cold temperatures aloft associated with an upper-level trough.
   However, both low- and mid-level winds appear to be too weak to
   support a more organized hail threat, and no probabilities have been
   introduced with this update.

   No other substantial changes have been made to the outlook. See
   various valid Mesoscale Discussions for additional near-term details
   on the severe weather threat.

   ..Gleason.. 06/29/2017

   .PREV DISCUSSION... /ISSUED 1159 AM CDT Thu Jun 29 2017/

   ...Central Plains...
   Multiple sub-regional corridors of severe thunderstorm development
   appear possible if not likely later this afternoon into tonight,
   although some details remain highly uncertain owing to last night's
   MCS that has mainly impacted northeast KS/northern MO. Considerable
   convective overturning and lingering cloud cover cast uncertainty on
   air mass recovery/resultant trajectories across northeast KS into
   parts of southeast NE/southwest IA vicinities. That said, some
   measure of air mass recovery and destabilization will occur through
   the afternoon.

   One sub-regional corridor of semi-focused severe-weather development
   would appear to be across parts of northeast NE into western IA.
   This should be semi-focused near a surface low/adjacent frontal
   corridor with aid of an east/southeastward-moving shortwave trough
   evident in water vapor imagery across SD. Deep-layer shear/buoyancy
   will be supportive of initial supercells capable of large hail.
   While low-level shear will not be overly strong, some tornado risk
   may exist particularly near the low/front. These storms should
   progress southeastward through the evening hours and pose at least
   some severe hail/wind risk tonight.

   Farther south, moist post-frontal upslope flow trajectories will
   support at least a few supercells across eastern CO this afternoon,
   with this development likely to continue (and possibly grow upscale)
   east-southeastward into west/southwest KS this evening, where
   damaging winds could become an increasing concern.

   One additional scenario of concern will be for some possibility of
   redevelopment near convective outflow/differential heating across
   eastern KS late this afternoon and evening. There appears to be a
   least some risk for large hail/damaging winds and possibly
   outflow/effective boundary-related tornado risk if/where storms
   redevelop.

   ...Lower MO Valley to Midwest...
   An MCV and outflow (along with differential cloud cover and heating)
   related to the overnight/early-morning MCS appears likely to focus
   downstream thunderstorm development and intensification across
   eastern MO into IL and eventually parts of IN later today. While the
   downstream air mass will not be overly moist and unstable by late
   June standards, weakening inhibition and moderate destabilization
   will support some semi-organized/sustained storm modes as winds
   gradually strengthen through a deep layer with aid of the MCV.

   ...Northeast States/lower Great Lakes...
   Somewhat stronger heating on the southern edge of thicker early-day
   cloud cover/lingering precipitation could allow for a few strong to
   severe thunderstorms later this afternoon into evening.

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