SPC AC 090539

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1239 AM CDT Sun Jul 09 2017

   Valid 101200Z - 111200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   MONTANA...EXTREME NORTHERN WYOMING...EXTREME EASTERN IDAHO...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
   THE NORTHERN ROCKIES AND NORTHERN PLAINS INTO PARTS OF NEBRASKA
   EXTENDING EASTWARD INTO THE SOUTHERN GREAT LAKES AND OHIO VALLEY
   REGION...

   ...SUMMARY...
   On Monday, severe thunderstorms will be possible from portions of
   the northern Rockies and adjacent Plains, eastward across parts of
   Nebraska into the Ohio Valley region. The greatest
   severe-thunderstorm risk will be across Montana and vicinity, where
   widely scattered severe wind and hail will be possible.

   ...Synopsis...
   A low-amplitude midlevel shortwave trough is forecast to track from
   the Pacific Northwest to the northern High Plains, with a downstream
   band of moderate quasi-zonal flow aloft extending eastward to New
   England. Rich moisture will reside over eastern parts of the central
   and southern Plains through the Mississippi Valley region and Great
   Lakes area -- as far north as a front (convectively augmented in
   some areas) stretching from parts of Nebraska to the southern Great
   Lakes region.

   ...Montana, Wyoming, eastern Idaho, the western Dakotas...
   Areas of strong diurnal heating are expected ahead of the shortwave
   trough, with trough-preceding ascent favoring increased convective
   coverage emanating from orographic circulations over the mountains.
   Convective activity will subsequently spread over the
   deep/well-mixed boundary layer across the adjacent Plains. Despite
   limited boundary-layer moisture, steep low/midlevel lapse rates will
   support around 500-1000 J/kg of MLCAPE and robust convective-scale
   upward accelerations. With 35-45 kt of midlevel flow across the
   added Slight Risk area, sufficient deep shear will support
   multicells and weak supercell structures. Deep inverted-V profiles
   suggest that widely scattered severe wind will be possible, with
   severe hail also possible in association with rotating updrafts.
   Given the relatively more predictable evolution of the shortwave
   trough, the Slight Risk area has been added. South of the Slight
   Risk area, weaker deep shear will exist, though deep and well-mixed
   boundary layers will support isolated severe wind gusts across WY
   and vicinity. Otherwise, more isolated severe risk will be possible
   across the western Dakotas through the period.

   ...Nebraska eastward to the southern Great Lakes and Ohio Valley
   region...
   Moderate to strong diurnal destabilization is anticipated south of
   the aforementioned front. Where the strongest deep shear is forecast
   to exist -- i.e., the southern Great Lakes and Ohio Valley region --
   the location and intensity of frontal ascent may be much more
   influenced by prior convection lingering into the beginning into the
   period. This casts considerable doubt regarding convective evolution
   through the period, though the potential for organized convective
   development/re-development with severe wind/hail could exist and
   linger into the overnight hours.

   Farther west through parts of the Midwest into Nebraska and
   vicinity, isolated to scattered thunderstorm development is
   anticipated near that portion of the front less influenced by
   antecedent convection. Despite areas of strong buoyancy, weaker deep
   shear is expected to limit severe coverage.

   For the aforementioned reasons, a broad corridor of Marginal Risk is
   presently depicted. However, some potential for an upgrade may be
   considered in subsequent outlooks -- particularly across parts of
   the southern Great Lakes region.

   ..Cohen.. 07/09/2017

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   NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z