SPC AC 071728

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1228 PM CDT Tue Jul 07 2020

   Valid 081200Z - 091200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY
   AFTERNOON INTO WEDNESDAY NIGHT ACROSS PARTS OF THE UPPER MIDWEST
   INTO THE MIDDLE MISSOURI VALLEY INTO CENTRAL PORTIONS OF THE
   PLAINS...

   ...SUMMARY...
   Clusters of thunderstorms capable of producing severe wind and hail
   are possible across parts of the Upper Midwest into the mid Missouri
   Valley and central plains late Wednesday afternoon into Wednesday
   night.

   ...Synopsis...
   Mid-level ridging appears likely to remain suppressed through this
   period, confined to the southern tier of the U.S., along a major
   axis from the subtropical eastern Pacific through the southern
   plains Red River Valley, and across the Gulf of Mexico/Florida
   Peninsula, south of broad weak troughing across the eastern Gulf and
   south Atlantic coast states.

   Within the westerlies to the north, the most prominent mid-level
   short wave troughs are forecast to progress east of the northern
   Rockies, along the central Canadian/U.S border area, and gradually
   inland of the British Columbia/Pacific Northwest coast.  More subtle
   perturbations on the southern periphery of this regime may progress
   across parts of the Northeast, and into/through northern portions of
   the central plains.

   In lower levels, a surface cyclone may gradually shift eastward
   across southern Saskatchewan and Manitoba, perhaps preceded by a
   secondary surface low migrating north of the central Canadian/U.S.
   border early Wednesday.  A trailing cold front may gradually advance
   east/southeast of the northern U.S. plains. 

   Seasonably moist boundary-layer air present across much of the
   central and eastern U.S., ahead of the front, may become supportive
   of moderate to strong destabilization with daytime heating.  It
   appears that this will contribute to scattered thunderstorm
   development across many areas.  However, potential for organizing
   convective clusters capable of producing severe wind and hail
   appears mostly limited to a corridor across the Upper Midwest into
   the middle Missouri Valley and central plains.

   ...Upper Midwest into Plains...
   Associated with the larger-scale troughing forecast to shift
   eastward along the central Canadian/U.S. border area, one
   significant smaller-scale impulse is forecast to accelerate from
   southern Manitoba/northeastern North Dakota into Ontario early in
   the period.  As it does, substantive weakening of initially moderate
   to strong (30-50 kt) south-southwesterly pre-frontal lower/mid
   tropospheric flow is forecast.  The magnitude of this weakening
   remains a point of model spread, and there may be some
   re-strengthening of southerly low-level flow, particularly Wednesday
   evening across the central plains into mid Missouri Valley region.

   Despite at least some uncertainty concerning the strength of the
   vertical shear, strong heating of a moist pre-frontal boundary
   layer, beneath the axis of a plume of warm initially capping
   elevated mixed-layer air, is expected to contribute to large CAPE by
   late Wednesday afternoon.  Various model output, including the
   convection-allowing guidance, suggest that the daytime heating and
   continued gradual mid-level cooling/height falls will weaken
   inhibition and allow storms to initiate late Wednesday afternoon. 
   One or two upscale growing clusters of storms appear possible into
   Wednesday evening, which could pose increasing potential for strong
   wind gusts, in addition to severe hail.

   ...New England into Hudson/Champlain Valley...
   Low-level moistening within weak surface troughing overspreading the
   region is forecast to contribute to moderate boundary-layer
   destabilization (CAPE of 1000-2000 J/kg) by Wednesday afternoon,
   particularly inland of New England coastal areas, across the higher
   terrain.  Aided by orographic forcing, and perhaps lift associated
   with a subtle short wave impulse, this probably will be accompanied
   by the development of scattered thunderstorms, perhaps in the
   presence of modest shear beneath westerly mid-level flow on the
   order of 30 kt.  A few strong storms are possible, some of which
   could pose a risk for locally strong surface gusts and hail.

   ..Kerr.. 07/07/2020

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