SPC AC 180558

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CDT Sat Jul 18 2020

   Valid 191200Z - 201200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   NORTHERN OH INTO NORTHWESTERN PA AND WESTERN NY...

   ...SUMMARY...
   Isolated strong to severe thunderstorms may occur Sunday from
   portions of the Great Lakes/Ohio Valley regions into the mid
   Mississippi Valley and parts of the Plains. Damaging winds and large
   hail should be the main threats.

   ...Great Lakes into the Ohio Valley and Western New York...
   An MCS may be ongoing at the start of the period Sunday morning
   across parts of Lower MI, with additional convection possible
   farther south into northern IL/IN. This MCS should decay quickly
   across Lower MI and vicinity as it encounters substantial convective
   inhibition with eastward extent across the Great Lakes region.
   Depending on its organization, it may still pose an isolated threat
   for damaging winds before it weakens/dissipates.

   In the wake of this morning activity, an upper trough is forecast to
   move east-northeastward from the Upper Midwest/Great Lakes into
   Ontario and Quebec through the period. A surface low should likewise
   develop northeastward across Ontario/Quebec through the day. A cold
   front trailing southwestward from this low across the Great
   Lakes/Upper Midwest should shift east-southeastward. Diurnal heating
   of a rather moist low-level airmass will likely foster moderate to
   potentially strong instability ahead of the front by Sunday
   afternoon. The stronger large-scale ascent associated with the upper
   trough should remain in Canada. Regardless, low to mid-level
   westerly flow is forecast to strengthen through the day across parts
   of the OH Valley into western PA/NY. A corresponding modest increase
   in effective bulk shear to around 25-35 kt will probably be
   sufficient for storm organization.

   Current expectations are for scattered storms to develop along/ahead
   of the front Sunday afternoon across northern IN/OH. Additional
   convection may advance eastward from southern Ontario across Lakes
   Erie/Ontario, and into northwestern PA and western NY by early
   Sunday evening. Clusters/small bows should the main storm mode. With
   steepened low-level lapse rates and enhanced low/mid-level flow
   across these areas, at least isolated damaging wind gusts may occur.
   Isolated instances of marginally severe hail also appear possible.
   Confidence has increased enough regarding this scenario to include
   15% severe wind probabilities and a corresponding Slight Risk from
   northern OH into western NY. Storms should gradually weaken Sunday
   evening farther east into central PA/NY as they encounter less
   instability.

   ...Mid Mississippi Valley into the Plains...
   Large-scale forcing aloft appears nebulous across the mid MS Valley
   in the wake of the previously mentioned upper trough ejecting
   northeastward away from this region. Even so, isolated storms may
   develop Sunday afternoon/evening along the decelerating cold front.
   Moderate to locally strong instability coupled with modest mid-level
   flow may support isolated strong/gusty downdraft winds and/or
   marginally severe hail.

   Farther west across the Plains, storms may form during the day along
   the stalled front in KS/NE, along/east of a lee trough over the
   southern/central High Plains, and in a post-frontal upslope regime
   across eastern WY and vicinity. In general, mid-level flow should
   remain rather weak along/south of the front, with instability and
   steepened low-level lapse rates supporting mainly isolated
   strong/gusty winds in loosely organized clusters. North of the
   front, mid-level west-northwesterly flow is expected to be somewhat
   stronger, particularly across eastern WY and vicinity. But,
   instability should be less owing to more limited low-level moisture.
   Net result is some potential for organized storms, including a
   supercell or two, with a isolated threat for both large hail and
   severe wind gusts. Additional, largely elevated storms may form late
   Sunday night into early Monday morning north of the front across the
   central Plains as a southerly low-level jet modestly strengthens.
   Marginally severe hail appears possible with this activity.

   ..Gleason.. 07/18/2020

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