SPC AC 230445

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1145 PM CDT Sat Sep 22 2018

   Valid 241200Z - 251200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MONDAY AFTERNOON
   AND EVENING IN A CORRIDOR ACROSS THE UPPER MIDWEST SOUTHWARD INTO
   PORTIONS OF THE MID MISSOURI VALLEY AND CENTRAL PLAINS...AND ACROSS
   PARTS OF THE LOWER OHIO VALLEY...

   ...SUMMARY...
   Strong thunderstorms may impact a corridor from the Upper Midwest
   into portions of the mid Missouri Valley/central Plains, as well as
   portions of the lower Ohio Valley, Monday afternoon and evening,
   accompanied by some risk for severe weather.

   ...Synopsis...
   While subtropical ridging becomes a bit more prominent across much
   of the southeastern U.S. through this period, models suggest that
   the mid-latitude westerlies may become more amplified across the
   northern tier of the U.S. and southern Canada.  This will include
   the evolution of one large-scale trough east of the
   Canadian/northern U.S. Rockies through the upper Mississippi
   Valley/Great Lakes/Ontario vicinity by 12Z Tuesday.  One short wave
   perturbation embedded within the cyclonic regime is forecast to be
   in the process of lifting north of the international border area as
   early as 12Z Monday.  It appears that this may be accompanied by
   modest surface cyclogenesis across northwestern Ontario, toward
   Hudson Bay.  A trailing cold front is expected to advance slowly
   southeastward through the upper Mississippi Valley, mid to lower
   Missouri Valley, and central  Plains, as one or two additional short
   wave perturbations dig into and through the base of the larger-scale
   troughing.

   At the same time, to the south and east of the troughing in the
   mid-latitude westerlies, models indicate that one or two
   increasingly sheared perturbations will accelerate northeastward
   through portions of the lower Mississippi and Ohio Valleys, around
   the northwestern periphery of the subtropical ridge.  These may be
   accompanied by the north/northeastward advection of seasonably high
   moisture content, initially confined to the lower Mississippi Valley
   and Southeast, through much of the middle Mississippi and lower Ohio
   Valleys by the end of the period.

   ...Upper Midwest into central Plains...
   Ahead of the southeastward advancing front, boundary layer moisture
   appears likely to be initially rather modest, but could increase
   through the lower 60s within a narrow plume beneath warm and capping
   elevated mixed-layer air.  As cooling associated with large-scale
   ascent and advection erode the elevated mixed-layer and stabilize
   mid-level lapse rates, models generally indicate the development of
   CAPE only on the order of 500-1000 J/kg by late Monday afternoon. 
   Furthermore, stronger low-level wind fields (at or above 30 kt at
   850 mb) are forecast to shift across and north of the  upper Great
   Lakes region during the afternoon.  However, 30-50 kt flow within
   the 700-500 mb layer may linger in a southwesterly belt near the
   axis of instability, and could still contribute to an environment
   conducive to vigorous thunderstorm development late Monday afternoon
   and evening.  Before boundary layer instability wanes due to the
   loss of daytime heating, and the progression of the front, a narrow
   broken squall line could evolve, which may pose at least some risk
   for severe hail and wind gusts.

   ...Lower Ohio Valley vicinity...
   Models suggest that low-level hodographs could become modestly large
   (850-700 mb flow up to around 30 kt) and clockwise curved across the
   region by midday Monday, on the leading edge of richer boundary
   layer moisture (including surface dew points near 70f) return. 
   Although this may be preceded by an area of convective precipitation
   associated with one mid-level perturbation, breaks in cloud cover to
   the southwest of this activity may contribute to moderate boundary
   layer destabilization (CAPE in excess of 1000 J/kg).  It appears
   possible that there could be a corridor where the environment
   becomes conducive to thunderstorms which could pose a risk for
   potentially damaging wind gusts, or perhaps a tornado or two, before
   boundary layer instability wanes Monday evening.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:      5%     - Marginal
   Hail:      5%     - Marginal

   ..Kerr.. 09/23/2018

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