SPC AC 101254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0654 AM CST Tue Nov 10 2020

   Valid 101300Z - 111200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE MIDDLE MISSISSIPPI VALLEY AND SOUTHERN LAKE MICHIGAN REGION...

   ...SUMMARY...
   Thunderstorms capable of producing damaging to severe winds and a
   few tornadoes are possible this afternoon and evening across
   portions of the Middle Mississippi Valley and southern Lake Michigan
   region.

   ...Synopsis...
   The upper-air pattern will continue to feature mean/large-scale
   troughing over the Rockies, with its broad cyclonic-flow field
   enveloping most of the CONUS between the Pacific Coast and
   Mississippi River.  That cyclonic flow will be traversed by several
   troughs of varying magnitudes, most notably:
   1.  A leading trough with a curving axis, presently extending from a
   vorticity max over the MT/ND border region across the NE Panhandle
   to another vorticity max near the KS/CO border, then southwestward
   across southern NM.  The southern part of this perturbation will
   eject northeastward across the central Plains to the upper
   Mississippi Valley into this evening, while the northern part forms
   a weak closed low over ND.  The southern lobe then may form a closed
   low as it continue to move northeastward, across northern WI and
   Lake Superior to adjoining parts of northern ON.
   2.  A complex trough accompanied by multiple vorticity lobes, and
   apparent in moisture-channel imagery from northeastern to southern
   BC, then southward over western/central WA and northern OR.

   At the surface, 11Z analysis depicted a slow-moving cold front from
   eastern Upper MI southwestward to a low over eastern IA and another
   over extreme eastern KS, with a faster-moving cold front across
   central OK, and west-central/southwest TX.  The KS low will ripple
   northeastward along the boundary as the primary frontal-wave low,
   causing it to back westward temporarily as a warm front before the
   low passes this afternoon.  The low will intensify, proceed
   northeastward over Lake Superior, and occlude as the southern
   vorticity lobe aloft catches up tonight.  By 00Z the cold front
   should extend from the low across southeastern WI, central/eastern
   IL, southeastern MO, central AR, and south TX.  By 12Z the front
   should reach central OH, western/mid TN, and LA.

   Meanwhile, T.S. Eta is forecast by NHC to begin moving northward
   from its present quasistationary presence off northwestern Cuba. 
   The now-compact system should remain over the southeastern Gulf and
   too far from FL to contribute tornado potential this period.  See
   latest NHC advisories a for tropical watches/warnings related to
   Eta, and track/intensity guidance. 

   ...Mid Mississippi Valley, southern upper Great Lakes...
   A line of thunderstorms is expected to develop during mid/late
   morning either 1) along the cold front, or 2) just behind it, but
   soon catching up to the front.  Either way, with access to
   increasingly surface-based effective-inflow parcels in the diurnally
   destabilizing warm sector, the convection should sweep east-
   northeastward to northeastward across the region in and near the
   outlook area, while backbuilding somewhat on the south end.  The net
   effect should be a band of strong-severe thunderstorms shifting
   eastward over the 15%/slight and 5%/marginal wind areas, before
   weakening this evening.  Additional convection may develop over
   eastern fringes of both the outlook area and modest supportive
   SBCAPE late this afternoon and this evening, in an initially
   separate warm-advection plume.  This activity may be overtaken by
   the front (and any remaining frontal convective band) tonight, while
   its inflow layer becomes more elevated, as it outruns surface-based
   destabilization.  

   The main concern across the region will be occasional damaging wind,
   along with the possibility of severe/50+ kt gusts in a few locales. 
   A QLCS or embedded-supercell tornado threat may develop as well,
   maximized where the parameter-space overlap between diurnally aided
   max buoyancy and strong low-level shear across eastern IA,
   northeastern MO, northern IL and southeastern WI.  This is east and
   northeast of the projected path of the deepening surface low, where
   both speed and backing of low-level winds should be relatively
   maximized with isallobaric forcing.  500-1000 J/kg peak
   preconvective MLCAPE and 45-55-kt effective-shear vectors are
   expected midday into early afternoon in that area.  

   Deep-layer lift and low-level instability should lessen with
   eastward extent, but only gradually, as warm and moist advection
   help to offset nocturnal boundary-layer cooling.  Given the strength
   of the ambient flow, progged presence of at least weakly unstable
   surface parcels as far east as portions of Indiana this evening, and
   potential for downward momentum transfer, both sets of probabilities
   have been expanded eastward spatially.  Forecast soundings as far
   east as the IND area between 00-03Z indicate the potential for
   surface dew points in the 60s F to contribute to around 500 J/kg
   MLCAPE, amidst 30-40 kt of 0-1-km shear and 40-50-kt kt
   effective-shear vectors.  Any remaining convection moving through
   this environment will offer at least a marginal severe threat.

   ..Edwards/Kerr.. 11/10/2020

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