SPC AC 201251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Mon Jul 20 2020

   Valid 201300Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
   THE WESTERN DAKOTAS TO NORTHERN KANSAS...

   ...SUMMARY...
   Scattered strong to severe storms capable of large hail and damaging
   winds are expected over parts of the northern and central Plains.

   ...Synopsis...
   In mid/upper levels, a belt of zonal to broadly cyclonic flow is
   expected to remain across the northern tier of the CONUS, from the
   Pacific Northwest across the northern Plains, Great Lakes and
   Northeast.  An embedded shortwave trough -- initially apparent in
   moisture-channel imagery over southwestern SK and western/central MT
   -- will pivot southeastward then eastward across the Dakotas through
   the period.  To its southeast, a weaker shortwave trough was
   apparent from an MCV over east-central NE to north-central/
   northeastern KS.  This perturbation should move eastward across IA
   and northern IL through the period, while gradually weakening.  A
   separate MCV over the eastern OK Panhandle should move slowly
   east-northeastward near the KS/OK border and southern KS to near
   southwestern MO by 12Z tomorrow. 

   At the surface, 11Z analysis showed a weak cold front from southern
   QC across northern NY, northwestern PA and central OH, to a low over
   central IN, then quasistationary westward over northern MO, becoming
   ill-defined amidst a large area of convection over northwestern
   MO/southeastern NE.  A secondary/reinforcing cold front was drawn
   across southwestern QC, the neck of ON, western Lower MI, and
   central IA.  The fronts should merge through the period, with the
   southern then combined boundary becoming quasistationary between the
   central Appalachians and IL.

   ...Northern/central Plains...
   Increasing deep-layer lift ahead of the shortwave trough, and a
   precursory MCV initially located over northeastern MT, should
   impinge on a diurnally destabilizing air mass over the Dakotas to
   encourage surface-based thunderstorm development later this morning
   into afternoon.  Timing is somewhat uncertain, given the subtle
   nature of low-level forcing and the presence of the precursory MCV,
   but some severe threat may develop within a few hours over the
   western Dakotas, followed by afternoon initiation in stronger
   heating and richer moisture over the central Plains. Sufficient deep
   shear and moisture will be present for a threat of damaging gusts
   and large hail. 

   A corridor behind and northwest of the ongoing lower Missouri Valley
   MCS is expected to destabilize through the day, near a wavy low-
   level moist axis extending from central KS to western ND.  Moisture
   will increase southward, with shear generally stronger northward. 
   MLCAPE values of 2000-3000 J/kg are possible beneath somewhat steep
   midlevel lapse rates, especially from western SD southeastward to
   the MCS outflow boundary over northern KS.  A few supercells may
   occur in the first few hours of the convective cycle, over the
   Dakotas, offering large hail and damaging gusts.  One or two MCSs
   may evolve upscale from development that occurs through this
   afternoon across the western Dakotas and and/or NE, shifting
   southeastward across parts of NE and northern KS this evening and
   tonight.  Any such convective evolution potentially offers a more-
   concentrated conditional wind threat than a 15% area would imply. 
   However, despite a strengthening consensus among synoptic and
   convection-allowing guidance for such growth, spatial and timing
   uncertainties remain too large to focus an area of greater
   unconditional probabilities within the broader swath. 

   ...Lower Missouri Valley to Tidewater region...
   Along and south/southeast of the combined front, a broad corridor of
   favorable low-level moisture and strong diurnal heating will weaken
   MLCINH and support isolated to scattered thunderstorms through this
   afternoon.  Sporadic damaging wind and isolated severe gusts may
   occur, with marginally severe hail possible over western parts of
   the swath.

   Buoyancy will be highly variable across this corridor, including a
   general lessening over the higher elevations in and near the
   Appalachians, though convection should develop over portions of that
   terrain as well.  Foci for lift will include the frontal zone and
   outflow/differential-heating boundaries.  Midlevel lapse rates, mid/
   upper-level winds and deep shear will be modest, generally limiting
   organization and keeping overall severe threat marginal and
   localized.  A relatively dense concentration of convection may
   continue for several hours in a zone of mesoscale ascent enhancement
   ahead of the NE/KS shortwave trough, with activity shifting east-
   southeastward across much of MO.  Overall organization of this
   convection will be limited, with messy mode; however, isolated
   severe gusts may be possible in embedded/water-loaded downdrafts or
   short-lived bowing segments.

   ..Edwards/Dean.. 07/20/2020

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