SPC AC 220058

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 PM CDT Tue Jul 21 2020

   Valid 220100Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL HIGH PLAINS AND OHIO VALLEY...

   ...SUMMARY...
   Severe thunderstorms are possible this evening across parts of the
   central High Plains and Ohio Valley.

   ...Central High Plains...
   The latest water vapor imagery shows a subtle shortwave trough over
   the central High Plains. Scattered strong thunderstorms are ongoing
   near the trough from southeastern Wyoming into eastern Colorado.
   Upslope flow is present across much of northeastern Colorado and
   southeast Wyoming where surface dewpoints are in the 50s F. This is
   contributing to moderate instability with the RAP estimating MLCAPE
   across much of the central High Plains in the 1500 to 2000 J/kg
   range. In addition, the WSR-88D VWP at Cheyenne shows a supercell
   wind profile with directional shear in the lowest 3 km above ground
   level and about 40 kt of flow in the mid-levels. This is creating
   0-6 km shear in the 40 to 50 kt range which should be favorable for
   severe storms this evening. Supercells will be capable of producing
   large hail and wind damage. The stronger multicells may also have
   wind and hail potential. The threat should continue for several more
   hours as the storms move eastward into western Nebraska and far
   northwest Kansas.

   ...Upper Mississippi Valley...
   The latest water vapor imagery shows a shortwave trough over the
   upper Mississippi Valley. At the surface, a low is analyzed to the
   southeast of Minneapolis with a moist airmass present from east of
   the low southward into eastern Iowa. The RAP is estimating MLCAPE in
   the 1500 to 2000 J/kg range along this corridor. Strong
   thunderstorms are ongoing just to the east of the surface low at the
   northern end of the instability axis. The storms are located just to
   the west of a low-level speed max which is enhancing low-level
   shear. For this reason, a marginal tornado threat will be possible
   with rotating cells for another hour or two. Hail and strong wind
   gusts will also be possible with this activity as it moves eastward
   across north-central Wisconsin.

   ...Ohio Valley...
   West-southwesterly mid-level flow is currently present across the
   Ohio Valley and southern Great Lakes region. At the surface, a very
   moist airmass exists from the mid Mississippi Valley extending
   northeastward into the lower Ohio Valley. Thunderstorms are ongoing
   within this moist airmass across eastern Illinois and western
   Indiana. Surface dewpoints ahead of the storms are generally in the
   upper 60s and lower 70s F, which is contributing to strong
   instability. The RAP is estimating MLCAPE near 3500 J/kg in
   southwestern Indiana which will fuel the convection for a few more
   hours. Although deep-layer shear is on the weak side with 0-6 km
   shear less than 30 kt, the instability and steep low-level lapse
   rates will be favorable for severe multicells. Wind damage and hail
   will be the primary threats. Severe storms could affect northern
   Kentucky and western Ohio before a weakening trend marginalizes the
   severe threat.

   ...Mid Atlantic...
   The latest surface analysis shows a very moist airmass in place
   across much of the eastern Seaboard. Surface dewpoints are generally
   in the lower to mid 70s F from the eastern Carolinas into the Mid
   Atlantic. MLCAPE is estimated by the RAP to be in the 3000 to 4000
   J/kg range. Scattered thunderstorms are ongoing along the
   northwestern edge of the stronger instability from northern Virginia
   into southern Maryland. Although deep-layer shear is relatively
   weak, the instability combined with steep low-level lapse rates may
   be enough for a marginal wind damage threat for another hour or two.

   ..Broyles.. 07/22/2020

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