Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1253 AM CDT Mon Jul 13 2020 Valid 141200Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE MID-MO AND UPPER MS VALLEY... ...SUMMARY... Strong to severe storms are possible on Tuesday from the central and southern High Plains vicinity to the upper Mississippi Valley. A broad upper trough over the north-central U.S. will pivot eastward toward the upper Great Lakes on Tuesday. Several shortwave impulses will rotate through the larger-scale trough, providing a belt of strong southwesterly flow across portions of the central Plains to the Great Lakes, and aiding in focused upper forcing for ascent. The main surface low attendant to the upper trough will be located over northern Manitoba Tuesday morning, with a surface trough extending southward into the upper MS Valley southwestward toward a lee low over the southern High Plains. A cold front will drop southward across the central Plains and eastward across the Upper Midwest during the period. Thunderstorms will likely be ongoing Tuesday morning across portions of the Upper Midwest vicinity as one shortwave impulse ejects northeast. Additional development is expected near the front during the afternoon near far eastern NE into IA/MN/WI. Isolated to scattered storms also are expected over the higher terrain of CO/NM and could pose a marginal severe threat during the evening as they move eastward across western portions of the central/southern Plains, possibly interacting with the southward-advancing cold front. ...Mid-MO Valley to Upper MS Valley vicinity... Convection will likely be ongoing across parts of MN/WI and perhaps IA as a decaying MCS shifts eastward across the region early in the period. This could have some impact on destabilization during the afternoon, but most guidance suggests the air mass should remain intact, or recover enough for redevelopment during the afternoon/evening along the cold front from southern MN into western IA. Southerly low level flow will maintain moist advection across the region, and coupled with evapotranspiration influences, dewpoints in the upper 60s to near 70 F appear likely. A plume of steeper midlevel lapse rates is expected to spread across IA toward southeast MN and far southwest WI during the afternoon, further aiding in development of strong instability. Warm temperatures in the 850-700 mb pose some capping concern, but strong frontal forcing should be sufficient to overcome whatever modest capping remains by late afternoon. Storm mode is also somewhat uncertain. Supercell wind profiles are forecast across the region, with backed low level winds resulting in enlarged, curved low level hodographs. However, upward development into clusters or bowing segments could occur rather quickly as the cold front surges east/southeast. If discrete convection can be maintained initially, supercells capable of all severe hazards will be possible. As a southwesterly low level jet increases during the evening, upscale development into an eastward propagating MCS is expected. ...Central/Southern High Plains vicinity... Thunderstorms will develop over the higher terrain of CO into northeast NM during the afternoon and spread eastward over the adjacent Plains by evening. Coverage of convection remains somewhat in question given the very warm and deeply-mixed boundary layer that will be in place. There is also uncertainty related to timing of the southward-surging cold front and what impact that may have on both storm development and maintenance. Most guidance does develop at least isolated, high-based cells capable mainly of strong downburst winds. A weakness in midlevel winds is evident in forecast soundings, however vertically veering wind profiles and increasing southwesterly flow above 700 mb will result in supercell wind profiles with 40+ kt effective shear amid very steep midlevel lapse rates. As a result, a few organized cells could also produce large hail. Given a large degree of uncertainty across the area, a Marginal risk will be maintained, but upgrades may be needed in subsequent outlooks. ..Leitman.. 07/13/2020Read more
from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day2otlk_0600.html
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