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Weather conditions on bar directly above are near Lakefront. Tabs under Moon Phases refer to O'Hare (official).

Archives for SPC Convective outlook are updated daily with a live map a the beginning of the article. Touch or click map to get possible update, and follow link at end of article for referral to NOAA/NWS Storm Prediction Center. Archives for Chicago's hourly weather data were updated 24-48 hours after end of each day prior to Dec. 2020.

Wednesday, December 8, 2021

SPC Dec 8, 2021 0830 UTC Day 3 Severe Thunderstorm Outlook

SPC 0830Z Day 3 Outlook Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0215 AM CST Wed Dec 08 2021 Valid 101200Z - 111200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE LOWER/MID MISSISSIPPI VALLEY INTO THE LOWER OHIO VALLEY... ...SUMMARY... Severe thunderstorms capable of producing a few tornadoes and scattered damaging winds appear possible Friday into Friday night across parts of the lower/mid Mississippi Valley, Ohio/Tennessee Valleys, and Southeast. ...Synopsis... A highly amplified upper trough centered over the Rockies Friday morning should advance eastward across the Plains through the day, eventually reaching the MS Valley by the end of the period. Rather strong low/mid-level southwesterly flow will be present ahead of the upper trough across much of the lower/mid MS Valley into the OH/TN Valleys. A surface low initially over the central High Plains is forecast to develop eastward across the southern/central Plains through the day. This low should continue northeastward while gradually deepening over the mid MS Valley, Midwest, and Great Lakes from Friday evening though early Saturday morning. Rich low-level moisture characterized by at least mid to upper 60s surface dewpoints will likely be in place Friday morning from central/east TX into the lower MS Valley and Mid-South. As both the large-scale upper trough and related surface low shift eastward, this moist low-level airmass is forecast to spread northward in tandem with a warm front into the mid MS Valley, OH/TN Valleys, and parts of the Southeast by Friday evening. A cold front attendant to the surface low will sweep eastward from the southern/central Plains to the vicinity of the TX Coast and MS River by the end of the period. ...Lower/Mid Mississippi Valley into the Ohio/Tennessee Valleys and Southeast... Even with the low-level moisture across these regions increasing through the day, generally modest mid-level lapse rates will tend to limit the degree of instability that can develop, especially with northward extent into the mid MS Valley, Midwest, and OH Valley. Still, most guidance suggests that MLCAPE will be sufficient (at least 500 J/kg) across a majority of the warm sector to support surface-based storms. A fair amount of uncertainty remains regarding the timing of robust convection. The 00Z ECMWF indicates that an embedded shortwave trough will overspread the lower MS Valley during the day, potentially supporting convective initiation earlier than other guidance would suggest. Regardless, thunderstorms will likely increase in coverage and intensity along/ahead of the cold front by Friday evening and continue eastward through Friday night, as strong forcing associated with the large-scale upper trough overspreads the warm sector. Deep-layer shear is forecast to become quite strong by Friday evening owing to the strength of the mid-level southwesterly flow. 50+ kt of effective bulk shear will easily support updraft organization. A mix of supercells and clusters may develop initially, but dominant convective mode should quickly trend to linear Friday night as thunderstorms grow upscale into one or more lines along/ahead of the cold front. Even though not much veering with height is forecast in the boundary layer due to mostly southwesterly flow, there should be ample speed shear, as the winds at 850 mb are expected to strengthen to 50-60+ kt Friday night. Strong 0-1-km SRH should support low-level rotation with any supercells or embedded QLCS circulations, and a few tornadoes may occur. Isolated to scattered severe/damaging winds also appear possible given the strength of the low-level flow, as it will not take much for convective downdraft accelerations to allow the already enhanced low-level winds to reach the surface. Primary uncertainties precluding greater severe probabilities at this time include the degree of boundary-layer instability, a large component of front-parallel flow in both low/mid levels, and the late timing of robust convective development. The northern extent of any severe threat in the Midwest/OH Valley is also quite uncertain, but most guidance suggests at least upper 50s to perhaps low 60s surface dewpoints may advance as far northward as east-central IL into IN and southwestern OH. This may be sufficient for low-topped convection to become surface based. The eastern extent of the severe threat will be tied to the location of storms along or just ahead of the front at the end of the Day 3 period, with expectations that at least a marginal/isolated severe threat will likely continue farther east into Day 4/Saturday. ..Gleason.. 12/08/2021 Read more LIVE: spc.noaa.gov
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