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Thursday, June 17, 2021

SPC Jun 17, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0257 PM CDT Thu Jun 17 2021 Valid 172000Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHEAST MN...SOUTHERN WI...NORTHERN PORTIONS OF IA/IL... ...SUMMARY... Severe storms are expected late this afternoon into tonight across parts of the Upper Mississippi Valley, from central/southern Wisconsin and southern Minnesota into northern portions of Iowa and Illinois. A few tornadoes, damaging wind gusts exceeding 75 mph, and hail up to the size of baseballs will be possible. ...Discussion... The only changes this forecast update are the following: 1) Added a 5-percent wind probability over parts of the central High Plains for late afternoon/early evening for the risk of isolated microbursts. Extremely steep lapse rates from the surface up to 450 mb will favor evaporation potential with any locally stronger cores that can develop. Slightly stronger mid- to high-level flow over northeast CO compared to areas farther south will aid in some eastward storm motion. 2) Lowered severe probabilities on the western edge of the previously issued SLGT and ENH risk areas over northern IA to reflect model guidance showing a lower probability for storm development over this region. 3) Minor adjustment to the northern part of the MRGL Risk over the Ozarks based on mid afternoon satellite trends. Elsewhere, the forecast remains on track and no other changes were made. ..Smith.. 06/17/2021 .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Jun 17 2021/ ...Upper MS Valley... Primary change from prior outlook is to expand greater probabilities farther north/east across parts of WI and IL, with the expectation of the wind threat persisting longer in this region tonight. A mid-level impulse/speed max over SD, responsible for morning ACCAS across the eastern portion of the state, will aid in ascent along a southeast-moving cold front. Thunderstorm initiation is expected across west-central WI into southeast MN as MLCIN wanes near the front during the late afternoon. Activity should eventually backbuild southwestward into northeast/north-central IA this evening, but uncertainty is greater with how far into central IA convection will ultimately become sustained owing to greater influence of the EML and persistence of a relatively hot/dry boundary-layer across KS/southern NE into southern IA. While guidance will probably be somewhat overdone with the degree of evapo-transpiration-induced moistening ahead of the front based on 12Z morning soundings and ongoing drought, large buoyancy with MLCAPE of 2500-3500 J/kg should develop within a confined corridor given steep mid-level lapse rates. The mid-level speed max (around 60 kts at 500 mb) will yield strong effective shear, especially with northern extent. This suggests initial supercells will have the potential to produce very large hail despite the relatively high freezing levels. Enlarging low-level hodographs as a southwesterly low-level jet strengthens this evening should maximize the supercell tornado threat between 22-02Z. Storms are expected to predominately coalesce and grow upscale into one or more bowing clusters with the potential to produce significant severe wind gusts (greater than 75 mph) and brief tornadoes this evening. The damaging wind threat may linger into the early overnight as strong low to deep-layer shear attempts to compensate for increasingly pronounced MLCIN. ...Ozarks to Lower Red River Valley.. A remnant MCV over southwest MO will drift southward across western AR this afternoon. Strong surface heating where dew points remain from 70-75 F will support large MLCAPE over 3000 J/kg. Isolated to perhaps scattered thunderstorms should develop southwest from southern MO towards the AR/OK/TX border area. A belt of 20-30 kt mid-level northeasterlies may provide adequate shear to support a couple loosely organized multicell clusters. Isolated damaging downbursts along with severe hail are possible during the late afternoon and early evening. Read more LIVE: spc.noaa.gov