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Sunday, June 20, 2021

SPC Jun 20, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Sun Jun 20 2021 Valid 201300Z - 211200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF EXTREME EASTERN IOWA ACROSS CHICAGOLAND TO NORTHWEST OHIO AND SOUTHERN LOWER MICHIGAN... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK...AND ALSO OVER PARTS OF GEORGIA AND THE CAROLINAS... ...SUMMARY... Scattered severe storms are expected across the Midwest into the lower Great Lakes/upper Ohio Valley. Large hail, severe/damaging winds, and a few tornadoes are possible. Damaging winds and a couple of tornadoes are also possible across the Southeast. ...Synopsis... The mid/upper-level synoptic pattern will become much more cyclonically curved, with height falls common across the northern/ central Plains and much of the Great Lakes by 12Z tomorrow. This will occur as a series of shortwave troughs moves southeastward to eastward over the northern/central Rockies, Plains, and Upper Midwest. This will include an MCV and associated perturbation now evident over the OMA area into northeastern KS, which should move eastward to the northwestern IN/western Lower MI region by 00Z. A strong shortwave trough -- apparent in moisture-channel imagery over the northern Rockies just north of the Canadian border -- will move southeastward and lose some amplitude, reaching CO by 12Z. The 11Z surface analysis showed a wavy quasistationary to warm front from south of Long Island across southern PA, northern parts of OH/IN, northern IL, and southern MN, to a low near ABR. A cold front was drawn from there southwestward across northeastern CO, and is expected to move southeastward across the central Plains and upper/mid Mississippi Valley through the period. Meanwhile the warm front should shift northward across northern IL and into southern/central Lower MI ahead of the low, which should cross northern lower MI and Lake Huron overnight. ...Midwest/Great Lakes... Multiple rounds of scattered to locally numerous thunderstorms are possible through tonight within a broad corridor from the mid/upper Mississippi Valley to the area between southern Lake Michigan and Lake Erie. All severe hazards are possible -- wind, hail (some significant where supercells can occur) and tornadoes -- with the most dense and widespread concern being severe thunderstorm wind. Predictability does not always improve closer to an event, and this is an example. A messy, conditional and very mesoscale-dependent scenario is evident, with convective trends from the prior overnight to this morning only augmenting the uncertainties. The main influences for now appear to be: 1. MCV-related convection, which has weakened over the last few hours across IA and northern MO as it moves into a pronounced low-level theta-e deficit and CINH maximum related to the next factor below. Still, the associated mass and thermal perturbations may reinvigorate convection and organize a severe MCS over IL into northern IN/southern MI, as it encounters a diabatically destabilizing and suitably moist boundary layer... 2. A separate, earlier area of convection and precip -- now mostly dissipated -- that left behind its cold pool across northern MO and parts of western IL. This theta-e deficit will be entrained into prevailing low-level flow downstream into eastern IA and northern IL, with mixing and diurnal heating potentially diluting its negative impacts on convective potential somewhat. 3. The cold front later this afternoon into evening, to the extent it encounters favorable pockets of relatively undisturbed boundary-layer air across IA/WI/MI/IL/MO. The amount and extent of favorable instability will depend strongly on recovery behind both ongoing convective factors 1 and 2 above. The front is likely to support development south of the MCV and cold pools, over part of northern MO and eastern KS, where instability will be strong but shear weaker than farther north and northeast. Given these factors and related uncertainties, only peripheral changes are being made to the existing outlook, pending both more clarity on mesoscale trends, and numerical guidance from 12Z onward that will better incorporate the effects of these cold pools and MCV influences than earlier progs did. ...Southeast, T.D. Claudette... Following a nocturnal relative min in both convective and severe (tornado, gust) potential, a ramp up in those is expected during the remainder of this morning and into afternoon across parts of GA and the Carolinas. See SPC mesoscale discussion 1030 for near-term details. The greatest tornado potential will be bounded by relatively low-theta-e air and cloud cover/precip with weaker shear to the north and northwest, weaker deep-layer winds and areas of stabilizing precip to the south, and veering of boundary-layer flow (reducing hodograph size) to the southwest. A well-defined mid/upper-level dry slot was apparent in moisture-channel imagery wrapping around the southern semicircle of the system, and was sampled well upstream by the 12Z JAN and LIX soundings. The eastern fringe of this slot may overlap areas where surface flow remains more southerly to easterly across the eastern semicircle, low-level moisture remains very rich, and deep-tropospheric vertical-shear vectors also will be aimed. In an environment of modest midlevel lapse rates, even subtle diabatic heating-related enhancement to low-level lapse rate will yield favorable buoyancy, with MLCAPE commonly around 1000-1500 J/kg and effective SRH 150-300 J/kg supporting supercell development with any sustained convection. [See NHC advisories for the latest track/intensity guidance, as well as any tropical watches/warnings, regarding Claudette.] ...Central High Plains... Isolated to scattered thunderstorms are possible this afternoon and overnight in two main episodes, cumulatively contributing to this expanded area of severe probabilities: 1. Diurnal development near and north of the frontal zone, where a combination of heating of elevated terrain and weak moist advection will contribute just enough buoyancy for some high-based thunderstorm potential, with MLCAPE in the 300-800 J/kg range. A well-mixed subcloud layer will support localized strong/isolated severe downburst potential. Strong mid/upper winds will contribute to around 35-45 kt effective-shear magnitudes (which are limited by lack of greater CAPE depth). 2. Overnight convection developing ahead of the northwest-flow perturbation, in a regime of both DCVA and low-level theta-e advection. Lack of more-robust moisture still will be a limiting factor, but an area of convection may develop and move southeastward across the outlook area, offering potentially damaging gusts and marginal hail potential. Though much of the boundary layer will stabilize diabatically, a relict diurnal mixed layer above that may support enough downdraft acceleration to push strong/locally severe gusts to the surface. ..Edwards/Broyles.. 06/20/2021 Read more LIVE: spc.noaa.gov
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