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Saturday, June 26, 2021

SPC Jun 26, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0746 AM CDT Sat Jun 26 2021 Valid 261300Z - 271200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL ILLINOIS TO PARTS OF LOWER MICHIGAN...AND OVER PARTS OF THE SOUTHERN HIGH PLAINS... ...SUMMARY... The greatest threat for severe thunderstorms today appears to be from central Illinois to parts of Lower Michigan, and over parts of the southern High Plains. Damaging wind and large hail are forecast in those areas, and a few tornadoes will also be possible, especially over the Illinois-Michigan corridor. ...Synopsis... An atypically amplified mid/upper-level synoptic pattern will persist for this time of year, dominated by: 1. A blocking ridge over the West Coast States and western Canada, with anchoring anticyclone centered over southern BC that will contribute to record heat in the Northwest this weekend; 2. A positively tilted trough, extending from a cyclone over southern Baffin Island and adjoining waters, across northwestern ON, the Red River of the North, central High Plains, western NM, and Sonora. A series of mostly low-amplitude (but still potentially important) shortwaves and vorticity maxima will be embedded in the southwest flow from NM to the upper Great Lakes, preceding the mid/upper trough. Among these will be a convectively induced/reinforced perturbation now moving northeastward from the lower Missouri Valley region across northern MO and IA. This feature should eject across northern IL and WI by this evening, perhaps phasing with a closely spaced MCV evident in composited radar imagery over southwestern MO. Another MCV is apparent just behind that, over north-central/ northeastern OK, and likewise should move northeastward toward IL through the period. At the surface, 11Z analysis showed a low over western IA, with wavy, quasistationary front across parts of WI and lower MI to another low over Georgian Bay on Lake Huron. A slow-moving cold front extended from the IA low southwestward across central KS to central NM. The IA low-pressure area will elongate northeastward and perhaps split, covering parts of WI, Lake Michigan and northern Lower MI by 06Z. The cold front should continue to drift southeastward toward a series of warm-sector outflow boundaries. The effectively blended baroclinic zone represented by the front and outflow boundaries -- aligned largely parallel to the southwesterlies aloft -- will be the major focus for convective potential through the period. That will include severe-thunderstorm threats for this outlook, and heavy rainfall noted in WPC products. ...IL-MI... Scattered thunderstorms in multiple modes are expected to move northeastward across this area from midday through the afternoon, offering damaging gusts, a few tornadoes and isolated severe hail. Enhanced low/middle-level flow is expected east and southeast of the midlevel MCVs/troughs, contributing to effective-shear magnitudes potentially reaching the 35-45-kt range, in support of organized multicells and a few supercells. A 40-50-kt LLJ and backed surface flow along boundaries will enlarge hodographs, yielding pockets of effective SRH in the 250-350 J/kg range. As such, any supercells or suitably oriented QLCS segments interacting with the boundaries may pose a locally magnified tornado threat. Mid/upper-level lapse rates will be modest, but a combination of diurnal heating near the boundaries, rich low-level moisture (e.g., surface dew points upper 60s to mid 70s F), minimal MLCINH, and boundary lift should ensure considerable convective development with access to an adjoining warm-sector corridor of 1000-2000 J/kg MLCAPE. Given the largely boundary-parallel flow aloft, training bands/clusters and messy/high-precipitation modes are likely to evolve over time. ...Southern Plains... Isolated to locally numerous thunderstorms in clusters are expected to form this afternoon on either side of outflow boundaries left behind by prior and ongoing convection across this corridor. These boundaries may shift back northward in parts of the area prior to the afternoon convective cycle. Strong-severe convection also may develop along a dryline that should mix into the southern South Plains and Permian Basin region south of the outflow/frontal zones. With all this activity, large hail and severe gusts will be the main concerns, with tornado potential marginal and conditionally tied to storm-scale/boundary processes. Stronger, more-sustained diurnal heating and somewhat greater lapse rates aloft are expected over this region, compared to farther northeast past the Ozarks and Mississippi Valley, but amidst weaker deep shear. Forecast soundings suggest favorable moisture with upper 50s to 60s dew points above the Caprock, and mid 60s to lower 70s across northwest TX into OK, will enable MLCAPE into the 1500-3000 J/kg range (locally greater). Where boundaries can back surface flow and heat strongly, effective SRH around 150-200 J/kg may develop, with effective-shear magnitudes 40-45 kt, supporting a supercell/multicell modal mix. South of the boundaries, modest deep shear and higher cloud bases (with deeper well-mixed subcloud layers) will foster more outflow-producing multicells. ..Edwards/Broyles.. 06/26/2021 Read more LIVE: spc.noaa.gov
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