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Tuesday, September 7, 2021

SPC Sep 7, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Tue Sep 07 2021 Valid 071300Z - 081200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM LOWER MICHIGAN INTO CENTRAL ILLINOIS AND NORTHERN INDIANA... ...SUMMARY... Damaging winds, as well as isolated large hail and a tornado or two, will be possible from Lower Michigan into central Illinois and northern Indiana. ...Synopsis... The CONUS portion of the mid/upper-level pattern is forecast to amplify through the period. The most important feature for this forecast will be a strong shortwave trough -- initially located across northwestern ON and north-central/northeastern MN, to the FSD area. An increasingly well-defined mid/upper cyclonic circulation -- now apparent in moisture-channel imagery over ON north of the MN Arrowhead -- is progged to become a pronounced closed vortex today. The resultant 500-mb low should move to near Lake Nipigon in ON by 00Z, with trough across WI to eastern IA. By 12Z, the low should deepen and reach northeastern ON southwest of James Bay, amid an expanding cyclone, with trough across northern Lower MI, northern IL and eastern MO. In the West, a strengthening anticyclone -- initially centered over NV -- will move slowly eastward to UT, while an attached ridge moves eastward across the Canadian Rockies and northern AB. Ridging also will extend from the high toward the Arklatex region and westward over central CA and the adjoining Pacific. To its west, a compact shortwave trough over the Pacific will eject northeastward and cross coastal WA/OR overnight. The preceding, pronounced patch of strong DCVA and warm advection will act with available low/middle-level moisture to support isolated thunderstorms over parts of northwestern OR and western WA tonight. At the surface, 11Z analysis showed a primary cyclone related to the mid/upper trough over northwestern ON, with an occluded front trailing southwestward to a weaker low near EAU. A cold front extended southwestward from there across southeastern NE, northern/ western KS and southeastern CO. A warm front -- drawn from the low southeastward across southern Lake Michigan and northeastern IN -- will move northeastward over Lower MI today. By 00Z, the cold front should extend from northern Lower MI across western IL, southeastern KS, west-central/southwestern OK, the southern TX Panhandle, and south-central NM. By 12Z, it should reach Lake Ontario, central OH, western KY, northern/central AR, southeastern OK, northwest TX, and far west TX. The front should be preceded by a strong-severe band of convection from the upper Great Lakes at least to parts of IL/IN. ...Upper Great Lakes to mid Mississippi Valley... Through the day, scattered to numerous thunderstorms should develop near/ahead of the cold/occluded front, organizing into a quasi-linear convective system with damaging to sporadically severe gusts as the main concern. Isolated severe hail also may occur, particularly with any early-stage or embedded supercells. Within a broader band of deep-layer lift, a band of strong thunderstorms (with elevated inflow) is noted, moving from northern Lake Michigan over northwestern Lower MI, north of the warm front. Please see SPC mesoscale discussion 1700 for coverage of marginal near-term severe potential therewith. Additional convection is likely to form through the remainder of the morning in the same plume of ascent, while building southwestward, and encountering increasingly unstable air nearer to the surface. Uncertainty regarding potential for surface-based inflow increases northward across northern Lower MI, where an antecedent cool/low-theta-e air mass north of the warm front will take longer to erode (and may not for surface-based parcels) in pre-cold-frontal warm advection. Still, given the strength of the ambient flow aloft and potential for momentum transfer to assist downdraft gusts through any shallow/stable layer near surface, will maintain the northern part of the outlook. With southward extent, the preceding air mass will have more time to destabilize from both theta-e advection and diurnal heating. By mid/late afternoon, a robust squall line is expected from eastern Lower MI to parts of northern/central IL. A narrow corridor of favorable southwesterly return-flow moisture (surface dew points in the mid/upper 60s F, with local values near 70) will combine with diurnal heating to yield a plume of MLCAPE in the 1500-2500 J/kg range over IL, decreasing northeastward to around 1000-1500 J/kg ahead of the line in MI. Deep bulk shear will be favorable, with effective-shear magnitudes in the 40-50-kt range. Despite southwesterly surface winds, sufficient veering with height and speed shear will exist to yield curved low-level hodographs and effective SRH in the 150-250 J/kg range, especially from northern IL northeastward, based on modified forecast soundings away from model-convection influences. This indicates the potential for embedded supercells and/or bowing mesovortices, with locally magnified severe-gust potential and at least marginal tornado threat. The severe potential should diminish after dark as the activity encounters a nocturnally stabilizing boundary layer, though an elevated warm-advection plume with moisture transport may help thunderstorms to persist eastward toward the lower Great lakes overnight. ...OK and vicinity... Thunderstorm coverage will become more isolated to widely scattered with southwestward extent along the front over MO and the southern Plains, in an environment of weaker deep-layer lift/shear and likely shallower frontal slope. Still, especially over parts of northern/ western OK, enough lift from the front and strong boundary-layer heating may develop to support afternoon/early evening multicell convection capable of isolated pulse-severe gusts/hail. Steep low- level lapse rates and well-mixed subcloud layers are expected beneath roughly 1500-2500 J/kg MLCAPE, enabling the marginal severe potential. ...Borderlands near TUS-ELP corridor... Isolated to widely scattered thunderstorms may produce locally strong-severe gusts this afternoon. Moisture-channel imagery indicates a stretched-out mid/upper perturbation from the southern TX Panhandle southwestward over parts of southeastern NM. Part of this feature should pivot southwestward across southern NM today, contributing large-scale lift atop a well-heated/mixed and favorably destabilized boundary layer. Lower-elevation surface dew points commonly remaining in the 50s F through diurnal mixing will contribute to peak preconvective MLCAPE around 800-1200 J/kg (locally/briefly higher). A channel of 30-50-kt northeasterly flow in the 500-350-mb layer may enable enough deep shear (30-40-kt effective-shear vectors and stronger cloud-layer shear) for some convective organization. ..Edwards/Gleason.. 09/07/2021 Read more LIVE: spc.noaa.gov
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