Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
519 ACUS02 KWNS 220451 SWODY2 SPC AC 220449 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1149 PM CDT Mon Jul 21 2025 Valid 231200Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY INTO WEDNESDAY NIGHT ACROSS PARTS OF CENTRAL AND NORTHEASTERN NEBRASKA...SOUTHEASTERN SOUTH DAKOTA...NORTHEASTERN IOWA...SOUTHERN MINNESOTA...NORTHERN WISCONSIN...UPPER MICHIGAN... ...SUMMARY... Organizing clusters of storms posing a risk for severe wind are possible across parts of the middle Missouri Valley and Upper Midwest into adjacent portions of the Great Lakes region Wednesday through Wednesday night. ...Discussion... Models continue to indicate further suppression of the northeastern Pacific mid-level ridging, with flow becoming more zonal and progressive across the British Columbia coast into the Canadian Prairies during this period. Downstream, it still appears that mid/upper flow will continue to intensify in a belt across Ontario through Quebec, between a significant trough and cyclone slowly migrating eastward across Hudson Bay and prominent ridging initially centered over the Ohio Valley. A cold front trailing the cyclone is still likely to advance more rapidly southward across the Great Plains, to the lee of the Rockies, than across the Upper Midwest and adjacent Great Lakes region. This general evolution has been forecast for several days, but spread has been evident within and among the various model output concerning the synoptic and, particularly, sub-synoptic developments, which persists in latest model runs. Among the details which remain characterized by sizable spread, the cold frontal progression through the Great Plains/mid Missouri Valley vicinity, the strength of a short wave perturbation progressing out of the Pacific Northwest through the central Canadian/U.S. border vicinity, and weak mid/upper perturbations migrating around the southern and northwestern periphery of the interior U.S. ridge, across the Gulf coast and southern Great Plains toward upper Mississippi Valley. These and other smaller-scale perturbations, with low predictability at this time frame, will considerably impact the convective potential for Wednesday through Wednesday night. ...Mid Missouri Valley into Upper Midwest/Great Lakes... While it still appears that the southern fringe of the stronger mid/upper westerlies will not extend south of a corridor from southeastern Minnesota through northern Wisconsin and Upper Michigan, models now generally indicate a belt of moderate south-southwest flow (20-40 kt in the 850-700 mb layer) in the lower/mid-levels will develop across the central Great Plains into the Upper Midwest, ahead of the weak perturbation migrating around the western periphery of the mid-level ridge. Beneath modestly steep lapse rates, but with generally warm profiles, it now appears increasingly probable that seasonably high boundary-layer moisture content, supportive of large mixed-layer CAPE, will extend along the frontal zone from the Upper Midwest/Great Lakes as far southwest as central/northeastern Nebraska by late afternoon. Coupled with the strength of the lower/mid-tropospheric flow, it appears probable that this environment will become supportive of one or two organizing thunderstorm clusters with potential to produce swaths of strong to severe wind gusts. However, based on late model output, including available convection allowing guidance, the potential evolution Wednesday through Wednesday night remains rather uncertain at this time. ...Northern Rockies into Front Range... In the presence of steep lapse rates, moistening easterly to southeasterly near-surface flow, beneath modest westerly flow aloft, may contribute to sufficient destabilization and shear to support widely scattered strong to severe storms. This may include evolving supercells near/just east of the higher terrain late Wednesday afternoon and evening. ..Kerr.. 07/22/2025 $$