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Saturday, September 11, 2021

SPC Sep 11, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1239 AM CDT Sat Sep 11 2021 Valid 111200Z - 121200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE CENTRAL HIGH PLAINS...AND FROM SOUTHERN MINNESOTA AND WISCONSIN INTO MICHIGAN... ...SUMMARY... Isolated severe storms are possible during the day and through the afternoon across parts of Upper and northern Lower Michigan, and from late afternoon into the evening across parts of the central High Plains. An isolated severe storm cannot be ruled out late tonight centered over southern Wisconsin. Numerous storms, mainly non-severe, are expected over much of Utah during the day. ...Synopsis... A shortwave trough with 40+ kt 500 mb winds will move eastward across MT and WY during the day, and across the northern Plains overnight. Ahead of this feature, a larger-scale trough will move across Ontario and into Quebec, with stronger northwest midlevel flow to 70 kt over the northern Great Lakes. Meanwhile, an upper high will shift a bit east into the southern High Plains, with a central plains ridge flattening overnight. At the surface, a prominent high will remain over the East, with low pressure developing over the central Plains. A cold front will move southward across MI, WI, IA and NE during the day, with afternoon convergence extending from Lower MI to southeast WY. An area of mid to upper 60s F dewpoints will exist along this front from NE across IA and into WI, resulting in an unstable, but mostly capped air mass. However, lower 60s F dewpoints will be sufficient for an uncapped air mass across parts of MI during the day. ...Eastern Upper and Northern Lower MI... Although early, sufficient instability is expected along a cold front between 15-18Z over Upper MI, with a few storms expected to form and continue into northern Lower MI. Wind profiles will favor cellular storm mode, and a few could produce severe hail or gusty winds despite warm temperatures aloft. Most of the storm coverage will eventually develop into Ontario, but an isolated trailing cell cannot be ruled out toward the thumb of MI. ...Northeast CO into western NE... Deep inverted-v profiles will develop over southeast WY, northeast CO, and western NE with strong heating. High-based convection is likely to form over WY/CO after 21Z, possibly producing severe outflow winds as it shifts into NE during the evening. As this activity interacts with higher dewpoints late, a few cells could produce brief hail. ...Southern WI and MN overnight... Forecast soundings during the day reveal a capping inversion, and it appears lift along the boundary will not be sufficient to initiate storms. Overnight, a strongly veered but strong low-level jet will exist with up to 50 kt over IA. Lapse rates aloft will be steep, and deep-layer shear strong with weak height falls into Sunday morning. While storm initiation is uncertain overnight, a conditional risk of large hail is great enough to maintain low severe probabilities for hail. Glancing warm advection with the low-level jet may be sufficient for isolated storms. ...UT... Scattered storms are expected from early afternoon through evening, with perhaps 500 J/kg MUCAPE developing with heating. Profiles overall appear moist, with only a few forecast sounding points yielding an appreciably deep inverted-v. Due to the number of daytime storms, a few strong wind gusts seem possible, but the overall setup does not appear to favor severe. ..Jewell/Bentley.. 09/11/2021 Read more LIVE: spc.noaa.gov