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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Friday, August 11, 2023

SPC Aug 11, 2023 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0756 AM CDT Fri Aug 11 2023 Valid 111300Z - 121200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MID/UPPER MISSISSIPPI TO LOWER MISSOURI/OHIO VALLEYS... ...SUMMARY... Severe thunderstorms capable of large hail, damaging to severe gusts and perhaps a tornado or two, are possible across portions of the Mid and Upper Mississippi Valley and Lower Missouri/Ohio Valley areas today. ...Synopsis... In mid/upper levels, an elongated anticyclone from central TX across the northern Gulf will strengthen slightly through the period. This will lead to net height rises -- but also stronger mid/upper- tropospheric gradient winds -- in a belt of northwest flow extending from the northern Plains across the southern Upper Great Lakes region. The main perturbation in this flow belt is a strong shortwave trough, apparent in moisture-channel imagery over SK, with a large field of difluent flow downstream across the northern U.S. Plains and Upper Midwest. The trough is forecast to reach MN and northwestern ON by 00Z, then eastern Upper to Lower MI and northwestern IN 12 hours later. A weaker preceding trough -- initially located from an MCV over northwestern WI south-southwestward to northeastern MO -- should shift eastward across most of Lower MI by 00Z. Farther south, even weaker (yet still potentially consequential), convectively induced/ enhanced perturbations were moving southeastward across AR, and from AL into portions of western/northern GA. At the surface, 11Z analysis showed a low over northern MN with cold front southwestward across western KS, to another low over the northern TX Panhandle. A warm front was drawn from each low east- southeastward, across OH and the Arklatex region respectively. The southern boundary was interrupted by outflow from an MCS over central AL, then became apparent again over southern/central GA. This front should move slowly northward through the day over GA/AL and into parts of SC. Meanwhile, the cold front will move eastward over the upper Mississippi Valley into this evening, then parts of Lower MI by 12Z tomorrow, in step with the eastward shift of the surface low to its north. ...Mid/upper Mississippi to lower Missouri/Ohio Valleys... An ongoing band of thunderstorms over parts of northern/western IL may continue to pose a threat of locally damaging to isolated severe gusts, as it moves southeastward through the reminder of the morning, south of the northern warm front. See SPC mesoscale discussion 1942 for near-term guidance. The trailing outflow boundary should shift back northward by early/mid afternoon over the southern IA/northern IL area. Isolated to scattered strong-severe thunderstorms, with some clustering, are possible this afternoon and evening from eastern KS and the Ozarks northward to the upper Mississippi Valley, starting this afternoon. Large hail and severe gusts both are possible from more-discrete, early-stage convection, and a conditional supercellular tornado threat may develop near a retreating outflow boundary across the southern IA/northern IL region. The severe threat should transition mainly to wind for any activity that can organize upscale and grow a forward-propagational, cold-pool-driven process. The most probable part of the outlook for a longer-lasting, organized MCS of that nature may be southern areas where inflow-layer theta-e is greatest: somewhere between eastern KS and either the Mid-South or lower Ohio Valley tonight. The western part of the greater boundary-layer moisture over central/ eastern KS will be characterized by strong diurnal heating/mixing and a deep boundary layer, beneath steep midlevel lapse rates supporting 2500-4000 J/kg MLCAPE. Though shear will be modest, strong upper/anvil-level flow should aid in storm organization. To the extent later guidance and mesoscale trends converge on a more-specific corridor, a channel of greater severe-wind probabilities may need to be drawn within the broader area. Farther north -- near and north of the outflow boundary from the morning activity -- stronger deep shear and weaker buoyancy are expected, but with instability still sufficient to support at least isolated strong-severe convection. MLCAPE should range from around 3000 J/kg just south of the boundary to 1000-2000 J/kg in parts of central/eastern MN and northern WI, under the strongest cooling aloft preceding the mid/upper trough. 40-50 kt effective-shear magnitudes are progged over the upper Mississippi Valley region as well. ...Southeastern CONUS... Isolated to numerous thunderstorms are possible in highly variable patches of coverage today, with damaging gusts as the main threat. Isolated, marginally severe (50+ kt) gusts also may occur. A broad swath of warm-sector air with rich low-level moisture (e.g., surface dewpoints commonly in the 70s F) will be heated diabatically to convective temperature, minimizing MLCINH and boosting MLCAPE to 3000-4000 J/kg. Weak, nearly unidirectional low/middle-level flow will serve two detrimental purposes: reduction of convergence and maintaining weak vertical shear. Still, given the large amount of moisture and buoyancy, water-loaded downdrafts will pose a local damaging- to severe-gust threat, and perhaps broader in scale if cold-pool aggregation/organization can build sufficiently upscale. Any such process(es) may yield one or two relative concentrations of potential today, depending on the downshear evolution of MCV/ trough-related large-scale lift and mesobeta-scale shear enhancements from the perturbations now associated with currently non-severe convective complexes over AR and AL/GA activity. However, potential still appears rather nebulously focused, and marginal in environmentally supported gust magnitude, for a 15% unconditional-wind area this outlook cycle. ..Edwards/Dean.. 08/11/2023 Read more CHECK UPDATE ZOOM GRAPHIC