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Tuesday, August 24, 2021

SPC Aug 24, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1128 AM CDT Tue Aug 24 2021 Valid 241630Z - 251200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY INTO TONIGHT ACROSS PARTS OF NORTHERN IOWA AND SOUTHERN MINNESOTA INTO CENTRAL AND SOUTHERN WISCONSIN... ...SUMMARY... A few additional clusters of thunderstorms may develop and organize across parts of the Upper Midwest into the middle Missouri Valley later today into tonight, posing at least some risk for hail and damaging wind gusts. ...Synopsis... Mid-level ridging will remain prominent across much of the southern mid-latitude and subtropical U.S. and western Atlantic through this period. While troughing is forecast to linger on the northwestern periphery of this ridging, across the Pacific Northwest coast vicinity, a fairly significant short wave trough within the westerlies is in the process of accelerating east-northeast of the Canadian Prairies and adjacent international border vicinity. As this perturbation approaches northwestern Ontario later today through tonight, mid-level heights may tend to rise at least subtly to the south of the international border, across much of the middle Missouri Valley through Upper Midwest. Increasingly warm elevated mixed-layer air is also forecast to continue advecting across the upper Mississippi Valley into the Upper Great Lakes region. In lower levels, to the south of a modest deepening surface cyclone over northwestern Ontario by late tonight, a cold front is expected to advance southeastward through the Upper Mississippi Valley and central Great Plains by 12Z Wednesday. ...Upper Midwest into mid Missouri Valley... Extensive, but generally weakening, thunderstorm activity is ongoing across a large portion of northern and central Wisconsin into southern Minnesota. This activity seems to have been generally supported by forcing associated with weak perturbations rounding the northwestern periphery of the mid-level ridging. As stronger forcing for ascent associated with the main upstream short wave within the westerlies tends to pass by to the west and north (mostly north of the international border) later today through tonight, subsequent convective developments remain much more unclear. In addition to the tendency for substantive warming (and increased capping) aloft, the pre-frontal southerly low-level jet and associated forcing appear likely to remain weak/unfocused. Seasonably moist air, to the south of the outflow boundary associated with ongoing convection, likely will remain characterized by large CAPE, beneath steep mid-level lapse rates, but the extent to which this will be able return to potentially better synoptic forcing near the Minnesota/upper peninsula of Michigan international border vicinity remains unclear. Additional subtle perturbations are evident upstream, crossing parts of the Great Basin and Colorado Rockies, including one now contributing to high based convective development across Nebraska. These will continue to progress into the crest of the larger-scale mid/upper ridging later today tonight. All of this probably will have some impact on potential for additional thunderstorm initiation later today through tonight. However, the latest output of the various models, including the convection allowing guidance, continues to exhibit large spread concerning if and where this may occur. At any one location probabilities for new thunderstorm development appear relatively low, but it does seem probable that at least one or two clusters of storms will develop somewhere across the Upper Midwest into middle Missouri Valley region later today or tonight. Instability to support convection with an appreciable risk for organizing and posing a severe weather threat seems most likely to remain focused along the remnant outflow boundary from ongoing convection. The western flank of this boundary may tend to slowly advance northeastward across parts of southern into central Minnesota later today, in response to the progression of the trough in the westerlies. So slight risk probabilities have been maintained across this general vicinity, with some extension westward and northwestward. However, the severe threat does appear rather conditional, and the confidence is low. ..Kerr/Lyons.. 08/24/2021 Read more LIVE: spc.noaa.gov
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