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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.


Sunday, August 21, 2022

SPC Aug 21, 2022 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook CORR 1 NWS Storm Prediction Center Norman OK 0314 PM CDT Sun Aug 21 2022 Valid 212000Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE OHIO VALLEY... CORRECTED FOR THUNDER LINE ACROSS SOUTH TEXAS ...SUMMARY... Isolated strong to briefly severe thunderstorms will continue into late this afternoon, with the greatest relative threat across parts of the Ohio Valley. ...20Z Update... The northern portion of the Slight Risk across OH has been trimmed in the wake of ongoing convection, though a threat for localized damaging wind may continue with additional rounds of storms late this afternoon into the early evening. The Marginal Risk has been trimmed across parts of north TX, NC, west TN, and northern MS/AL, based on observational trends and the progression of ongoing convection and related boundaries. Portions of coastal NC have been added to the Marginal Risk, where isolated damaging wind gusts will be possible ahead of an eastward-moving line of storms. See MCD 1708 for more information on the threat across coastal NC. Otherwise, no changes have been made, with a Slight Risk remaining in place across parts of the Ohio Valley. See the previous discussion below for more details. ..Dean.. 08/21/2022 .PREV DISCUSSION... /ISSUED 1120 AM CDT Sun Aug 21 2022/ ...Synopsis... Mid/upper ridging remains generally suppressed across the southern mid- and subtropical latitudes of the eastern Pacific into parts of the Southwest, and across the northern Mexican Plateau and Gulf Basin into the Atlantic. Much of the U.S. will remain under the influence of a weak branch of westerlies, well downstream of large, deep troughing centered over the mid-latitude Pacific, roughly between 140-170 W. This regime includes mean ridging across the Pacific coast into the Rockies, with an embedded short wave trough approaching the Pacific Northwest coast. Downstream, broad mean troughing prevails east of the Rockies, with the most substantive embedded short wave troughing forecast to continue slowly digging east-southeast of the mid/upper Mississippi Valley toward the Appalachians. Another weaker perturbation likely will very slowly progress across the southern Great Plains. Milder and, particularly, drier air has already infiltrated much of the northern Great Plains into Upper Midwest, and will gradually overspread much of the Ohio Valley, while a similar environment remains entrenched across much of New England. Seasonably high moisture content will continue to gradually become increasingly confined to parts of the Southwest, and Gulf into southern and mid Atlantic Coast states, by the end of the period. ...East of the Rockies... The lingering seasonably moist air, coupled with weak mid-level inhibition and large-scale forcing for ascent, is already contributing to considerable pre-frontal thunderstorm development across parts of the southern Great Plains into the Ohio Valley, lower Great Lakes region and Mid Atlantic. This will continue, with a further increase in storm coverage with boundary-layer heating through late afternoon. While the remnants of elevated mixed-layer air might enhance potential for localized downbursts across parts of north central and northeastern Texas by late afternoon, lower/mid tropospheric lapse rates across most areas are generally weak. Deep-layer mean flow and shear is also rather weak, but heavy precipitation loading and downward mixing of 20-30 kt southwesterly to westerly flow in the 850-500 mb layer, to the south and east of the digging short wave trough, might be enough to contribute to locally damaging downbursts in stronger storms, mainly across parts of the Ohio Valley. ...Southwest... Boundary-layer heating and mixing are forecast to remain somewhat modest for the time of year, with the stronger heating and deeper mixing mostly confined to the lower Colorado Valley. However, this may be sufficient, given the relatively moist conditions, to support sizable CAPE by late this afternoon. Aided by 10-20 kt northerly mid-level flow, if thunderstorms can cluster/consolidate sufficiently along the Rim late this afternoon, there appears to be potential for convection to gradually propagate off the higher terrain (and particularly toward the Colorado Valley) accompanied by a risk for strong surface gusts into this evening. Read more CHECK UPDATE ZOOM GRAPHIC
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