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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.

Monday, August 21, 2023

SPC Aug 21, 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 0742 AM CDT Mon Aug 21 2023 Valid 211300Z - 221200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE INTERIOR WEST MAINLY DURING THE AFTERNOON...AND PARTS OF THE UPPER MIDWEST AND LOWER/MIDDLE TEXAS COAST OVERNIGHT... ...SUMMARY... Locally severe thunderstorms are possible across parts of the Interior West mainly during the afternoon, and parts of the Upper Midwest and lower/middle Texas Coast overnight. ...Synopsis... The middle/upper-level pattern over the CONUS will remain dominated by a large-scale anticyclone, which had a 500-mb high of 604 decameters just north of TOP, on last night's 00Z manual chart analysis. This high is expected to remain at or above 600 decameters through the remainder of the period, while drifting erratically around the TOP/STJ/MKC vicinity, before shifting south and weakening somewhat on day 2. North of the high, ridging should amplify somewhat over the Dakotas today, making the flow over the Upper Great Lakes more northwesterly. Meanwhile, a broad but weak mid/upper cyclone -- initially centered just offshore from SFO -- is expected to move little before 12Z tomorrow, but devolve into an open shortwave trough, as another low digs southeastward just offshore from Vancouver Island. In between the low and high, a broad channel of fast flow aloft and low/middle- level moisture will persist today across the Desert Southwest and Great Basin, becoming southwesterly across the northern Rockies. Farther southeast, a well-defined low/middle-level trough, with some potential to develop into a tropical cyclone, will move westward from the central Gulf, reaching the lower TX Coast near the end of the period or early day 2. See latest NHC tropical weather outlooks for more on that system. At the surface, 11Z analysis showed a cold front from northern ME across the Lower Great Lakes to northern IN, becoming a quasistationary to warm front from there over northern IL, southern/ western IA and southern SD, to a low near TOR. This front should move northward as a warm front over eastern SD, southern MN and parts of IL/WI, as the low migrates east- northeastward to east-central/northeastern SD by 12Z tomorrow. ...AZ-MT corridor... Isolated to scattered thunderstorms are expected to pose a marginal severe-wind threat this afternoon into evening, within a long plume from the southern AZ border, up through the eastern Great Basin and Star Valley/Teton/Yellowstone corridor to parts of southern MT. Though the remains of TC Hilary are expected to continue ejecting northward and becoming less distinct today, the enhanced southerly flow belt farther east will continue to yield seasonally strong deep-layer southerlies, as noted above. This should enable downward momentum transfer to the ground, especially in more intense/ persistent cells within a lengthy, broken plume of surface-based convection developing in the diurnal heating cycle. Heating should be stronger and more persistent along the eastern rim of the favorable moist plume, which is approximately represented by this outlook. Peak/preconvective MLCAPE should range from around 300-800 J/kg over the UT-MT part of this corridor, to 1000-1500 J/kg across AZ. ...Upper Midwest... Widely scattered to scattered thunderstorms are expected to develop this evening and overnight, moving east-southeastward across the outlook area while offering isolated, marginal potential for severe hail. After 00Z, as the upstream boundary layer decouples and a southwesterly LLJ develops/intensifies, associated isentropic lift of an increasingly moist air mass to LFC should occur, amidst elevated frontal forcing northeast of the surface boundary. Time series of forecast soundings suggest that, above a stable surface- 850-mb layer and away from convection, continual steepening of low/middle-level lapse rates will occur. This should result mainly from elevated low-level warm advection, but also, slight cooling in midlevels. Elevated MUCAPE in the 1500-2500 J/kg range may develop, collocated with 40-50-kt effective-shear magnitudes. This can support organized thunderstorms with hail, though potentially expansive, messy storm mode may limit local duration/magnitude of the hail threat. ...Portions of mid/lower TX Coast... Isolated supercells may reach the immediate coastal areas on ether side of CRP the last few hours of the period (late overnight/early morning). Though uncertainty remains regarding the evolution of the Gulf trough into a potential tropical system (per NHC forecasts), a belt of enhanced low-level gradient flow should strengthen along its northern rim. This process may expand low-level hodographs enough to support storm-scale rotation in cells approaching the coast in the last few hours of the period, as a start to the more extensive/ inland (but for now still marginal) tornado potential discussed in the day-2 outlook. ..Edwards/Jewell.. 08/21/2023 Read more CHECK UPDATE ZOOM GRAPHIC