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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, September 4, 2023

SPC Sep 4, 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 0747 AM CDT Mon Sep 04 2023 Valid 041300Z - 051200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE NORTHERN PLAINS... ...SUMMARY... Severe gusts and large hail are expected over parts of the northern Plains this afternoon and evening. ...Synopsis... West of a cut-off cyclone lingering south of Atlantic Canada, the mid/upper-level pattern has become substantially more progressive. High-amplitude ridging will shift eastward through the period, from a Great Lakes/Tennessee Valley axis to the Appalachians and St. Lawrence Valley. This will occur as heights fall across much of the central CONUS and Mississippi Valley in relation to two ejecting perturbations: 1. A strong trough (itself a formerly cut-off Pacific Coast cyclone), now readily apparent in moisture-channel imagery from southwestern MT southward over western UT to near LAS. Although this trough will lose some amplitude through the period, it still will present a seasonally formidable wind profile aloft as it moves east-northeastward across the north-central Rockies toward the northern Great Plains today, with a strongly cyclonically curved jet core of 60-80 kt at 250 mb and 40-50 kt at 500 mb still apparent by 00Z from the central Rockies to the Plains in WY and the western Dakotas. By 12Z tomorrow, the trough should become more positively tilted, extending near a MOT-PHP-LIC axis. 2. A weak, presently cut-off cyclone centered near Lake Texoma, devolving to an open wave today as the larger perturbation encroaches upon the central/northern Plains. The southern trough will shift east-northeastward, reaching near a line from SGF-TXK-LFK by 00Z, and near MLI-UNO-SHV by 12Z. At the surface, 11Z analysis showed a weak cold front from southern MB to a low near MBG, south-southwestward over eastern CO. A stronger cold front/surge was apparent over eastern MT into central WY, and will catch up with the leading boundary later today. The low should reach southeastern MB by 00Z, connected by the cold front to a secondary/newer low over the SD Badlands region, with front southeastward over south-central CO. By 12Z, the SD low should move to northeastern SD or adjoining extreme western MN. This low will anchor a small wave cyclone along a cold front extending northeastward over the MN/ON Boundary Waters region, and southward to southwestward over eastern NE, central KS, northwestern OK, the TX Panhandle, and northeastern NM. ...Northern Plains... Scattered to locally numerous thunderstorms are expected to form mainly behind the cold front, but along and perhaps slightly ahead of it as well, depending on laterally expansive extent of a complex that may evolve from initial activity over WY and move rapidly northeastward. Large hail and sporadic severe gusts are possible from early-stage, relatively discrete convection, with the threat becoming wind-dominant when and where cold pools aggregate from the early convection and build a forward-propagational process. Timing/location of that evolution from more discrete elements (including supercells) to an MCS is uncertain, but the 15%/"Slight" area still appears to represent the greatest unconditional wind threat. A fairly sharp gradient of severe-wind potential in particular may evolve, and some part of the current 15% area may need an upgrade today if mesoscale details become clearer and 12Z/onward numerical guidance focuses potential better. In the meantime, the 15%/wind area has been expanded somewhat to allow more room for MCS growth, especially on the north and east ends. Activity initially should be somewhat high-based atop 40s and 50s F surface dewpoints over WY, moving into increasing moisture and buoyancy with northeastward extent. Forecast soundings reasonably indicate well-heated/mixed, deep subcloud layers and 500-800 J/kg MLCAPE in the immediate preconvective environment over WY and western SD. This will develop amidst strengthening deep shear, with 40-50-kt effective-shear magnitudes possible along and behind the front. This environment will support strong-severe downdrafts, maintenance of hail from relatively discrete storms to the surface, and outflow-aggregation potential. 60s F surface dewpoints downshear over northern SD and central/eastern ND will support a corridor of MLCAPE in the 1000-1500 J/kg range, enabling maintenance of any forward-propagating cluster into a nominally stabilizing near-surface environment that still may support severe-gust potential. Isolated significant-severe (65+ kt) gusts are possible conditionally almost anywhere in the corridor. However, such a hazard now appears most probable in a zone bounded by greatest confidence of maturing upscale organization on the southwest side (northwestern SD) and most-impactful evening stabilization on the northeast end (central ND). ...Portions of Ozarks/Mid South... Widely scattered to scattered, multicellular thunderstorms should develop today over portions of AR and move northeastward, with localized wind damage, isolated hail near severe limits, and perhaps a marginally severe gust or two possible. Though low-level forcing appears modest over his region, a combination of antecedent weak MLCINH, rich low-level moisture (upper 60s to mid 70s F surface dewpoints), and strong diabatic heating will support development, with aid from outflow/ differential-heating boundaries. Some midlevel cooling is also expected over the area as the trough approaches, resulting in a net diurnal steepening of low/middle-level lapse rates, supporting preconvective MLCAPE commonly in the 2000-2500 J/kg range (locally higher). In a nearly unidirectional deep-layer wind profile, a channel of relatively fast (40-50-kt) southerly flow may develop east of the trough over parts of AR this afternoon, aiding in storm organization despite modest (25-35-kt) effective shear. Convective organization should diminish this evening amidst regional stabilization from both outflow and diabatic near-surface cooling. ..Edwards/Leitman.. 09/04/2023 Read more CHECK UPDATE ZOOM GRAPHIC
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