Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 26 2023 12:32:51 ACUS01 KWNS 261232 SWODY1 SPC AC 261230 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0730 AM CDT Tue Sep 26 2023 Valid 261300Z - 271200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF NORTHERN AND EASTERN ILLINOIS THROUGH WESTERN AND SOUTHERN INDIANA AND ACROSS SOUTHERN NEW MEXICO INTO WEST TEXAS... ....SUMMARY... Widely scattered strong thunderstorms are possible today across parts of the Midwest into lower Ohio Valley vicinity, and across parts of southern New Mexico into west Texas. Some of these may be accompanied by hail and gusty winds. ....Synopsis... As an initially broad and deep cyclone undergoes considerable further weakening near the northern British Columbia coast, associated large-scale mid-level troughing may gradually lose some amplitude while continuing to progress inland across the Pacific Coast and Canadian/northern U.S. Rockies today through tonight. It does appear that one vigorous embedded short wave impulse will accelerate toward the Pacific Northwest coast, accompanied by a more modest and compact surface cyclone late this afternoon into early Wednesday. This short wave has been preceded inland by at least a couple of other significant perturbations. One of these has already slowly begun to suppress a prominent downstream blocking high, initially centered near southern Hudson Bay, though the elongating center of higher heights may maintain considerable strength across southern Hudson/James Bays into the St. Lawrence Valley through this period. Beneath the mid-level high, a sprawling surface ridge (centered over Quebec) will maintain a considerable influence across much of the nation to the east of the Rockies. Seasonably high moisture content will generally remain confined to areas along and south of a weak surface front across southern Atlantic and eastern Gulf coastal areas, downstream of an elongating cyclonic perturbation migrating north-northeast of the Yucatan Peninsula, and merging into weak larger-scale troughing over the Gulf of Mexico. ....Southern Rockies into Texas Big Bend... Within the weaker southern branch of westerlies, and in advance of the significant inland migrating northern branch troughing, a weak short wave trough is forecast to slowly turn across and southeast of the Four Corners vicinity during this period. As it does, associated forcing for ascent may augment convective development, particularly storms initiating in response to heating across the mountains of southern New Mexico by early afternoon. It appears that moisture across this region will be sufficient to support initially sizable CAPE, beneath modest westerly to northwesterly flow, at least in upper levels. Stronger storms may become capable of producing marginally severe hail, then a few strong to severe gusts while propagating southeastward off the higher terrain into an unstable and more deeply mixed boundary-layer through early evening. ....Coastal Oregon... Forcing for ascent within the exit region of a strong jet streak (including 70-90 kt around 500 mb) may contribute to an organized, vigorous pre-frontal convective band, associated with the surface cyclone approaching the Pacific Northwest. However, with the cyclone forecast to occlude prior to reaching the coast near/after 27/06Z, appreciable inland boundary-layer destabilization appears unlikely, and stable near-surface lapse rates may tend to mitigate the potential for downward mixing of severe surface gusts as the convective line progresses inland. ....Florida Peninsula... Inland destabilization may support fairly widely thunderstorm development this afternoon and evening, with high precipitable water content supporting heavy precipitation loading and some potential for locally strong surface gusts. However, given the initial large degree of saturation in the thermodynamic profiles and the weakness of the ambient mean flow, peak gusts seem likely to remain mostly below severe limits. ....Indiana/Illinois vicinity... To the southwest of the lingering blocking mid-level ridging, a weak mid-level low is forecast to continue slowly digging from parts of the Upper Midwest toward the lower Ohio Valley. It appears that this will maintain at least broad, weak inverted surface troughing on the southwestern periphery of the Quebec surface high, which is forecast to become a focus for scattered thunderstorm development today beneath modestly cool mid-level temperatures (around -14 C near 500 mb). Beneath an evolving mid-level dry slot, mixed-layer CAPE may exceed 1000 J/kg by peak afternoon heating, which may be sufficient to support small to, perhaps, marginally severe hail in stronger cells. Due to the generally modest to weak nature of the deep-layer mean flow and shear, the potential for severe wind gusts appears low. However, an isolated, brief tornado might not be entirely out of the question, where surface convergence can become locally enhanced in close proximity/beneath the mid-level low. ...Kerr/Squitieri.. 09/26/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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