Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 22 2023 12:51:02 ACUS01 KWNS 221250 SWODY1 SPC AC 221249 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Fri Sep 22 2023 Valid 221300Z - 231200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL AND NORTHERN HIGH PLAINS AND OVER THE COASTAL SOUTHERN MID-ATLANTIC/TIDEWATER REGION... ....SUMMARY... Severe thunderstorms are possible across parts of the central and northern High Plains this afternoon and this evening. A few tornadoes are also possible over the coastal southern Mid-Atlantic/ Tidewater region overnight into tomorrow morning. ....Synopsis... A temporarily cut-off, synoptic-scale, mid/upper-level cyclone covers most of the northwestern CONUS at this time, and is centered over the OR/NV border region. The associated 500-mb low is forecast to pivot east-northeastward across southern ID today, reaching over or near the Bighorn Basin of north-central WY by 12Z tomorrow. In the southeastern quadrant of that cyclone, a shortwave trough was apparent in moisture-channel imagery over portions of UT, and should eject northeastward to central/southeastern WY by 00Z. Overnight, that perturbation should pivot northward and weaken across the MT/ND border vicinity. Meanwhile, a deep-layer cyclone east of the southern Atlantic Coast -- classified as Potential Tropical Cyclone 16 -- continues to organize. This feature is forecast by NHC to move ashore eastern North Carolina tomorrow, likely as a tropical or subtropical cyclone. Refer to NHC advisories for the latest specific forecast track/intensity information and related tropical watches/warnings. At the surface, 11Z analysis depicted a low between GCK-LHX, with a quasistationary frontal zone northeastward over eastern NS, and an outflow boundary eastward over central/eastern KS. This low should weaken today while another develops in a regime of lee troughing over eastern CO. The low should move into northern KS or southern NE by the end of the period, with a cold front southwestward over southeastern CO, and a stationary to slow-moving warm front northeastward over eastern NE and northern IA. ....Northern Plains... Widely scattered to scattered thunderstorms are forecast to form over higher terrain of northeast WY early this afternoon, moving northeastward through a relatively narrow corridor of favorable low-level instability/buoyancy to support surface-based supercells. Large/damaging hail, some exceeding 2 inches in diameter, is expected, along with isolated severe gusts and slight potential for tornadoes. Boundary-layer winds over the region will have a substantial easterly component through most of the day, supporting a combination of favorable processes: * Warm and moist advection, * Upslope lift to support development in tandem with heating of higher terrain, * Enlarged low-level hodographs, and * Storm-relative winds in the inflow layer of maturing convection. Meanwhile, large-scale ascent/cooling aloft, and deep shear, each should strengthen from midday through afternoon as the shortwave trough approaches. 40-50 kt effective-shear magnitude and at least 150-200 J/kg effective SRH are forecast, with enough cooling aloft to support 800-1200 J/kg MLCAPE. Low LCL (especially for this area) and related moist boundary-layer conditions support some tornado potential, while analogs and 2D hail models applied to forecast soundings reasonably generate significant hail. MUCAPE will remain favorable even after the initial round of convection, thanks to additional cooling aloft, suggesting any storms that form closer to the trough still may pose a large-hail threat, even over a somewhat stabilized boundary layer. Activity should spread/expand northeastward across much of the north-central Plains tonight. Additional development is possible in the elevated warm-advection zone over parts of the mid Missouri Valley, and ahead of an MCV over parts of IA, with mainly a marginal hail concern. ....Coastal southern Mid-Atlantic/Tidewater... Potential for supercells and a few tornadoes will increase overnight as PTC 16 approaches the coast. In the meantime, the overland boundary layer will remain north of the baroclinic zone accompanying the northern/western part of the cyclone, and as such, too stable for surface-based convection through most or all of the daylight hours. However, this evening and overnight, the frontal zone should shift inland. Persistent theta-e advection with intensifying low-level flow will increase moisture and steepen low-level lapse rates across the outlook area from southeast to northwest. As that happens, buoyancy will increase, generally being largest along immediate mainland coastal areas around the eastern NC sounds, over the sounds, and across the Outer Banks. MLCAPE in the 500-1500 J/kg range is possible, despite midlevel lapse rates only slightly greater than moist adiabatic. Enlarging and well-curved hodographs are forecast, with effective SRH commonly 300-500 J/kg, and isolated values near 700 J/kg. This parameter space should support tornado potential with any sustained supercells in the northern through eastern parts of the circulation. Based on instability guidance and the latest forecast timing/track of the low, some minor reshaping of the outlook lines was done to reduce tornado potential in the unfavorable sector southwest and west of the projected track, and to include more of southeastern VA the last few hours of the period. ...Edwards/Kerr.. 09/22/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .