Severe Weather outlook
Storm Prediction Center
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SPC Sep 15, 2024 1300 UTC Day 1 Convective Outlook
SPC 1300Z Day 1 OutlookDay 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Sun Sep 15 2024 Valid 151300Z - 161200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL AND NORTHERN HIGH PLAINS... ...SUMMARY... Thunderstorms capable of isolated severe gusts and hail will be possible today across parts of the central and northern High Plains. ...Synopsis... In mid/upper levels, a blocky longwave pattern will remain over the CONUS, with amplifying troughing in the West and a Rex configuration in the East. Cyclonic flow already covers most of the land west of the Rockies. That will continue but with backing of the flow aloft, as a cyclone now centered over Vancouver Island strengthens and digs south-southeastward down the West Coast. By 12Z tomorrow, the associated 500-mb low should be located near OAK, with strongly difluent flow and modest ridging over the High Plains from eastern NM to MT. The southern part of a weak shortwave trough -- evident in moisture-channel imagery from UT to eastern AZ -- should cross CO today and reach the eastern border of WY to near GLD by 12Z, likely enhanced from this evening onward by convectively generated vorticity. A mid/upper anticyclone now over the Lower Great Lakes will stretch eastward and open up to that direction around the end of the period. The base of the eastern pattern block will be composed of a broad, nearly zonal height weakness anchored by: 1. The decaying remains of Francine, long merged with a mid/upper low, and forecast to drift erratically over the Mid-South while weakening further; 2. An intensifying, deep-layer cyclone with low-level manifestation as a deepening wave cyclone along a frontal zone, offshore from the Carolinas. The 11Z surface analysis showed a wavy, quasistationary frontal zone attached to a strengthening low located roughly south of EWN and east of SAV. The front extended across northern FL to the western FL Panhandle and southwestern AL. A remnant low was drawn well inland over northern MS, connecting to the Gulf boundary via an occluded front. These features should move little through the period, but with further weakening of the western low, and intensification of the eastern one. Elsewhere, a lee trough and dryline either overlapped, or were drawn within less than 100 nm apart, near the eastern border of WY, eastern CO, the northern/ western TX Panhandle, and east-central/southeastern NM. The dryline should shift somewhat eastward today amid diurnal heating. ...Central/northern High Plains... Isolated to widely scattered thunderstorms are possible across a large part of the Rockies, Intermountain West, and Desert Southwest today. Some of this activity will mature atop deep, well-mixed boundary layers of desert valleys and the High Plains, with non-zero potential for a strong-severe gust. The most concentrated/organized potential for gusts near to above severe limits, as well as marginal hail, still appears to be over portions of the central High Plains into the Black Hills and vicinity, near the lee trough/dryline. East of that, low-level moisture will be relatively maximized in a return-flow airmass of predominantly continental origin, with a north-south belt of 50s to mid 60s F surface dewpoints. Where the western part of that moisture coincides with strong surface heating, and underlies the eastern part of the EML, a north-south corridor of MLCAPE in the 1500-2500 J/kg range should develop, with 500-1500 J/kg extending into the more deeply mixed regime nearest the lee trough. East of the trough, considerable veering of winds with height is expected, with some low-level hodograph enhancement potentially moving 0-3-km SRH to near 150 J/kg, but lack of stronger midlevel flow will temper overall shear. ...Coastal NC, extreme eastern SC... Many progs deepen the low-level cyclone and transition it to subtropical or tropical character through the end of day 2, while eroding the frontal zone's baroclinicity; however, inconsistency among models is high. The most aggressive among the synoptic and CAM guidance with the speed of the transition and approach to the coast (e.g., 06Z GFS and 00Z ARW high-res window respectively) show favorably large low-level shear, but barely have surface-based effective inflow onshore at the end of the period, with greatest convective coverage and buoyancy still offshore. As such, the associated coastal tornado threat still appears too low and conditional to outlook until day 2 (after 16/12Z). Potential for faster/deeper development and shoreward translation of the low will continue to be monitored for onshore supercell threat the last few hours of day 1. See latest NHC tropical outlook for guidance on overall development potential with this system. ..Edwards/Mosier.. 09/15/2024
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SPC Sep 15, 2024 1630 UTC Day 1 Convective Outlook
SPC 1630Z Day 1 OutlookDay 1 Convective Outlook NWS Storm Prediction Center Norman OK 1128 AM CDT Sun Sep 15 2024 Valid 151630Z - 161200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS... ...SUMMARY... Thunderstorms capable of producing isolated severe gusts and hail may occur today and tonight across parts of the northern/central Plains. ...Northern/Central Plains... An amplified upper trough along/near the West Coast this morning will further evolve into a closed upper low over northern CA and vicinity by late tonight. Ahead of this feature, modestly enhanced south-southwesterly mid-level flow will be in place over much of the western states and Rockies through the period. Although low-level moisture is expected to remain limited over much of this region, isolated strong to severe gusts may occur with any thunderstorms that can develop in a deeply mixed boundary layer favorable for gusty downdraft winds. Latest guidance continues to suggest that a relatively greater concentration of severe potential will be focused across parts of the northern/central Plains late this afternoon through tonight. Across this area, a weak surface lee trough and greater low-level moisture to its east should support weak to moderate instability with daytime heating. Steep mid-level lapse rates should also aid updraft accelerations/intensities, with around 20-30 kt of deep-layer shear supporting modest thunderstorm organization. Isolated severe hail may occur with initially more discrete convection, but a tendency for clustering should lead to a relatively greater wind threat with time as convection spreads generally east-northeastward through the evening. Regardless, the overall severe threat from northeast CO into western NE/SD and vicinity should remain fairly isolated/marginal given the modest deep-layer shear. Low-level warm advection is forecast to increase across much of the northern/central Plains tonight as a southerly low-level jet strengthens. A surface warm front is also expected to lift northward over ND late tonight into early Monday morning. With steepening mid-level lapse rates overspreading the northern Plains in tandem with increasing low-level moisture, ample MUCAPE should be available. Most guidance shows at least isolated convection developing across parts of ND tonight in this favorable thermodynamic environment. Although deep-layer shear may remain fairly modest, it may still be sufficient for updraft organization and an isolated threat for severe hail with any elevated convection that can form. Have therefore expanded the Marginal Risk northward to account for this potential. ...Coastal Carolinas... Latest NHC forecast continues to indicate some potential for sub-tropical or tropical development of a surface low currently off the SC Coast. Regardless of development, it still appears that the more moist/tropical low-level airmass over the Gulf Stream will struggle to advance inland over coastal portions of NC/SC through the end of the Day 1 forecast period early Monday morning (16/12Z). Accordingly, tornado potential with any low-topped supercells on the north to northeast periphery of the surface low should tend to remain offshore until Day 2/Monday (after 16/12Z). ..Gleason/Moore.. 09/15/2024
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