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Saturday, October 9, 2021

SPC Oct 9, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Sat Oct 09 2021 Valid 091300Z - 101200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE EASTERN DAKOTAS AND MINNESOTA... ...SUMMARY... Severe hail, damaging gusts and a few tornadoes are possible today into this evening over parts of the eastern Dakotas and Minnesota. ...Synopsis... In mid/upper levels, a positively tilted synoptic trough will shift eastward across parts of the western CONUS and Great Plains this period, with two strong/embedded shortwave troughs: 1. A northern perturbation -- initially located over portions of WY and southeastern MT -- is forecast to move eastward to the western/ central Dakotas this evening. A closed 500-mb low should develop and move east-northeastward overnight to near the ND/SD/MN border confluence. 2. A southern perturbation -- apparent in moisture-channel imagery over northern CA -- should pivot southeastward across the Sierra and western/central NV by 00Z, amplifying considerably. By 12Z, this trough should extend from southwestern CO to southeastern AZ, connected to the northern shortwave trough by a southwest/northeast- oriented vorticity banner over the central High Plains. The 11Z surface analysis showed a quasistationary frontal zone from northwestern ON across northwestern/west-central MN to a weak low near PHP, then northwestward over southeastern MT. A dryline was drawn from the SD low southward to another low in the GLD/IML area, then across western KS, the eastern TX Panhandle, and the Permian Basin. A weak cold front was evident from the central Plains low into central CO. The latter low should move northeastward and gradually deepen today, linking with the northern Plains frontal zone over eastern ND by 00Z. A cold front by then should extend from the low southwestward to eastern CO, with warm frontogenesis occurring southeastward across southern MN to central IL. The low should strengthen further and move northeastward to the southeastern ND/northwestern MN area by the end of the period, with warm front across western/central WI and cold front over western IA, central KS and northeastern NM. ...Northern Plains to upper Mississippi Valley region... Scattered thunderstorms -- perhaps in multiple episodes -- are forecast to develop near the front in central/eastern SD and move northeastward across the outlook area. A blend of convective modes is expected, from discrete to densely clustered, multicellular and supercellular. Damaging gusts and large hail are expected, and a few tornadoes may occur from either discrete or MCS-embedded supercells. A dominant complex may evolve late afternoon into evening and move northeastward into MN, with a dominant wind threat, though marginally severe hail and a QLCS tornado threat may persist. A broad area of precip and embedded thunderstorms is evident from south-central to northeastern SD, within which isolated hail- producing convection may erupt this morning as large-scale ascent/cooling increase aloft, and moist advection continues into the area mainly above the surface. However, the main round of activity with severe potential should develop from midday into afternoon northeast of the low, where low-level convergence will be relatively maximized. This area will be along the northwestern rim of a channel of favorable low-level moisture (dew points generally upper 50s to low 60s) aligned parallel to and near the warm front. A midlevel dry slot wrapping around the southern part of the mid/upper trough may extend far enough north and northeast into the outlook area to thin or clear out abundant, antecedent cloud cover and enhance heating, but given cloud-cover progs from several models, this is more uncertain. In a scenario with only muted heating, MLCAPE should range from around 250-500 J/kg over southeastern ND and adjoining northwestern MN, to 1000-1500 J/kg near the southern rim of the outlook over south-central/southeastern SD. Backed near-surface winds and southwesterlies aloft will lead to favorable shear, with effective-shear magnitudes 45-55 kt over the near-frontal warm sector and effective SRH in the 150-300 J/kg range. This should support a mix of convective and severe modes, but with wind becoming more common as an arc or line of strong- severe thunderstorms organizes and moves into MN tonight. Activity will encounter gradually weaker instability northeastward. Isolated large hail also may occur from elevated thunderstorms just north of the warm front as far east as parts of WI, with a lobe of the outlook extended accordingly. ...Eastern NC... Broad cyclonic flow aloft is apparent around a very slow-moving mid/upper trough located over WV, western VA, the Carolinas, to northeastern FL. This trough should drift eastward and amplify through the period, resulting by 12Z tomorrow in a closed (or very nearly closed) 500-mb low over the Atlantic, roughly east of JAX and south of HSE. An associated surface low -- initially over the Atlantic well east of CHS - may retrograde northwestward toward the NC coast through the period, with NHC monitoring for signs of subtropical development. [See NHC tropical-weather outlooks for latest details on that potential.] Regardless of whether the low ever acquires a name, low-level flow above the surface may increase substantially over the Outer Banks if the cyclone gets closer, enlarging boundary-layer hodographs. The greatest low-level buoyancy will remain offshore, and considerable questions exist regarding the track/intensity of the low and amount of overland destabilization. At this time, too many uncertainties linger to introduce an unconditional outlook area for supercell/ tornado potential overnight, but the scenario will be monitored. ..Edwards/Gleason.. 10/09/2021 Read more LIVE: spc.noaa.gov