SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0746 AM CDT Thu Oct 22 2020 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF EASTERN IOWA AND SMALL PARTS OF ADJOINING ILLINOIS AND WISCONSIN... ...SUMMARY... Severe thunderstorms with hail and damaging gusts are possible this afternoon and early evening over parts of eastern Iowa and small parts of adjoining Illinois and Wisconsin. A tornado threat may develop as well. ...Synopsis... In mid/upper levels, broadly cyclonic flow over the western CONUS and High Plains will shift eastward to cover most of the Plains States and Mississippi Valley through the period. The dominant shortwave involved is evident in moisture-channel imagery over the Yellowstone region of MT/WY/ID. This perturbation should pivot eastward by 12Z tomorrow to western MN, southeastern SD and central/southwestern NE. A small but pronounced southern-stream perturbation over western NM is forecast to reach the eastern NM line with TX by 00Z, then southwestern OK and northwest TX near the end of the period. At the surface, 11Z analysis depicted a wavy, quasistationary front from southern New England to northern IN, becoming a warm front across central IL, northern MO and northeastern/west-central KS, to a combined lee-side/frontal-wave low over southeastern CO. A dryline was drawn from that low across the western OK Panhandle and extreme eastern NM, to the TX Big Bend region. The low should split into two components: 1. The western one shifting southward across the south-central High Plains in step with the propagation of the intersection of the cold front and lee trough. 2. A frontal-wave rippling northeastward across northern KS and southeastern NE today, with a closed low forming over IA this afternoon. The eastern low should move northeastward across WI, Lake Michigan, northern Lower Michigan, and northwestern Lake Huron overnight. The warm-frontal zone to its east likewise will cross the eastern upper Great Lakes and the lower Great Lakes. By 00Z, the cold front should extend from the low across northeastern KS and the OK Panhandle. By 12Z, the cold front should reach northern/western IL, central/southwestern MO, southern OK, the Permian Basin area of west TX, and south-central NM. The front will overtake the dryline from north-south. ...IA and vicinity... Scattered thunderstorms should form this afternoon along and behind the cold front, and near the surface low, offering a threat for locally damaging to severe gusts, sporadic hail, and a conditional tornado threat. Elevated/morning convection across this region should move away through midday, behind which the warm sector and its 60s F surface dew points -- now over portions of KS/MO -- will spread northeastward across the outlook area. Convection will become probable this afternoon as deep lift increases (including direct frontal forcing and cyclogenesis-related low-level mass convergence), and the warm sector destabilizes from a combination of diurnal heating and theta-e advection. A deep troposphere (EL above the 200-mb level) also will contribute to favorably large buoyancy, despite a lack of optimal lapse rates aloft, with preconvective warm-sector MLCAPE values reaching the 1000-2000 J/kg, locally/briefly higher. Strengthening mid/upper winds and relatively backed low-level flow near the low and warm front will yield supercell-favorable effective-shear magnitudes of 35-45 kt and effective SRH in the 250-500 J/kg range, based on forecast soundings. However, with forcing ramping up quickly, largely front-parallel flow aloft, and CINH being substantially weakened over much of the area essentially at once, relatively discrete storm types may be short-lived. Sustained supercell evolution/longevity is in question. The forecast parameter space in a small area near and east of the low otherwise would favor a greater unconditional tornado probability, if there were: 1) more evidence for favorable storm mode, and 2) less potential for the cold front to undercut convection forming along it. Related mesoscale trends will be monitored through the day. Otherwise, thunderstorms are expected to develop this evening and tonight along and behind the cold front as far south as eastern OK, with isolated, marginally severe hail possible. ...West TX... Isolated to widely scattered thunderstorms may develop late this afternoon near the dryline, west of the Caprock, and move eastward off the Caprock while offering isolated severe gusts/hail. Large-scale ascent related to approaching southern/stream shortwave trough, combined with an antecedent EML, will contribute favorable midlevel lapse rates. Strong diurnal heating and the related well-mixed boundary layer will minimize MLCINH for a couple hours across the outlook area, contributing to peak MLCAPE in the 1500-2000 J/kg range. Despite a lack of stronger mid/upper-level flow, directional shear will be pronounced, contributing to 30-40-kt effective-shear magnitudes. Multicells, with transient supercell characteristics, will be possible. Nocturnal cooling/stabilization of the boundary layer beneath the EML will lead to weakening of the convective/severe threat eastward across the Low Rolling Plains this evening. ..Edwards/Leitman.. 10/22/2020
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