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Friday, August 20, 2021

SPC Aug 20, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Fri Aug 20 2021 Valid 201300Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE UPPER MIDWEST TO LOWER MISSOURI VALLEY... ...SUMMARY... Damaging winds, large hail and a few tornadoes are possible with episodes of thunderstorms today and tonight, from parts of the Upper Midwest to lower Missouri Valley. ...Synopsis... The CONUS portion of the large-scale mid/upper-level pattern will feature a mean trough in the West, with two strong shortwaves (one exiting, one entering): 1. The initial shortwave trough -- evident in moisture-channel imagery from central MT to western CO -- is forecast to eject northeastward across the central Rockies, central High Plains and northern Plains through 12Z tomorrow. Though deamplifying, this will remain a strong perturbation for the rest of this period. By 00Z the trough should extend from southeastern SK across the western/central Dakotas to the NE Panhandle. By 12Z, its axis should lie near a line from Winnipeg southward along the Red River of the North to near FSD. 2. An upstream trough -- initially positioned over coastal BC between the AK Panhandle and Vancouver Island -- will dig southeastward, forming a closed 500-mb low over the interior Pacific Northwest by the end of the period. To the east, a broad area of modest cyclonic flow is apparent around a trough located over extreme southern QC, northern NY, and southwestward through the central/southern Appalachians. A closed, cut-off low is progged to develop tonight over WV, while ridging builds to its north from Hudson Bay across eastern ON. The associated cold-core cyclone and trough will influence motion and evolution of Tropical Storm Henri over the Atlantic, per latest NHC discussions, but with no associated threat for organized severe storms through day-1. At the surface, 11Z analysis showed an area of low pressure elongated on a frontal zone from southeastern MB to central/ northeastern SD, with a cold front southwestward across southwestern NE. The low is expected to consolidate over central ND, then move northeastward along the frontal zone, across the northwestern corner of MN and over adjoining southeastern MB this evening. By 00Z the cold front should extend from that low across western MN, northwestern IA, southeastern NE, and south-central KS, to a low near the Raton Mesa and CO/NM border. By 12Z, the front should reach the MN Arrowhead, western WI, northeastern MO, southeastern KS, to the trailing low over the northern TX Panhandle. ...Upper Midwest to lower Missouri Valley... Scattered thunderstorms should develop today and tonight, at first in a blend of discrete and banded modes, then evolving into one or two dominant quasi-linear swaths of convection with southward growth through the evening. As this activity moves eastward to northeastward across the outlook area, all severe modes (damaging gusts, large hail, and a few tornadoes) are possible. Animated satellite and radar imagery have shown good continuity with a field of large-scale lift and associated precip just ahead of the southern portion of the shortwave trough. This UVV plume is apparent from northeastern CO across the NE Panhandle to southeastern SD, and should shift northeastward in step with the motion of the parent perturbation. A relatively maximized zone of severe-wind threat may develop across parts of eastern SD, southeastern ND and western MN, as the zone of relatively maximized large-scale ascent approaches the front, and in combination, impinges on a warm sector that could destabilize favorably from both theta-e advection and diurnal heating. Prefrontal surface dewpoints in the upper 60s to lower 70s F remain common, except over central NE, where a now-decayed MCS has left a mesoscale, low-level theta-e deficit that should advect/deform northward and become narrower in zonal extent. Given that antecedent boundary-layer disruption, enough uncertainty lingers to preclude an unconditional "enhanced"-level/30% upgrade at this time, related to: 1. How much of both heating and warm/moist advection will occur, and how fast, behind the morning clouds/precip, and 2. Support for robust cold-pool organization on the mesoscale, given a very deep layer of southerlies from just above the surface through upper levels. The southerlies will contain favorable bulk shear, but also, will be aligned strongly parallel to the quasi- linear corridor of convective forcing. Mesoscale trends and shorter-fused progs will be assessed for more confidence in such a wind area, if needed, during the day. Regardless, the axis of greatest wind potential appears displaced eastward from that of hail. The latter should be maximized earlier in the convective cycle, when the potential is greater for discrete to semi-discrete supercells. As such, within the broader categorical outlook, these probabilities have been refined accordingly. Low-level shear will be favorable for maintaining a swath of 150-250 J/kg effective, inflow-layer SRH during both phases, increasing some after dark as the LLJ increases (but QLCS mode also becomes more dominant). This will support potential for a few tornadoes throughout the afternoon and evening, including as trailing convection develops near the front and builds southward into the western IA/eastern NE/northeastern KS/northwestern MO area. A more-pristine boundary layer will exist over those areas, with rich low-level moisture and still-favorable frontal lift, but also, displacement from stronger mid/upper forcing and the gradual nocturnal surface cooling as counterbalancing influences. ..Edwards/Gleason.. 08/20/2021 Read more LIVE: spc.noaa.gov
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