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Thursday, August 19, 2021

SPC Aug 19, 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 Thu Aug 19 2021 Valid 191300Z - 201200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL ROCKIES/FRONT RANGE AREA ACROSS PORTIONS OF THE CENTRAL AND NORTHERN PLAINS... ...SUMMARY... Scattered severe thunderstorms are expected from the central Rockies/Front Range area across portions of the central and northern Plains this afternoon and tonight. ...Synopsis... The dominant upper-air feature for this forecast period will be a cyclone now evident in moisture-channel imagery over the northern Great Basin and parts of the northern Rockies, centered near BOI. The closed low is forecast to devolve through the period as the parent trough moves eastward and remains strong. By 00Z, the 500-mb trough should extend near a line from LVM-SLC-LAS-SAN. By 12Z tomorrow, the trough is progged to become neutrally to slightly negatively tilted and less amplified, along an axis from near BIL-CAG-FMN. Meanwhile, a broad area of height weakness and associated cyclonic flow is apparent over the Great Lakes, with circulation center apparent over northern Lake Huron. As that feature moves slowly eastward through the period, the remnants of former T.D. Fred will move eastward over south-central New England today. A series of small shortwave troughs/vorticity maxima aloft, in a corridor from the inland Mid-Atlantic down the Ohio Valley to southwest TX and northern MX -- will support general thunderstorm potential. While very isolated/localized severe gusts may occur, the potential appears too unfocused and randomized for an unconditional risk area in that swath, at this time. At the surface, 11Z analysis showed a cold front extending from a low over southeastern MB southwestward across eastern ND, south-central SD, and the NE Panhandle, becoming quasistationary to a frontal-wave low over east-central CO. The front should stall today from northwestern MN to north-central SD, and may retreat westward somewhat over western portions of SD/NE as the low deepens somewhat and moves northeastward over eastern CO. The front should advance eastward/southeastward again tonight across the central Plains, but not much over ND due to frontal-wave cyclogenesis there. ...Central/northern Plains... Satellite and composite radar imagery indicate an arc of large-scale ascent and sporadic related convection now over portion of WY, western CO and southeastern UT. As this belt of lift shifts east-northeastward over CO and more of WY from midday through the afternoon, it will encounter a diurnally destabilizing airmass near the front, and across the warm sector to its east, with favorable moisture. Scattered strong to at least isolated severe thunderstorms are forecast as a result, initially over higher terrain in the southwestern parts of the outlook area, including the mountains of central CO to southern WY and the Palmer/Cheyenne Ridges. More convection should form through the afternoon, spreading northeastward from the Front Range vicinity and expanding across the central/northern High Plains. Early-stage convection may evolve into a mix of supercells, multicells, and coalescing bands/clusters. Damaging gusts, large hail and a few tornadoes all are possible from this activity. A corridor of southeasterlies at and just above the surface will both augment deep shear and advect/transport moisture into the region through the afternoon. To some extent, this should offset mixing enough to modulate dewpoint reduction during the afternoon near the Front Range (and over the connecting west-east divides), as diurnal heating and deeper lift erode MLCINH. Meanwhile low-level convergence will be maximized near and north of the surface low. Activity developing in the region will encounter greater moisture with eastward extent across the High Plains, amidst steep low/ middle-level lapse rates. This should support 1000-2000 J/kg peak preconvective MLCAPE (locally/briefly higher). Strengthening, somewhat difluent mid/upper-level southwesterlies are expected along with the belt of large-scale lift, leading to increasing deep shear with time (and from west-east) across the outlook area. Backed near-surface winds north/northeast of the low will enlarge low-level hodographs, aiding tornado potential in any supercells that can mature in that regime. One or more clusters may evolve upscale tonight across portions of the area from western/ central NE to central SD, persisting eastward into the favorable moisture transport and storm-relative winds of a 45-50-kt LLJ. The main adjustment to the "slight" and "marginal" areas for this outlook cycle is to allow more room for that potential extending into the overnight hours, before activity weakens below severe levels. ...Southern New England... As the remains of Fred proceed eastward, a small corridor of juxtaposition between relatively backed near-surface winds and surface-based buoyancy will shift across southern New England, southeast of the surface low. Even with weak, nearly moist- adiabatic lapse rates aloft, the rich boundary-layer moisture (dew points 70s F) will support MLCAPE in the 800-1500 J/kg range, amidst weak MLCINH. Overall weakening of low-level winds above the surface will continue with this system, reducing hodograph size compared to previous days, but with pockets of 150-200 J/kg effective SRH still possible. Overall, the tornado potential appears marginal, and should wane from west-east through midday as surface winds veer. ..Edwards/Kerr.. 08/19/2021 Read more LIVE: spc.noaa.gov
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