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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Saturday, June 19, 2021

SPC Jun 19, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Sat Jun 19 2021 Valid 191300Z - 201200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE CENTRAL PLAINS...LOWER MISSOURI/OHIO VALLEYS...MID-ATLANTIC...AND CENTRAL GULF COAST... ...SUMMARY... Damaging thunderstorm wind and isolated large hail are possible from portions of the central High Plains to the Mid-Atlantic. A tornado threat also exists over parts of the central Gulf Coast States to southwestern Georgia, east of the inland track of Tropical Storm Claudette. ...Synopsis... In mid/upper levels, the main belt of northern-stream westerlies will become more cyclonic through the period from the northern Rockies to the Northeastern CONUS, as a series of shortwaves lead to height falls. As this occurs, the strong and persistent anticyclone to the south, over the Desert Southwest, will shift southward slightly, with the 500-mb high approaching the southern border of AZ by the end of the period. In between, a perturbation initially apparent in moisture-channel imagery over UT will move eastward, reaching southern WY and the CO Western Slope by 00Z. This trough should move over parts of the central Plains overnight, potentially with some convective vorticity reinforcement. Farther east, a strong shortwave trough will move from its present location over the western Lake Superior region and WI across the rest of the upper Great Lakes, with some weakening expected by 00Z as it reaches the Lake Erie vicinity. The trough should pivot eastward from there to southern New England by 12Z tomorrow. A weak mid/upper low will remain over east-central/southeast TX in the CLL area, as the perturbation accompanying T.S. Claudette ejects northeastward across MS/AL. The 11Z surface analysis showed a cold front across portions of southeastern ON and Lake Ontario, which should move eastward/ southeastward into parts of NY and New England today into tonight. The wavy frontal zone extended through a low over southern WI, southwestward across extreme southeastern NE to another low near the southern part of the CO/KS line. Frontolysis is expected along the High Plains portion of the boundary today, though easterly flow components to its north and relatively maximized low-level moisture will persist. ...Central Plains... Widely scattered to scattered thunderstorms should develop this afternoon over western parts of the outlook area, from the NE Panhandle across parts of eastern CO. As this activity moves roughly eastward through the late afternoon and early evening, it will encounter greater moisture and intensify, offering the threat for damaging gusts and large hail. The wind threat will increase/maximize when upscale cold-pool aggregation and related forced ascent can occur, while the foregoing boundary layer remains deep and well-mixed, favoring downdraft acceleration. As such, the potential for significant/65+ kt gusts is maintained over parts of the High Plains. As aforementioned height falls occur from this area northward, winds aloft will become more difluent and strengthen slightly, contributing to favorable deep shear. Meanwhile, large-scale ascent and low-level mass response will increase with the approach of the UT perturbation, leading to increased convergence north of the remnant front, near a lee trough. Destabilization will occur aloft with the DCVA and in the boundary layer from strong diurnal/diabatic heating, leading to steep lapse rates, with peak MLCAPE in the 2000-2500 J/kg range. This will support the initial development and upscale growth. Eastward extent of the nocturnal wind threat into lower elevations and greater MLCINH (but also a strengthening southerly LLJ) is uncertain, and largely dependent on depth/strength of the MCS cold pool. ...Lower Missouri/Ohio Valleys... Widely scattered to scattered thunderstorms in clusters are possible this afternoon and evening, offering damaging gusts and severe hail. Aggregated outflow from the prior day's and night's convection has left a boundary across parts of northeastern KY and southern IN/IL, to near STL, and west-northwestward across northern MO. Isolated severe hail may be noted this morning with elevated convection north of the boundary, though the supportive west-southwesterly to westerly LLJ branch should weaken over the next few hours. Despite weak shortwave ridging behind the upper Great Lakes trough, strong low-level moisture/heating are likely along and south of the boundary, which may drift back northward over parts of MO. A boundary-parallel corridor of MLCAPE in the 3000-4000 J/kg range is possible, amidst surface dew points from the upper 60s to low 70s F, weak CINH, and a deep troposphere to aid in the development of deeply buoyant profiles. Although forecast soundings suggest modest deep shear, localized low-level shear/vorticity enhancement by the boundary, and well-mixed subcloud layers on the warm side, may aid storm organization as well. Low-level warm/moist advection may help some of the convection to persist at severe levels tonight across the Ohio Valley region. A relative minimum in severe potential may exist between this regime and that over the central Plains; however, confidence in that is not high enough yet to carve out lower unconditional probabilities. ...Mid-Atlantic/Northeast... Widely scattered to scattered thunderstorms are expected to form through this afternoon over western parts of the outlook area, especially over portions of western/central PA toward northern MD, and move eastward with the threat for strong-severe gusts and large hail. Some guidance indicates the potential for upscale clustering over the lower DE Valley region and/or NJ before activity moves offshore. Large-scale lift is expected to increase over the region ahead of the upper Great Lakes shortwave trough, spreading ahead of the cold front. Meanwhile, low-level destabilization will occur diurnally, steepening deep-layer lapse rates with favorable moisture in place. An area of 500-1500 J/kg MLCAPE should result, amidst favorable deep shear. Although low-level flow will be weak and veered, a 45-55-kt 500-mb speed max should shift over the region, contributing to effective-shear magnitudes in the 35-45-kt range. Organized multicells and isolated supercells are possible. A separate area of thunderstorms should develop this afternoon across portions of northeastern New England, along or ahead of the cold front, offering isolated damaging gusts and marginally severe hail. A zone of regionally maximized large-scale UVV -- preceding a compact mid/upper cyclone and related shortwave trough now located over western QC southeast of James Bay -- should spread over the region atop a destabilizing boundary layer with diurnally minimized MLCINH. A well-mixed boundary layer will support potential for hail/gusts to reach the surface, beneath 500-1200 J/kg MLCAPE. Effective-shear magnitudes around 30-40 kt will support some storm-scale organization. ...Gulf Coast... See tornado watch 284 and related mesoscale discussion for the near-term tornado potential with what now is T.S. Claudette. This highly asymmetric and strongly sheared cyclone is forecast by NHC to turn northeastward, then east-northeastward across MS/AL through the period. [See NHC advisories for latest specific track/intensity forecasts and tropical watches/warnings.] The tornado potential should remain displaced well away from the center, across those portions of the outer eastern semicircle that can destabilize sufficiently to support sustained supercells. That condition will remain most probable relatively close to the coast, both in persistent convergence/convective bands initially located over southwestern AL and the FL Panhandle, southward over the Gulf. Isolated discrete supercells also are possible east of the bands. Theta-e advection and diurnal heating should destabilize the boundary layer across the FL Panhandle, southeastern AL and perhaps southwestern GA as favorable low-level shear/hodographs spread eastward through the outlook area. The tornado threat should diminish this evening, both with inland/northeastward extent and with time, as Claudette continues to gain distance from the Gulf, flow veers to its south, and the most-favorable buoyancy and wind profiles become more displaced from each other. ..Edwards/Broyles.. 06/19/2021 Read more LIVE: