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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.

Friday, July 28, 2023

SPC Jul 28, 2023 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Fri Jul 28 2023 Valid 281300Z - 291200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF SOUTHEASTERN SOUTH DAKOTA TO THE SOUTHERN LAKE MICHIGAN VICINITY... ...SUMMARY... Severe storms are most probable from southeast South Dakota across Iowa, northern Illinois and southern Wisconsin this afternoon and evening, with corridors of damaging wind and large hail. Other scattered strong to severe storms are expected from Ohio into Virginia, and across the northern High Plains. ...Synopsis... In mid/upper levels, a persistent, strong anticyclone will remain centered over the southern Rockies, with ridging eastward across the Mid-South and southwestward over northern Baja. The northern-stream belt will start the period mostly zonal, but start to buckle some through the period as a series of mostly low-amplitude shortwaves traverses a re-amplifying eastern CONUS mean trough. Ridging also will amplify from the 500-mb high north-northwestward over the northern Rockies. These processes will yield a belt of difluent, northwesterly to west-northwesterly flow over the northern Plains, Upper Midwest, Upper Great Lakes, and lower Ohio Valley. At the surface, 11Z analysis showed a cold front across northeastern ON to Lower MI, where it was broken up by an area of convective outflow. A warm front became apparent farther southwest over parts of northern IA, and southern MN, to a low in the SUX area, with the cold front extending westward over northern NE to northern WY. The low should migrate eastward across the WI/IL border region to near TOL by the end of the period, along the aggregate baroclinic zone from synoptic and convective influences. ...North-central Plains to central Appalachians... Scattered to locally numerous thunderstorms are possible through the period in this corridor. The main concern remains upscale growth of one or more organized thunderstorm clusters into MCSs with organized rear-inflow jet(s), cold pool(s) and attendant severe wind. Early- stage, relatively discrete thunderstorms also should produce large hail -- perhaps significant/2+ inch diameter in western parts of the outlook area, where deep-layer lapse rates and low-level hodographs will best favor hail-producing supercells. In the area from central/southeastern SD to northern IL, strong diurnal heating will steepen low-level lapse rates amid 60s and low 60s F surface dewpoints, combining with steep midlevel lapse rates to yield MLCAPE in the 3000-4500 J/kg range. This will support rapid development and upscale growth of convection once the cap is broken, likely preferentially near the baroclinic zone and surface low where low-level convergence should be maximized. Deep shear will be favorable for early-evolution supercells as well as bowing structures during upscale aggregation, with effective-shear magnitudes in the 45-55-kt range. Current unconditional probabilities don't depict a substantially denser, greater-magnitude wind threat that *could* develop somewhere within the broad swath of the "slight" and "enhanced" lines. The "enhanced" area simply represents what still appears to be the best apparent environment for such an event, but still may shift around in position and shape today, as mesoscale/convective trends and shorter-fused progs warrant. This uncertainty is partly because of mesoscale questions regarding the influence (and possible downshear intensification of) ongoing activity and its boundaries, as well as substantial differences among both CAM and synoptic-scale guidance on convective coverage/organization later today, in that more-ideal environment from the Plains to the Corn Belt. ...Northern High Plains... Widely scattered to scattered thunderstorms should form this afternoon over the higher terrain of southwestern MT and northern WY, moving eastward the east-southeastward. Isolated severe gusts and marginal hail will be possible. This projected path conforms both to: 1. The gradual anticyclonic curvature of ambient/mean flow near the mid/upper ridge, and also 2. The orientation of a supportive, boundary-layer moist plume whose axis should remain across southern MT into western SD. Near that moist axis, which should feature 50s F surface dewpoints, diurnally steepened low/middle-level lapse rates will contribute to 500-1000 J/kg MLCAPE, a well-mixed boundary layer, and minimal MLCINH ahead of the orographic initiation regime by mid/late afternoon. Surface flow should veer with time from northerly to easterly and southeasterly, maximizing storm-relative winds in the inflow layer, though absolute speeds may be weak. Still, strong veering with height will contribute to 45-55-kt effective-shear magnitudes. Some of this activity may persist well into tonight and portions of the Dakotas with a sporadic wind threat, as it encounters increasing moisture and continued favorable storm- relative winds above a nocturnally stabilizing near-surface profile. ...Northern ME... Widely scattered to scattered thunderstorms are expected to develop along/ahead of a cold front over southern QC, upshear from this region, during the afternoon. Some of this activity may cross northern ME before weakening, with the potential for isolated, marginally severe gusts and/or hail. Forecast soundings suggest a combination of diurnal heating and sufficient moisture (e.g., mid-60s F surface dewpoints) will offset modest mid/upper-level lapse rates and yield around 1000 J/kg MLCAPE. Nearly unidirectional deep-layer wind profiles will limit low-level shear, but with strong flow in mid/upper levels, effective-shear magnitudes around 35-45 kt reasonably appear in modeled wind profiles, prior to convective influences. As such, organized multicells and isolated supercells may occur. ...AZ... Isolated severe gusts are possible again this afternoon from thunderstorms propagating westward to northwestward over portions of southern AZ -- mainly developing over orographically favored areas early, then perhaps on a couple of subsequent outflow boundaries. IR satellite imagery indicates extensive cloud cover from earlier convection over MX is breaking up, and substantial diurnal heating once again is expected across the region. MLCAPE up to about 500 J/kg should develop as enough moisture is maintained in a deep, hot, well-mixed boundary layer that supports locally intense downdrafts. ..Edwards/Broyles.. 07/28/2023 Read more CHECK UPDATE ZOOM GRAPHIC