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Weather conditions on bar directly above are near Lakefront. Weather conditions with Moon are O'Hare (official).

Saturday, July 17, 2021

SPC Jul 17, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1124 AM CDT Sat Jul 17 2021 Valid 171630Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF THE NORTHEAST STATES AND MIDDLE ATLANTIC AS WELL AS THE NORTHERN THROUGH CENTRAL HIGH PLAINS... ...SUMMARY... Thunderstorms with damaging wind are expected over a portion of the Northeast/Mid-Atlantic today. Other storms with damaging wind and large hail are possible this afternoon/evening across parts of the northern through central High Plains. ...Northeast through Middle Atlantic... Surface analysis as of mid-day shows a warm front from MA westward through southern NY to a weak surface low over Lake Erie. A cold front extends southwest from the low through OH. The warm sector south of these boundaries is very moist with 70F dewpoints, and mostly clear to partly sunny skies are already occurring from the Middle Atlantic area into much of PA, southern New England into MD, VA where MLCAPE between 1500 and 2500 J/kg is likely this afternoon. A positive-tilt upper trough is currently moving through the southern Great Lakes and OH. A couple of MCVs are embedded within the flow just downstream of this feature including one over northeast OH and another moving into western NY. A belt of 35-40 kt southwesterly winds in the 800-500 mb layer accompanying the upper trough will spread east through the northern portion of the warm sector contributing to up to 30-35 kt effective bulk shear. Storms developing within pre-frontal convergence bands and over the higher terrain will gradually intensify as they continue east through a destabilizing boundary layer. Though the stronger winds aloft will remain north of the warm sector, sufficient vertical shear will exist to support some organized structures. Some storms may evolve into line and bowing segments capable of producing mainly damaging wind during the afternoon into the early evening. See swomcd 1304 for more information. ...Northern High Plains this afternoon into early tonight... Gradual background height rises will occur over the northern High Plains as the midlevel ridge expands northeastward from the central Rockies. Despite the height rises, subtle speed maxima will crest the ridge and move southeastward over the Dakotas. Strong surface heating along a lee trough will support widely scattered, high-based storms this afternoon from northeast CO across eastern WY to eastern MT. The storms will spread southeastward this evening into richer low-level moisture from northwest KS northward across the Dakotas, but also into increasing convective inhibition with cooling surface temperatures toward sunset. Effective bulk shear of 30-40 kt could be sufficiently strong for some supercell structures, but low-level shear will generally remain weak where storms are more probable through this evening. Overall, the scenario appears supportive of a few severe outflow gusts and isolated large hail this evening, and some of the storms could persist as a cluster or two into tonight across NE/KS as warm advection strengthens. ...Southern Plains through late evening... A couple of remnant MCVs are moving slowly southeastward over KS/NE, and these features may persist through the day. Boundary-layer dewpoints in the upper 60s to mid 70s will support strong buoyancy this afternoon (MLCAPE of 2000-3000 J/kg) along and south of a stalled front and the mesoscale outflows associated with ongoing morning convection (driven by warm advection). Though vertical shear will remain modest at best, weak but veering winds with height should support a few clusters of southeastward-moving storms capable of producing isolated strong/damaging outflow gusts. ..Dial/Jirak.. 07/17/2021 Read more LIVE: spc.noaa.gov
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