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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, July 17, 2021

SPC Jul 17, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0256 PM CDT Sat Jul 17 2021 Valid 172000Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE MID-ATLANTIC AND NORTHEAST AS WELL AS THE NORTHERN AND CENTRAL HIGH PLAINS... ...SUMMARY... Thunderstorms with damaging wind will continue over a portion of the Northeast/Mid-Atlantic this afternoon and evening. Other storms with damaging wind and large hail are possible trough the evening across parts of the northern and central High Plains. ...20z Update... No changes have been made to the outlook. Scattered strong to severe storms capable of damaging gusts are ongoing across parts of the Mid-Atlantic and Northeast this afternoon. This activity is expected to continue for several more hours across Severe Thunderstorm Watch 382. Reference MCD 1307 for more short-term severe thunderstorm information. Additional isolated strong to severe thunderstorms are possible through the evening across parts of the northern and central High Plains. Latest mesoanalysis and forecast sounding analysis shows moderate inhibition is still impacting the High Plains. Additional heating should continue to erode capping and at least isolated convection is expected through this evening. Damaging gusts and large hail may accompany the strongest cells. ..Leitman.. 07/17/2021 .PREV DISCUSSION... /ISSUED 1124 AM CDT Sat Jul 17 2021/ ...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. Read more LIVE: