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Wednesday, July 14, 2021

SPC Jul 14, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0248 PM CDT Wed Jul 14 2021 Valid 142000Z - 151200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE UPPER MIDWEST... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHEAST... ...SUMMARY... A few tornadoes, scattered damaging winds, and large hail still appear likely across portions of the Upper Midwest this afternoon and evening. Isolated damaging winds will also remain possible across parts of the Northeast. ...20Z Update... Have expanded and increased tornado probabilities across northern and central IA. A diffuse warm front is becoming established over this region, with an outflow boundary from earlier storms also present. Recent VWPs from KDMX show strengthening flow in the 0-2 km layer, with 0-1 km SRH around 150 m2/s2. Effective bulk shear of 40-50 kt is expected to persist for at least the next few hours, and supercells should develop along/near the warm front by 21-22Z. A favorable discrete storm mode initially and enhanced low-level shear suggest a threat for a few tornadoes this afternoon, mainly across portions of northern/central IA along the warm front. Isolated large hail may also occur given moderate instability and strong deep-layer shear. Later this afternoon and evening, storms will probably congeal into one ore more bowing clusters, with more of a scattered damaging wind threat. For more information on the near-term severe threat across this area, see Mesoscale Discussion 1281. Severe probabilities have been trimmed across parts of MN and WI behind a loosely organized MCS moving eastward this afternoon across southern WI and northern IL. Although there will be some potential for additional strong to severe convection behind this leading activity, observational trends and short-term guidance suggest any appreciable organized severe risk will probably focus across IA into southern WI and northern IL through the evening along/south of a warm front. Additional information on the short-term severe risk across northern IL and southern WI can be found in Mesoscale Discussion 1282. Elsewhere, only minor changes have been made to severe probabilities across parts of the Northeast/Mid-Atlantic for isolated damaging wind potential. For more details on the isolated damaging wind threat across these regions through the rest of the afternoon, see Mesoscale Discussions 1283 and 1284. ..Gleason.. 07/14/2021 .PREV DISCUSSION... /ISSUED 1132 AM CDT Wed Jul 14 2021/ ...IA/WI vicinity... Complex forecast scenario as ongoing MCS over eastern IA/southeast MN continues to result in substantial cloud cover/rain-cooled air across the area. Some uncertainty remains regarding whether this MCS will strengthen with time as downstream heating/moistening takes place. Current trends suggest at least some continued risk for strong/possibly severe winds and isolated severe hail will continue into early afternoon across southwest WI. In the wake of the morning MCS, redevelopment of severe thunderstorms later this afternoon/evening is still anticipated along and north of the residual outflow boundary as warm advection increases with a strengthening low-level jet. RAP forecast soundings and 12z HREF guidance depict a favorable parameter space for supercells with severe hail/wind, as well as some potential for a tornado or two. Upscale growth should occur within a few hours of initiation, resulting in damaging winds becoming a more prominent risk. Overall, no changes were made to the current risk areas and probabilities, although this may be necessary at 20z as the effects of morning convection and amount of air mass recovery become clearer. ...Northeast CO... An MCV was located over western CO this morning and will move slowly east today, providing modest large-scale ascent for afternoon thunderstorm development. A moist boundary layer beneath steep midlevel lapse rates will lead to sufficient instability for a risk of isolated severe storms, with damaging gusts and severe hail possible as storms move into the adjacent northeast CO plains through this evening. ...Northeast States... Isolated thunderstorms this afternoon may pose a risk for strong/locally damaging gusts as weak/pockets of moderate buoyancy develops within a weakly sheared environment. Ascent associated with a midlevel shortwave trough from eastern NY/northeast PA will contribute to scattered thunderstorm development, with a few stronger multicell storms capable of strong downburst winds. ...NV/UT... Strong heating will result in a deeply-mixed boundary layer again today, but generally weak vertical shear as the area remains south of the stronger midlevel flow. Thunderstorms developing this afternoon will have the potential to produce strong outflow winds, a scenario supported by 12z CAM guidance. Overall, only modest adjustments to the MRGL risk area were made with this outlook. Read more LIVE: spc.noaa.gov
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