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SPC Aug 20, 2019 1300 UTC Day 1 Convective Outlook

5 years 11 months ago

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Tue Aug 20 2019 Valid 201300Z - 211200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EXTREME SOUTHEASTERN IOWA AND NORTHEASTERN MISSOURI TO WESTERN INDIANA AND SOUTHERN ILLINOIS... ...SUMMARY... The greatest severe-storm threat will be destructive winds from an ongoing complex of thunderstorms, which is forecast to continue southeastward at least across portions of Missouri, Illinois, and Indiana, as well as remaining parts of Iowa. ...Synopsis... In mid/upper levels, a belt of seasonally active northern-stream westerlies will continue across the northern 1/2-2/3 of the CONUS, while a high shifts/erodes westward from the southern Plains toward the southern Rockies. An elongated cyclone across northern portions of QC/ON/MB and adjoining parts of Hudson Bay will split, with the western portion digging southeastward across northern ON. The trailing strong shortwave trough will approach northern MN and Lake Superior late in the period. Meanwhile, a pre-existing shortwave perturbation over the western IA/southern MN region -- with reinforcing/convectively generated vorticity to its southeast -- will move southeastward to Indiana by 00Z then across the Ohio Valley into parts of the central Appalachians by 12Z. At the surface, 11Z analysis depicted a cold front offshore from most of New England, transitioning to a wavy/quasistationary front from northern MD across northern WV, southern OH, southern IN, becoming a warm front over central IL and a small part of southeastern IA prior to being overtaken by an ongoing MCS. The Ohio Valley States part of this front should move little before being overtaken by the MCS. Meanwhile, a separate cold front -- initially analyzed from northwestern MN and southeastern ND across southwestern SD and northeastern WY -- will move southward across much of the central Plains and Upper Midwest this period. By 12Z, the front should be positioned from Lake Michigan across northern MO, northern KS and east-central CO. ...Corn Belt...mid Mississippi Valley... An expansive, severe wind-producing MCS will continue to organize and offer damaging, possibly significant-severe wind across the mid Mississippi Valley and toward the lower Ohio Valley through the early-mid afternoon. Please refer to SPC watch 602 and related mesoscale discussions for the latest near-term guidance. With a favorably moist and diurnally destabilizing air mass projected downshear, within and south of the warm-frontal zone, the distance this MCS will travel before weakening is uncertain. It will depend to a great extent on internal cold-pool characteristics such as rear-inflow jet organization and lifting related to eventually decreasing depth of the density current along and shortly behind the gust front. In the meantime, continued lateral expansion of the cold pool and related forced ascent will encourage additional convective development along the margins of the MCS, especially in any bookend vortices, while the greatest severe-wind potential should continue to be in the bowing portion either side of its apex. Diabatic heating and rich low-level moisture, beneath favorable midlevel lapse rates, will help to boost preconvective MLCAPE to the 3000-4500 J/kg range ahead of the complex. ...Central Plains... Widely scattered thunderstorms are expected to develop late this afternoon onto evening, initially over higher terrain in the post-frontal moist/upslope-flow regime. This activity should proceed southeastward across parts of western/central NE this evening and tonight, offering large hail and severe gusts. Some of the hail may become especially destructive, exceeding two inches in diameter. Additional convection may form tonight ahead of the downshear progression of early storms, as well. Uncertainties exist regarding convective coverage, especially after about 03Z and over eastern parts of the outlook area; however, any sustained convection in this environment will be capable of an organized severe threat. Although some weaknesses will exist in the midlevel wind profiles, strong veering with height will contribute to favorable deep shear, with effective-shear magnitudes in the 35-45-kt range. This will favor supercell characteristics. An axis of strong buoyancy will set up across western through central/southeaster NE, with the width of the favorably unstable air mass increasing eastward. Rich low-level moisture -- with surface dew points commonly in the mid 60s to mid 70s F -- will underlie steep lapse rates in support of MLCAPE in the 3000-4500 J/kg range. ..Edwards/Marsh.. 08/20/2019 Read more