SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1252 AM CDT Thu Jul 12 2018 Valid 121200Z - 131200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF THE MID MISSOURI VALLEY THROUGH PORTIONS OF THE UPPER MIDWEST... ...SUMMARY... Thunderstorms may pose a risk for strong wind gusts and perhaps some hail late this afternoon and evening in a corridor across parts of the mid Missouri Valley into the Upper Midwest. Additional strong storms accompanied by a risk for potentially damaging wind gusts are possible across South Carolina and adjacent portions of the south Atlantic Coast region. ...Synopsis... Models indicate that the stronger mid-latitude westerlies will remain generally zonal and confined to latitudes near/north of the Canadian/U.S. border through this period. Within this regime, a vigorous short wave trough and associated fairly deep cyclone are forecast to progress across northwestern Ontario into Hudson Bay by late tonight. It appears that a trailing cold front, now advancing southeastward into/across the northern Plains, will stall and weaken across the Upper Midwest/upper Great Lakes region and northern Plains. Downstream, the westerlies may become increasingly split, with troughing in the weaker southern branch amplifying east of the northern/middle Atlantic coast. An influx of cooler/drier low-level air associated with this feature may advance southward to the east of the central/southern Appalachians, and perhaps south of the lower Ohio Valley into the Tennessee Valley. Otherwise, mid/upper subtropical ridging will prevail across much of the central and southern tier of the U.S., with a prominent embedded mid-level high gradually shifting across and east of the middle Mississippi Valley. Seasonably strong heating and residual monsoonal moisture may once again support considerable afternoon and evening thunderstorms across much of the Southwestern deserts into the southern Rockies. Thunderstorms may also become widespread in a corridor along the front across the northern Plains into the upper Great Lakes region, with at least somewhat more scattered thunderstorms possible across the Gulf and south Atlantic Coast region. ...Middle Missouri Valley into Upper Midwest/Great Lakes region... Models indicate considerable modification of the plume of elevated mixed layer air emanating from the Great Basin, in the wake of the aforementioned vigorous short wave trough and associated surface cyclone. However, beneath the remnants of this plume, a corridor of strong surface heating near/just south of the stalling surface front may once again contribute to moderate to large CAPE. This will be aided by the lingering presence of mid 70s+ surface dew points along the front. Given this instability and weakening mid-level inhibition, considerable thunderstorm development appears possible late this afternoon into this evening. Generally south of the mid-latitude westerlies, mid/upper forcing to support activity remains somewhat unclear, but a perturbation now over eastern Colorado could augment convection across the middle Missouri Valley late this evening as it progresses around the subtropical high. Although west southwesterly deep layer mean flow may remain somewhat modest, 30 kt in the 850-500 mb layer may be enough to contribute to the evolution of organizing convective systems accompanied by a risk for strong surface gusts, before convection weakens overnight. ...South Carolina and adjacent portions of Georgia/North Carolina.. Seasonably strong surface heating may yield a relatively deeply mixed boundary layer characterized by moderately large CAPE by late this afternoon, ahead of the southward advancing surface front. Aided by forcing for ascent along/ahead of the front, there appears at least some potential for an upscale growing cluster of thunderstorms, supported by consolidating cold pools. Although embedded within generally weak wind fields, a northerly component at mid/upper levels is expected to contribute to an eventual general southward propagation, with the surface cold pool perhaps becoming strong enough to support potentially damaging surface gusts into this evening. ..Kerr/Wendt.. 07/12/2018
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