SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0255 PM CDT Thu Jun 10 2021 Valid 102000Z - 111200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS THE WESTERN DAKOTAS INTO FAR NORTH-CENTRAL NEBRASKA... ...SUMMARY... Scattered severe thunderstorms are likely late this afternoon and tonight across much of the northern and central Plains, especially across eastern Montana, the Dakotas, and Nebraska. All severe hazards are possible, including very large hail, wind gusts above 70 mph, and a few tornadoes. ...20Z Update... ..Northern/Central Plains... Overall forecast outlined in the previous outlook remains valid, with a very strong instability across the northern and central Plains supporting severe thunderstorms late this afternoon through tonight. Recent 18Z LBF sounding sampled an impressive thermodynamic environment with a 100-mb mean mixing ratio of 17 g/kg and over 3500 J/kg of MLCAPE. An initially discrete supercell mode is still anticipated, with all severe hazards possible. Upscale growth into one or more convective line is then expected, with the resulting convective lines pushing eastward/southeastward across the northern and central Plains. Strong wind gusts, some possibly over 70 mph, will be the primary severe threat overnight. Only change to the previous outlook was to extend the 45% wind probability/Moderate Risk southward into far north-central NE. This change was made based on trends within the most recent guidance. ...Northern Wisconsin/Upper Michigan... Recent issued MCD #906 discussed the short-term severe potential across northern WI and the western Upper MI. In this area, widely scattered to scattered thunderstorms are possible. The strongest storms are most likely near the WI/MI border, where shear is maximized, and a few isolated instances of marginally severe hail and/or damaging wind gusts may occur. ..Mosier.. 06/10/2021 .PREV DISCUSSION... /ISSUED 1149 AM CDT Thu Jun 10 2021/ ...Northern/central Plains including Dakotas/Nebraska/E Montana... An active severe-weather day is expected across the region late this afternoon and tonight and have upgraded parts of the western Dakotas to a categorical Moderate Risk, with much of this regional risk expected to persist well through the late-night hours. 12Z upper-air analysis/soundings essentially sampled near-record moisture content by early/mid-June standards across the High Plains in terms of mean mixing ratios/850mb dewpoints, including locations such as Dodge City/North Platte/Rapid City/Bismarck/Glasgow Montana, although mid-level/700 mb temperatures were notably warm (near-record values) at these locations as well. Thunderstorms should develop initially this afternoon into early evening near the surface low and adjoining frontal segments over eastern Montana and the western Dakotas. Supercells capable of all hazards (tornado, large to giant hail, severe wind gusts) are expected in the first few hours of the convective cycle. This should be followed by a transition to one or more organized convective complexes with a dominant destructive-wind hazard, with isolated severe hail and possibly some tornado threat. During and after that transition, very damaging gusts to hurricane force may be observed through the evening and possibly the early overnight hours. Very steep mid-level lapse rates will overlie a corridor of moist/theta-e advection and diurnal heating south of the warm front and east of the low/cold front. This will contribute to MLCAPE strengthening into the 2000-3000 J/kg range (locally higher), amidst intensifying deep shear (effective-shear magnitudes reaching 35-50 kt). Low-level shear should be maximized along the warm front and east of the surface low across eastern Montana and western North Dakota. Farther south across portions of South Dakota/Nebraska and perhaps northeast Colorado, high-based supercells should form late this afternoon/early evening near the dryline/lee trough, offering significant hail and severe gusts. However, this activity may evolve upscale more quickly to a QLCS configuration with a dominant wind threat. Deep, well-mixed sub-cloud layers will support maintenance of both severe hail and gusts to the surface during earlier, relatively discrete stages, but also, foster quick cold-pool aggregations and rapid expansion. Forced ascent of a foregoing boundary layer containing 60s to near 70 F surface dew points, with MLCAPE commonly remaining above 2000 J/kg even well after sunset, will help to drive the severe-wind threat southeastward over much of Nebraska tonight. ...Northern Wisconsin/Upper Michigan... Widely scattered thunderstorms -- some in clusters -- are expected to develop by late afternoon/early evening within a weakly capped air mass characterized by favorably rich moisture, but modest deep-layer shear. The lake breeze, localized convergence zones and outflow boundaries will also be influences. The combination of large-scale lift ahead of the Ontario shortwave trough with boundary layer heating will steepen low/middle-level lapse rates favorably, contributing to MLCAPE in the 1500-2500 J/kg range south of the Superior lake-breeze front. Isolated instances of severe hail/wind will be possible as storms develop south-southeastward. ...Arkansas/ArkLaMiss... Scattered thunderstorms should redevelop through the afternoon into evening particularly on the western/southern periphery of the lingering convective complex over the ArkLaMiss vicinity at late morning. Locally damaging downbursts of water-loaded cores will be the main concern from a severe-weather perspective. An outflow/differential-heating zone related to the decaying overnight convection -- and oriented with a substantial component parallel to the northwesterly midlevel flow vectors, should provide most of the early focus for this redevelopment. Diurnal heating of a very moist boundary layer -- with surface dew points in the 70s F and PW near 2 inches -- will foster peak MLCAPE in the 2500-3500 J/kg range, in a very deep troposphere. Weak vertical shear will tend to limit organization to pulse characteristics with multicell modes, although a few transient supercells could occur with storms interacting with the outflow/differential heating boundary. ...West/southwest Texas... A few late-afternoon thunderstorms again are possible over higher terrain of the Davis Mountains/Big Bend area, and more conditionally, the adjoining segment of the dryline, offering locally strong-severe gusts and hail. Strong capping will make convective development more improbable with northward and eastward extent. The lack of more-robust deep-layer winds (and of stronger vertical shear) should keep convection multicellular in character and slow-moving, remaining close to its genesis area. Hot, deep, well-mixed boundary layers will contribute to strong wind gust potential aside from some hail risk. ...Carolinas... A weak mid-level wave and somewhat stronger mid-level westerlies should influence storm development and organization to a degree this afternoon within a moist/unstable air mass. Some of the stronger storms may produce wet microbursts capable of locally damaging winds. Read more LIVE: spc.noaa.gov
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