Labor Day Severe Threat
One model forecast showing initial thunderstorm development late in the afternoon. (HRRR/6 p.m.)
Morning observations placed a surface low over central Kansas with a frontal boundary draped northeastward into lower Missouri Valley. Winds along and just north of the boundary were out of the east. Along and south of the boundary was a moist and increasingly unstable air-mass. This front and associated warm sector should ease north a bit through Labor Day, setting the stage for likely thunderstorm development, including some severe, by mid to late afternoon. Radar imagery showed only scattered drizzle and light rain shower activity across Nebraska, while satellite imagery showed some northwestward advancement of clearing within the warm sector.
Most computer model forecast data are in agreement with the overall setup and target area. The frontal boundary is projected to be oriented from north-central Kansas into southeastern and east-central Nebraska by mid-afternoon. With temperatures rising into at least the upper 80s to lower 90s and surface dew-points in the low to mid-70s, the air-mass from northern Kansas into southeastern Nebraska will become moderately to strongly unstable. Model guidance all agrees with a sizable area of at least 2000 to 3000 J/kg MLCAPE, juxtaposed with at 40 to 50 knots of effective bulk shear. 14z mesoanalysis showed a large area of 40 to 60+ knots of such shear on both sides of the Nebraska/Kansas border. Given forcing along the front and seasonably strong wind fields aloft, the setup favors the development of at least isolated severe thunderstorms later in the afternoon.
12z 4km NAM forecast 0-1km EHI early Monday evening.
The data supports elongated hodographs with considerable turning in the 0 to 3 km layer. Considering the wind fields and elevated CAPE values, initial thunderstorms that form can easily produce damaging winds and large hail, perhaps locally very large. There does exist a somewhat elevated tornado threat, particularly with any cells that remain discrete and interact with the boundary, where winds should remain strongly backed to the east through early evening and 0 to 3 km storm relative helicities locally exceed 200 m2/s2. That zone is unanimously showed on the models across southeastern Nebraska and possibly far northeastern Kansas. The overall threat zone extends of south-central/southeastern Nebraska into far northern Kansas through early afternoon, shifting east into southwestern Iowa and northwestern Missouri during the evening. Into the evening, the threats should transition more to heavy rainfall with flash flooding and damaging wind swaths with storms that merge to form quasi-linear line segments.
