Sept. 4th-7th Severe Threats

[Quincy]

I think this thread is at least marginally supported by the expected setups coming through the Labor Day holiday weekend. I want to stress that each of these four days is looking like relatively low-end in terms of severe potential (not particularly widespread or significant), but since each day has a shot and a day or two could be a bit more robust, we can focus attention in one specific thread. I will venture a prediction that at least 3/4 of these days will prompt a SLGT risk outlook from SPC. Friday already has a SLGT risk.

Overview:

Ridging around much of the eastern half of the United States will prevail, while a trough currently pivoting through the Northwest translates eastward. The setup on Friday includes a surface low already positioned over the northern High Plains with further development likely in the coming days. For Friday into Saturday, modest height falls and a surge of moisture air (60s dew-points) ahead of the developing surface low should favor at least isolated severe thunderstorms, primarily across the northern Plains into the Upper Midwest. As shortwave energy reaches the western North Dakota region, it should swing north into southern Saskatchewan/Manitoba with a trailing cold front moving through the Plains and portions of the Upper Midwest. The severe threats on Sunday and especially Monday are a bit unclear and will come into better focus after seeing how the system is evolving. One of these days (Fri-Mon) could go ENH risk, if things were to line up just right. There are caveats, which preclude higher confidence in significant and/or widespread impacts through the holiday weekend.

Red flags/uncertainties:

One of the glaring issues here is that the strongest forcing and steepest mid-level lapse rates will likely lag west of the more favorable thermodynamic fields through the period. In fact, the overlap of favorable shear/instability looks rather small in aerial extent on Friday and Saturday, with shear also tending to be toward the marginal side for supercell favorability. For Sunday into Monday, the whole setup becomes more complicated as a secondary low could develop along the cold front and become a player from the central Plains into the mid-Missouri Valley. During that time, mesoscale details will likely change (perhaps significantly) from what current forecast models are displaying.

With all of that said, I'll leave further specifics to the reply section below, as this setup should continue to evolve in the coming days. Given the time of year and the fact that severe weather threats have been quite limited for some time, I think it will be good to collect some thoughts and reports here.

[Quincy]

A bit of a dud day today. Heights were neutral and even rising a bit into the afternoon. A lot of cloud debris and ongoing convection to limit destablization. Tomorrow actually looks fairly similar, so perhaps another day that underperforms.

 

Still looking ahead with at least some enhanced (no pun intended) potential on Sunday and Monday. Sunday may be a fairly typical Iowa 2015 event with storms quickly forming a line with damaging winds being the predominant threat. 

[Quincy]

Severe Thunderstorm Watch in effect until 1 a.m. MDT for portions of the northern Plains. Thermodynamic parameters are modest at best, though. Could see a few isolated severe thunderstorms further east in Minnesota as well.

[Jim Marusak]

http://www.spc.noaa.gov/products/md/md1763.html

 

well, we are looking out for the potential for severe weather now here in MN and Superior West in Ontario. but given the current satellite representation, I'm personally wondering if maybe the best chance is north of the metro. but we do have some sun here in STP attm after a couple of rounds of showers between 9a and noon.

[Quincy]

Winds remaining modestly backed to the S and locally SSE across much of Iowa. Warmer 700mb temperatures resulting in some capping, although surface temps in the 90s should have eliminated most of any lingering CIN by this point. Weak forcing for ascent as the best kinematic support is displaced a bit to the northwest.

 

Favoring a heavy rain/localized flooding event for Iowa, but if a discrete cell could get going, especially where winds are a bit backed, could see a brief supercell. CAMs have been all over the place with convective evolution in the short-term, but once things go, they should light up pretty fast given strong instability in place and decent lapse rates, especially across western Iowa into far eastern Nebraska.

[Quincy]

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. 15z 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.

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.

[Chinook]

localized supercell-tornado outbreak in southeast Nebraska--several storm reports in Nebraska, Kansas, South Dakota today