Sept. 4th-7th Severe Threats
NAM computer model forecast for Sunday night.
I think this update 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.
