A fairly impressive and springlike setup has become apparent over the northern Plains Tuesday. In fact, the upper level fields appear to resemble a fairly springlike pattern for the next 7 days or more, with frequent deep western US troughing and occasional short-wave troughs speeding through the mid-level jet above a fairly moist MT airmass in the low levels. One major difference between now and a typical springtime setup, however, is lapse rates. Since we're coming out of summer, mid-level temperatures are very warm (h5 temps, in particular) as a result of months of constant energy input with little to knock that down. Lower-level temperatures are not as warm, so no real EML plumes are present.
A more-or-less synoptically forced dryline looks to move east pretty quickly Tuesday as it transitions into a cold front; however there is noticeable uncertainty in the longitudinal placement of the boundary by 00Z from the NCAR ensemble, with differences of 100 miles between members present. The N-S orientation should help favor cellular storms, and a combination of good mid-level flow and a stiff low level jet will provide for fairly high-end shear for supercells and tornadoes.
Right now the highest threat appears to be in the eastern half of SD where more moisture and instability will exist compared to up north, and it looks like capping will suppress storms south of there. However, a warm front will be draped southeastward from the dryline across SE SD, ext SW MN, NW IA, and maybe NE NE. Low-level thermodynamics will be quite high for late September there, so if an isolated storm manages to break through the cap, it could go bonkers. However, the convection-parameterizing models have been rather quiet south of SD. The CAM NAM has shown a few "bubbles" in NE, but not on the warm front itself. The NCAR ensemble also has nothing but white space south of I-90, so such a contingency seems quite unlikely. My money would be on C/E SD.
A more-or-less synoptically forced dryline looks to move east pretty quickly Tuesday as it transitions into a cold front; however there is noticeable uncertainty in the longitudinal placement of the boundary by 00Z from the NCAR ensemble, with differences of 100 miles between members present. The N-S orientation should help favor cellular storms, and a combination of good mid-level flow and a stiff low level jet will provide for fairly high-end shear for supercells and tornadoes.
Right now the highest threat appears to be in the eastern half of SD where more moisture and instability will exist compared to up north, and it looks like capping will suppress storms south of there. However, a warm front will be draped southeastward from the dryline across SE SD, ext SW MN, NW IA, and maybe NE NE. Low-level thermodynamics will be quite high for late September there, so if an isolated storm manages to break through the cap, it could go bonkers. However, the convection-parameterizing models have been rather quiet south of SD. The CAM NAM has shown a few "bubbles" in NE, but not on the warm front itself. The NCAR ensemble also has nothing but white space south of I-90, so such a contingency seems quite unlikely. My money would be on C/E SD.
