High Plains Drifter

Tuesday the 20th is a work off-day for me, thus I plan to make it a chase day since there is a reasonable opportunity at a supercell within a half day’s one-way drive. All along, it looked like Iowa was the place to be for Tuesday the 20th… and it still does… however, Northwest Iowa is not a reasonable target for me from Dodge City. I’ve already done the Dodge City to Sioux City drive once this season… I think one time is enough, LOL. Since there is a reasonble opportunity for supercells closer (still 4+ hours away), I will opt for a closer play. Pretty strong zonal flow aloft with a short-wave trough embedded in the westerly flow will allow for organized storms in two areas it appears: Northwest to North-Central Iowa… and Northeastern Colorado. The better wind profiles and CAPE/moisture will be in northwest Iowa, which is where the best opportunity for tornadoes will be. The cold front will move south into western KS during the day tomorrow… however, the front will stall out given the strong zonal flow aloft. North surface winds in northeastern Colorado early in the day will turn to northeast then east by late afternoon/early evening… increasing shear and allowing post-frontal low level moisture to advect west. Storms should develop in terrain favored areas like the Palmer Divide and the Cheyenne Ridge. I am shooting for a target around Akron, CO at this point.

Below is the 24hr forecast from the NAM model valid 6pm MDT tomorrow (850mb Theta-E and wind). Note the warmer colors in the image (indicative of greater instability) in northeastern Colorado with easterly component winds advecting higher theta-E into northeastern Colorado.

For those who know how to read Skew-T (thermodynamic) diagrams, here is one for a location near Holyoke, Colorado (a 24hr forecast valid 6pm tomorrow) . Holyoke is in extreme northeastern Colorado about 30 miles south of where I-76 and I-80 merge.

90/55°F surface parcel yeilds about 1600 J/kg of CAPE. 0-6km shear is forecast to be about 40-45 knots by 00z, which is sufficient for supercell structures. Both the NAM and the NAM-WRF generate decent storms by 00z in this area with storms continuing well into the evening with high QPF values at 06z.