Broad upper low, situated over the far NW US, pushed eastwards during the afternoon hours and onwards. A sharp 100-120 kt high-level jet also translated to the east over the N-C Rockies with the favorable left exit region approaching our area of interest over NW Dakota at 19Z onwards. The airmass composite image was helpful in pinpointing the position of the jet max as a sharp moisture gradient evolved (roughly extending from Oregon to Wyoming). Initiation over far NW Dakota also awaited the approach of the “ozone-rich” airmass (purple) as it overtook the eastward progressing dryline between 19-22Z, fostered by reports of surface dewpoints in the mid 50s beneath rapidly drying mid/upper levels.

The first image captured at 1900Z:

and the second one at 2200 Z with initiation already underway/building SE-wards:

Helge