NWS Spokane and Pendleton have been quite busy issuing a series thunderstorm watch/warning intermixed with flash flood and red flag warnings for the past week-or-two. Complimentary to these statements have been local storm reports that have featured some perplexing spotter reports of funnels clouds in NE Oregon and hail the size of “SweeTarts” to “half dollars”. BTW, when was the last time you saw a half dollar, let alone hail the size of a half dollar? And how much do you have to crave sugar to identify hailstones sizes using an aggregate of pastel colored sugar that comes in a tube?
The root of active convection has been a stubborn cutoff low off the N. Cal/Oregon coast that has stalled under the quiescent summer regime with mid-latitude jet well north toward Alaska nominal dynamics to push the disturbance. The inland northwest being upstream of the cut off low, has seen a barrage of moist air being advected up from the North American Monsoon with dewpoints the last 12 days this month seemingly quite high (close to 60F) across eastern WA and the Idaho panhandle. When this moist air interacts with atmospheric instability, either through strong daytime surface heating, cool air aloft, and/or associated uplift mechanism (e.g., topography, weak disturbance), ingredients for convection and thunderstorms come into alignment.
The NWS Storm Prediction Center highlighting the northern swatch of the inland Northwest with a slight risk of severe weather on Friday including the potential for “large hail” and “supercell thunderstorms”.
The cause for concern is the holy grail for a classical severe weather outbreak by inland NW standards. This includes: (1) strong instability with the upper-level low that features cold air aloft moving inland and overriding a very warm surface layer, (2) frontal uplift with a short-wave disturbance as the upper-level low passes through the area, and (3) an extremely moist atmosphere with integrated water vapor (total # of water in the column of air if you were to condense it into liquid) nearing 1.5 inches. The latter is due in part to a broad advection of moisture from what was once Hurricane Fabio that petered out several hundred miles south of southern California. The configuration of the larger-scale flow has allowed this moisture to track northward into California on a bee-line for the inland Northwest. A few comedians have poked fun at the name of this hurricane, but let’s wait for a bit before we throw stones.