Recipe for an Unsettled Week

Our numerical forecast models are the backbone of modern weather forecasting, but sometimes you can just look at an analysis an know what's coming.

That was the case this morning when I pulled up this satellite image and 500-mb height and absolute vorticity analysis.

In particular, there is not only a long-wave upper-level trough over the western United States, but another one upstream over the Gulf of Alaska.  This upstream trough is also strongly diffluent, meaning that there is strong diffluence or spreading of the flow on the downstream side of the trough.  Further, the trough is negatively tilted, which means that the trough axis isn't oriented meridionally (i.e., along a meridian) but instead along a SE-NW axis.

In such a configuration, the strongest cyclonic vorticity advection lies in or near the base of the trough axis, which results in the trough digging (i.e., plunging southward) as it moves eastward.  In the configuration above, that means this trough will essentially dig into the downstream trough, reinforcing it and extending the round of cool, unsettled weather for the western United States.

All of this is terrible news for the Tour of California, which was formerly held in February, but is now being held in May in the hopes of better weather.  In a quote that sums up the situation perfectly, Andrew Messic, President of AEG sports, the presenter of the race said, "Two years ago, when we decided to move the race from February to May, we did it so we wouldn't get rained on, which was successful, but now we have snow."

I-80 traffic cam just west of Truckee, CA.
Source: CalTrans

Stage 1, which was scheduled for yesterday and would have been a spectacular race around Lake Tahoe was cancelled.  The start for today's Stage 2 has been shifted from Squaw Valley (~6000 ft) to Nevada City, CA (~2500 ft) and will not longer go over Donner Pass.