Highlight: Focus is on the West…Not an “atmospheric river” pattern BUT models clobber West Coast of North America with -PNA generated heavy rain!
Fig. 1-3: The GFS identifies a south-moving West Coast of North America storm track in the 15-day forecast. The stormy 11-15-day forecast is New!
Discussion: Are there any “atmospheric river” patterns ahead for the U.S. West Coast? No, but that doesn’t mean excessive precipitation is prevented. The issue is the East/Northeast Pacific SSTA pattern. Super warm SSTA northwest of Hawaii and a cool pool of SSTA in the Gulf of Alaska generate a parallel upper air forecast causing a digging upper trough to form just off the West Coast over the next couple weeks. The result? More heavy precipitation in the 5-day forecast for Southwest Canada and into Washington (Fig. 1). The 6-10-day forecast has shown heavy precipitation into California which is upgraded to excessive (Fig. 2). New to the forecast is more excessive precipitation into California days 11-15 (Fig. 3). There is no direct connection to the tropics ALTHOUGH the Madden Julian oscillation (MJO) shift toward the Dateline is definitely helping. If this precipitation verifies as shown by GFS this a game changer event for the long-term drought affecting the West Coast. The GFS is likely overdone but the wetter trend is the big news. The upper air pattern identifies the West Coast trough but somewhat cutoff from tropical influence (Fig. 4). A strong -PNA pattern locks in so the stormy (and cold) pattern is likely to last well through December (Fig. 5).
Fig. 4-5: The GFS 15-day upper air forecast across the Pacific Ocean and the persistent strong negative Pacific North America index forecast.