Wednesday, October 20, 2010

Beware when the atmosphere is in outlier mode

It appears that a major large-scale flow transition will occur over the next few days, one that is bound to produce high-impact weather over the western United States.

It's always good to be alert when the atmosphere is in "outlier mode."  We've been in outlier mode the last few days with the strong Rex Block, but we're going to be in a different sort of outlier mode in a couple of days.

The 108 h GEFS forecast from 18Z 20 October (from PSUs E-wall) clearly shows a "honking jet" (to use a scientific term) over the eastern Pacific poking its nose into the western US.

Strong cyclonic and anticyclonic height anomalies flank this jet.  Often, we look for unusually deep cyclones to identify when exceptional weather may occur, but in this case, neither of these anomalies is all that impressive.  Instead, it is their meridional collocation that yields an exceptionally strong Pacific jet.

In addition to major changes in surface sensible weather, this is a recipe for nasty terrain-induced gravity wave activity.  It will not be a fun time to be flying across the western United States.

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