Monday, October 1, 2012

Jelewat Mischief

If you've been following the forecasts for this coming week, especially Wednesday, you've probably noticed they have changed dramatically.  There are two reasons for this.  One is that cold air is moving into the northwest United States.  The second is that Typhoon Jelewat, which we discussed in the previous post, is causing all sorts of mischief, influencing the amplitude of the jet stream, and ultimately affecting whether or not that cold air barrels into northern Utah.

On Friday, Jelewat was located just east of Taiwan, but it subsequently moved across Okinawa and Japan and presently sits just south of the Kamchatka Peninsula.

Although it has weakened, Jelewat is expected to undergo extratropical transition and become a powerful extratropical (non-tropical) cyclone as it approaches the western Aleutian Islands early tomorrow.

Cyclones of this type are known to wreak havoc with the jet stream through the strong northward transport of warm air combined with warming produced by condensation of the moisture-laden tropical airmass.  This frequently initiates a phenomenon known as downstream development whereby the jet stream downstream of the cyclone amplifies into a series of high amplitude troughs and ridges.  Indeed this is the case by Wednesday when there is a highly amplified jet-level flow pattern over the Gulf of Alaska and northern North America.

This amplification, however, is very sensitive to Jelewat's track, intensity, and precipitation structure after it undergoes extratropical transition.  The more recent forecast runs are amplifying the flow more so that the cold air that was previously expected to remain to our north surges into northern Utah on Wednesday.  Thus, instead of highs near 80, we're looking at what will probably be a blustery afternoon with strong north and northwest winds and temperatures dropping into the 60s.

All of this is a prime example of why weather forecasting requires comprehensive global observations.

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