Monday, December 12, 2005

Ridge Retrogression Underway, at Least for a Few Days

Last week we expected an event involving discontinous retrogression of the western North American ridge position, from its location along the USA/western Canadian coast this past weekend (part of a split flow structure), to around 140W. This process is occurring as I type (with the north Pacific jet being shunted northeast), and most model solutions from today such as the GFS and CDC ensembles, as well as the single ECMWF run, are now indicating this shift back into the eastern Pacific by about this coming Thursday. Downstream ramifications include baroclinic storm development across the central and northern Plains.

However, also per most model solutions, this retrogression looks to be relatively short lived. Tied to yet another east Asian cold air outbreak, I agree with the eastward movement back toward the west coast by early next week, but with large amplitude. Thus perhaps more bitterly cold arctic air can be expected to come back into much of the country by this coming weekend, along with the possibility of a second storm development across the central USA. I would expect the cold air to begin moving east early next week.

During week 2, many models again break down the PNA ridge, and have generally westerly flow across much of USA. As discussed before, that notion is not consistent dynamically with the current circulation state. We expect the tropical convective forcing to remain across the Indian Ocean and Indonesian region, suggesting the continuation (with perhaps additional reduction) of below average westerly flow globally throughout the atmosphere. A scenario that would be possible is another discontinous retrogression event on the order of days 10-14. As always, details and timing for any insights such as this are unclear. However, this would raise hopes of precipitation along the USA west coast starting days 7-10 (next week), and a welcome warm-up in temperatures over at least much of western half of the country.

For the DDC CWA, I think we need to contine minitoring the magnitude of the cold air for this upcoming weekend, as well as precipitation chances. I would expect the coldest air (relative to climo) to remain north of us, and any decent precipitation to stay to our southeast. Warmer with little chance of precipitation looks reasonable for much of next week.

Ed Berry

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