Monday, December 27, 2010

Cold And Damp This Week

Almost all the weather attention this morning is focused on the Northeast blizzard, which has stranded thousands of Holiday travelers. There are a lot of photos and information about the storm and its impacts at:
The weather action will quickly shift out West this week. The top panel shows precipitable water over the Pacific and indicates another large moisture plume developing from west of Hawaii toward the continental Northwest US. The middle panel is from the 11 pm forecast run of the U of AZ WRF-GFS model. The figure shows model forecast accumulated precipitation ending at 11 pm on the 29th of December. The WRF predicts the first part of the storm will mostly impact the higher elevations of southeast Arizona. The precipitation amounts in the Catalinas are quite large - the WRF has been fairly accurate, albeit somewhat wet, for the Catalinas forecast of the past several storms. So, we should watch the evolution of this event - the model is forecasting very strong, upslope winds at 850 mb from the south-southwest into the Catalinas. The bottom panel shows this morning's 12 UTC NAM forecast of total precipitation for the same period as the middle panel. The NAM has the precipitation further north and west than does the WRF, indicating a slower onset in the NAM forecast. Looking further out in the NAM forecast, I note that it does not forecast 1000-500 mb thicknesses to drop as low as does the GFS. So snow levels may pose a bit of a forecast challenge, as the GFS takes them very low. Regardless, an interesting week on tap for Western US weather watchers!

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