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HMT forecast discussion for 26 Dec 2007

Happy Holidays to all....plenty white out here and more on the way. Not the 2006 version, but nice. As for HMT, a fast-moving wave on Friday followed by another Saturday, then ridging for awhile until potentially things get interesting late next week, perhaps by Thu/3 Jan, with more of a sw flow system. This could then usher in an interesting period of several waves through the following ~7 days.

Current set up has a mean trough over the eastern Rockies with an upper level ridge off the West Coast. Shortwave troughs digging down the east side of the ridge have been amplifying over CO/NM and this will be the case with the next couple of waves. But they do affect the ARB area as they quickly move by. The next more moist one Friday with another close behind, coming out of the northwest. These do not look as strong as the previous IOPs, and so are not expected to produce nearly the amount of precip. Still, over 48-72h period ending Saturday night (29 Dec) suppose one could certainly see more than an inch of precip at Blue Canyon. Some models (the Canadian Global, for example) are more exuberant with this event, but most roughly similar to the GFS/NAM. Don't think these are worthy of an IOP, especially since 3 better events are in the books. Also, more interesting potential system exists for later next week.

Decent agreement in the GFS ensembles from 00z on the event beginning on Thu of next week (3 Jan). This is then followed by other waves in the GFS as the overall mean trough retrogrades and a more zonal-looking flow sets up into the West Coast. Canadian Global ensembles have more spread, with tendency towards a more prolonged event in several members, perhaps beginning a day or so later than in the GFS. Latest 12z ECMWF supports the overall GFS 12z deterministic solution with a high-amplitude trough approaching the ARB on Thu/3 Jan then weakening as it passes by Fri/4 Jan, but followed by a strong zonal flow into the area. One wonders if beginning later next week we set up a prolonged period of several events, similar to what we had just before Christmas, but potentially (at least with the first event) with more sw flow ahead of it. Such a prolonged period of wet weather is seen in the NAEFS (North American Forecast System, 00z runs) meteogram for Sacramento, which has precip in the mean every day beginning 3-4 Jan and continuing through 10 Jan.

ed szoke NOAA/CIRA/GSD