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HMT forecast discussion for Tuesday 29 January 2008 (entered 20z/Tue)

Now that the upper level trough/low that was lurking off the California coast all of last week has moved inland the pattern has changed to a more zonal flow across the Pacific with now a low-amplitude upper level ridge well off the West Coast north of Hawaii, with fast-moving systems that approach the ARB from the wnw with limited moisture. A very weak closed low has been left behind near Hawaii, and associated with this is a pool of tropical moisture east of the upper low and Hawaii. The other persistent tap to the tropics is near the Dateline, with each of the fast-moving waves tapping into some of this moisture, but then losing this connection as they approach the West Coast. The one forecast for Thu/Fri of this week is doing this right now as the wave itself moves just south of the Aleutians. All of the upcoming events will be fast-moving and marginal for an IOP, and all look at this point to be all snow at the elevation of the BLU radar. The most promising wave in terms of total precip may be the Thu-Fri one of this week, with more spread on the weekend storm, and then even more spread for anything of substance at all for next week.

Precip from the next system just now beginning to move into the ARB, but it is a colder storm with the 15z temp at KBLU at only 22F. The freezing level is quite low, near 2000 ft or so, rising perhaps to 4500-4800 ft or so by late Tue before droping again, and this will be all snow at BLU. The 00z GFS predicts 0.9" at BLU with the precip ending by 18z/Wed with 0.3" at SAC, and the latest 12z/GFS is similar in both timing and precip. Less in the 00z/ECMWF and the 12z/ECMWF with 0.23 at SAC and 0.5-0.6" at BLU with similar timing to the 00z/GFS. More at higher elevations in the 12z/NAM with 1.25" at BLU but only 0.25" at SAC. The ensemble mean of our 12z 3 km runs has a few points up to an inch for the 24-h ending 12z Wed.

The next wave follows quickly behind the current storm. In the 00z GFS light precip begins in the higher elevations of the ARB about 15z/Thu, and really doesn't pick up until 00z/Fri 1 Feb and continues until about 15z/Fri though lingering light precip longer, with 1.4" at BLU and 0.4" at SAC. The 12z/GFS has 1.7" at BLU and 0.5" at SAC with similar timing to last night's run. More also than today's storm in the 00z/ECMWF run with 0.65" at SAC and 1.3" at BLU for totals with similar timing to the GFS. The 12z ECMWF is quite similar to the 00z run, with a heavy precip period 00-12z/Sat. Even more in the 00z/Global Canadian model with about 2" max in the ARB and the precip lingering longer on Friday. The 12z/NAM has similar timing to the GFS with ~1.75" total at BLU, and about 0.5" at SAC. The ensemble mean of our 12z 3 km runs has areas of 1-1.5" amounts in the ARB for the 24-h ending at 12z/Fri/1 Feb, so also better than the current event, with individual runs around 2" by 12z/Fri. Precip begins in our 3 km runs as early as 09-10z/Thu in the higher elevations of the ARB. Looking at the NAM BUFR soundings this system also starts out cold with the freezing level at 3500 ft at SAC, rising to around 5000 ft or so by Thu evening (though lower at BLU with upslope cooling), so again barring something unexpected this system should also be all snow at the radar site at BLU. WSW flow picks up to about 50 kts at 700 mb by late Thu which provides good orographic forcing for the higher elevations of the ARB. There was discussion as to whether this event would be IOP-worthy during the telcon, with general agreement on the scenario above, but Dave felt that the snow could get wet enough to potentially cause radar problems, and being it was a fast-mover with limited precip, would not be of interest for an IOP.

The 3rd wave is stronger with the precip in the 00z GFS beginning 12-15z/Sat in the ARB. The wave takes a perfect track in the 00z/GFS with the heaviest precip 18z/Sat to 09z/Sun, and precip ending totally later on Sunday/3 Feb. Totals from the 00z/GFS for this event are 0.7" at SAC and 1.5" at BLU. There is much less though in the ECMWF 00z run with only about 0.25" at BLU, with the system not quite as concentrated or tracking as perfectly as in the GFS. Likewise the 00z/Global Canadian model is much weaker, even more than the EC run with a more southern track. Right now this system also looks to be snow at BLU unless it deepens more. The 12z/GFS just in is now following this much weaker trend, with a more southern track to the upper level wave and very little precip in the ARB from this event, so quite a change from last night's run. Consensus on the telcon was that this system will probably now be weaker than the Thu-Fri event.

Following the weekend storm, in the 00z/GFS the next wave dives a little farther inland Tue-Wed/5-6 Feb as the upper level ridge builds off the West Coast, so most of the precip misses the ARB. Similar in the 12z/GFS, with just a tad of precip Mon/4 Feb and Wed-Thu/6-7 Feb as the waves pass more to the east into a mean trough position near the central CONUS. Then dry through most of the rest of the 15-day forecast as fast-moving waves continue to miss just to the north of the ARB until Day 15 (13 Feb) when another fast-moving wave takes a more southerly track In the 12z/GFS also dry through 13 Feb but with the pattern changing to a deeper trough over the Pacific. It would not take much of a shift, however, to keep the ARB in the track of fast-moving waves from the nw, and this is seen in the 00z ECMWF and Global Canadian models with a little more precip for a couple of waves after the weekend (2-3 Feb) storm, but right now the chance of any of these waves being IOP-worthy would not be great. Likewise, while there is variation in the ensembles, nothing to indicate any huge event in the longer range.

ed szoke NOAA/CIRA/GSD