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HMT forecast discussion for Wednesday 30 January 2008 (entered 20z/Thu)

Mean low-amplitude upper level ridge near 150W with fast-moving systems heading across the Pacific in a zonal flow pattern. Some subtropical moisture lurking near 180W, then east of this is a weak upper low near Hawaii, with another area of tropical moisture from Hawaii eastwards near and south of 20N. The systems moving quickly across the Pacific are tapping some of the moisture west of Hawaii through the one this weekend, but only as they move across. So given their limited moisture and rapid speed ending up with systems Thu-Fri and Sat-Sun that will be cold in the ARB with snow at the BLU radar site, and precip perhaps approaching max amounts of 3" for the first event and probably less for the second. Maybe a tap into the moisture east of Hawaii for a possible wave midweek next week, but by then the upper level ridge is forecast to build a bit and shift eastward slightly, sending this system more to the north and east of the ARB in the current model runs. Possible changes though towards the following weekend (9-10 Feb) and longer term.

The current system is ending with the ESRL gages showing the following totals: 1.5" at Big Bend and 1.2" at Alta, not sure of the 0.6 or so registered at BLU (telcon suggested this may be an unreliable amount with a heated tipping bucket gage). The next system is still on pace to start about 12z/Thu and go until about 18z/Fri. The NAM is pretty moist but with a strong terrain gradient, putting out a total of 2.5" at BLU and about 0.5 at SAC, vs. 0.6 at SAC and 2.25 at BLU from the 12z/GFS. In the 12z/ECMWF 1.2" at BLU and 0.5" at SAC. So a stronger system than the current one, but still cold, with all snow at the BLU radar site, though likely a bit warmer than the current one with the high on Thursday near 32 according to the latest forecast from the SAC WFO. HPC forecasts have a max close to 3" just to the nw of the ARB, in a similar spot to where the NAM is predicting over 3" of precip. About 2-2.5" max in our ensemble mean forecasts from the 3 km runs, and like the NAM, 1-2" more to the nw of the ARB in the higher terrain there.

The next system in line starts up about 18z/Sat and winds down the next day around 21z/Sun, with the GFS putting out ~1.4" at BLU and 0.6" again at SAC. In the 12z ECMWF 1.1" at BLU and 0.5" at SAC. So not quite as strong in the current forecasts as the Thu-Fri storm, and also cold again, with the snow level below BLU. Barring some change this would probably be considered not appropriate for an IOP. HPC is forecasting far less with this event, only 0.66" max amounts in the ARB.

Longer range into next week still looking like the upper level ridge inches a little closer to the West Coast, sending the next wave more to the east, but brushing the ARB with some precip perhaps on Wed/6 Feb. Note though that this system is forecast to have a possible tap into the moisture that is now east of Hawaii, and so if this should come in a little farther south, could be a storm of interest in the ARB, but right now this does not look to be the case.

Looking further into the longer range, by 240h (Sat/9 Feb) the ridge breaks down with a very zonal flow across the Pacific, which could lead to warmer systems hitting the West Coast. While this could still result in fast-moving systems affecting the ARB, perhaps as early as 9-10 Feb, the temperature with these would be warmer, so a better chance of getting rain down below the BLU radar site. There is a considerable amount of spread in the 10-15 day longer range period (ending Fri/15 Feb), however.

ed szoke NOAA/CIRA/GSD