HMT Forecast 1/12/2008 19:40 UTC
The minor system we have noted in earlier forecasts is now passing north of the ARB region. This morning, minor precipitation was observed primarily north of I80 with radar echoes 35-40 dbz at times with surface reports of light snow in the higher elevations of the ARB and light rain to the north of the ARB in valley areas. With temperatures right around freezing at the higher elevations of the ARB, I would not be surprised to see the snow switch over to rain/drizzle as the day progresses. All model guidance shows the passing system will simply graze the area leaving trace to minor precipitation amounts and end by this evening (03utc 1/13, Sunday).
Looking at the water vapor imagery over the Pacific we see the current system is related to the wave observed yesterday nearest the coast. This is now associated with a good vorticity center located at 49N 130W. The second circulation center mentioned yesterday has now occluded with a circulation at 35N 167W and is spinning up an associated rotation at 41N 154W. This has a good topical connection, as it has all along, extending back to Indonesia. This plume of moisture might impact the ARB in a minor way next week as we will see in the discussion below.
The models and the SREF probabilities show no precipitation after 03utc 1/13 with the current event leaving us with a dry ARB for certain out to 00utc 1/16 (Wednesday). During this interval the GFS builds a strong ridge off the coast and the moisture plume that was mentioned yesterday is seen to approach the coast with progressive short wave on or about 06utc 1/15 Tuesday only to have it retreat out to sea (drawn into a 554 low the EC model puts at 32N 147W) before it can reach the ARB leaving the area dry. The ridge continues to build in the GFS with a 584 ridge axis at 44N 130W extending down to 25N 124W, this appears to reach its maximum level on or about 09utc 1/16 (Wednesday). At about this same time a cold air surge is forecast to drop from Canada and push west possibly approaching the ARB from its east. Again, this should have no impact on HMT operations.
Looking out to the long range, the GFS shows a possible surge of moisture coming down the coast from the north, likely from the second circulation now out in the Pacific that has been driven over the high ridge, traversed just south of the Aleutians, around the Gulf of AK and then down the west coast. It is not anticipated that this will be an "optimal" moisture plume if this scenario pans out and is downplayed in the GFS 12ut run over the earlier 06ut run. Beyond that, the ensembles present a roughly 50% chance of precipitation in the ARB on or about 1/22 (Tuesday) associated with a cold air outbreak (both Canadian and GFS ensembles). The bottom line is that after today's minor event, models are consistent in forecasting an extended dry period with perhaps some minor activity associated mainly with cold air outbreaks. The near term is dominated by a sharp off-shore ridge that shuts off the possibility of atmospheric "rivers." Ensembles show the possibility of the ridge retrograding but staying persistent out as far as the end of this month.
Dan Birkenheuer ESRL/GSD