HMT Forecast 2/14/07
The outlook for any significant weather in the near term remains nil. However, next week has two promising events which might provide grounds for another IOP.
This morning the Pacific high pressure is located at 33N 140 W and directs tropical plumes from a good source at 140E 10N. This is aligned with OR just off the CONUS coast as we have been following over that past few days in these discussions. Today the plume brings some moisture to the ARB by Friday 16utc. A weak atmospheric river with poor roots to the tropics develops at 12 utc 16 Feb and is reinforced by a second moist plume sweeping down from the N. These give rise to a weak precip episode now forecast by the GFS to produce a total of 0.27 inches between 18utc 2/18 and 06 utc 2/20 (last night's run had this 2x higher, about 0.54 inches). Later next week there appears to be a better chance of an event.
By 18 utc 2/21 a better moist tropical flow elongates and sweeps by the ARB. This is oriented E-W and extends back to a very high moisture plume centered over 170 W. The GFS appears to work this into a precipitation event that totals about 2 inches of precip beginning 06 utc Thurs 2/22 and ending around 06utc 2/24 Saturday.
Both events though not ideal atmospheric moist plume events do bear watching with time, especially the second one.
This morning's ensembles underscore the above model indications. The 2/18 storm indicated at 18ut initiation has a high probability. About 50% of the ensembles put the freezing line over the ARB at the onset. After this the second event described above has a better than 50% chance of occurring between 2/22 00ut and should be over by 2/24 12utc. Beyond that there is a low chance (30-40%) of an event about 2/26 and one that might carry higher probability near March 1. These ensemble probabilities have been fairly consistent over the past several days. Beyond March 1, there is poor consensus in the ensembles at this time.
To summarize, I still see the ARB remaining dry until this Sunday that might begin a weak precip event that may not reach IOP criteria. However, I now have more confidence in another possible IOP criteria event occurring about 2/22 (Thursday, a week from tomorrow).
Dan Birkenheuer - ESRL/GSD