HMT Forecast 2/13/07
Again, the immediate threat of precipitation over the ARB is minimal. This discussion focuses on the GFS moisture plume forecasts and the GFS ensembles looking for the next possible event, far down the road.
This morning's NAM produces only 0.04 inches of liquid equivalent in the ARB by 00utc 2/17 (Saturday) (total run accumulation). The GFS produces 0.03 inches by 00ut 2/14 and then additional amounts totaling 0.1 inches by 06utc 2/20 (Tuesday of next week, again total run accumulation). So the immediate forecasts are for a mostly dry ARB.
As opposed to yesterday the plume off the Oregon coast is now driven more N to Juneau, AK by 2/18. Later in the run a low spins up at 40N 130W and drives a good part of a plume into S CA. This corresponds to increased ensemble precip probabilities for S CA in this timeframe but only moderate chances exist that this will affect the ARB to any extent. After this the E Pacific remains dry. Monday 2/26 00utc a plume brushes by the coast but does not make it far inland. A more intensive intrusion inland but by a weaker surge of moisture is associated with a potential event on 3/1. It should be noted that NONE of these scenarios appear to be good atmospheric river events.
The ensembles have a low chance of precip 2/18 12utc and given the model forecasts I would anticipate this to be nothing but light showers. A 50% chance is forecast associated with the moisture surge S of the ARB on or about 2/22 12utc through 2/24 00utc. This bears watching especially if the trajectory of the moisture shifts to the N. The potential event on 3/1 appears to approach the area from the N and might have greater chances of producing more than 1.0 inches of precip, but it is too early to tell.
The bottom line is that at least up until Monday 2/19, I expect no precipitation events to reach IOP criteria.
Dan Birkenheuer - ESRL/GSD