HMT Forecast Discussion 1945z M 3 March 2008
The possibilities of an IOP for next week are looking better this mrng. The GFS hi-res tendency from late last week to not build the upper ridge along and off the West Coast this week as strongly as most of the GFS ensembles and the ECMWF, seems now the more favored scenario for later this week. As a result, the strong westerly flow (up to 190kts in the GFS 250mb analysis) now extending from east Asia to near the dateline appears likely to extend eastward to the West Coast by the end of next weekend (8-9 March). The details of this are still uncertain. The most likely scenario appears to delay serious pcpn over the ARB until Sunday night 9 March. An earlier weaker system, the first one to give pcpn to CA, seems likely to affect mainly the north coast as it weakens into the ridge late Friday or Saturday.
The GFS hi res from 12z this mrng and the ECMWF from last night indicate a strong system affecting northern CA, including the ARB, on Monday. The GFS would argue for 2-3" pcpn with this storm. However, the diversity in the GFS ensembles visible on the Penn State Ewall argue that it is too early to regard this as a sure thing. Further, as pointed out in the discussion, the GFS is a little earlier than the ECMWF with this system. In addition, the GFS often errs on the side of being a bit too early in bringing in pcpn to the Sierra with collapsing upper ridge situations such as this one. Given these considerations, this does not appear to be as predictable (or as strong) a situation overall as gave rise to IOP4 on 4 Jan, but it is certainly worthy of close attention. Snow levels in this situation seem likely to be in the 6000 ft range in the ARB, so there is potential for a weak rain on snow situation in the ARB (it looks to me as if there is not much snow cover below about 4,500 to 5,000 ft in the ARB; the more reliable word from California is that the serious snow pack starts at about 4,800 ft elevation).
Looking farther ahead, Klaus Weickmann thinks that, with convection reorganizing over Indonesia at present, we may see a retrogression of the long-wave pattern later in week 2 and 3 as this convection begins to impact the midlatitudes. A hint of this is seen in the ECMWF 240h, which has the subtropical ridge off the west Coast. This could lead to retrogression of the mean trough to back over the West with an implication of colder storms tracking SEwd from the Gulf of Alaska. Klaus's confidence in this scenario did not seem too high.