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HMT forecast discussion for Monday 3 December 2007

Big storm currently underway in the Pacific Northwest with destructive winds and heavy rains from northern California through Washington. In California around a half inch of rain south all the way to near Santa Rosa, 3 inches up the coast at Honeydew, appears to be nothing of note in the HMT area at this point. Max winds at Blue Canyon at this point not quite 30 kts but way stronger to the north. The main shortwave trough now located near 43N/140W, with another wave back at 40N/165W, all south of an elongated closed low sitting in the Gulf of Alaska. A plume of moisture stretches al the way back to Hawaii. The easternmost shortwave is heading towards Vancouver later tonight, with the trailing one arriving a little farther south later tomorrow, by which point the plume of moisture gets detached some from the main system and sinks south, with what is left of it on Tuesday aimed pretty much at the HMT area. This should bring some echoes into the area perhaps as early as 0600 UTC tonight but mainly 1000-2200 UTC on Tuesday. Right now still looks like the most over the higher terrain areas would be in the order of 0.25", but this all depends how much is left in the plume and imagine this could be tricky for the models. The hi-res WRF NMM window has spots of over an inch, but the ARW much less. The ECMWF from 0000 UTC is also more moist, as is the Canadian run, but consensus favors lower amounts. Note though that current graphical forecasts out of the Sacramento WFO have up to 0.45" of precip max in the HMT area.

Once the plume passes, the next system still on track for later Thursday through Friday (6-7 Dec). The wave that becomes this system is just now coming into the northern Pacific, and is forecast to begin to amplify as it passes beneath the retrograding Gulf of Alaska trough later Tue into Wed. At the same time a separate upper level wave to the southwest of this system becomes detached from the main flow and closes off to the northwest of Hawaii by Wed. Tne main significance of this is that it pumps some tropical moisture north into the progressive northern system, although how much of this connection remains intact by Thu/6 Dec as it deepens and heads towards the coast is up for question. In terms of detailis so far on this next system, the 0000 UTC runs were, overall, more favorable for a decent event than the latest 1200 UTC runs. For example, the GFS had up to ~2.5" max over the HMT area, similar to the Global Canadian run, but the 12z GFS and Canadian runs are down to about an inch. The main difference is a faster moving system in the latest runs compared to the 0000 UTC runs, so less time spent with good s-ssw 700 mb flow. Too early to sort out these details but we should note that the start time in some runs is as early as 1500 UTC on Thursday. Overall the ECMWF 0000 UTC run is somewhat faster (and with less precip) than the GFS or Canadian, while the NOGAPS drops the system farthest south eventually. The 1200 UTC ECMWF is quicker to end the event than the 0000 UTC runs, with an accumulated total precip max of ~0.6" in the HMT area. The UKMET 1200 UTC run is also more progressive than the 0000 UTC run. And finally the NOGAPS is a llittle faster with the 1200 UTC run though still dives the system farthest south by Sat/Sun. Ensembles from 0000 UTC are somewhat split between faster and slower motion on the GFS, leaning more towards more progressive on the Canadian ensembles. The latest 1200 UTC GFS ensembles have an event ranging from not much precip to around 2 inches in the HMT area, with the main period from 1800 UTC Thu/6 Dec to 1800 UTC Fri/7 Dec. Bottom line at this point is a potential event still on tap, could begin in earnest (if earnest occurs!) by 1800 UTC on Thu, and trend is to end it sooner than in the earlier runs, probably mostly over by midday Friday. We'll have to see if this trend continues or if we go back to a somewhat slower solution, plus need to monitor the amount of moisture tap into the system near Hawaii.

Looking farther ahead, most models have a break from ~9-12 Dec, but then the threat of systems out of the wnw with the upper level ridge far enough to the west for the period 13-18 Dec. Consensus is towards progressive shortwave troughs digging into the Intermountain West with the threat of enough digging before they hit California to produce an event.

ed szoke NOAA/CIRA/GSD