So here’s what I’m thinking for the big "storm" tomorrow. The basic idea is that we are all going to see some white, like we did on Christmas Day, through the morning hours. Assuming the precipitation falls at a significant clip, which it should, all of us should see some white on the ground as well. That said, increasing southerly flow in the 1,000-2,000′ elevation (20-30 mph) is sure going to be fighting against the precipitation rate. So this should be a snow event where elevation makes a tremendous difference in snow totals. Notice I’m forecasting possibly 2-4" at the top of the West Hills, but maybe almost nothing down IN Portland "proper". Put another way, if you want to go sledding tomorrow or build a snowman, I think you will probably need to be at least 500′ or above.
or another…I doubt we’ll have any travel issues on the main city roads/highways. Only in the higher hills will you find slush and then totally snow covered roads as you get well above 500′.
The reason I don’t think we’ll have the 3-7" the NWS is forecasting are these:
1. Airmass is pretty much the same (except for overnight cooling tonight) overhead that we had Christmas Day. That was also a very marginal event, but produced a widespread dusting with only about 1 hour of steady moderate precip.
2. Precipitation appears to be quite light until early afternoon, not the real heavy stuff that could drag the 32 degree line down to the valley floor. Yes, the precip. gets quite a bit heavier as the cold front approaches mid-late afternoon, but much stronger southerly wind gets going above the surface at the same time (notice 00z MM5-Cross-Section).
3. Our RPM model here says the same thing…slightly too warm, especially after 10am.
If we don’t get sustained and moderate precip in the morning, then we could end up with nothing in the city…hopefully that won’t be the case.
I think we all agree on this…it’s going to be a dramatic warmup with a gusty southerly wind after 4pm. The snow level jumps up to 2,000′ and then stays above there until early Sunday.