After all these days, we finally have an agreement in the models for the critical Tuesday night through Wednesday forecast. I’ve put in a slide show which takes the ECMWF, GEM, GFS, & NAM for 18z (10am) Wednesday on the same map projection. Note that they all are much closer on a surface low moving up against the Oregon/Washington Coastline by Wednesday morning. The NAM has finally turned warmer with 850mb temps in the -7 to -8 range tomorrow, although that’s still slightly colder than the GFS/ECMWF.
So what does it mean?
1. The National Weather Service has issued a Winter Storm Watch for much of Northwest Oregon and Southwest Washington for this storm. I think that is a little much, but they do need to err on the side of caution since that’s their job. Probably a good move since it’s only a WATCH, not a WARNING. That means if it becomes obvious there is only rain coming in the lowlands, they can just change it to a Winter Weather Advisory for the hills.
2. All of these models are very marginal for snow here in Portland, especially the GFS. That said, even the ECMWF/GFS 850mb temps around -4 to -5 with 6-10 hours of easterly flow could change us over to snow. Note the 12z WRF-GFS sounding for 1am Wednesday morning. That’s REALLY close to snow. Most important, I told myself I wouldn’t make the “December 29th mistake” again. When it’s marginal, I’m going to tell the public so instead of just taking them along with my “weather gambling forecast” they can be prepared in case I’m slightly off.
3. Due to that, I think it will probably not stick Tuesday night and Wednesday morning for most of the metro area, but it’s going to be very close! Luckily it should happen mainly at night too. And this forecast is subject to change the next 24 hours if models come any farther south with the surface low.
Where could we get significant accumulation of snow? The central/eastern Columbia River Gorge (10″+ possible). Longview and northward in Southwest Washington (4″+), and the western parts of Washington/Columbia counties, and maybe Scappoose/St. Helens too. A little bit of the “Forest Grove Effect”, where cooler air pools up against the east side of the Coast Range. Those spots could see 4″+ as well.
Quick warming on all models Wednesday afternoon. A south wind with 850mb temps above zero is 50 degrees easy. Gusts could be over 40 mph depending on the low placement. Interesting to note the NAM-GFS from UW is much stronger and farther south with the low than NOAA’s version of the NAM.
Oh yeah, as for the next 24 hours, I haven’t changed my thinking much:
REST OF TODAY: Snow showers pick up later…increasing southerly wind lifts sticking snow level up around 1,000′. Highs near 40
TONIGHT & TUESDAY A.M. COMMUTE: Snow showers, snow level doesn’t lower much below 1,000′, so 1-2″ new on the hills closer to 1,000′ and above. No freezing in the lower elevations. That’s because of the mild southerly wind continuing to blow.
TUESDAY: Mixed rain/snow showers all day, sticking snow only up around 1,000′ or even a little higher by afternoon…1-3″ on the higher hills only.
Chief Meteorologist Mark Nelsen