An exciting morning with lots of heavy snow in the hills, and snow in the very lowest elevations. It’s really adding up in the hills; I have 6″ on the ground here at 1,000′, and I see reports up around 10″ in other hills higher up. The heavy showers ALMOST overcame the strong southwest wind, which advects in warmer air off the Pacific, what an epic battle! Here’s the forecast for those of you just tuning in for the basics:
Snow and mixed snow/rain showers continue through the day, although they die down quite a bit this afternoon.
The sticking snow level rises slightly again like it did yesterday afternoon, so roads in the hills that have some light snow should only get better as the day goes on.
Evening commute should be fine for metro area highways and most roads too.
TODAY: Rain/snow showers in the lowest elevations. Still, light dusting possible briefly anywhere before noon. Another 2-4″ near/above 1,000′…it’s getting pretty deep up in the hills. Be aware that’s a forecast of nothing at sea level to 2-4″ at 1,000′. You don’t just suddenly get 2-4″ at that higher elevation and nothing 200′ below, amounts will vary between those elevations AND, MORE IMPORTANT, depend on precipitation intensity. So yes, 500′ could still get somewhere between zero and 4″ possibly.
THIS EVENING (Through at least 10pm): Rain/snow showers, turning to steady rain/snow later…most sticking snow at/above 1,000′ through our 10pm show. Light accumulations during this time on the hills, this will be the calmest period.
OVERNIGHT THROUGH 7:00 AM: Winter Storm Watch is still up, and NWS will probably upgrade to a Winter Storm Warning mid-afternoon to be safe. I still think we’ll escape a big snow event here in Portland, but it’s going to be VERY CLOSE. So I’ll forecast 1-3″ of heavy wet snow possible anywhere from Wilsonville north through Clark County (all of the metro area) between Midnight and 5am. No snow for Wilsonville south to Salem. Once again, gut feeling is we get little or nothing here, but it’s safest to forecast (Dec 29, 2009) a few inches in the dark of night.
This still looks much better in far Western Washington County (Forest Grove, Banks, maybe Hillsboro too), Columbia County (Vernonia, Sacappoose, St. Helens), and Longview north through SW Washington. The Gorge is just going to get hammered! Possibly 4-8″ west of Hillsboro and Scappoose/St. Helens and Longview. Vernonia has a lot of snow on the ground now, and could see another 8-12″! Just out of our viewing area, north of Longview, 8-12″ is likely, a historic storm up there. In the Gorge from Cascade Locks to The Dalles, I could see a total of 18-24″ by Wednesday afternoon.
AFTER 7:00 AM TOMORROW: The plan is for a quick warmup to rain. High around 45. NWS has a High Wind Watch up for the Central/Southern Willamette Valley. This MIGHT be tomorrow’s big story, but not all of our models have such strong wind. The Gorge stays snowy tomorrow.
Now, on to the technical stuff:
A very challenging forecast continues. I’m pretty happy the overnight forecast worked out well with slightly warmer temps keeping most of the sticking snow above the lowest elevations. Lots more showers through mid-afternoon, then models indicate a brief break before steadier precipitation heads in this evening. Satellite loop clearly shows the approaching system heading towards us. This one will be far juicier with 1.00-1.50″ precipitation between this evening and midday Wednesday! The areas that see most or all of that fall as snow are just going to get hammered. That includes Cascades, central/eastern Gorge, and the North Coast Range plus foothills (as mentioned in the forecast above).
We finally have pretty much perfect agreement in the models for the overnight/Wednesday morning system. A surface low will track to right around Astoria by 10am, then fall apart later in the day over Washington. Perfect setup for a historic snowstorm just to the east and north of the center. That means SW Washington REALLY gets nailed, as well as the North Coast Range of Oregon and Willapa Hills. With this setup, as the low approaches the coast tonight, we lose our south wind during the evening hours, and then over the metro area it goes calm through part of the night, then a light SE or E wind picks up towards daybreak. Two things to point out: one is that it’s not a cold east wind coming through the Gorge and it won’t be very strong, so we don’t have one of those setups with a gusty gorge wind and temps dropping all night. In fact it’ll be a “mild” west wind through the Gorge until midnight. What’s really going on is during this period? The flow of warmer air moving in with the southwest wind stops, allowing the near/below freezing temps to extend in a column from the 1,000′ freezing level all the way down to the surface. This is most likely during the very heavy precipitation midnight and beyond. Assuming the low positioning is correct, this ends by 7am as strong southerly wind punches through overhead, raising the snow level way above 2,000′ (eventually 4,000′ or more by midday). Here’s the sounding off the WRF-GFS at the coldest forecast point for both Portland and Salem (around 4am): Isothermal (near 32 degrees) profile at Portland, but well above freezing at Salem in the lowest 1,500′. Click for a better view, or your eyes are going to hurt. 
IF our models are incorrect and the low decides to come into Tillamook or even slightly farther south, we get a ton of snow here like we currently expect for Longview to Olympia, but I see no reason to go for that solution. If the low is any farther north, that just shortens the period in which we could get snowfall in our neck of the woods. So, with all that info I like what our RPM model and WRF-GFS show for snowfall. First, from 4pm today through 4pm Wednesday:
Notice we are right on the line here, the Gorge and SW Washington get dumped on. Here’s another issue, the GFS and WRF-GFS keep marginally cold air in place over the Columbia River Gorge through tomorrow night and Thursday as another system moves through, note the 4pm tomorrow through 4pm Thursday snow forecast:
The MM5-NAM doesn’t do that since it has a much stronger low center tomorrow running into eastern Washington, wiping out the cold air eastside.
Speaking of that MM5-NAM, it’s the one model that has a huge southerly pressure gradient through the Willamette Valley as the low tracks by to the north. Note the map:
If this actually occurs, there will be a damaging windstorm in the valley tomorrow, possibly as far north as Portland. This shows 11-12 millibars just from Longview down to Salem with perfect east-west orientation of the isobars. Other models are not as intense and we have the 00z runs to see what happens. Interesting to note that there is a High Wind Watch out for the central/southern Willamette Valley tomorrow; that alone would cause us to go into BREAKING WEATHER mode, but this little pesky snow thing is drawing the newsies attention elsewhere.
Unless some surprise occurs, I probably won’t post again until 9pm or so. It’s going to be a long day in the newsroom!
Chief Meteorologist Mark Nelsen
Posted by Mark Nelsen 
FOX 12 Weather Blog 