For those of you just wanting the forecast details tonight, here are the highlights:
1. Ice all over the place tonight as temps have dropped to or below freezing with some areas of wet roads from the earlier snow showers. This continues overnight, and we may add a dusting of snow over the top of that for the morning commute. Expect a none to 1/2″ additional snowfall in the late night and very early morning hours.
2. Tomorrow we gradually thaw out with snow showers here and there, picking up some in the evening. After noon tomorrow, most (or all) sticking snow should be confined to the hills and above through Tuesday. By that I mean once we get past noon tomorrow, the vast majority of metro area roads will be in the clear through Tuesday evening at least.
3. Snow showers turn to mixed showers, and then mainly rain showers Tuesday. On Tuesday the sticking snow level should be around 1,000′ in the morning, and up to around 2,000′ by evening.
4. Winter Storm Watch is up for the eastern Columbia River Gorge. Technically it’s only for The Dalles east, but that’s because that forecast area comes from the Pendleton NWS. Cascade Locks and Hood River are likely to see even more snow than that (6-12″) but are under the Portland NWS forecast area. They haven’t issued any watches or warnings yet. That happens once in awhile. Either way, I’m increasingly confident there could be a heavy snowfall in the central/eastern Gorge Tuesday night and Wednesday. Even our warmest model shows it just barely cold enough.
5. Significant snow is also possible Tuesday night-Wednesday north of Longview in SW Washington. Too early for amounts, but at least 2-4″
Now on to the details…I had lots of time (probably too much time) to look at just about every piece of meteorological data I could find this evening. My brain is fried, but here goes…
TONIGHT: Our RPM model (the only one doing so) keeps insisting a batch of showers (the one over the North Coast right now?) moves eastward into the metro area before sunrise with a Trace-1″ accumulation. It’s done that for several runs. That’s why I added the possibility for the metro area.
MONDAY/MONDAY NIGHT: The coldest atmosphere of this whole event is over us now, and we gradually begin to warm overhead, especially after 4pm tomorrow. All models (even the NAM) show moderate southerly wind developing not only overhead but at the surface as well later tomorrow. It goes all night tomorrow night and into Tuesday morning. I noticed the 25kts. of that onshore flow on the WRF-GFS just overhead, and our RPM goes breezy as well. This is due to a fresh batch of arctic air spinning up a surface low off Vancouver Island. Lowering pressures to the north induce the south/southwest wind here. Due to this, I feel the snow level tomorrow night lifts off the Valley floor (as you see in the forecast). I doubt there will be any freezing Monday night. This shows up well in cross sections as the zero degree line gradually rises. And the image below shows little or no sticking snow from now through Tuesday afternoon in the lowlands.
TUESDAY: More of the same, frequent showers may just turn to mostly cloudy with light rain at times as the atmosphere overhead continues to GRADUALLY warm. GFS, RPM, WRF-GFS, and ECMWF are all too warm for sticking snow here with a decent onshore flow. They all show 850mb temps between -4 and -7. Yes, I know the NAM has -7 to -8, but that’ significantly warmer than the -10 to -12 it was showing just 24 hours ago. But several inches of fresh snow above 1,000′, especially early.
TUESDAY NIGHT & WEDNESDAY: This is where things become a MAJOR pain. That stupid NAM! It has a deep surface low tracking into Central Oregon, pulling strong easterly and northerly flow right down over us at the same time plenty of precipitation arrives. Almost a perfect snowstorm here in Portland and Salem too; huge in the Columbia River Gorge. But the ol’ GFS comes in with the exact same low pressure movement during the same period, bringing a surface low to the Washington Coast. It seems SLIGHTLY farther south, but it doesn’t matter for the western valleys; warm air floods in during this period. Even this “warm” GFS appears it’s just far enough south with the low center for 32 degree temps all the way down to the surface in the central/eastern Gorge, so I pulled the trigger on that one and will make a graphic for it.
For those of you agonizing over the thought “how models can be so far apart?”; keep in mind that it’s only one surface low location that’s maybe 150 miles off from one model to another. Compare the upper-level patterns on those models and they are only slightly different. It just so happens that we live in a spot where the actual weather can be far different based on these two different solutions. Hey, it’s only 1 hour until the 00z ECMWF!!!
Chief Meteorologist Mark Nelsen