I think I can smell a slow-roasting “forecast bust” as I come into the weather kitchen this evening. The slow cooker will be working overtime tonight and tomorrow morning…
The National Weather Service and at least one or maybe two other forecasters in town (as of the early evening shows) are calling for at least some sticking snow tomorrow evening down here in the lowest elevations in the city. I think that’s very unlikely; my reasoning is below.
Here are the highlights:
1. Rain arrives in the metro area and north Willamette Valley around midday tomorrow, sometime between noon and 2pm. It should fall as rain with snowflakes POSSIBLY mixed in just about anywhere. Temperatures peak around 40 and stay there all the way through late tomorrow evening. I don’t expect a temperature drop back down towards freezing.
2. I don’t expect sticking snowfall anywhere in the metro area; the only POSSIBLE exception could be out around Banks and Vernonia if temps don’t warm too much. Other than that area; up against the east slopes of the Coast Range, you’ll need to be up around 2,000′ to find sticking snow tomorrow afternoon. Even the central and eastern Columbia Gorge looks very marginal for snowfall late tomorrow afternoon; I-84 will probably be slushy at worst.
3. Starting late tomorrow night through Thursday, we’re back in the cold showers and clearing periods pattern; almost exactly what we saw over the weekend. That means each morning any of us could see snow mixed in with the rain, but sticking snow Wednesday and Thursday morning will probably stay in the hills, closer to 1,000′.
4. Put this together, and you can see we still don’t see a return to winter (lots of snow or a deep freeze) here in the lowlands of western Oregon or SW Washington. Just more of the late winter cold showers that occasionally mix with snow.
Now, on to the technical chit-chat:
I wasn’t scheduled to work today, but after seeing the snow talk ramping up, I figured I should work and take some other boring weather day off. Plus, we have NASCAR on right now so no early shows and plenty of time to check out all the other sources of weather information. Clearly I had too much spare time today…
We have two separate forecast “snow issues” in the next 3 days.
A. Pre-Frontal steady precipitation from midday tomorrow through tomorrow evening.
This doesn’t look nearly cold enough for snow to me; it doesn’t even look that marginal compared to some past events. In fact if the NWS Discussion hadn’t mentioned it this morning, I would have just said “40 degree rain” and not even mentioned the snow possibility for tomorrow:
1. Surface temps are too warm and models don’t show the usual “evaporative cooling dip” when we have a dry airmass and moisture starts moving in (December 29, 2009).
2. No mesoscale model shows snow accumulating at the lowest elevations
3. Significant warm advection to a bit above freezing in the 1,000-3,000′ layer. Note the 1.3km WRF cross-section. Not only is the 0 degree (celsius) line heading towards 850mb, but the +5 line lifts above the surface! That goes along with the meteogram above and our RPM text data above showing we hang right around 40 from midday tomorrow through late evening.
4. No cold and dry air coming through the Gorge to keep chilly air in place. All I really need to say is “1-2 millibars easterly flow tomorrow and it was 46 degrees at The Dalles today”.
5. In deference to the “Forest Grove Effect” with an approaching low pressure system and slightly cooler air banking up against the Coast Range, I said we could get some stickage out in places like Banks and Vernonia, but I’m not to encouraged that even that will happen.
B. Cold Post-Frontal Showers late Tomorrow Night through Thursday
This is pretty much the same as what we just went through this past weekend. The old “showers and clearing periods” pattern. Most of us at the lowest elevations saw nothing.
I should say that I am somewhat impressed by Wednesday morning’s precipitation intensity. This looks similar to what brought us lower than expected snow levels on Saturday morning. In fact our RPM shows all of 0.3″ snow over Portland at that time (around 7am Wednesday).
So THIS period appears to be the best chance for snow showers to lower elevations. The Wednesday morning commute will probably bring some heavy snow showers for all of us, but still, anything other than a brief dusting will remain above the lowest elevations with afternoon temps up into the lower 40s.
If you live in the hills up around 1,000′ or higher, 1-3″ is likely again Wednesday morning and again 1-2″ Wednesday night and Thursday morning. Shower intensity really backs off Wednesday night too, we saw what happened Sunday morning compared to Saturday, even fewer spots saw a dusting for that reason. Lots of snow covered roads (again!) for Wednesday and Thursday mornings in the hills.
If you live 1,500′ or higher? Could be sticking snow anytime after 10pm tomorrow night through Thursday evening, with most of the accumulation Wednesday. 6-10″ at that elevation easily. Here is the 3 day snowfall forecast from our RPM model.
Beyond Thursday, upper level ridging moves overhead through the weekend for significantly warmer 850mb temps, so seasonal March highs in the 50-55 degree range will return.
Here are the 12z GFS and 12z ECMWF 850mb ensemble charts…huge disagreement on either warmer ridging or more cool troughs in the next 10-14 days. Look at those lines all over the place, especially on the ECMWF! We’ll see which way it goes.
Chief Meteorologist Mark Nelsen