Snowy Afternoon & Evening Tuesday, But Only For Some In Lowlands

January 25, 2021

6pm Monday…

We have one more “close call” with snow late tomorrow afternoon & evening, then continued cool and wet through the first few days of February.  Tomorrow will be dry through early afternoon, with a gusty east wind picking up after noon.  Temperatures reach the lower 40s. Then a wet weather system moves inland the 2nd half of the day. I expect precipitation to start sometime right after noon in the Coast Range and central Willamette Valley, then spread north into the metro area around 4pm. That leads to…

TUESDAY AFTERNOON & EVENING HIGHLIGHTS

  • THE VAST MAJORITY OF THE METRO AREA & LOWLANDS WILL NOT SEE STICKING SNOW TOMORROW AFTERNOON
  • BUT EXPECT A SNOWY MESS THROUGH THE COAST RANGE LATE TOMORROW AFTERNOON & EVENING. Watch out if you are headed that way! At least 3″ of snow is likely over Wilson River and Sunset Summits. Possibly down into the Van Duzer corridor between Lincoln City and Willamina as well.
  • Where the precipitation is heavier (far west metro up against Coast Range) we see a decent chance for at least a dusting.  That’s anywhere west of Hillsboro or North Plains.  It’s possible Forest Grove, Banks, Chehalem Mtn. pick up more than a trace.  Timing is 4pm-10pm.
  • We think there won’t be enough precipitation and/or temperatures won’t be cold enough for sticking snow (or even in the air) for just about all other areas.
  • Whatever happens will be done by midnight, then it’s back to light showers for Wednesday with temperatures in the 40s

TECHNICAL INFO

Models are in better agreement this evening. The general idea is that dewpoints drop into the 20s by midday due to a mild downsloping easterly wind (heading toward low pressure offshore). Temperatures with good mixing move into the 41-45 degree range in the lowlands. Then as precipitation arrives, evaporative cooling drops everyone at least a few degrees.

Generally the highest resolution models, which have the best topography, are forecasting no snow in the lowest elevations over and east of I-5. That’s because temperatures don’t drop down close to freezing tomorrow afternoon. That’s partly due to a breezy easterly wind (keeping air stirred up) and/or very light precipitation in those areas. There is no cold airmass east of the Cascades heading our way; the Columbia River Gorge is not “in play” this time around.

But west of I-5? Specifically close to the Coast Range? Cool air piles up a bit more, wind is light, and precipitation is much heavier. That can drag snow all the way down to the valley floor. Sometimes I’ve called this the “Forest Grove Effect”. I first noticed it early in my career. It can be breezy & 36-40 degrees in Portland. Yet calm, snowing, & 32-33 degrees way out against the Coast Range. I think that’ll be the case 24 hours from now. At this moment the HRRR shows this best…I like the temperatures for 5pm as well.

But the GRAF and RPM also forecast no snow. The contour pictured here is the 18z GRAF; it did very well with yesterday’s “event”. By the way, the ECMWF is accumulating snow for us with temps above 35 degrees…unlikely.

The (usually) reliable WRF-GFS at 1.33km resolution can see all the hills. It says forget it for anyone east of I-5, even up at 1,000′ or higher! This event is about LOCATION, not so much ELEVATION.

Regardless what happens in the late afternoon through early evening, slightly warmer air overhead should change anything to mainly rain showers after 10pm. Although there won’t be much precipitation by that time anyway. I’m guessing PDX goes from 43 tomorrow afternoon, down to 37-39 at 5-7pm, then back up to 39-40 by late evening.

We’ll see how this all plays out!

Chief Meteorologist Mark Nelsen