First Alert Weather Mornings ahead; cold showers dump wet snow in spots

8pm Wednesday…

March is almost here! We’ve got a Leap Day tomorrow, then the first weekend of March 2024 ahead. Now some years in late winter or very early spring we get mornings with sloppy/wet snowfall in the lowlands of NW Oregon or SW Washington. Last year it occurred several days in the first week of the month. Notice the highs/lows and traces of snowfall at PDX last March. Highs only reached the mid-upper 40s, but little/no overnight freezing

The last day of February and first few days of March THIS year look the same. First, just the TL;DR version for those of you not that interested in details.

KEY POINTS

• Next 2-3 mornings the sticking snow level dips very close the valley floor west of the Cascades. During the coldest part of the day (2am-8am), IF a heavy shower moves through your neighborhood, a quick 1-2″ (or more?) of snow could accumulate. This could happen ANYWHERE, especially Friday/Saturday mornings.
• If you want snow, elevation helps a bit because it’s colder as you go uphill, but it’s really about where the heavy showers form/move. Weather models don’t handle details in this pattern well. For example, they don’t know that at 7am Friday heavy showers drop snow over Beaverton at 200′, but none pass over Sandy at 1,000′ (just an example). We just know that showers will be roaming around. Don’t get fixated on elevations
• If you live near/above 1,000′, you are just about guaranteed at least a dusting each of the next 3 mornings.
• Thursday (tomorrow) morning is a little different. Steady rain later tonight turns to a heavy/wet snow for many of the communities east of I-5 and up in elevation a bit. Expect a sloppy dusting or up to 3″ in some of those spots such as Sandy, Estacada, Damascus, Boring, Corbett, hills around Oregon City, etc…
• Regardless of what happens each morning, temperatures jump into the 40s every afternoon for clear roads. People like me that work afternoons/evenings aren’t really affected by this weather pattern.
• We will see lots of “vigorous” showers each day for a mix of sun/hail/rain/snow/thunder showers. That’s like what we saw on Monday. Any location could suddenly get a dump of hail/snow at mid-afternoon if a heavy shower passes over.
• There is no cold air on the way, by that I mean no widespread freezing. Don’t worry about pipes, pets, plants, etc…

The weather fun is being brought to us courtesy of a chilly upper-level trough out in the Pacific. Here’s the view up at 18,000′ tomorrow

By Sunday the trough is weakening and has split up a bit

Last year’s close calls with morning snow were under the same setup; a cold upper-level trough just offshore on March 5th

No official measurable snowfall in Portland last March, but I had 4 days with snow up at 1,000′ at home just eat of Corbett

You can sure see the cold/unstable air approaching behind this evening’s cold front. Look at those “popcorn” looking clouds to the west!

The airmass tomorrow morning actually isn’t as cold as Fri/Sat mornings (just -3 to -4 at 850mb). Tomorrow morning’s snow possibility is what we call an “anafront”. That’s when steady/heavy precipitation falls BEHIND the cold front, dragging the sticking snow level 1-2,000′ lower than one would expect. In the past, these anafronts have brought a quick snow dump in the lowlands. Often it’s the lowlands up against the western Cascade foothills. The fresh 00z GRAF model has increased the snowfall forecast, although it seems a bit aggressive pushing 1/2″ deep into the eastern half of Portland and even downtown. We will see!

This WRF-GFS forecast through 4pm tomorrow is a bit messy with all those wind arrows on top. But you see those 1-2″ forecast totals down into foothills of eastern Multnomah and Clackamas counties

I love this graphic…the WRF-GFS cross-section for the next 96 hours (4 days). I’ve drawn in the zero degree (C) isotherm; you can see the lowering freezing level each morning. Notice it’s coldest Friday and Saturday mornings

Then the sounding for Friday morning at 7am from the same model. This is a “valley floor snow” sounding with temps at/below freezing just about to the surface. In this setup ALL showers are just snow in the lowest elevations. But whether it sticks in any one location depends on intensity and duration.

So IF showers are still going both Friday and Saturday mornings, ANYONE could see sticking snow as they pass overhead. Or, you could live closer to 1,000′, no showers pass during the coldest part of the day, and you get almost nothing. See why you shouldn’t get hung up on elevation? It always helps because it’s colder, but it’s only one factor in this weather pattern.

That’s it for this evening, I’m off for a quick little ski vacation and some Dune time the next couple of days, but the rest of the weather team should have this showery weather pattern handled well.