metrosnowfcst

A very cold airmass continues to linger over the Pacific Northwest today.

Portland’s low of 24 is the coldest March low since I was in college (1989).  That’s a long time; quite a cold spell!  I see The Dalles made it down to 6, which is the all-time coldest March temperature there.  But now we’re done with the really cold nights since clouds move into tonight.

Quick Summary

Between now and Wednesday night we transition from this really cold and dry stuff to a more typical “cold March shower weather pattern”.  That’s when we get snowflakes in the lowlands in the mornings, then by afternoon it’s in the 40s and you’ve forgotten about it as those change to rain showers.  The next morning the higher hills have a wet/slushy dusting again before melting etc…you get the idea.  That’s the pattern through early Saturday.  In that setup it’s very tough to get sticking snow to sea level, or below 500′ elevation.

But late tonight and Wednesday, during the transition to the “milder” weather pattern, we have a pretty good chance of seeing at least a littlesticking snow all the way down into the lowest elevations

Snow Headlines Metro Tonight

WEDNESDAY AM COMMUTE IN METRO AREA & SALEM/ALBANY

  1. Snow showers, a trace to 1/2″ accumulation
  2. Many roads will remain bare since snow could be very light
  3. Expect a few snowy roads in spots, but many of us should see good driving conditions for the commute

WEDNESDAY DAYTIME & EVENING COMMUTE

  • Snow showers continue and may turn to steady snowfall
  • Through the daylight hours it’ll be too warm for snow to stick to roads (strong March sun energy coming through the clouds)
  • Not much additional accumulation

WEDNESDAY EVENING (after 6-7pm)

  • Snow showers continue
  • Temps cool to near/just above freezing so sticking snow is possible again
  • Nothing to 1″ additional snow, could even stick to roads again IF temps drop down to freezing and snow keeps falling.

TOMORROW NIGHT & THURSDAY

  • Snow showers, change to mainly rain showers after morning commute
  • Little or no freezing of roads
  • Dusting to 1″ on hills, little or nothing in lowest elevations

HOPEFULLY YOU’RE GETTING THE IDEA THAT MUCH OF THIS MAY BE “CONVERSATIONAL SNOW” IT MAY NOT HAVE MUCH EFFECT ON OUR LIVES, BUT WE’LL SEE PLENTY OF IT IN THE AIR AND ON OUR LAWNS.  WE’LL ALL BE TALKING ABOUT IT (WHINING?).

TECHNICAL DISCUSSION

We’ve only seen measurable snow three times in the past 20 years at the Portland NWS Office.  That shows you how rare it is.  Last time was 7 years ago.  It’s MUCH easier to get sticking snow up around 1,000′ and above and there will be plenty of that the next three days.

All models in pretty good agreement bringing the moisture north for snow late tonight through Wednesday evening, then the flow turns westerly and showers move in off the Pacific tomorrow night through early Saturday.

ECMWF Snow Accumulation

Once again the ECMWF and GEM models are “wettest”, showing up to .40″ precip by tomorrow night over the metro area.  It’s interesting that none of them generate much precip through the morning hours, but then some sort of convergence in the afternoon/evening as westerly flow begins to take over.  Most precip is the 2nd half of the day, but in March that means it’ll be above freezing.  The GEM is still producing sticking snow at temps above freezing tomorrow.  I’m quite confident it’ll be cold enough in the morning with 6 millibars dry/cold flow still coming through the Gorge.  But then it goes flat by sunset.  From that point forward we’re into the usual onshore-flow cold shower pattern.  Plenty of evaporative cooling to go around through early afternoon due to the cold easterly flow.  So if we get steady/moderate snow in the afternoon hours it could stick anywhere in the metro area, although roads would still be too warm at that point.  The HUGE question for tomorrow evening is whether snow keeps going after sunset and we get 1-2″ in the central/east metro area.  The WRF-GFS is a bit too warm through the entire day/evening and only generates a little snow for us; most areas under an inch.  It also shuts of snowfall around sunset.

wrf_snow_4pm-4pm

Beyond tomorrow evening I think models are overdoing snow forecasts, they typically do in these onshore-flow type patterns.  I’ve been burned by that several times.

By the way, the ECMWF is back on top folks!  This is the 2nd event in the past two weeks where it was the first to show snowfall over us.  A few days ago all other models were dry and keeping all that to the east/south.  Then they fell into line.  I’m just obsessed with model competition; a bit weird I know.

That’s it for now; see you on TV this evening!

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