Quick "surprise" snow event possible (probable?) in Portland tonight

[Mallow]

With moderate offshore flow, a cool airmass in place, a cold, snowcovered Columbia River Gorge, cool air aloft, and a band of moderate precipitation moving in this evening, things are looking somewhat favorable for a borderline snow setup in Portland tonight.

The 4km-mesoscale-model sounding for 7PM tonight shows the threat pretty well:

Obviously we'd need things about 1°F colder in the lower levels to assure snow, but that should be easy enough I think. After all, here's a comparison of actual 10AM temperatures aloft (per the KPTV tower just west of downtown Portland) vs. modeled (per 4km forecast sounding over PDX).

1,043' sensor...

Forecast: 2.1°C

Actual: 1.6°C

1,473' sensor...

Forecast: 2.1°C

Actual: 0.3°C

1,818' sensor...

Forecast: 2.0°C

Actual: 0.2°C

Satellite imagery shows good upper level diffluence over northern Oregon, so continued cloud/precip development seems a given:

http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?wv8km_enhanced+12

[csnavywx]

With moderate offshore flow, a cool airmass in place, a cold, snowcovered Columbia River Gorge, cool air aloft, and a band of moderate precipitation moving in this evening, things are looking somewhat favorable for a borderline snow setup in Portland tonight.

The 4km-mesoscale-model sounding for 7PM tonight shows the threat pretty well:

Obviously we'd need things about 1°F colder in the lower levels to assure snow, but that should be easy enough I think. After all, here's a comparison of actual 10AM temperatures aloft (per the KPTV tower just west of downtown Portland) vs. modeled (per 4km forecast sounding over PDX).

1,043' sensor...

Forecast: 2.1°C

Actual: 1.6°C

1,473' sensor...

Forecast: 2.1°C

Actual: 0.3°C

1,818' sensor...

Forecast: 2.0°C

Actual: 0.2°C

Satellite imagery shows good upper level diffluence over northern Oregon, so continued cloud/precip development seems a given:

http://www.atmos.was...8km_enhanced+12

Very low static stability or moist-neutral profile above 750mb there too. That'll no doubt help your cause, provided there's some decent omega in that layer. Might be one of those situations where it starts briefly as rain, then flips to snow when precipitation rates pick up enough to get significant diabatic cooling through melting going.

[Mallow]

Very low static stability or moist-neutral profile above 750mb there too. That'll no doubt help your cause, provided there's some decent omega in that layer. Might be one of those situations where it starts briefly as rain, then flips to snow when precipitation rates pick up enough to get significant diabatic cooling through melting going.

Exactly.

We saw a similar situation early yesterday morning, but things were generally about 3-4°F warmer, and it was rain or a rain/snow mix in most places. Northeast Portland, and western suburbs, did get snow out of that. And as you said, it was a rain-to-snow situation with cooling due to evaporation and melting once precip intensity increased.

3-4°F should make all the difference this time around.

[Mallow]

Another thing to note is that even though it appears as "warm" today as it was before our previous event, that is misleading. Radiational warming at the surface has contributed to the apparent warmth today, and the depth of that "warm" air is much shallower than it was with the last event.

The warmth with the last event was due to the mixing down of an airmass that had undergone downsloping, so the warm layer was much deeper (probably about 2-3,000' deep). That warmth was partially offset by the dryness of the air (also due to downsloping). In addition, the southerlies aloft brought in "warm" air above the 3,000' level, so there was a lot of warming to be had all around with the last event.

In this case, the apparent warmth is mostly at the surface, due to radiation. This can be seen on the KPTV tower temperatures:

http://www.wrh.noaa.gov/pqr/tower_temps/tower_temps.php?&units=F

Notice with the previous event (very beginning of the plot) that the warmth was through the depth of the layer, and it was 35°F even at 2,000' in the "free air". Today, though there is probably still a bit of warming to be seen, temperatures up higher in the "free air", where the impact of surface radiation absorption is small, it's still below 33°F.

[Mallow]

BTW, clouds have already moved in and solar radiation is substantially reduced. I hate to sound "folksy", but it just looks like a snow sky out there to me.

[Mallow]

Also, 18z NAM is definitely wetter. Shows well over 0.25" of precip during the prime period.

[Mallow]

http://radblast-mi.wunderground.com/cgi-bin/radar/WUNIDS_map?station=RTX&brand=wui&num=15&delay=15&type=N0R&frame=0&scale=0.759&noclutter=0&t=1292793122&lat=45.52&lon=-122.68&label=Mallow%27s+Work&showstorms=0&map.x=400&map.y=240&centerx=454&centery=168&transx=54&transy=-72&showlabels=0&severe=0&rainsnow=0&lightning=0&smooth=0

[baroclinic_instability]

Your weather near the Pac NW is crazy. Some of the hardest in the U.S. to forecast, IMO.

And is that a stuve? I like the Washington suite of guidance, they have some well run high res models. They also try to sell it for a ridiculous amount of money to clients.

What kind of DOT response can they expect there? I know from events last year they were quite ill-prepared, including a pretty well-known event that crippled the region.

[Mallow]

Your weather near the Pac NW is crazy. Some of the hardest in the U.S. to forecast, IMO.

And is that a stuve? I like the Washington suite of guidance, they have some well run high res models. They also try to sell it for a ridiculous amount of money to clients.

What kind of DOT response can they expect there? I know from events last year they were quite ill-prepared, including a pretty well-known event that crippled the region.

Ya, it's a stuve. It's the default from the UW wx page.

In "surprise" events like this, the response will be miserable, and people will get stuck.

[Indystorm]

The Pacific Northwest and California have currently been off to a roaring start this season I presume due to LaNina. Have a friend from the flatlands of IN who moved to Vancouver WA who is amazed by the orographic and topographic influences on the weather out there, particularly in the Columbia River gorge.

[Mallow]

The Pacific Northwest and California have currently been off to a roaring start this season I presume due to LaNina. Have a friend from the flatlands of IN who moved to Vancouver VA who is amazed by the orographic and topographic influences on the weather out there, particularly in the Columbia River gorge.

Ya, but we really have had crap in the way of wintry weather so far this year. A dusting in late November is all we could muster so far.

[Mallow]

There was a dusting of snow in many eastern areas and suburbs of Portland. I saw snowflakes out the window of my office downtown. Otherwise, the event wasn't as good as I was hoping.