Wednesday, November 26, 2014

Lowland Snow Threat on Friday Afternoon and Saturday

There is a four letter word one tries to use sparingly in Seattle: SNOW.

The mere mention of this solid form of water causes our local media to go wild and anguish to furrow the brows of Seattle mayors.

But there is a chance for light lowland snow on Friday afternoon and Saturday morning, but uncertainties (as always the case with western Washington/Oregon snow) are substantial.   However, the ingredients will be in place, the only question is whether they will combine in the right way.

So what are the ingredients we are looking for?  First, you need cold can't get snow in the western lowland without that!  And you need precipitation.    We have two separate shots at getting the combination.   Let me tell you about it.

First Friday afternoon at 1 PM.   Take a look at the sea level pressure, lower atmospheric temperature (at about 3000 ft above sea level), and surface (10-m) winds.  Blue and purple indicate air cold enough for snow.  A strong cold front is pushing southward into western Washington.  This front will have substantial precipitation with it, and much colder air behind. There will be a period after frontal passage when both cool air and precipitation will be in place.  And precipitation will be heavy enough that cooling due to melting may help bring down the snow level.

The cold front will be aided by an approaching weak upper level trough (see 500 hPa weather map, around 18,000 ft)

The UW WRF model (24h amounts ending 4 PM Friday) is forecasting some snow over the lowlands with the fron, with LOADS of snow in the Cascades (up to a foot)

Then the next day brings the main upper trough swinging through, a trough that would cause upward motion and some precipitation (snow).  4 AM Saturday morning is shown.

The surface chart indicates that the air would be plenty cold for snow...the question is precipitation.

Here is the 24h snowfall ending 4 PM Saturday. Light stuff in the lowlands EXCEPT for heavier snow on the NE side of the Olympics as strong northeasterly flow from the Fraser River Valley collides with the Olympics.  You will feel sorry for those poor folks in Sequim and Port Angeles.  No golf.  And several inches in the Cascades.

Snow forecasting is very difficult around here because it is hard to get cold air and moisture together in the right way.  But both ingredients will be available during this period and there is certainly the potential for light lowland snow.

But there is a lot of uncertainty in this forecast.   For the first event, we are on the edge for temperature, but precipitation will be there.  For the second, the temperature will be cool enough, but precipitation is in question.   This is not an unusual situation for snow around here.

Don't forget about the potential for strong northeasterly winds pushing out of the Fraser across the San Juans and then against the Olympics.  Gusts could reach 40-60 mph.  Here are the sustained winds for 1 AM on Saturday morning...35 knots hitting the northern San Juans.  I would not be surprised if some folks there lose power.

And there is something else.   The ground and road surfaces are very warm right now, particularly after a warm fall and the near 60F temps of the last few days.  Seattle wisely put a number of road temperature sensors in a few years ago, what do they show?

 Here is a plot of current air and road temperatures (in the boxes) around Seattle. Yikes!  The road temperatures are in the mid to upper 50s!  The elevated sections will cool relatively quickly 10-20F in a day, but road surface in contact will the ground will cool far more slowly.  Light snow will tend to melt quickly even without any pretreatment.

1 comment:

pacnwwxfan said...

It appears to me that the main threat of accumulating snow in the lowlands is associated with the passage of the upper trof on Saturday morning. It looks like we still maintain a westerly component to the wind flow after initial cold front passage on Friday evening.

While it certainly is possible to see a few flakes mixed in, it might be a tad too warm in the lowlands for meaningful snow given that the airmass is still maritime influenced (overnight Friday into early Saturday). And then the true cold air comes in on Saturday morning in conjunction with trof passage.