Late-Season Snow Hits The Pacific Northwest

I’m back! My sincere apologies for being AWOL the last month. I have, however, been working hard on WeatherTogether’s weather models, though I’m having some connectivity issues uploading them to the site since I’ve reformatted my code to download more, but smaller grib files for my plots (grib files are the files with the raw model data). The plots will still upload but often don’t finish to completion for the entire model run. I hope to get that sorted out in the next week or so… I’ll keep you updated.

Anyway, we saw some late-season snowflakes around the region yesterday and early this morning due to heavy showers spinning off a very cold upper-level trough. Because air temperatures were in the mid 30s and surface temperatures were in the 40s, most accumulations were fleeting and snow began melting soon after the shower ended, but it was still fun to see some post-spring-equinox flakes in the air.

Our setup was classic for late-season snow, with a deep and extremely cold upper-level trough directing cold, moist, air into the area. The surface is rapidly warming by this time of the year, but because the atmosphere is much more transparent to incoming solar radiation, it can keep its wintertime chill past the spring equinox, especially at upper levels. In fact, the 500 mb temperatures (shown below) over the area that were associated with this trough were some of, if not the, coldest 500mb temperatures of the year.

The cold upper-level trough over the area at 5 am 3/24/2018 as modeled by the UW 12Z WRF-GFS model. Graphic shows the 500 mb temperatures, heights, and winds.
Credit: UW Atmospheric Sciences

In a scenario like this where the upper atmosphere is extremely cold while the surface is only “chilly,” the atmosphere is turbulent and ‘unstable.’ This is because air parcels cool at a specified rate as they rise (9.8^oC/km if the air is dry, 6.5^oC/km if it is saturated), but if the atmosphere cools at a faster rate than these specified rates, then rising air parcels remain warmer (and therefore less dense and buoyant) compared to their environment and continue rising. The result is convection in the form of cumuliform clouds, which can include heavy showers and even weak thunderstorms.

Yesterday evening’s 00Z sounding (aka: weather balloon) from Quillayute on the Washington Coast shows these “lapse rates” between 8 and 8.5 ^oC/km for the lower to mid portions of the atmosphere, meaning that saturated parcels are buoyant. Based on the sounding climatology at Quillayute, these values barely fell short of breaking the records for this date. Not surprisingly, late March, on average, has the highest low-level lapse rates of the entire year for the Pacific Northwest due to mild surface temperatures while upper-level temperatures remain cold.

Sunday/Next Week

No lowland snow flurries are in the forecast for the foreseeable future, as a ridge over the Northeast Pacific will be close enough to prevent any upper-level troughs from dipping down into the region.

However, a moist system will ride over the top of this ridge on Monday and bring an extended period of lowland rain and mountain snow to Western Washington and Northwest Oregon from Monday morning to Tuesday night. 48-hour precipitation totals from Sunday evening to Tuesday evening are extremely impressive for this time of the year, with parts of the Washington Cascades potentially receiving over 5 inches!

Freezing levels should start near 1500 feet Sunday evening and will only rise to 3000 feet by Monday evening. By Tuesday evening, freezing levels will be closer to 6,000 feet, so if you were planning on calling in sick to work so you could hit the slopes, make sure you are sick on Monday.

Here’s one more picture of the snow yesterday from Brie Hawkins at Little Bear Creek, Woodinville, WA. Check out her weather page at Little Bear Creek Weather!

Thanks for reading and have a great rest of your weekend!

Charlie