Forecasting Snow Amounts In A Maritime Environment,
With Diverse Topography


The Fall and Winter seasons are usually a cool, cloudy and damp time for the Seattle area, but this past Fall/Winter the weather was cold, unusually cold and snowy. A particularly cold day in December provides the perfect scenario of the 3 major factors that all make forecasting snow and snow amounts difficult in Seattle and Western Washington. The three factors are Topography, a Maritime Climate, and the Puget Sound Convergence Zone.

On a given December day, a meteorologist in Seattle can expect to have cool and more than likely damp conditions, however this day is a bit different. A modified-artic air mass moved in the night before, as a dry-cold front pushed its most Western grip of cold air into the region. An artic front is one of the two major ways the area can see snow. The 1000-500mb thickness charts show heights across the region are going to drop to the 522gpm line, which is well below the "standard" mark for the rain/snow line. The front from the night before helped to clear the skies out, so conditions will be very cold in the morning. However, the winds should be westerly today which usually means a forecaster can count on the onshore flow from the Pacific to help keep the temperatures up enough to avoid snow. A short wave trough is going to move through in the morning, well before the day begins to slowly "warm" up. All conditions point to snow, but that is very unusual for the area, and the amount of snow is critical. What should a forecaster do from here?

Forecasting snow and snow amounts is never an easy task for a meteorologist, no matter what the conditions are. Even in areas where snow is part of the climate, it can be a tough forecast to nail based on tropospheric conditions and the amount of climactic and topographic variability over your forecast region. It can become even harder to forecast snow amounts when the area is a normally mild Maritime Environment like Western Washington, and Seattle. Throw the extremely diverse topography into the mix, and you've got a tricky forecast on your hands. The Pacific Northwest has it all and because of that terrain will see it all.

Heavy snow in Western Washington is not a common occurrence, however it can and does happen. Seattle for example on February 1st and 2nd in 1916 had 21.3 inches of snow in a 24-hour period with a total storm accumulation of 29 inches. Other areas in Western Washington saw between two and three feet with snow drifts has high as four to five feet. (Mass, Ferber Lackmann, Patnoe 1992) Overall on average, Seattle will see roughly two snow events a year resulting in two or more inches. Here again there is much variability, ranging from 64" in 1968, to nothing to a trace for a majority of the years between 1945-1990. (Mass, Ferber Lackmann, Patnoe 1992) Variability is the key because of the topographic and climactic differences within the region. The Maritime air, the regions rich Topography, and what is known as the Puget Sound Convergence Zone all play a major role in why we see or don't see snow.

Meteorologist Rich Marriott is a former Avalanche and Mountain Weather Forecaster/NWS Forecaster for the Northwest Avalanche Center, and for the past 19-years has been a main Meteorologist at KING5-TV in Seattle. Marriott says forecasting snow and winter weather in Western Washington is a true challenge, and "the weather in the Northwest can be just plain annoying, and sometimes it can make Kansas look good!" Marriott says one of the biggest forecasting challenges with snow and snow amounts, comes from the Maritime Climate. In general the 540gpm line is the height line that can represent the rain/snow line. Throughout the Northwest, it is usually as low as the 522gpm line. The average daily temperature in Seattle doesn't drop below freezing, so it takes a cold weather event to get the temperatures down enough for snow. Even then, if there is enough cold air for snow to fall, the relatively warm water can keep much of the area from seeing anything but rain.

The Second of the two major ways the area can see snow is with a cold front, and very cold onshore flow. A cold enough flow will usher in Maritime air that will force the relatively mild air usually in place up. When that happens the area can see snow, but the snow events are usually not uniform even over a short distance. The influence of the water and the already mild air in place means snow can fall over one neighborhood, and literally a few blocks closer to the water residents will only see rain. That Marriott says can make it very hard to forecast snow, and pinning down the exact snow amount for specific areas can be in his words "nearly impossible."

Marriott says the biggest forecasting challenge is dealing with the regions topography. As a shortwave or cold front comes through Western Washington, the system will deal with many topographic features, all of which will make the precipitation outcome vastly different across the area. One area in the Convergence Zone (discussed below) could see .08" of precipitation, and another on a local rural hillside could see .02". Depending on temperatures, that could mean the difference between 2" and 8" of snow, and there is no method for pinpointing that. The state itself has two different mountain ranges the Olympics, and the Cascades. Both of which contribute to widespread rain shadow effects, East of the Cascades and in the city of Seattle itself. To add to that, the region has even smaller scale topography that can influence, just like the major ranges, the amount of moisture precipitated from a storm. The city of Renton is just East of Seattle. During one of the snow storms this past winter, small scale orographic lifting on regular city hills helped dump 3" of snow in the city, while 2 miles away near the water at a marginally lower elevation only rain fell. After taking all of the different topographic features into consideration, another mesoscale feature comes into play, and that's the Puget Sound Convergence Zone.

Marriott says synoptic scale models in general, do not have a good handle on mesoscale snow events like the events described here. The models have an enough tougher time pinpointing the convergence zone. The PSCZ happens when low level coastal winds are from the northwest, producing a low level trough and convergence zone in the lee of the Olympic Mountains. (Mass, Ferber Lackmann, Patnoe 1992) The winds funnel around the Olympics and pinpoint an area from Downtown Seattle, Northward to the city of Everett about 30 miles North. The PSCZ can produce or enhance low elevation snowfall over the Sound, even in conditions when temperatures aren't cold enough for snow. The PSCZ can produce showers heavy enough to encourage greater diabatic cooling due to melting and evaporative cooling to bring the snow level down to the surface. (Mass, Ferber Lackmann, Patnoe 1992) For example, on April 2nd of this year just a week before this paper was written, the PSCZ caused yet another snow event above 2" in an environment where the forecast only called for a small wet mix of rain/snow. The city of Bellevue, also just East of Seattle, saw 4.5 inches of snow as a result of heavy showers and diabatic cooling, caused by the Puget Sound Convergence Zone. Marriott says there have been multiple rain and snow events, where the models completely missed the PSCZ, and says "at one point the Convergence Zone gave us six inches of snow, and we didn't even know it was happening." The bottom line is that the region is tough to forecast how much snow and where snow is going to fall, but there are some ways to make it easier.

People who live in Seattle are use to a relatively warm Maritime climate, and are not use to the snow. Even just a couple of inches can cause major traffic, commercial and educational problems. Most school districts in Seattle won't take the chance when snow falls, and school is canceled. That's why Marriot says it's so crucial to get the forecast right. He says the synoptic scale models do a great job at giving the overall pattern, but in such a mesoscale climate, a mesoscale model like the MM5 through the University of Washington is the best bet. He also says Skew-T soundings and forecast soundings are critical for the profile of the atmosphere now and in the future. Forecast soundings are especially important when it comes to the timing of artic fronts.

Most importantly, it is crucial to know the area. Understand that in a post frontal weather pattern, especially with Northwesterly winds, the Puget Sound Convergence Zone is a real possibility. Even with cold High pressure cells from Canada, the Northwesterly flow can change a potential rain/snow mix event into a major snow event for a relatively large area. The last and best piece of advice is to understand which areas have the potential, like the higher hills, to see the most snow and which areas have the potential to see the least. There is no precise science to forecast exact amounts, but understanding the region will play a big role getting a correct forecast.


"Snow Storms Over The Puget Sound Lowlands" Garth K. Ferber, Clifford F. Mass, Gary M. Lackmann, and Michael W. Patnoe 1992.

Rich Marriott: Former Avalanche & Mountain Weather Forecaster for the Northwest Avalanche Center/NWS. Current Meteorologist for KING5-TV Seattle.