A winter storm blanketed the Pacific Northwest this week and more is expected in the coming days. The National Weather Service (NWS) – Seattle tweeted on February 9th:
6.4″ inch of snow at Sea-Tac yesterday. New daily record and 2nd most in a single day of the past 20 years. Only exceeded by 6.8″ on January 18. 2012. #wawx
Responding to a question about how many times it has snowed on February 8th in Seattle, NWS pointed out that “since records began 6.4″ in 2019, 2.9″ in 2014, 1.0″ in 1916, 0.4″ in 1957 and 0.1″ in 1904.” Seattle averages just under 6 inches of snow during the winter according to SeatteWeatherBlog.com. It snowed while I was at a weather conference in Seattle several years ago, and the city was clearly excited and nervous about it. The stunning views of snow-capped Mt. Rainier may give you the perception that Seattle is a snowy place, but it is not. The weather set-up this week allowed for snow-supporting, cold air in Seattle (graphic below), but there is typically a competing factor that limits snow. What is it?
In order to answer to this question, we have to review some basic concepts that I recently shared with my students at The University of Georgia. The first concept is something called heat capacity. It is a measure of the amount of heat required to raise the temperature of a body or substance by one degree. If this quantity is given in term of unit mass, it is called specific heat. The specific heat or heat capacity of water is very relevant for understanding why Seattle is not as snowy as places further inland or at higher elevations.
The U.S. Geological Survey Water School website has an excellent discussion of the heat capacity of water. It points out that
water has to absorb 4,184 Joules of heat for the temperature of one kilogram of water to increase 1 degree celsius (°C). For comparison sake, it only takes 385 Joules of heat to raise 1 kilogram of copper 1°C.
Such basic characteristics explain why walking on a hot sandy Georgia beach with bare feet may be uncomfortably warm in May even though the water is still quite cool. Sand requires much less heat to warm up than water. The same “physics” also explains why the peak of the Atlantic hurricane season is usually in September even though the seasonal ”starts” June 1st. If your kids start bugging you to go to the pool when it opens on Memorial Day and you know the water is too (personal experience), just hit them with some specific heat formulations.
Due to its heat-retaining properties, water also tends to hold heat longer as the transition from summer to winter occurs. This physics coupled with the geography of Seattle is critical for explaining the relative lack of snow there. According to the emergency management website of the city of Seattle,
Most of the time Seattle’s winter weather is controlled by the Pacific Ocean which remains relatively even in temperature throughout the year. Occasionally, however, cold air from the interior of the continent pushes into the Puget Sound region and causes dramatic cold spells, ice and snow.
In other words, winter weather in Seattle is regulated by its proximity to the Puget Sound and Pacific Ocean. According to Jordan Steele writing for King5.com, the water temperatures of these bodies are typically around 48-52 degrees F (see graphic above for current temperatures in degrees C). This limits the number of sub-freezing days. With the most recent winter storms in the Pacific Northwest, the February 7th NWS discussion indicated an area of low pressure in the Northwest Territories and high pressure in the Yukon area. These conditions allowed sufficiently cold air to move into region offsetting the moderating influence of the nearby water bodies.
Seattle is certainly known for its rainfall, but it is making headlines with some of its highest snowfall totals in over 70 years. Ironically, the snow cover itself amplifies the already cold temperatures. It will be interesting to see how much additional snow falls there in the coming days. While NOAA or National Weather Service is always my preferred location for climatological date, the website at this link has some very useful statistics on the climatology of snow in Seattle.