Forecasting rain totals in Washington state, particularly around Seattle, can be a challenge due to the topography of the state. On 19 November 2020, while the skies were clear across most of the country, the west side of Washington state instead experienced rain (Fig. 1). The Cascade mountain range, which runs through vertically through the middle of the state, confine most of the showers induced by the moist Pacific air to the western edge of the state. However, the exact location of these showers comes down to an even smaller orographic (aka mountain induced) effect.
Figure 1: Radar Reflectivity at 1351 UTC (5:51 am PST) on 19 November 2020. Source: UW-Madison Department of Atmospheric and Oceanic Sciences
The Olympic mountains, which are smaller than the cascade mountain range in both size and height, are located on the Olympic Peninsula directly west of the city of Seattle. The mountains have differing effects depending on the direction of the wind, resulting in either the Puget Sound convergence zone or the rain shadow effect. When the wind is flowing from the southwest towards the Olympic mountains, it is largely forced upward, cooling and condensing the moist pacific air and causing it to rain on that side of the mountain. As a result, the air that flows down the mountain contains less moisture than the air that was not forced up the mountain, causing a rain shadow (Fig. 2).
Figure 2: Puget Sound Convergence Zone and rain shadow caused by the Olympic Mountains in Washington state. Source: The Seattle Times
When the wind is flowing from the Northwest, it flows around the Olympic Mountains, and converges on the other side. This area of convergence forces the moist air upwards, resulting in a thin line of showers such as the one that can be seen over Puget Sound at 5:51 am PST (Fig. 1). Due to both of these effects, an accurate wind forecast is essential to estimate both the location and total amount of rain that falls. While the initial observations at 1400 UTC (6:00 am PST) on 19 November 2020 show winds with a westerly flow near the Washington coast (Fig. 3a), the wind shifts to become southwesterly six hours later (Fig. 3b).
Figure 3: Wind barbs and pressure contours at a.) 1400 UTC (9:00am PST) and b.) 2000 UTC (3:00pm PST), and the c.) 1-day observed precipitation for 19 November 2020. The red circle highlights the rain totals on the windward side the Olympic mountains, while the pink circle highlights the rain totals due to the convergence zone. Source: NOAA/NWS/SPC
As a result, the majority of the rainfall totals in the Seattle area for 19 November 2020 are due to the convergence zone present just north of Seattle earlier in the day and on the windward side of the Olympic mountains from the wind shift later in the day (Fig. 3c).