What month of the year is the wettest? Why it is this month - November.
Of all 12 months in the year, November is the wettest throughout Western Washington. It may seem like December would be the wettest since fall ends and winter begins, but November is number one.
Here are some Western Washington 30-year average November and December precipitation statistics:
Dec. (in inches)
November is also the number one month for frequency of flooding in Western Washington. Some of our biggest flooding events on record have occurred in November.
In 1990, Interstate-5 went underwater in the Centralia/Chehalis area for the first time, and in 1995 which was the second time for I-5 in Lewis County.
In 2006, Mt. Rainier National Park suffered substantial damage with recovery still underway today.
In all these cases, there was little or no snow in the mountains. Heavy amounts of rain drive flooding in Western Washington.
Why is November the wettest month? There are a number of reasons:
- The Pacific jet stream tends to spend more time at our latitude during the month.
- The air mass is still relatively warm versus December, meaning the air mass can hold more moisture and hence it rains more when weather systems embedded in the jet stream move ashore.
- With Western Pacific tropical storms more frequent in the month, such as the super typhoon that pounded the Philippines last week, more subtropical moisture can be involved in weather systems that cross the Pacific and move onshore, producing heavier rain amounts.
So have your umbrella handy during this month. It will slowly start getting drier next month.