clouds — Seattle's Past and Present Weather and Climate in Charts and Graphics

Precipitation data updated on June 30. This is the final update to this post.

The spring and early summer of 1993 was memorably wet for some of us who have called Seattle home for many years. That was the one year where I belonged in a van pool to go to and from work. each day. One dreary morning following a series of similar days in June or July, several people in the van were complaining about the weather. I can’t recall if I was one of them or just in my mind agreeing with that sentiment. I do remember a fellow vanpooler and native of Seattle chastising the complainers and suggesting this weather was normal for Seattle and telling us if we didn’t care for it, we should move. I had mixed feelings but did not completely disagree with her. Nevertheless, that was my 10th summer in the city, and I hadn’t recalled any of the others being so miserable weatherwise.

We appear to be experiencing a similar wet and cool spring and early summer in 2022. It has brought up memories of that gray early summer of 1993. I was just speaking to a neighbor yesterday about the very thing though she hadn’t lived in Seattle in 1993. What triggered my discussion was looking at the daily rain total from the day before and seeing we had set a record for that June day. The previous record was in place since 1993.

So how does this wet spring and early summer compare to a similar period from 1993? In all honesty, I didn’t know if 1993 was our wettest spring since 1984, my first year in Seattle, let alone for the recorded history of spring weather at Seattle-Tacoma International Airport (KSEA) going back to the 1940s. But it was, for me, a memorable wet period, as is this spring, so let’s compare.

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Precipitation

Figure 1 shows the cumulative precipitation traces for calendar periods starting on October 1 of one year through September 30 of the following year recorded at SeaTac airport. This period is often considered the water year on the U.S. west coast due to our seasonal weather patterns of a strong wet season usually followed by a strong dry season. The midpoint of this multi-calendar year span is April 1 which roughly represents the transition period from wet-to-dry seasons.

Figure 1. Cumulative precipitation traces for water years 1948-49 through 2021-22 at Seatlle-Tacoma International Airport (KSEA).

I’ve highlighted two specific annual traces: the 1992-93 and the 2021-22 water years. The first obvious thing one notices is that since October 1 of last year, this has been one of Seattle’s wettest periods. It is in the top five wettest water years so far. Ironically, two of the other 74 traces in the top five category are also recent water years, 2015-16 (#3) and 2016-17 (#1).

A second obvious feature is that in the first six months of water years 1992-93 and 2021-22 leading up to April, it has been far wetter this water year (2021-22) than 1992-93. On April 1, SeaTac Airport had already seen 35.8 inches of rain during the previous six months. On April 1, 1993, SeaTac Airport had seen 21.35 inches of rain in the previous six months. This is a difference of 14.45 inches, or more than two average November’s worth of rain. Memory plays tricks on us, but I’d argue that up to June 10, this water year has been much drearier than 1993.

There are two boxes on Figure 1. These boxes encapsulate the period between April1 – June 10 for each trace being compared, 1993 (red) and 2022 (blue). The total amount of precipitation for each period is similar, 8.94 inches and 8.83 inches respectively. The typical Seattle summer dry spell in 1993 began on or around July 30. It remains to be seen when this will occur in 2022.

Figure 2 shows the same data as Figure 1, cumulative precipitation at SeaTac for the years 1948-2022 except the it shows precipitation for the months of April through August. The values have been zeroed to March 31 for comparison purposes. Plotting this shows just how wet this spring has been. Aside from 1948, 1993 and 2022 have been two of the wettest springs recorded at SeaTac Airport. The spring of 1993 ended up being the second wettest spring recorded to-date at SeaTac. The spring of 2022 ended up being the fifth wettest.

Figure 2. Cumulative precipitation since April 1 at Seattle-Tacoma International Airport (KSEA) for the years 1948 through 2022.

Figure 3 is a dot plot which ranks the precipitation periods from April 1 to Jun 17 and is analogous to Figure 2 showing the values for all traces between 1948 to 2022. In mid-June, the springs of 1993 and 2022 were virtually tied for 2nd place. But the latter half of June 2022 was relatively dry and the final tallly of April-June precipitation for 2022 dropped the year to fifth place on Figure 3. June 1993 ended up having .68” inches more of precipitation than June 2022.

Five of the top ten wettest years for this April-Jun period have occurred since 2010. At mid-month it was six. The 2017 period dropped to #12 on the list by the end of the month.

Figure 3. Ranking of spring-summer precipitation at Seattle-Tacoma International Airport (KSEA).

Figure 4 shows the same data displayed in Figure 3 but lays the data out in chronological order to see if any trends over time are evident. Any trending appears to be mild, but there appears to be a slight increase in spring and early summer precipitation in recent years.

Figure 4. Cumulative spring-summer precipitation trends, April 1 through June 30, KSEA.

Temperatures

This spring has been much cooler than normal since April. How do the temperatures of Spring 2022 compare to Spring 1993?

Taking a look back at April through June daily average temperature departures from normal (30Y average for 1961-90) shows that spring and early summer was normal to slightly warmer than normal. The average April daily temperature was about 1˚F warmer than 30Y normals. The average May daily temperature was about 4˚F warmer than long-term normals. And the average June daily temperature was just about normal.

Contrast this with this spring and early summer (to-date). This year at SeaTac airport, the average daily temperature in April was -4˚F, the average May daily temperature was -4.6˚F, and the average June daily temperature (to June 17) is -2.4˚F. These are averages across an entire month. Of course there were nice days during these periods, even some warm one, but in general it has been a very cool spring and early summer. These differences are shown in Table 1.

Table 1. Monthly Average Temperature Deviation from 30-Year Normals

Figure 5 shows the daily temperature departures for the period from April 1 to June 30 for 1993. A 7-day average line plot is included.

Figure 6 shows the same data for this year through the most recent data point.

Figure 5. Daily temperature departures for April through June 30, 1993.

Figure 6. Daily temperature departures for April through June 18, 2022.

Looking back, spring and early summer of both 1993 and 2022 were wet and gloomy. Some people like the rain and clouds. Come spring and summer I’m ready for light, increased sun, sunrises and sunsets, and warmer temperatures to separate from our normal 6 month wet and cool winterspring climate.

But this spring and early summer has been much cooler than those of 1993. However, the current spring / early summer did get a boost of warmth in the last two weeks of June.

It’s also been notably darker in terms of solar radiation compared to recent years (I don’t have similar data for 1993).

Based on this, barring any sudden change in weather for the last two weeks of June, I’d have to say this year takes the cake for weather gloominess.

Updated: December 29, 2021, 6:30 AM PDT

Figure 1 updated with most recent data.
Figure 2 has been updated. Note that the minimal value on the x-axis begins at 320 MJ/sq.meter. Figure 3 has been updated. Note that the minimal value on the y-axis begins at 300 MJ/sq.meter. Figure 4 has been added to show the daily temperature departure for 2021 to date.
Figure 5 has been added to show the daily temperature departures since 2000 at SeaTac airport.

If it seems like it has been pretty dark and rainy this autumn and early winter in Seattle you would be right. I’ve already posted about the high level of precipitation we’ve seen in Seattle and the extended Western Washington area since the start of the 2021-22 water year on October 1. The areas surrounding Bellingham and Vancouver B.C. have been especially hit hard with rain.

But it has seemed very dark in recent months as well, and by dark, I mean low levels of light. Of course all this rain comes in leaden skies. And looking at solar radiation data collected near Husky Stadium and Union Bay (47.66, 122.29) in Seattle confirms this has been the darkest start to winter in the past ten. Washington State University operates a solar collection station in this area and exposes the processed data to the public on their web site. A link to this data is listed at the bottom of this page.

Figure 1 shows the cumulative daily totals of solar radiation (MJ/sq.meter) from October 1 through December 31 for 2013 through the present year. At this writing we are only partly through December this year. But it is clear that this year has been quite dark compared to the other years. Lower traces indicate lower levels of solar energy measured. This year’s trend line is the heavy line. The Seattle area began with a normal October, but around mid-month the rate of cumulative solar energy slowed considerably.

I choose a start date of October 1 since this is the traditional start date of the Northwest water year, the time of year when storms begin to blow off the Pacific with increasing frequency. This date provides a good start or “zero point” for the start of the wet season.

**Figure 1.** Solar radiation measured at Seattle’s Union Bay data collection site.

Figure 2 was added on December 15. This figure is a dot plot showing the total cumulative solar radiation received at WSU’s Solar Station in Seattle. The dot plot is sorted by total solar daily accumulation for the stated period.

This year’s accumulation of sunlight since the start of October is marked by the black dot in the lower left corner. It is substantially lower than the sunnier years at winter’s start. It is substantially lower than the median value for all years shown. The median value is shown by the dash vertical line.

Looking at the years’ positions on the y-axis, there does not appear to be any pattern to the order for the very limited number of years for which data exists.

**Figure 2.** Dot plot showing cumulative solar radiation sorted by year.

Figure 3 essentially shows the same data as Figure 2. In this bar chart, the years are in order along the X-axis (horizontal). It’s a little easier for the mind to see the lack of any pattern by time in this small sample size – we are more often used to seeing date-format data along the horizontal axis, increasing in time from left-to-right. It’s also clear to see how low sun levels have been this Oct-Dec period compared to recent years. All years show data through the date in the title.

**Figure 3**. Cumulative Solar Radiation, Oct 1 through December, sorted by years, 2012-21.

In addition to lower solar radiation levels in Seattle since roughly mid-October, it has been a relatively cool autumn and start to winter. September had mare days that were cooler than normal than days warmer than normal. Same for October. November was pretty mild with a series of atmospheric rivers coming in from the tropics. Those brought warmish, moist air with them. And plenty of flooding in parts of the Pacific Northwest and southern British Columbia. December has been relatively colder than most recent Decembers. This can be seen in the large number of days where average daily temperatures have been below 30Y climatic normals.

Figure 4 shows the daily temperature departures for 2021 up through the most recent date.

**Figure 4.** Daily Average Temperature: Departure from 30Y normals. Click to enlarge.

Figure 5 shows similar data as Figure 4, daily average temperature departures from 30-year normals going back to January 1, 2000. It appears in recent years, the trend is back towards the climatic normals of the past 30 years after a warming period for several years in the mid-2020s.

The 30Y normal daily average temperature reference values for all years except 2021 are based on the years 1981-2010. In 2021, the comparison is against the normal daily temperature range from 1991-2020. It’s possible that the 1991-2020 30Y normals cycles have increased from the previous 1981-2010 30Y cycle and this year’s lower trend is simply reflecting that change - departures may be comparing against a high reference line..

This data is for the National Weather Services’s Seattle-Tacoma International Airport site.

**Figure 5.** Daily normal temperature departures for SeaTac airport from 2000 to present. Click to enlarge.

SOURCE DATA

Washington State University AgWeatherNet: http://weather.wsu.edu

NOAA/NWS Climate Data Seattle/Tacoma https://www.weather.gov/wrh/climate?wfo=sew

FULL DISCLOSURE

I am not a meteorologist, climate scientist, data scientist, geologist nor hydrologist. I am simply a (retired) engineer who has some familiarity with numbers, basic statistics and probability statistics who enjoys looking at readily available public data and trying to make sense of things. I enjoy building data visualizations from data I find much like others enjoy working daily crosswords or sudoku puzzles. Local weather, climate and hydrology science are complex subjects. Take what you read and find here with this in context.

1993 vs. 2022