rain

1993 vs. 2022

in ,

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.

 

Click to enlarge.

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.

Click to enlarge.

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.

Click to enlarge.

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.

Click to enlarge.

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

 

Click to enlarge.

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.

 

Pacific Northwest autumn 2021 rains...

in ,

Updated: December 1, 8:00 AM PDT

This post has been substantially updated on Wednesday morning, December 1.

  • One chart has been replaced by five (5) charts with further explanations and comparisons.

  • Data has been updated to include precipitation totals through November.

  • Vancouver’s annual precipitation totals have been corrected to state the 1981-2010 30Y average for Vancouver International Airport as 46.8 inches. In the original blog post, it listed the 30Y annual average for Vancouver’s Harbour climate station site. This site has a considerably higher average than the airport.

  • On Figures 2 thru 5 a new reference line has been added for the recently released 1991-2020 climate precipitation normals for Seattle-Tacoma International Airport. As you can see, the most recent 30Y normal reference line show it has been wetter in Seattle in recent decades.

All figures subject to correction and, if required, will be noted.

Click on any chart to expand.

The Pacific Northwest almost always experiences rainy Novembers. This year precipitation rates have been high, if not extraordinary. Heavy flooding occurred in northwest Washington and southern British Columbia in mid-month. Since then, rains have levelled off a little but are accumulating nonetheless. This weekend we are expecting more rain in Seattle. Bellingham and Vancouver will likely see even more.

So how does this start to the rainy season compare to others? The charts below shows several comparisons:

Figure 1 sets the table for this data and subsequent charts and shows the cumulative precipitation traces at Seattle-Tacoma International Airport (SeaTac) for the water years 2002-03 through 2021-22 (to-date). Water years are calendar constructs which help visualize the wet season precipitation totals vs. dry season in the Pacific Northwest. Typically a water year begins on October 1 of a given year and runs through September 30 of the following year. Looking at cumulative precipitation traces in this time frame usually show distinct wet / dry season patterns.

Figure 1 also highlights the 2006-07 water year. Seattle experienced an extraordinary level of precipitation in November 2006, the most of any traces for SeaTac in this set of data. The blue box in this and subsequent charts highlights for comparison the level of precipitation versus the first 61 days of the 2006-07 water year and corresponds to the months of October and November.

Figure 1. Record-setting Oct-Nov at Seattle-Tacoma International Airport, 2006.

Figure 2 highlights the amount of precipitation SeaTac has received so far this water year. As mentioned in the updates, the 1991-2020 climate normal reference trace has been added to this and subsequent charts.

Figure 2. Seattle-Tacoma International Airport cumulative precipitation for Oct-Nov, 2021.

Figure 3 shows the accumulated precipitation trace (red) for Bellingham International Airport. Bellingham is approximately 90 miles north of Seattle and was hard hit by the mid-November flood.

Figure 3. Oct-Nov accumulated precipitation for Bellingham International Airport compared to SeaTac totals, 2021.

Figure 4 shows the accumulated precipitation (blue) trace for the Vancouver International Airport weather station. Vancouver is approximately 50 miles north of Bellingham. In the last few days of November, Bellingham has seen more rain than at Vancouver’s airport climate station.

Figure 4. Vancouver International Airport and Bellingham International Airport cumulative precipitation totals for Oct-Nov, 2021.

Figure 5 compares the accumulated precipitation at Vancouver’s airport in it southwest corner to its Harbour climate station located near Stanley Park. This station is near the core of downtown Vancouver. The station at Vancouver’s Harbour has seen far more rain this water year than at the airport.

Figure 5. Vancouver International Airport and Vancouver Harbour precipitation totals for Oct-Nov, 2021.

Preliminary observations:

  • Seattle and Bellingham have had similar precipitation totals through this period though they arrived at this point in slightly different manners. Over the past 48-96 hours Bellingham has received substantially more rain than Seattle.

  • The precipitation totals at Vancouver International Airport for October-November are similar to Bellingham’s.

  • The precipitation totals witnessed at Vancouver’s Harbour climate station far exceed totals in the Washington cities and at Vancouver’s airport. I don’t know if this level of disparity is typical. However, with regards to the comparison with Washington cities, Vancouver typically sees approximately 46.8 inches of rain per year; Seattle sees – with the new 30 year climate normals – about 39.3 inches of precipitation per year.

Sources:

[1] NOAA / NWS Climate\Observed Weather: https://www.weather.gov/wrh/climate?wfo=sew for SeaTac and Bellingham data.

[2] Canadian Government Historical Climate Data: https://climate.weather.gc.ca/historical_data/search_historic_data_e.html

 

ABOUT THIS BLOG / 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.

A winter to forget...

in ,

The past year (and then some)

This is my final post on this cold snap / winter topic. First, I think the weather is changing and winter really is exiting. Second, as a subject, it has become a little tired. So, here goes. 

I suspect the extended coolish pattern we’ve experienced over the past three months has finally had its run. I’m not a meteorologist nor a climate scientist. Just someone working on my writing and data presentation skills. It just seems that we’re approaching the end of a pattern. The longer term forecasts seem to point towards more normal temperatures.

No doubt, we will still get cooler-than-normal days. Just as we’ll get warmer-than-normal days. But I think the persistent patterns are breaking down. Looking at those cooler-than-normal patterns on the right of Figure 1 shows that up until about March 9, we were still seeing strings of continuous cool days. However, the last few days have been mild, and the temperature deficits have been shrinking since about early January.

A couple of things seem clear from Figure 1: 

  • This coldish spell was persistent and long — almost 3-½ months long (with a few brief interruptions);
  • it was preceded by at least 11 months of predominantly warmer-than-normal weather; and
  • the change from one state to the other was quite abrupt in early December. It was as though a switch was flipped.

Figure 1. Temperature departures for the past 15 months. Click to enlarge.

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Temperature Deviations - A longer comparison (2000-2017)

Figure 2 shows similar temperature deviation data to Figure 1, except it covers a much longer stretch of time backwards. The red zones depict the approximate stretch of winter (Dec-Feb) for each annual period.

I mentioned in the section above that prior to the recent three month cold snap, Seattle experienced 11 months of warmer-than-normal temperatures. Well, looking at a longer timescale backwards, the stretch of warmer weather goes back at least about three years, aside from occasional brief spikes of temperatures in the opposite direction.

For most of this period, specifically the dozen or so years extending from 2000 through 2012, the daily differences from normal temp appear to be more evenly distributed between short warmer and cooler periods. The year 2011 appears to have a long stretch of cooler temperatures — more in the early spring-to-fall time frame rather than winter. And the two year period of 2003-04 appeared warmer than normal in winter as well. I recall during this time thinking to myself that after 20 years, I was finally becoming accustomed to cool, rainy Northwest winters and they weren’t as bad as they previously seemed to me. It turns out, I may had just been experiencing a milder series of back-to-back winters.

Figure 2. Comparison of temperature deviations over time (2000-2017). Click to enlarge.

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What about the rain

The temperature discussion above focuses on the traditional winter months of December through February. In Seattle, our average winter day temperatures don’t differ much than late fall or early spring temperatures. We have mild winters, mostly and coolish springs. In fact, I’ve always considered the period between mid-October to mid-April as one long winterspring season. This time period nearly coincides with the first half of the west coast meteorological water year which typically runs from October 1 to September 30 each year. Seattle, and much of the northern West Coast has a wet season and a dry season. So I’ll use the Oct-Sep water year as a time period to discuss this winter in terms of precipitation.

Figure 3 shows the accumulated precipitation since the beginning of the water year. This year’s line is the blue dashed line. The orange line represents the climatic averages from the years 1981-2010 from SeaTac International Airport. Clearly, we are experiencing a very wet rainy season. So far — and the rainy season for this year is not over  — we have had about 11 more inches of rain than normal since October 1 of last year.

Figure 3. Cumulative precipitation since October 1, present year and historical. Click to enlarge.

So we're having an extraordinarily wet year. Big deal — some years are wet; some are dry. However, are these wet years becoming more frequent? I don't know if we have enough data to establish this claim for the long run, but recent years seem to be.

Figure 4 shows more recent years in the same chart as Figure 3. Last year, the 2015-16 water year, is the heavy black line. Last year was also an exceptionally moist year, the wettest of the previous 14 years shown. The medium gray lines reflect the water years: 2014-15; 2013-14; 2012-13 and 2011-12, the years precluded from the 1981-2010 average. The light grey lines represent the nine preceding years before 2011-12 since I have the data handy and they give some balance to the discussion.

The chart in Figure 4 illustrates:

  • Though it has been very wet since October and we are approach last year’s surge of rain, it is unlikely we will catch it. We are currently about 5 inches behind and are slowly approaching our drier months. Back-to-back years of this flow of water lends understanding as to why many are griping about this winter;
  • This year’s totals are in a tight race for second place of the wettest years of the past 15 years. Today, we are about even with 2006-07, which produced a notably wet start to winter [1];
  • Of the six (6) years since the 1981-2010 climate reference period, three are near the reference average and three are well above it (assuming the 2016/17 water year keeps accumulating precipitation at even the climatic average rate). Simply put, in an annual accounting of rain, we have been in a pretty wet period since 2010.

Figure 4. Cumulative precipitation based on the water year calendar, multi-years. Click to enlarge

So, what happens if you take the averages of the most recent past six water years, not included in the climatic data range, and compare them to the historical averages. Keep in mind that the 2016/17 water year is not even half way completed. This is shown in Figure 5. There has been a significant bump in the amount of precipitation we've seen recently. From the chart, it appears most of this bump has occurred in the months of October, February and March. September also shows an uptick in precipitation as well, but not as pronounced as those three. One could also make the case that summers are slightly drier (seen by the flattening of the blue curve in comparison to the orange one for these months).

Figure 5. Comparing the past six years to the climatic average. Click to enlarge.

 

Seattle has received over 246 inches of rain since October 1, 2011. Climatically, Seattle would be projected to receives about 214 inches of rain during this period. We've received nearly 34 inches of additional rain in the past six years. Table 1 illustrates this clearly.

This is almost an entire year's worth of additional rain squeezed in those six years. Much of that additional precipitation has occurred in the past two years.

Actually, it is worse than that. I included 2011-12 in Table 1 since it is not included in the 1981-2010 historical averages. I wanted to include all recent years not included in the historical average in an exploration to see where the data took me. 

Since Table 1 shows the 2011-12 water year to match essentially the historical average for the period, you can leave that year out of the discussion. Our near additional year of rain has been squeezed into the past five years. And — again — this water year is not half over yet.

In this context, one may like the rain but should also understand the weariness people are experiencing and the accompanying complaints.

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Shades of gray

Aside from rain, the most common characteristic of Seattle winters (and wintersprings) is the near constant cloud cover. So, how did we stack up with regards to cloud cover? I’ll use the first half period of the water year for the time scale, since this is traditionally when the clouds roll in.

We began the water year with one of our rainiest Octobers. This was followed by a normal November, typically our rainiest month. So that in itself a sign that it might be shaping up for a pretty dismal winter. However, if you look at Figure 3, the rains softened in December. This can be seen by looking at the slope of the blue curve in December. Further, the last half of January saw little rain. In fact, December and January were cold, but not too rainy. If the skies were not completely clear, we experienced some sun during those months. More than usual, I’m guessing, without looking deep into the data. 

The rains returned in February at an accelerated rate. February 2017 was one of our wettest Februarys. The month of March, to date, has been dismal as well.

The National Weather Service gives a rating for sky cover for each day based on an 11-point scale, going from 0-10. In maybe an oversimplification, a day rated 0 is clear with no clouds. A day rating of 10 has 100% cloud cover. We can sum the scores for each day since October to see how this year compares to other years.

Figure 6 shows this. So far, this water year has been cloudier than all of the previous 10 annual periods except for one. But, with this chart it doesn't appear to be much.

Figure 6. Seattle Cumulative Sky Cover ratings, 2016-17 water year. Click to enlarge.

Figure 7 illustrates this method of comparison with a close-up of the data shown in Figure 4. So far, through March 10, SeaTac Airport has accumulated about 50 more sky cover points than the average for previous 10 years. These additional sky points don't buy you an upgrade; only a slight downgrade, unless you love clouds.

However, spread evenly, 50 sky points divide by 161 days (Oct 1 to Mar 10) account for only about .3 sky points per day. Or, another way of thinking things, the maximum sky cover points any day can have is 10. Divide 50 sky points by 10 points per really cloudy day and we have had roughly 5 extra really cloudy days more than average during this time period.

Figure 7. Cumulative Sky Cover rating points for annual periods running from October 1 through September 30. Click to enlarge.

A simpler way of observing the cloudiness of the past 5-½ months is to look at the total number of cloudy days in a table and compare this year’s numbers to past years. Table 2 shows  this comparison. 

To date, Seattle had about a 68% chance of seeing an all cloudy day this past wet season. Normally, in recent years, this would be a 64% chance. This winter, Seattle also had about an 8% chance of seeing a pretty clear day. In the recent past, we would have a 12.5% chance of seeing a reasonably clear day. The chances of a partly cloudy day are essentially the same for this year and the recent past.

So, yeah. It’s been a bit cloudier than normal; though not that much more. Still, every little bit adds up.

 

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Final thoughts...

Yeah, this winter in the Pacific Northwest has been pretty dismal. More dismal than normal. More dismal than most. The coldish streak was a bit long — I grew very weary of it. But the low temperatures were not that cold. Consider the Midwest or Northeast in any normal winter. Our coolish winter temperatures this year were still pretty moderate in comparison.

We had snow a couple of times. Deep enough for some fun and some nice photos. Not too much to cause trouble for commuting in the lowlands, though the mountain passes got hit hard at times.

This area doesn’t seem to suffer too often from daily extreme temperatures, humidities, or even heavy rains. We get strong winds blown in off the North Pacific occasionally, but we don’t see hurricanes or tornados often. 

Where we see extremes, it seems to me, is that we settle into long, long patterns, whether long heat streaks; long coldish streaks, long rainy streaks, and — for me the most dreaded — endless cloudy stretches. 

This winter combined most of those long streaks. I’m happy to see it ending and am looking forward to longer days, brighter skies and warmer temperatures. 

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References:
[1] City That Takes Rain in Stride Puts on Hip Boots; Yardley, Wm.; New York Times; November 27, 2006.
[2] Data Sources NOAA NWS Seattle Local Climate site (click on text for link).

Charts and Graphics Notes:
Data from NOAA/NWS Climate site was post-processed in Microsoft Excel for Mac 2016.

Figures 1-7 were produced using DataGraph 4.2.1 software from Visual Data Tools, Inc. and available on the Apple App Store. A terrific product that keeps improving — support good engineering and design.

Base charts were copied to Adobe Illustrator CS6 for post process refinement and scaling.

Tables 1 and 2 were developed in Adobe InDesign CS6.