precipitation

San Francisco's rainy season...

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California is in the midst of a wet and snowy winter. It sounds as if more is due this coming week. Although the copious amounts of rain and snow have wreaked havoc in some places, after several years of drought, this influx of moisture was very much needed.

 Not much in the way of charts to show here on this initial post on California. But I’ll post two charts. The data comes from the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI) site where you can download historical climate data records for most major weather stations in the United States.

 The data shown below is for San Francisco International Airport and ranges from July 1, 1945, through the present. I plotted these charts using a water year calendar. The water year calendar is a convention often used when plotting precipitation or snow data because on the West Coast these are typically cyclic or seasonal events. Most precipitation events fall during 4-6 months of the year beginning in October. This is true up and down the coast, though the precipitation season typically longer in higher latitudes. Therefore, the water year calendar typically runs from October 1 through the following September 30 to encapsulate the data for an entire season, which traditionally spans across two standard calendar year-dates. So, for the current water year, data is included from Oct 1, 2022, through September 30, 2023. This span is referred to the 2023 water-year (WY), as nine months of it exist in 2023.

 I disregarded the early data period from July 1, 1945, to September 30, 1945, since it only represents a small part of the 1945 WY.

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 Figure 1 is called a dot plot and it shows the total accumulation of precipitation for each WY from 1946 to 2023. The current water-year is only partially complete. The data has been sorted showing the rainiest water seasons on top. I’ve highlighted the current 2023 WY in red. I’ve also highlighted the previous nine water seasons in blue. You can see much of the period between 2014 and 2021 were dry years though 2015-16 were close to the mean for total rain. The 2022 and 2019 seasons were also relatively close to the mean for annual precipitation. The 2017 WY, now six years past, was quite wet (for San Francisco). This year is so far close to the mean, but we are only five months into the water-year. But those are the wettest five months.

 
 

Figure 2 is a short animation (~30 sec.) which shows a cumulative precipitation trace across the water year of each year from 1946 to 2023 (through March 2, 2023). It can be seen from this chart that year-to-year, (a) the basic pattern is the same (lots of rain early followed by a long dry spell until October, and (b) the total amount of annual rainfall varies widely. And playing the small animation through there doesn’t appear to be a year-to-year trending pattern other than what was mentioned in the previous paragraph.

Helpful hint: Clicking on the gear wheel in the lower playback element allows you to slow or spead the animation playback rate.

Figure 2. Precipitation traces for San Francisco International Airport, 1946-2023 (current). Traces are displayed one at a time and follow the water year calendar starting on October 1 and ending the following September 30. The water year number takes on the year value for the January to September dates.

On Seattle-Tacoma precipitation traces, Part 1...

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This post will display a history of water-year cumulative precipitation traces from Seattle-Tacoma International Airport. A water-year defined here crosses parts of two calendar years and runs from October 1 of year 1 to September 30 of year 2. For instance, the water year 1972-73 runs from October 1, 1972 to September 30, 1973.

The reasons for using water years is simple. The Puget Sound area can be thought of at a higher level of having two basic seasons: a wet season running from October through March when most of our annual precipitation falls, and a dry season which runs from April through September.

Figure 1 is the baseline chart and shows cumulative precipitation traces at Seattle-Tacoma International Airport from 1948-49 through the start of the 2021-22 water years. The data comes from the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS). These data extracts normally list daily precipitation totals. I’ve summed these values up for each water year.

Figure 1 also shows some other graphic elements. In the lower left corner, a blue box appears highlighting the first two months of the water year. The reasons for this box is simple. We just completed this two month period for the 2021-22 water year. It was a very wet period in Seattle and the Pacific Northwest in general. I placed this box on the chart to allow for easy comparisons of this year’s heavy rains with other years’. As you will see later, this year’s start to the water year was one of the wettest since the late 1940s, but not the wettest.

Two other key elements added to the baseline chart include a 30Y climatic normal reference line for two climate periods. These periods are the 1981-2010 climate normals for precipitation at SeaTac and the 1991-2020 climate normals. These are standard 30 year daily averages published by NOAA/NWS. As you can see, the more recent 30 year normals show a higher level of daily precipitation than the 1981-2010 period, adding up to a little more than 2 inches additional rain per year at SeaTac.

One final observation is that precipitation rates vary widely within each annual trace and between the final annual totals of all traces. The wettest years had roughly 51-52 inches of rain over the course of 12 months, the driest years had less than 25 inches over a 12 month period.

Figure 1. Precipitation traces for each water year from 1948-49 through 2021-22 (to the current date).

I only have two full water-year traces from the 1940s. These are shown in Figure 2. Interestingly, both years began the first two months with similar trace patterns. After about mid-December, each trace line diverges for the remainder of the year, one towards the high side of annual precipitation totals, the other towards the low end. At a glance, and without performing the statistical calculation, it appears as if the final year-end totals span the first or second standard deviations of all year-end precipitation totals.

Figure 2. Cumulative precipitation traces for available SeaTac data in the 1940s.

The 1950 traces in Figure 3 show that the start of the wet seasons varied widely during that decade and that, in general, it was a decade when annual precipitation totals hewed towards current climate normals or were generally higher in precipitation.

Figure 3. Cumulative precipitation traces for available SeaTac data in the 1950s. Tracks in the 1950s trend towards years with greater precipitation.

Figure 4 shows the 1960s traces were distributed about evenly above and below current climate normals and tend to not have extraordinarily wet or dry annual totals.

Figure 4. Cumulative precipitation traces for available SeaTac data in the 1960s. Precipitation tracks in the 1960s hover more closely to 30Y climate tracks about evenly above and below these normals.

Figure 5 shows the 1970s traces. It appears most years in the 1970s began relatively dry. From December on, the annual totals varied widely. This might be the decade with the widest and most uniform spread between very wet and very dry years.

Figure 7. Cumulative precipitation traces for available SeaTac data in the 1970s. Half the 1970’s traces hover close to the climate normals. The remaining traces are biased towards both extremes: wet years or dry years.

Most annual traces in the 1980s hewed close to current climatic normals, though biased on the dry side of those normal. Figure 6 shows a few years were substantially drier by year’s end.

Figure 8. Cumulative precipitation traces for available SeaTac data in the 1980s. The 1980’s traces tend to be biased towards drier than climatic normal years. The also tended to start the years tightly around climatic normals.

The precipitation traces for the 1990s, illustrated in Figure 7, appear to be evenly distributed around the current climatic norms much like the 1960s traces. However, the 1990s traces are dispersed more widely, reaching towards the wettest and driest annual limits for all of the traces. Collectively, the 1960s and 1990s traces in the early part of each year exhibit pretty similar patterns, hewing close to current climatic normals except for one particularly dry autumn in each subset.

Figure 7. Most years began with fairly normal precipitation levels except for perhaps two. By years’ end, the traces were evenly, if broadly, distributed around current climatic normals. This decade saw the most varied ditribution of precipitation totals at the end of the year.

Figure 8 shows the traces for the 2000s. The year end precipitation totals tend not to trend towards extremes and are maybe slightly biased towards drier years. But the start of the wet season (October-November) varied widely for this period. Some of the wettest starts to winter began here and even a drier one as well. Many other decade charts showed must tighter distributions around the current climatic normal lines.

Figure 8. Cumulative precipitation traces for available SeaTac data in the 2000s. Traces appear to show wide autumn variations then trending towards an even distribution around climatic norms at years end.

The 2010s year-end precipitation totals trended towards wetter years. These traces are illustrated in Figure 9. This decade, along with four very wet years in the 1990s might explain why climatic normal curves jumped for the 1991-2020 period. The starts of the water-years in the 2010s also trended towards wetter Octobers and Novembers in most years. The 2010s also hosted some very long, dry summer periods.

Figure 9. Cumulative precipitation traces for available SeaTac data in the 2010s. Trends show wide variation in the autumn months and a bias towards wetter year-end totals.

The 2020s are too new to host many precipitation traces. In fact, we’ve only completed one year of this decade and only started a second water-year trace. In Figure 10, the single, complete year hewed very closely towards climatic normals after a wet late winter and early spring. This was followed by a very long and dry late spring and summer period.

The start of its second year has been very wet.

Figure 10. Cumulative precipitation traces for available SeaTac data in the 2020s. The 2020-21 year followed current climatic normals closely. The current year has begun very wet.

Conclusions and Notes

Looking at the traces, I’m not really sure I’m seeing any convincing trending decade-to-decade. The only long-term trending I’m seeing is between the climatic normal lines where wetter late winters and early springs seem to be trending. But these two lines are drawn from only two sets of averaged daily data. I would be more convinced if other 30-year climatic normal periods were plotted here, say for the 1951-80, 1961-90 and 1971-2000 periods. I may do so for Part 2 of this blog entry.

Sure, the 2010s appear to be quite wet. But that decade was preceded by several decades of precipitation traces distributed pretty evenly around the climatic normal curves. And the 1970s and 80s trended towards the drier side of the distributions. They, in turn, were preceded by a pretty normal series of plots in the 1960s and, again, wetter trending in the 1950s. Who knows what’s in store for this decade. It’s way too soon to tell.

And this second point brings up an important third one: the arbitrary definition of a decadal dataset. I used the common calendar dates to group my datasets and plots, begin each set of plots with a year ending in a zero and enclosing the dataset with a trace from a year ending in a nine. But I could have just as easily chosen decadal sets such as 1974-83; 1984-93; 1994-03; etc. Who knows what trends we might detect then?

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I think I’ll plot a Part 2 post to this topic. It will take some time due to the need to post-process the data. I have data preceding the 1950s going all the way back to the 1890s for the Seattle area. However, this data is typically from other locations, including the Portage Bay area in North Seattle and Boeing Field in South Seattle. These areas are 10-20 miles from Seattle-Tacoma Airport.

I think too, I will plot climatic normal curves for other 30-year periods to see if any trending is detected within those longer-term datasets.

That’s all for now.

Pacific Northwest autumn 2021 rains...

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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.