SUMMARY

Seattle has had a very warm and dry Fall season so far in 2022. This is a quick analysis of historical rainfall and temperature trends, putting the current weather in context. Water Year 2023 (WY2023), starting 01-Oct 2022 has been among the driest five recorded. Looking at full seasonal rainfall, starting the Extended Water Year (EWY*) on 01-Aug, shows that EWY*2023 is the driest and among the warmest Fall Seasons in a century.

Get Data

Data is from NOAA Climate data Online from 1921 onward. Since there was no single contnuous monitoring station with all the data, I made the data by combining measurements from several stations.

Data analyzed consist of daily precipation (inches), and min and max temperatures (in degress F). To get a full data set where the data overlap values are averaged.

To get a century of data need to use multiple data sources.

Data are from SEATTLE TACOMA AIRPORT, SEATTLE BOEING FIELD, SEATTLE PORTAGE BAY, BREMERTON, SEATTLE UNIVERSITY OF WASH, & KENT (stations: USW00024233, USW00024234, USW00024281, USC00450872, USC00457478, and USC00454169) covering 1921-01-01 to 2022-10-21.

Here is a look at the some of the raw data. The average of the points are shown as well, giving an indication of the fidelity of the process used later in this analysis.

Min Temperatures 1957

Max Temperatures 1957 and 2014

Rainfall Data 1957 and 2022

Here is the rain data for 1957. The mean rainfall which is used in later analysis is also shown.

We can get a look at all the rain data here.

Notice that the max rainfall occurs 2008-12-21 with 8 inches of rain

To get tidy the data are combined and where dates overlap numbers are averaged.

Here is the same plot for this averaged data. Notice that some of the more extreme points are reduced. For instance the max rainfall now occurs 2007-12-03 with 4.36 inches of rain

Standard Water Year (WY) Rainfall

The first analysis is to generate water year cumulative sums for Water Year (Water Years (WY) traditionally start on Oct-01 in North America, with the year indicated by the year when the Water Year ends (meaning we are currently in WY2023).

This shows that WY2023 (Starting 01-Oct) is among the driest years with the last 100 years, but it is not yet the driest.

We can also plot a Histogram of the cumulative rainfall.

This shows that for a standard 01-Oct start date WY 2023 is the fifth driest year in a century. For three other years there was zero recorded precipitation.

## # A tibble: 102 × 2
##    WATER_YEAR TOTAL_PRCP
##         <dbl>      <dbl>
##  1       1926     0     
##  2       1953     0     
##  3       1992     0     
##  4       1987     0.0075
##  5       2023     0.01  
##  6       1988     0.0358
##  7       1934     0.0367
##  8       1973     0.048 
##  9       1975     0.0575
## 10       1979     0.088 
## # … with 92 more rows

Extended Water Year (EWY*) (01-Aug to current date)

Seattle’s month of minimum rainfall is August, so we can also look at totals starting 01-August as a way to gauge “Summer and Fall” seasonal waterfall. This is a better indicator of stress of local plants, for example.

## # A tibble: 102 × 2
##    WATER_YEAR TOTAL_PRCP
##    <fct>           <dbl>
##  1 2023            0.162
##  2 1975            0.342
##  3 1994            0.923
##  4 1953            0.932
##  5 2013            1.21 
##  6 1988            1.23 
##  7 2003            1.48 
##  8 1943            1.57 
##  9 2018            1.71 
## 10 1926            1.89 
## # … with 92 more rows

So out of 100 years, EWY*2023 (Water Year starting 01-Aug) is the driest in a century.

Here is how the ten driest and warmest years look in terms of rainfall. It’s interesting that some of the other warmer years, e.g 2021, had fairly high cumulative rainfall. So there is not necessarily a strong correlation.

October 1975 was an another extremely dry October, with no substantial rainfall until Oct 20.

Appendix: Min Temperature Analysis

Climate Change is primarily driven by the green house effect (i.e by trapping radiated thermal energy). Since the green house affects radiatvie cooling, its effect is primarily observed on overnight low teperatures. We can see this in the data for this year in contrast to earlier years.

We can compute a summary statistic by taking the average low temperature for the first 13 days of each October since 1962.

## # A tibble: 102 × 2
##    WATER_YEAR TMIN_AV
##         <dbl>   <dbl>
##  1       1989    53.1
##  2       2021    52.1
##  3       2015    51.9
##  4       1964    51.7
##  5       2023    51.7
##  6       1948    51.4
##  7       2016    51.1
##  8       1959    50  
##  9       1968    49.9
## 10       1941    49.8
## # … with 92 more rows

2023 has had the sixth warmest overnight lows 100 years.

It’s also helpful to look at a graph of the data.

While there is a trend in this data (of about a degree) it is not high confidence (small compared to year to year changes).

Another way to look at the data is with a histogram. It also reveals we are on the warm extreme of the historical distribution.

Conclusions

According to this historical analysis of weather data CY2022 (and particularly EWY*2023, with Extended Water Year* defined as starting at 01-Aug) has been the driest and among the warmest in a Century.

For the Extended Water Year (starting 01-Aug) EWY*2023 is so far the driest in a Century. Other dry years have had had very dry Octobers, with 1975, 1926, 1953 particularly standing out as having been dry until late October.

Temperature data show a warming trend of overnight low temperatures, but it is small compared to year to year fluctuations and therefore is not high confidence. This is consistent with previous analysis.

If someone wanted to do more work on this, extending to additional weather stations would be super interesting.

@WinstonOnData