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altasnob
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PostTue Sep 20, 2022 12:52 pm 
gb wrote:
It used to be that we would get summer systems coming out of the gulf of Alaska every two or three weeks. Thus, the amount of rain we would get in the summer would be fairly consistent month to month in Seattle (or any other NW weather station). What we get now are stalled low pressure systems along the coast which bring up subtropical moisture and hence allow for thunderstorm development and periodic heavy rains.(think tropical weather patterns).
What's your cite for this? Personal observation or peer reviewed study? Does it matter (for the plants) where the moisture is coming from? Wouldn't what really matters is just how much total moisture we get, combined with other factors like temperature and humidity?
gb wrote:
And therein lies your problem. You are listening to Cliff Mass.
I don't listen to Cliff Mass. I don't listen to anyone. I am looking for the data, and the methods people used to come up with analysis of that data, like jaysway did above (which from my memory, seems to mirror what I have read from Cliff Mass on this topic). The data is what it is.
gb wrote:
Perhaps Cliff Mass hasn't lived here long enough to have an understanding or doesn't know how to look at statistics.
Cliff Mass got his PHd from UW in 1978, longer than I have been in WA and I was born here. Not sure when he became a UW professor.

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gb
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PostTue Sep 20, 2022 1:07 pm 
jaysway wrote:
I'm waiting for a large model to finish running at work so I have a bit of downtime (hence this post and my one in the lazy reports thread) and decided to paste the data into Excel and then do a quick trend analysis in Python. Caveats: I'm not an expert on time series, hydrology, or climate change. My preferred method for trend detection is the Mann-Kendall test which is non-parametric and detects for increasing or decreasing monotonic trends compared to the null hypothesis of no trend. This test assumes that the data do not exhibit serial autocorrelation. I summed June, July, and August precipitation values as a proxy for summer precipitation totals from the Seattle/Tacoma airport and present it below in a time series from 1945-present. I'm assuming in the dataset that "T" means trace, so I replaced those values with 0s, and that "M" means missing, so I imputed those by month using monthly means. On the plots I included the trend test results as well as the Theil-Sen best-fit line which is more robust to outliers than linear regression. Here are the results:
The p-value on the Mann-Kendall test is 0.16, which basically means that if we assume that there is no trend in our dataset, we would detect a trend as or more extreme as the one we did about 16% of the time. Since our prefined p-value threshold was 5%, that means that we no not detect any increasing or decreasing trend in summer precipitation totals for the Seattle/Tacoma airport from 1945-2021. The lower the p-value, the more confident we can be that we ought to reject our null hypothesis of no trend, i.e. our data is incompatible with our null model of no trend. In practice, p-values should be weighed holistically with effect sizes and other evidence when conducting hypothesis testing - see the American Statistical Association's statement on p-values for more information. I did a quick autocorrelation plot to confirm that the time series does not exhibit serial autocorrelation, if it did we could use Hamed and Rao's variance correction in the Mann-Kendall test.
Out of curiosity, I also repeated the above analysis for just June, July, August, and September precipitation totals:
Note that I simply repeated my above analysis and did not correct for multiple comparisons. One key thing to note with trend analysis is that it is sensitive to the time period, thus one of the easiest ways to lie with statistics when it comes to trends is changing the starting time period. Since 1945 is the beginning of the dataset this is where I began all of my analyses. Another way to lie with statistics here is to p-value hack by trying lots of different comparisons and hoping that eventually we will find ones that give us the results we are hoping for, ex. for this analysis trying different months, different ways of defining "summer precipitation," trying different weather stations. My takeaway from the above analysis: while a bit interesting, the above is woefully incomplete and cannot tell us very much (or anything, really) about how climate change has affected summer precipitation in Seattle. Let's leave that to the climate scientists smile.gif. A quick note on climate change, which again is far from my area of expertise: human-caused climate change is an undeniable scientific fact. There is a lot of variability in how climate change affects different geographies. It rarely makes sense to point to specific anecdotes or single datapoints as evidence for or against climate change. While it's less sexy, trend analysis and other techniques that shed light on long-term patterns are key for understanding the effects of climate change. I typically put no more stock in someone saying "this heat/lack of rain is evidence for climate change" than someone saying "this higher-than-normal snowfall is evidence that climate change doesn't exist." Neither are helpful. It's best to think about climate change probabilistically and consider that natural variability and climate change work in concert to affect weather patterns.
A more telling statistic would be sky cover on a monthly and daily basis. But you have to pay a lot of attention not to the mean, but to the distribution over a period of time. The more days we get with sunny skies (particularly talking in the mountains, here) the more it dries out between rains. Perhaps an easy way to look at this is Cascade glaciation, where the greatest decrease in ice mass is along and east of the Cascade Crest. The Dakobed is my example where precipitation on an annual basis is probably little changed, but the loss of ice mass is much greater than in the Cascades nearer the crest. The reason for this is that spring comes earlier and the critical, high sun-angle months of May, June, and July have far more sunny days than they had in the recent past. Cascade glaciation and moisture retention in vegetation has historically had a lot to do with whether onshore flow or offshore or neutral flow are occurring. Mountain areas historically accumulated a good deal of moisture from wind driven fog and light drizzle. (In The Olympics one study showed that this amounted to 30% of total moisture in the Hoh River area.) So how often do mountain sites have onshore clouds to elevation now - there are probably no such statistics over a longer period? But the Dakobed is a classic example of an evident decrease in cloudiness especially in those peak high angle sun months and long days when snowmass loss can reach 8-12" in a single day. The Dakobed was at one time favorably situated for Cascade Glaciers primarily because of convergence about Glacier Peak, but also valley convergence from the White River. But now the favorable precipitation is overcome by late spring and summer sunny skies. The Stuart area is another example of dramatic ice loss.

jaysway
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gb
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PostTue Sep 20, 2022 1:13 pm 
neek wrote:
altasnob wrote:
I wish that rather than people just saying, I remember it used to rain here and now it doesn't, people would post the actual amount of rain in Seattle in August/September for the last 100 years. It is hard for me to find this data in graph form.
I'm pathetic at finding and analyzing weather data, but here's a first take, using data grabbed from https://www.weather.gov/wrh/Climate?wfo=sew and pasted into a spreadsheet.
Seattle area, August rainfall totals, inches
Seattle area, August rainfall totals, inches
This graph is actually telling in that although the average (including high extremes) may be rather constant, a quick look at the graph you posted shows much more spread and more groupings of extreme lows in the past 20 years.

rossb
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gb
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PostTue Sep 20, 2022 1:18 pm 
altasnob wrote:
gb wrote:
It used to be that we would get summer systems coming out of the gulf of Alaska every two or three weeks. Thus, the amount of rain we would get in the summer would be fairly consistent month to month in Seattle (or any other NW weather station). What we get now are stalled low pressure systems along the coast which bring up subtropical moisture and hence allow for thunderstorm development and periodic heavy rains.(think tropical weather patterns).
What's your cite for this? Personal observation or peer reviewed study? Does it matter (for the plants) where the moisture is coming from? Wouldn't what really matters is just how much total moisture we get, combined with other factors like temperature and humidity?
You can do your own research if skeptical. Go to the climate portal and go back and read year by year the summaries of recent years. If you need to go back further, contact the NWS, they will provide you with additional older summaries. I got a lot of data from the NWAC (at NWS) from a couple of statistical snowpack studies I did regarding avalanche case studies. Have you ever read any of those annual climate summaries? You can get daily rainfall records and temperature records. Looking at averages as Mr. Muss does is very misleading. You miss sky cover and the number of days with rainfall. I gave you a link to get started on your personal research.

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gb
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PostTue Sep 20, 2022 1:30 pm 
philfort wrote:
I wish there were an easy way to download that data instead of just having to use the website. btw, the last 3 years (2019-2021) have had significantly above average precipitation for September (which surprised me).
But all it takes in one big rainfall event (good for fires) to skew those averages. I recall a trip to Tapto Lakes around September 24th of 1994 where on a four or five day trip rain was forecast on the last day. I was little concerned, but was totally drenched on the walk out. A look at NWS data (after the fact) showed that it rained 7" in 24 hours at Baker. eek.gif This was probably about the greatest one day September rainfall in Washington. The "average" rainfall for that September was much above average...... lol.gif

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Snowshovel
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PostTue Sep 20, 2022 1:32 pm 
Mr Muss is actually Dr Mass. A little respect for an actual professional versus rank amateur would be nice.

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gb
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PostTue Sep 20, 2022 1:43 pm 
Snowshovel wrote:
Mr Muss is actually Dr Mass. A little respect for an actual professional versus rank amateur would be nice.
Mr. Muss deserves as much credit as he should get given his attempt at politicization of climate information. He has an agenda and should keep his website free of such politics. He would have a lot more respect in the community.

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Snowshovel
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PostTue Sep 20, 2022 1:57 pm 
Isnít your post against forum rules regarding politics?

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gb
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PostTue Sep 20, 2022 2:06 pm 
Snowshovel wrote:
Isnít your post against forum rules regarding politics?
Isn't it you and Altasnob who are trying to make it political? I said that earlier to Altasnob when he attempted to make a post about extremely dry conditions political. Do you disagree that it is very dry and that until we get rainfall we will to a greater or lesser degree be dealing with smoke and possible additional fires? Is that your point?

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Snowshovel
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PostTue Sep 20, 2022 2:11 pm 
My entire contribution here has been to correct you that it is Dr Mass.

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philfort
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PostTue Sep 20, 2022 2:21 pm 
gb wrote:
Isn't it you and Altasnob who are trying to make it political?
Asking for data when people are presenting anecdotes is not political. However, calling Cliff Mass "Muss" - I'm not even sure what this is supposed to mean (can someone explain?), but it seems in the same vein as "libtard" or "repubican"? And I usually tend to ignore things people who use those terms say.

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Cyclopath
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PostTue Sep 20, 2022 2:24 pm 
Anyway, let's get back to talking about what's going on in nature as it relates to hiking.

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gb
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PostTue Sep 20, 2022 2:54 pm 
altasnob wrote:
Thanks for all the above posts. Very interesting. I guess we would also have to plot temperature because even if it is not raining that much less in the summer here, if it is also warmer then it would exasperate the dryness for the plants.
That is exactly accurate, and that is what I addressed when I spoke about the decline of glaciers in the Dakobed Range, which by comparison with west side glaciers is defacto evidence of the tendency for more sunny days and lower average humidties (this latter is essentially redundant). So are the Soil Conservation Services Western Drought Indexes. Drought Index Discussion.

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thunderhead
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PostTue Sep 20, 2022 5:03 pm 
Some good analysis and charts in this thread. Good work guys and gals! I think we can say that western wa is experiencing no significant trend to maybd a minor decrease in summer precipitation totals, and with high confidence say there is no trend in annual precipitation totals. The freezing level however is climbing, so snowfall is trendind downwards especially in the lower part of the typical snowfall band, and thus earlier meltout, more runoff, and likely a minor increase in late summer soil dryness.

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philfort
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PostWed Sep 21, 2022 10:30 am 
June 21 - September 21, driest on record: https://twitter.com/NWSSeattle/status/1572636841739964416/photo/1

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