September 29, 2023

The Implications of El Nino for Northwest Autumn and Winter

There has been a lot of speculative talk about the implications of El Nino for our winter, with some claiming a certainty of drought and other afflictions.

So let's try to determine reality from hype.

Let me begin by noting that the latest forecasts are for at least a moderate El Nino this winter: GUARANTEED!

Looking at many El Nino model predictions, all have temperatures at least 1°C above normal in the tropical Pacific.  Some more than 2°C above normal.  (see forecasts below for the sea surface temperature anomaly from normal for the Nino 3.4 area).  Folks...this is about as certain as these things get.

With a certain El Nino in store, let's find the implications of El Ninos on our weather (and the rest of the U.S.), by averaging (or compositing) the NOAA Climate Division Data for major El Ninos of the past 40 years.  

An important insight is that the connection between El Nino and our weather depends on the season.

For autumn the effects are more subdued.   Here are the differences from normal of autumn (Oct-ber-December) temperatures during El Nino years.   Modestly warmer than normal (by 0.5 to 1°F) over western Oregon and Washington, and about half that over eastern Washington. Colder than normal over much of the rest of the country


In contrast, there is very little impact on precipitation in the Northwest.  Perhaps slightly drier than normal over the western slopes of the Cascades.  So there is NO reason to expect any kind of drought over the Northwest during the fall rain season.... our big water collection period.  California and the southeast typically get a bit more than normal during El Nino falls.


To make doubly sure you aren't too worried about autumn drought from El Nino, here is the average Palmer Drought Severity Index (PDSI) for El Nino autumns.   NO signal over the Northwest.



What about after January 1? 
 The El Nino signal generally strengthens then.

Here is the temperature difference (anomaly) from normal for January to March.   A very clear warm signal over the Northwest, which generally decreases snowfall.


The January-March precipitation signal is weak east of the Cascade crest and is pretty localized to the western slopes of the Cascades, which gets less precipitation (down around 3 inches)--which is relatively small compared to the 60-100 inches often observed on the slopes.


So what is the bottom line for expected snow impacts of El Nino for the entire winter? 

Below is a NOAA analysis of the difference from average snowfall for the 10 strongest El Nino winter and all El Ninos.    The Northwest tends to get less snow by as much as 10 inches over the mountains.  Considering that much of our high terrain gets hundreds of inches a year, the El Nino impact is modest.


So the bottom line is that the El Nino influence should be modest, with most of the impacts after January.    And keep in mind that not all El Nino years follow the above patterns.   El Nino is only one factor influencing atmospheric evolution over the planet.








7 comments:

  1. This morning, BLI recorded its coldest September temperature, and its first September temperature in the 30s, since 2019.

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    Replies
    1. In the 2014/15 El nino event the sst off the west coast were well above normal aka the blob, how will the warm blob effect the Mt snow pack this season?.

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  2. In the mid-Atlantic, El Nino years are when we get big Nor' Easters and snow dumps up to 2 feet. If you live on the I-95 corridor, you are on the edge of the rain-snow transition so its often heavy and wet. During strong El Nino's it tends to be a bit warmer so are more likely to get rain but moderate El Nino's are when we get the big snow accumulations.

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  3. Sounds to me you're predicting a perfectly NORMAL Fall and Winter. I assume when you say Normal you mean average. And when you say differential (anomaly) from Normal you mean from the average. And of course the average is just a whole bunch of anomalies added together. So plus or minus a degree or two C, or a few feet of snow IS exactly the Normal. Basically nothing we'd ever notice if not for all this fancy tech. And I'm good with that!

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  4. Does El Nino have any effect on the likelihood or frequency of extreme fall/winter weather events in the PNW (e.g., heavy rain storms or wind storms)?

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  5. To delve into the effects of El Nino upon snowfall, does the moderate decline in seasonal snowfall at altitude have a sort of snowballing effect upon water supply? (no pun intended)

    What I mean by this is that for every 1000 ft of elevation you gain west of the Cascade crest, acreage within the water catchment area falls off exponentially. There's a lot more land at 3000 ft than 4000 ft, more at 4000 ft than 5000 ft, and so on all the way to the permanent snowfield altitude below Camp Muir.

    If snowfall at some subalpine 5000 ft location is down a nominal 10%, is the diminishment of total snow water content more significant than 10% because the snow level rose 1000 ft and eliminated good snow catchment acreage in the Canadian forest zone?

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