El Niño and the Winter Outlook 2023-24

What will this winter bring?  In a previous blog post, found here, we talked about how El Niño could impact our fall weather.  Now we will take it a step further and examine the winter.

In case you haven't heard, there is a 100% chance of an El Niño this winter!  This is guaranteed.  And there is an 80% chance the event will be classified as a strong El Niño.  Below is the typical pattern associated with an El Niño winter.  Typically the polar jet stream stays well to our north and east while an active Pacific Jet Stream sends wet weather into the west coast (especially California).  But as you'll see later that is certainly no guarantee.

Typical Jet Stream Pattern associated with El Nino

While the impacts locally are not the same for every El Niño winter,  the winters favor warmer than normal temperatures.  We will talk about precipitation and snow later.

So how is the current El Niño looking?  As the graphics below show, there is plenty of warmer than normal water along the Central and Eastern Equatorial Pacific.

Average Sea Surface Temperature Anomalies Sep 24-Oct 21, 2023 (left image).  Specific anomalies for Nino regions (right image)

The Niño 3.4 region already has an anomaly of 1.6C (3 F).  Anything 1.5C or warmer would classify as a strong event if this intensity maintains itself for three or more months.

The model predictions are calling for not only for these warm waters to maintain themselves, but to get even warmer through the remainder of the fall and early winter

Model predictions of Nino 3.4 Sea Surface Temperature Anomalies through JJA (June, July, August 2024)

Why does this warm water matter when it is so far away?  The why can be found in this September 29th blog post from the Climate Prediction Center (CPC), found here.

Since 1980, composite temperature and precipitation anomalies of Strong El Niño winters (shown below) reveal that on average, Strong El Niño winters result in warmer than normal temperatures, while precipitation has been slightly above normal in Central Washington, and slightly drier than normal over SE Washington into the south Idaho Panhandle.

Left Image: Average temperature anomalies of the 1982-83, 1991-92, 1997-98, 2009-10, and 2015-16 Strong El Niño Events.  Right Image: Averaged Standardized Precipitation Anomalies for those same events

It is important to mention here that this only includes five events (small sample size).  Due to the influence of climate change, there is some reluctancy to put as much weight into earlier cases.  However, the maps below show all strong events (8 in total) since 1950.  Let's begin with temperature:

Temperature anomalies observed from previous Strong El Niño events

And now precipitation:

Precipitation anomalies observed from previous Strong El Niño events

Here are the key takeaways from previous Strong El Niño Winters

• Every event since 1980 has been warmer than normal
• Precipitation varies significantly.   Some years have been wet, some dry, and some near normal.

So, what do the official outlooks call for this winter?  The CPC is calling for an 89% chance that temperatures will finish near to above normal (56% above normal, 33% normal), and only a 11% chance of the winter finishing cooler than normal as shown below.

Climate Prediction Center Dec 2023 - Feb 2024 Temperature Outlook.  The green icon on map is over Spokane, WA with the pie charts on left side of graphic showing various temperature and precipitation probabilities for the period

Note the bottom left pie chart which is the precipitation forecast.  Consistent with previous cases, there isn't much weight put into any one category, with all outcomes carrying nearly equal weight.

The warmer than normal temperatures themselves tends to reduce snowfall amounts for the lower elevations.  Below is a map of % of normal snowfall for ALL El Niño Winters from 1950-2015

Percent of normal snowfall averaged over all El Nino years from 1950-2015

As you can see, just about every town on average sees below normal snowfall.  Want to fiddle with this page some more?  Well you are in luck, an interactive version can be found here.  Each location when clicked on will bring up a chart showing average snow amounts for all three ENSO States (El Niño, La Niña, and Neutral).

There are two important things to point out regarding snowfall for the Inland NW.  

1. The Palouse area and Spokane area historically received the greatest reduction in snow during El Niño events compared to normal.  Since we are heading into a strong El Niño, we examined previous events for Moscow and Spokane (shown below).  
2. Communities from the East Slopes of the Cascades up into the Okanogan Highlands including Leavenworth, Chelan, Winthrop, and Republic show much more variability with amounts. 

Let's start with Moscow, ID.  Every strong El Niño event since the 1970s has brought below normal snow, with most years only receiving around 20-30" versus a normal near 50"!

For the Spokane International Airport, EVERY strong El Niño event since 1950 has brought below normal snow except 1965-66 which was near normal.

And for Wenatchee, snow amounts are quite variable.

If you live in North Central Washington, the presence of El Niño won't really help much regarding the seasonal snow prediction.  Because of this there is higher uncertainty with the snow forecast for these areas.

Interested in reading more about how El Niño can impact snow over North America?  The Climate Prediction Center recently published a blog post going into more detail, found here.

Finally, while El Niño plays a role for the winter as a whole, short term variations in the weather (lasting 1-3 weeks) commonly occur as other atmospheric variables impact the weather pattern.  These can have impacts over the region including strong winds, flooding, cold temperatures, and a variety of winter precipitation types including snow, sleet, and freezing rain.  With that in mind, it is always wise to be prepared for winter weather.  In fact, winter weather already struck parts of the region on October 27, 2023 creating slick roads as these images from WSDOT showed on I-90 near the Four Lakes area.

Oct 27th, 2023 - Photo courtesy of WSDOT on X (Twitter)

Oct 27th, 2023 Photo courtesy of WSDOT on X (Twitter)

Major pattern change this week - Snow and Cold!

 *Oct 24th Update to Snow Probability Map animation embedded below*

Have you heard?  Big weather changes are coming this week.  If you have not winterized your outdoor faucets and irrigation, NOW is the time!  And do you have travel plans this week?  Roads may be impacted by snow and/or ice, especially up over the mountain passes.  Of course with the varying terrain of the Inland NW, the snow part of the forecast will vary by location.  But regarding the cold everyone will be impacted.  Let's dive into the details.

The setup

What is the pattern that will cause this cool down? A high amplitude ridge to our west will allow a cold trough to settle over the region.

12z GFS 500 mb heights and anomalies valid Oct 23-27th, 2023

First hazard: Snow

A low pressure system swinging through the region will bring rain and snow to the region.  Here is one model depiction showing a band of rain and snow swinging through late Tuesday into Wednesday.  Green is where the model forecasts rain, blue is snow, and the darker the shading the heavier the precipitation rate. As cooler air moves into the region, most of the precipitation is progged to fall as snow on Wednesday.

12z NAM Nest forecast of precipitation type 5 AM PDT Tuesday - 5 PM PDT Wednesday (Oct 24-25) 

Of course this is just one model solution.  And for the valleys, part of the challenge with this storm is temperatures.  Given it is early season, the ground temperatures are warmer compared to a typical winter storm.  This makes the snow amount forecast extra tricky for many of our towns.  With air temperatures near or above freezing for many lowlands during the bulk of the event, snow for a portion of the event may mix with rain, or fall as a wet non-accumulating snow.

To help illustrate this, here is the chances for freezing temperatures for 8 AM, 2 PM, and 8 PM on Wednesday according to the National Weather Service's National Blend of Model (NBM).  And then finally 8 AM Thursday is shown.  Orange and and yellow shading is where chances are highest.

8 AM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 AM Wed (Oct 25th)

So Wednesday morning, the best chances for freezing temperatures are elevation dependent with the best chances north of I-90 except the lowest elevations along the Okanogan and Columbia Rivers. There is also a good chance of freezing temperatures around the Spokane and Coeur d'Alene areas (especially higher hills).

2 PM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 2 PM Wed (Oct 25th)

By this time, the chance of freezing temperatures is mainly in the mountains, as well as part of the Highway 2 corridor from Waterville to Davenport, as well as around the Sandpoint area.  Thus for most of the lowlands snow that falls Wednesday afternoon may have a harder time accumulating.

8 PM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 PM Wed (Oct 25th)

By 8 PM Wednesday, chance of freezing temperatures expand over much of the region, except for lower elevations south of I-90.  But by 8 AM Thursday, nearly all of the Inland NW is expecting freezing temperatures except for possibly the Lewiston area.

8 AM Thu chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 AM Thu (Oct 26th)

An ensemble system which incorporates multiple forecast model solutions gives forecasters a better idea of confidence in snow amounts.  Here is an animation of various snow amounts being reached over the region.

***Updated 8 AM PDT Oct 24th***

For the latest maps, please head to https://www.weather.gov/otx/winter

There is concern for area with fall foliage that any areas that receive moderate to heavy amounts of wet snow could bring down tree branches with local power outages.

Next hazard: Widespread hard freeze

This system will usher in much colder temperatures this week, with widespread freezing temperatures.  For those that still have tomatoes or other plants that are still thriving, time to take action!  Have you winterized your outdoor irrigation?  Also it's time to take action!  Here is a look at the forecast temperatures this week

As the graphic states, also protect livestock and pets and dress warmly if heading outdoors.  Get those gloves and winter coats ready!  Big changes are coming this week.