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.

A Very Scary Cloud over Puget Sound

A line of clouds moved through western Washington yesterday that was intimidating and scary. 

Take a look at an amazing image taken around 12:15 PM yesterday by Murphy McCullough. A bizarre convoluted line of clouds stretched to the horizon.

A lone cruise ship was heading into the terror.....would it survive?

Murphy was a brave soul and he took an extraordinary video, which is shown below.

Particularly terrifying were the torn-up, fractured clouds hanging out of the general cloud feature.  Surely the work of some Lucifer-like creature.

This video is real--not the creation of AI or some Hollywood disaster flic.   Just to prove this to you, here is a view from the Seattle PanoCam, located on the Seattle Space Needle at 12:10 PM

You can see the cruise ship heading into the murk. Reminds me of the movie "Final Countdown" in an aircraft carrier moves into such a feature and comes out at a different time.

Ten minutes later, the line is approaching Seattle and the ship is gone behind a curtain of rain....or is it something more ominous?

So What Was It?

        A strong convective line, made up of thunderstorms and heavy showers, was moving through and the ominous feature you see is called a shelf cloud.

Let's begin by showing you the radar image right before the images above (12:07 PM).  Red is very heavy rain or hail.  Orange and yellow are just heavy rain.  You see the corrugation of the feature....another indication of a powerful line.

The shower line was made up of a series of strong cumulus convective elements, including thunderstorms. 

 Leading the strong storms was a cool outflow of powerful winds....called a gust front.  The gust front pushed air up ahead of it, producing a shelf cloud (see schematic below)

The flow at the leading edge of the gust front--with strong, cold sinking air behind and warm, moist upward-moving air in front of it-- can be turbulent. leading to tendrils of upward motion that produce unusually fractured clouds.   That is what they are called fractus clouds!

And when the line went through, the weather got very interesting.  A profound drop in temperatures, an increase in winds, and, of course, heavy precipitation.  Check out what happened as the line passed across the UW (see below).

And the weather fun is not over.  An intense mini-low moved across the Oregon/Washington border, bringing strong winds to the northern Oregon coast (see forecast map for 2 AM today).

 Enjoy the weather.