It’s many people’s favorite time of the year: football season, crisp, cool nights, pumpkin patches, hay rides and of course fall foliage. Autumn can be spectacular around here, but the quality of the foliage season can vary from year to year.
WHAT MAKES FOR A GREAT FOLIAGE SEASON?
1) A WET EARLY GROWING SEASON
If dry conditions prevail in the spring and early summer, there is an increased risk of leaves dropping before they reach peak color in the fall. This year, we DID have a pretty wet period from April-June.
2) SUNNY DAYS AND CLEAR NIGHTS LATE IN SUMMER AND EARLY IN AUTUMN
3) CALM, SUNNY DAYS IN OCTOBER
What we don’t want in October is a bunch of rainy, windy days, especially later on the month. Those conditions will help pull down leaves and lead to a muted foliage season. In the first half of the month, cool and dry days and nights would be ideal. Welp:
Early October is looking pretty warm overall.
SCIENCE OF FALL FOLIAGE
Why do leaves change color? It’s all about chlorophyll. That’s what gives leaves their green color. But leaves have other pigments as well and as daylight decreases in the fall, chlorophyll quickly depletes. This allows those other pigments such as yellow, red, orange, etc to show up.
CURRENT FOLIAGE STATUS
We are not seeing much color yet locally. Pockets here and there but overall it seems like we already running behind schedule a bit.
Colors are most noticeable in areas well north of us, mainly in the mountains.
Peak colors typically occur in mid-to-late October around here. Sometimes we see a peak around October 16-23, sometimes more like October 22-31. I suspect we’ll see the latter this year.
It’s still (astronomical) summer and it’s going to feel like it for the next handful of days with warm, humid weather in the forecast. It’s WAY too early to put out an official winter forecast. But I thought I’d share a few thoughts on the upcoming winter anyway. With the important caveat:
THIS IS NOT A FORECAST.
THE OFFICIAL WINTER FORECAST WILL BE ISSUED ON AIR AND ONLINE AROUND NOVEMBER 1
Ok, now that we have that out of the way. This post will not dive into ALL the factors that I surely will be looking at when I compile the forecast in 6 weeks. I’ll just touch on a couple here.
#1 The Winter Is Likely To Feature A Weak El Nino
Remember, El Nino is a phenomenon having to do with water temperatures in the Pacific Ocean and how they compare to average. El Nino is a warming of the waters in the eastern Pacific, near the equator. What does this have to do with our weather?? Well, the oceans and the atmosphere are one big system and what happens in the oceans can influence how the atmosphere behaves.
The strength of an El Nino is important. Weak El Ninos tend to result in more “surprises” and more variance than strong ones. In other words, the range of outcomes is greater than during a winter featuring a strong El Nino.
The location of the warmest water (compared to average) is also important. A “standard” El Nino sees the warmest water banked up against South America. But there is a special flavor of El Nino called “modoki”, which sees the warmest water farther west.These 2 flavors of El Nino often produce winters over the US that are quiet different. Odds favor a “modoki” this winter.
#2 The “QBO” Or Quasi-biennial Oscillation Is Going From VERY Negative (Easterly) To Near Neutral
Say what? The QBO refers to the oscillation of river of air at very high altitudes in the atmosphere. It switches directions at regular intervals and has been shown to have an impact on weather patterns in the northern hemisphere winter. It was very “negative” or easterly last winter through this summer but is trending back toward neutral.
Again, so what?
Winter Outlooks Are Largely Based On Looking At The Past
Making a seasonal forecast is tough but thankfully we have lots of data to look at., including detailed records of the past. So, we look for years with similar setups and see what happened. For the purposes of this post, I’ll just look at past years with weak modoki El Ninos and a QBO trend similar to this fall. When I do the “real” forecast, I will be looking at much more than this.
Temperatures (averaged together) during winters with weak modoki El Ninos:
Finding years with a weak modoki El Nino AND a QBO trending from very negative back toward neutral is tough. Only found a few. Average them and you get this:
Using these factors alone, the winter of 1968-1969 is the best overall match. Temperatures that winter:
Pretty cold winter. Snow was nothing to write home about. A bit below our average in the lower/mid 60s actually.
Again, the forecast will come out sometime around November 1. This just gave you a little peek behind the curtain. I love the challenge of the winter forecast and look forward to trying to beat the atmosphere this year!