Snow Advance Index

The Snow Advance Index is a feature that is not always the best indicator for how our winter will be. The basic principle is that the advancement of snow cover in October is closely linked to the phase of the North Atlantic Oscillation.

North Atlantic Oscillation

The North Atlantic Oscillation (NAO) measurement is a KEY FEATURE needed to help get snowstorms on the East Coast of the United States. For example, the NAO is currently negative, which helps to slow down and aid in the development of big Nor’easters like the one we are having this week (October 26 – 27, 2021)

Really, though, the negative NAO (NAO-) is not the key to these storms, it is as the NAO flips back and forth between the negative and positive (NAO+) that usually signals a storm

Anyways, when the Snow Advance Index shows a great increase, the NAO *COULD* end up being more negative for the winter.

So what is the Snow Advanced Index Looking Like?

Snow Progression

The index is actually below where I would like to see it, but it is higher than last year!

The NAO Has been running Negative for a long while now. We will see if this can continue into November and Beyond! That would be a good sign for offsetting the La Nina impacts. (The line is below the 0)

Arctic Oscillation

Another factor for colder conditions is the Arctic Oscillation! As noted yesterday, when the Polar Vortex gets disrupted, it can turn negative! WE NEED THE NEGATIVE (if you like snow)! That is a better indication for more snow in the east! It has been running back and forth between positive and negative phases. We will see how this works out, but it looks like it is heading to negative again so the colder long-range seems to be linked to this as well! (Observed in black and red the various model predictions)

I am not super convinced we get non-stop help from the Arctic Oscillation this year, but I do expect a lot of variabilities which could help us at times be colder and have a chance of winter weather.

In Summary

I do not think we have much to count on from the Snow Advanced Index, North Atlantic Oscillation, or Arctic Oscillation. The negative NAO has been encouraging, but that alone will not set us right for winter weather.

The Polar Vortex is not something new and makes visits into the Lower 48 states every winter. The news media LOVES to over hype when it comes into play for the United States. Additional names to this Arctic region airmass could be the Polar Express (the streams of air that bring in the Vortex, or at least portions of it), or the circumpolar vortex.

What is it exactly?

The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles. (that comes straight from the NWS) In the winter it strengthens.

When it is very circular, it is considered stable and the jet stream is usually very strong. Storms likely will rip across the United States and the coldest air stays “locked” in Canada.

Disruptions

If you want there to be colder air masses in the United States, you would want the Polar Vortex to be weaker than normal so that storm systems can disrupt and dislodge the colder air into the United States. Another thing that can help to split and disrupt the Polar Vortex is something known as Sudden Stratospheric Warming. This is warming above the vortex that can “split” the vortex in some situations. When it splits, this also will bring colder air into the lower latitudes. This does not always mean the United States! The Warming and Splitting can take some time and air masses and weather patterns can take some time to respond.

SO WHAT!! What about our winter?

Two things I am noticing this fall! The first is that the Polar Vortex appears weaker! Remember, this will allow for the influence of storm systems that could dislodge the colder air and give us cooler weather. I am also ALREADY SEEING THIS HAPPENING!

If you have been following me on the My Facebook “Speculations” I mentioned some “signals” that we could see some cold early. It was SPECIFICALLY related to the disruptions being forecast by models for November. The disruptions started showing on models two weeks ago and are now happening. It looks like this disruption pattern could come in a few waves in November! Some is coming into play next week, and some other long-range speculations show the middle of the month. Just because we get disruptions does not mean we will get cold here. That mid-month intrusion seems pointed towards the middle part of the country, and maybe down into Texas! Remember, when you get past 5 to 7 days in forecasting, the forecasts become much more speculative as the models may be failing to pick up on some things. HOWEVER, these Polar Vortex disruptions do seem likely, again, because the vortex is so weak. And, while in this stage, we could see some below normal spells in November! Snowflakes would not surprise me at some time. However, sustained cold air may be a challenge, as it should be, since it is so early in the colder months.

The BIGGEST concern I have for below-normal snow this year is the presence of La Niña

What is La Niña?

La Niña is a cold pool of water in the equatorial Pacific region. When the equatorial Pacific is Warm (El Niño) this region can supply a strong sub-tropical jet stream and usually a better chance of big snowstorms due to the amount of moisture. Although El Niño winters are not particularly cold, if the timing is right, we can get some whoppers! YES, 2016 was an El Niño year, and we got the last HUGE storm then.

La Niña, on the other hand, tends to make the Jet to the north push milder air into our region. We are often in the Battle Zone between colder to the north and warmer to the south when storms come along. The storms tend to turn to a mix or rain if they have any wintry weather in them at all. In general, we end up being slightly above normal with temperatures. The Pacific Northwest tends to get blasted with rain and Mountain snows (Like the absolute bomb cyclone of a Low from this last weekend). That does not do us much favors for keeping it cold in the east, as we get a ridge of High Pressure (Southeastern US warmer high pressures) pumping warmer air here.

Typical La Niña Pattern:

So this is Factor One for the 2021 – 2022 Winter weather outlook!

You can see in the National Weather Service’s Winter Outlook that they are weighing heavily on the La Niña pattern. If you have been watching the weather the last week or so, you can see that this is what the pattern has looked like!

And, I have to say, we have had a horrible set of winters when there is aLa Niña! I saw a meteorologist show that 60% produced below normal snow for the area.

My Thinking:

La Nina will have an early impact that will be offset by some other factors! So it WILL have an impact, especially through January.. but I think it will become less of a factor later in the year.

Also, there are a few reasons why we may see some early sneak peeks of winter in November and December, but they are unlikely to be long-lived.

That concludes part one of the Winter Weather impacts for 2021 – 2022 Loudoun Weather Winter!