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TERRY'S TAKE ON WINTER 2021-22...


Cedar Rapids Iowa April 10th, 1973


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THE 2021-22 WINTER OUTLOOK

With the arrival of October, my thoughts are increasingly drawn towards the winter ahead. This is an exciting time of year for me as there is nothing more challenging than forecasting the complex interactions that drive winter storms. Individual snow events are only on my radar for a few days (perhaps a week), and yet their implications often remain unknown right up to the very end. That said, if it's that difficult to forecast a snowstorm over a period of days, you can only imagine how hard it is to to make a seasonal forecast for winter encompassing three months. I'm the first to point out that I'm a meteorologist, not a climatologist and this type of outlook is on the fringe area of my expertise. Making a winter forecast in early October is fraught with peril so keep in mind what you are about to read is nothing more than an educated guess. If it doesn't pan out, neither you or I should lose any sleep over it.


Having laid down my qualifier, I do want you to know that I have given the topic of winter countless hours of thought and will supply you with solid arguments that support the theories I'll identify below.


THE SOUTHERN OSCILLATION

The first place I always begin a long range winter forecast with is by determining the state of the upcoming ENSO. ENSO stands for (El Niño-Southern Oscillation) The ENSO is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. During periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific warm or cool by anywhere from 1° to 3°C, compared to normal.


This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third one called ENSO-neutral.


So we've established the fact that El Nino and La Nina events are essentially sea surface temperature anomalies that exist near the Equator. Strong events, whether they be associated with El Nino or La Nina, flood the atmosphere with unusual amounts of energy that in turn control weather patterns in a somewhat predictable way, especially in the winter and spring. Strong ENSO events can have global implications with catastrophic results that range from excessive precipitation to drought. Temperatures are significantly altered as well. Weaker versions of ENSO are far less predictable and can be influenced by other factors such as atmospheric blocks and sea surface temperatures in other parts of the globe. With 70% of earth covered in water, sea surface temperatures play a large role in our overall climate.


The first order of business then is to figure out the state of this years ENSO and its strength. With unanimous consensus, all of our climate models indicate a developing La Nina is in the works (a build-up of cold sea surface water in the Equatorial Pacific), so that mystery is relatively clear. What isn't is the duration and intensity of the La Nada. The tropical Pacific constitutes a large geographical region making it tough for models to accurately pinpoint how cool the water will be, the spatial extent of it, and how long it remains in place. This graphic from NOAA shows the cold water associated with a La Nina (in blue) already in place over all of the central and eastern Pacific.


That certainly resembles the example of what NOAA uses to define La Nina below .

Forecasters at NOAA expect further cooling with the La Nina steadily strengthening through the fall and lasting through winter of 2021-22.

Predicted model plumes show a weak to moderate La Nina at its peak during the months of November through January before weakening come spring.

Another point worth mentioning is that ENSO can be measured and the location of the coolest water has important implications. The large expanse of water that encompasses ENSO is divided into boxes that have designations. Having a weak or moderate La Nina in the central Pacific is generally considered more favorable for a normal or colder than normal winter with normal to above normal snowfall. The central Pacific is considered region 3.4. Currently the western and central regions are forecast to have the coolest sea surface water departures during winter.

During a weak to moderate La Nina, the polar jet is frequently aimed at the upper Midwest which can at times bring significant outbreaks of cold air. However, there are intervals where it retreats allowing brief but welcome breaks from the cold. Overall though, the signal is for near to slightly colder than average temperatures assuming all things are equal.

This graphic measuring moderate La Nina's in ENSO 3.4 (which is indicated this year) shows the cold centered over the upper Midwest through the Great Lakes and into the Northeast.

Winter precipitation is typically about average, perhaps a bit above normal from my area east. Individual storm tracks not known at this time and can cause notable variations.

For all La Nina's this is what average snowfall looks like. Again, where individual events set-up can skew the averages. Moderate La Nina's tend to be a bit snowier than average.

Below you can see average La Nina winter temperature anomalies over the period 1949-50 through 2017-18. In my area 9 winters were colder than average, 7 near average, and 6 warmer than average. 2008-09 and 2000-01 were the coldest and 2011-12 was hands down the warmest.

THE QUASI-BIENNIAL OSCILLATION (QBO)

Another factor that can give us a clue about the winter ahead is the QBO (Quasi-Biennial-Oscillation). Considered one of the most remarkable phenomena in the Earth's atmosphere, the QBO originates in the stratosphere high above the equator where strong zonal winds blow in a continuous circuit around the Earth. The QBO oscillates back and forth blowing in an east to west direction for a few months, becomes neutral and weak for a few months, and then reverses blowing in a west to east direction. The whole cycle takes about 26 months to complete. The direction and intensity of the QBO plays an important role in the evolution of the winter pattern. It's been shown that winters with strong negative values during winter significantly increase the chances of cold and snow here in the central U.S. The majority of the climate models are indicating at least a weak to moderate QBO during the winter months.

The 700mb upper air pattern during winters with a weak to moderate La Nina and an east QBO looks like the bottom right panel. High pressure resides off the west coast and the mean trough is centered over the upper Midwest. Good if you like winter. The west QBO on the left is a much warmer solution and is not forecast.

Winters where the QBO averaged negatives of -10 to -15 C. December through February, had temperature anomalies that looked like this. The QBO ensembles shown above are leaning that way.

When the QBO averages -15 to -20 the anomalies are even colder.

TELECONNECTIONS

That leads us into another area of climate drivers known as teleconnections. There are several that I discuss regularly, especially during winter when their skill levels are greatest. The problem with teleconnections are that they are difficult for models to forecast beyond 7 to 14 days. At this distance from winter, we can only make broad assumptions as to what phase they will be in at any point in the winter based on the trends we've discussed above, in particular sea surface temperatures. Consistently warmer or colder than normal water temperatures have proven tendencies for a specific type of teleconnection to form and perhaps persist much of the winter.


One that I will rely heavily on when the time comes is the MJO (Madden Julien Oscillation). Essentially the MJO is a disturbance that moves eastward through the global tropics roughly every 30-60 days. The convection it generates feeds back energy which passes through one of eight geographical regions known as phases. Each phase is known to produce specific regions of above and below normal temperatures and precipitation across North America at a specified time of year.


During the heart of winter, phases 8, 1, and 2 are the holy grail of cold. Winters where the MJO is consistently in those phases tend to be harsh and snowy. Phases 4, 5, and 6 are the opposite. Below is the MJO phase diagram from the winter of 2017-18. If you follow the dotted lines you will see that from December 23rd to January 5th the MJO traveled through phases 8, 1, and 2. The resulting period was one of the coldest on record in many parts of the nation.

In fact, that period in the Midwest was the coldest ever measured in many areas. If it wasn't #1 in a specific region it was in the top 3.

Look at the anomalies while the MJO was in phase 1/2. The MJO is a great tool when it comes to determining trends and pattern changes.

When you see the MJO forecast to enter the cold phases with any degree of amplitude during the heart of winter look out. Now of course, MJO forecasts don't go out that far yet so it's a mute point unless you consider the correlation that a cold May leads to a cold December. The thought is that the high amplitude MJO that creates the chill in May, months later results in an MJO cycle that is in a cold phase when December arrives. If the theory holds, combined with the evidence above, this December has the potential to be cold allowing the winter to get off to a fast start. Here's the extent of the below normal temperatures this past May 2021. The MJO was amplified in cold phases.

I'm going to take this a step further too. Cool May's generally lead to a warm October's and I see that's happening this year. When October's are warm in the central and eastern U.S., analogs support a cold December in the Midwest and Great Lakes. Here's the 30 day temperature departures from the EURO weeklies through October.

Other key teleconnections exist and one to watch this year will be the PNA (Pacific North American Oscillation). When there's a large pool of warm water off the west coast of North America as there is this year, it often supports a positive PNA. This produces a persistent ridge over the western U.S. and a downstream trough in the central or eastern part of the country. That supports cold air intrusions and opens the door for respectable winter chill. See the warm SST off N. America below.

The Pacific North America pattern typically remains in a given phase anywhere from a few days, to a few weeks, and has a reputation for being unpredictable which makes it an unknown factor this early in the game. If you want cold, you want to see the PNA in a positive state. Many times the +PNA occurs in conjunction with the colder phases of the MJO. Below you can see the orientation of the jet stream with both the positive and negative phases and why the phase matters.

The last two teleconnections that are major pattern drivers are the AO (Arctic Oscillation) and the NAO (North Atlantic Oscillation). The Arctic Oscillation plays a vital role in driving the climate of the mid-latitudes and is particularly useful in long range forecasting. The AO is one of Earth’s most important atmospheric climate cycles. It occurs over the Arctic and influences mid-latitude weather patterns over the entire globe.


On average, there is consistent low pressure centered over the North Pole. The core of this low pressure is located near the stratosphere and is known as the polar/vortex. It tends to be much stronger in winter than it is in summer, and thus the influence of the Arctic Oscillation is greater in winter than it is in summer.