Why Was the Winter of 2013-14 so Cold and Snowy

Kevin Birk and Ricky CastroNWS Chicago- Romeoville, IL

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What is an “Average” Winter(1981-2010)

December-February Average Temperature

24-27° Ave daily highs start around 40 and dip to near 30 in January,

then recover back to near 40 by the end of February.

Ave daily lows start in the low to mid 20s and dip to the mid teens in January, then recover back into the low to mid 20s by the end of February.

December-February Average Precipitation

4-6 inches

December-February Average Snowfall

Around 29 inches

Average Yearly Total Snowfall (July-June)

Around 37 inches

What Were the Stats This Winter(2013-14)

Temperatures in Chicago

3rd coldest meteorological winter (Dec-Feb) with a seasonal ave temperature of 18.8° (6.5° below ave)

Coldest December to March period on record in Chicago with an ave temperature of 22°.

60 of 90 days had high temperatures of 32° or colder.

25 days saw low temperatures below 0°.

Lowest temperature was -16° (-20°s in the suburbs)

Total Snowfall (July-June)

82 inches (225% of normal and the 3rd most ever).

What Caused This?

The behavior of the Polar Jet Stream during the winter season largely determines how cold and snowy the region will be. A Jet Stream is a river of wind that blows horizontally. It is

concentrated in a relatively narrow and shallow stream in the upper troposphere of the earth (around 30-35,000 feet).

A Jet Stream will be in place near areas of strong temperature contrasts at earths the surface. Jet stream winds can reach speeds in excess of 250 mph in the winter.

The Jet Stream The Jet Stream pattern can be more zonal (direct west to

east flow) in nature, or it can be meridional (north to south flow). It can even transition from one to the other. Generally when the Jet Stream pattern is zonal over the area,

conditions tend to be mild and drier during the winter.

However, when the Jet Stream buckles into troughs and ridges, the weather pattern becomes more stormy.

Winter storms tend to form just downstream of the trough in the upper level jet stream.

The atmosphere is the coldest where the jet stream forms a trough, and warmest under the ridge.

The Jet Stream The strength and placement of the winter Polar Jet Stream can

vary significantly from day to day during a season, and can also be very different from one winter to another.

As a result, some days during a winter can be very cold and snowy, while others are dry and mild.

Similarly, one winter can be bad (Snowy and very cold), while the next winter may be relatively nice (mild with less snow).

Although it is typical to have cold and snowy periods during the winter, it is unusual to have such persistent cold and snow throughout the entire season.

Winters with persistent Cold and Snowy periods tend to have a jet stream pattern that is “locked” in place geographically (less variable day to day).

The Jet Stream Pattern this Past Winter A Persistent and very strong large scale ridge in the Jet Stream

pattern set up across the North Pacific, Western North America and Alaska.

Lead to near record warmth across Alaska and the western CONUS.

The strong ridge out west buckled the Jet Stream far southward forming a trough across the central and eastern CONUS.

This pattern allowed for air masses originating from northern Canada and even Siberia to drop southward across much of the region.

The combination of this extreme dip in the jet stream and presence of very cold air also set the stage for numerous winter storms with heavy snowfall.

The Jet Stream Pattern Winter 2011-12

In contrast to this past winter, March of 2012 was extremely warm with little snow.

The Jet Stream pattern here favored ridging across much of the country.

Lead to record shattering warmth across much of the central and eastern CONUS.

However, across Alaska, the Jet Stream formed a strong trough, which produced very cold conditions there.

What Impacts the Behavior of the Jet Stream?

The behavior of the Jet Stream during a season can be altered by many things. However, over the past few years, these have played a large roll in setting up the preferred placement and strength of the Jet Stream across North America.

The Eastern Pacific Oscillation (EPO).

The Arctic/North Atlantic Oscillation (AO/NAO).

Each of these oscillations have alternating positive (+) and negative (-) phases. The phases are defined by differences in atmospheric pressure patterns between the high and the mid latitudes, which in turn drive the strength and placement of the jet stream.

Upper Level Pattern Near AlaskaAlso known as East Pacific Oscillation (EPO)

Low Pressure near/over Alaska

(+EPO)

Tendency this winter was for higher pressure near Alaska (-EPO)

• Persistent strong high pressure over and near Alaska (-EPO) acts to drive deeper cold southward from Canada (Jet Stream trough over the CONUS). • This occurred this

past winter.

• However, persistent strong low pressure over and near Alaska (+EPO) can result in Pacific air masses moving across the U.S., favoring mild conditions. • This occurred in

winter 2011-12 and 1st half of winter 2012-13.

High Pressure near/over Alaska

(-EPO)

Temperature anomalies

Pressureanomalies

Temperature anomalies

Pressureanomalies

Jet Stream

Jet StreamJet Stream

Jet Stream

Eastern Pacific Oscillation (EPO)

Tendency this winter was for higher pressure (-EPO) near Alaska

Figure shows the upper level pressure and Jet stream pattern from Dec 2013 through Feb 2014.

• Higher than normal pressure has dominated near Alaska and this helped buckle the jet stream into a large trough across the central CONUS.

• This is a weather pattern associated with a strong –EPO.• Some of the previous coldest

winters have occurred during this type of pattern.

A north to south dipole pattern of atmospheric pressure anomalies of opposite sign across the North Atlantic and Arctic.

Consists of the positive and the negative phases shown in the image below.

• Negative phase favors more arctic outbreaks with a Jet Stream trough and colder than normal conditions and more snowfall across the eastern half of the country.

• Positive phase favors Jet Stream ridging or a zonal Jet and hence warmer and drier conditions across the eastern half of the country.

North Atlantic and Arctic Oscillation (NAO/AO)

North Atlantic and Arctic Oscillation (NAO/AO)

Although the NAO/AO were not big players in the cold this past winter, during March of 2013 a –NAO set up and produced a large trough in the Jet Stream over the eastern CONUS.

Cold and snowy conditions resulted from this pattern. A similar pattern occurred in the winter of 2009-10 and in December 2010 through January 2011.

-NAO signal

Jet Stream trough Develops over east CONUS

Conclusion Knowledge of the Jet Stream is very important in day

to day forecasting of storm systems. However, the behavior of the Jet Stream over a longer period of time can also determine how bad a winter will be.

No matter the cause, when a large Jet Stream trough forms over an area, cold and more active weather can be expected. If the pattern locks in place for an extended period, this can

result in a very prolonged period of cold weather with little relief.

In contrast, when the Jet stream ridges or is more west to east (zonal), relatively mild and drier conditions result.

The main challenge in longer range forecasting, is being able to predict where these troughs and ridges in the Jet Stream will set up, and hence whether an area will be cold or mild.

THANKS FOR READING!

If you have any questions,email us:nws.chicago@noaa.gov

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