00:24 Mexico Earthquake Makes Waves in Pool A powerful earthquake in southern Mexico Tuesday turned a swimming pool into a wave pool in Mexico City.

At a Glance A split temperature story is expected this winter: cold in the East and warm in the West.

February is anticipated to be the coldest of the three winter months.

A developing variation of El Niño and a weak polar vortex are the cold culprits.

Prolonged November cold and early snowfall are helping to raise the odds of a persistently cold winter in the East, according to the latest outlook from The Weather Company, an IBM Business.

The United States has had its most expansive November snow cover in almost three years due to a plunge of the jet stream centered over the central U.S.

This persistent upper-level trough has led to above-average snowfall not only where snow is expected along the northern tier of the country, but also throughout much of the Plains.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/snow_cover_anom_1113b.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/snow_cover_anom_1113b.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/snow_cover_anom_1113b.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Snow cover departure from average on Nov. 13. Blue squares indicate where more snow has fallen compared to average. (Rutgers Global Snow Lab) (Rutgers Global Snow Lab)

The colder air and increased snow cover may set the stage for frigid weather to take over a good section of the country this winter.

The overall trend calls for a colder-than-average winter in the eastern half of the country while the West remains warm, according to The Weather Company's latest winter outlook.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/wsi_winter_1116.png?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/wsi_winter_1116.png?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/wsi_winter_1116.png?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > December-through-February temperature outlook from The Weather Company, an IBM Business.

During each of the three months of meteorological winter – December, January and February – the temperature pattern is expected to be similar, with colder air on the East Coast and across the South while the warmer-than-average weather remains in the Northwest.

The December outlook calls for the coolest weather, relative to average, to be in the Northeast and the southern half of Texas while the Northwest remains well above average.

Warmer-than-average temperatures are forecast to expand to much of the West Coast and the northern Rockies in the first month of 2019. The core of the colder-than-average air is expected to shift to the Gulf Coast and much of the Southeast.

The harsher half of winter appears to be the second half of the season, with a good chunk of the Northeast and Great Lakes experiencing far-below-average temperatures in February. The cold air will likely continue across the Gulf Coast while expanding into the Plains.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/wsi_february2019_outlook.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/wsi_february2019_outlook.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/wsi_february2019_outlook.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > February temperature outlook from The Weather Company, an IBM Business.

Meanwhile, well-above-average temperatures are expected to return to the Pacific Northwest in February.

These are seasonal or monthly averages, meaning that we'll all have periods of both warmer and colder weather this winter, but the above maps show what kinds of temperatures will dominate.

(MORE: Four Odd Things We Saw in November's Cold Snap )

The Brains Behind the 'Brrr'

A few other factors will weigh heavily on how the winter evolves.

A Wilder Jet Stream

The first factor that might change how winter shakes out is the strength of the polar vortex.

November's big trough across the eastern U.S. gives us some idea that the polar vortex – the semi-permanent low-pressure system that can reconfigure our nation's weather pattern – will be weaker this winter.

At first glance, you would think this is a good thing if you like a quiet winter, but a weaker polar vortex allows for more arctic blasts to surge southward.

(MORE: What Is the Polar Vortex? )

This is because air that circulates around a stronger polar vortex – again, a low-pressure system – maintains its momentum around the polar region. A weaker polar vortex is vulnerable to being thrown out of whack.

One signature of a weaker polar vortex is a "blocked-up" flow of weather from west-to-east, which is what we're seeing in November. This clogged pattern has allowed for repeated bursts of cold air to flow in from Canada.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/negative_nao.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/negative_nao.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/negative_nao.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > A blocky pattern, a signature of the negative phase of the North Atlantic Oscillation (negative NAO), is increasingly likely this winter.

"Now that we are seeing significant blocking here at the end of November, this gives us more confidence that a 'weak vortex' winter is more likely than not, and we are more comfortable increasing the magnitude of cold in our winter forecast," Crawford said.

This more variable jet stream idea is also supported by our current place in the solar cycle. It is thought that we are now within a year of the least active part of the 11-year solar cycle, meaning less heat is being directed at our equator.

Less heat in the equatorial region generally means there is less of a temperature gradient between the poles and the equator, which leads to a weaker polar vortex and more opportunities for cold outbreaks in the U.S.

Developing Modoki

Secondly, a regional specialty variation of El Niño is maturing in the Pacific, and it will set up the jet stream in a pattern that will favor dumps of cold air into the eastern U.S.

This particular variation of El Niño, known as Modoki El Niño, is similar to a regular El Niño but is a relocation of the warmest waters to the central Pacific Ocean.

As a reminder, a typical El Niño is declared once water temperatures in the Eastern Pacific Ocean reach 0.5 degrees Celsius above average and the warmth is sustained for at least three months.

(MORE: Here's How Much El Niño Usually Influences Winter Temperatures in the U.S. )

Water temperatures are currently warm enough, but have not been warm for a long enough period of time, to declare a regular El Niño. Still, temperatures in the central Pacific, where the Modoki variation of El Niño is born, are approaching 1 degree Celsius above average, and the atmosphere is beginning to respond.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/menso_1114.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/menso_1114.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/menso_1114.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Water temperatures are running warmer than average across the central Pacific Ocean. Zones typically monitored for El Niño are outlined with boxes. Current as of mid-November.

A Modoki El Niño generally brings bulges in the jet stream over the northwestern Pacific and drives a plunge in the jet stream over the eastern half of the U.S.

The atmosphere and its weather patterns often begin to change ahead of when scientists mark the beginning of a new phase of El Niño.

"The index [that measures the atmospheric response to El Niño] has been slowly increasing during 2018, with a big jump over the last couple of months, representing the onset of this new event and helping drive a colder pattern change in the U.S. in October and November," said Dr. Todd Crawford , chief meteorologist at The Weather Company.

This ocean-atmospheric pattern is expected to mature throughout the winter. A signal for colder weather grows from November through February in both computer models that develop a composite of winters with a similar El Niño state and in long-range climate models. This supports the idea of a colder back half of the season.