Weather Science

Into the deep freeze: what large-scale climatic changes mean for Rochester weather

Mid-January has arrived which means a couple of things in the Northeast. College students reluctantly return to the grind of classes, football season is coming to an end and the winter cold has arrived in full force. Yes indeed, the bone chilling cold returned last week from a long hiatus and is back with a vengeance.

Recently, (although not so much this week), Arctic air from Canada caused frigid temperatures in locations such as International Falls, Minnesota (-35?F) while cities such as Boston and New York barely made it out of single digits. Although it may appear that subzero temperatures can seemingly come out of the blue, forecasters have the ability to predict the severity of the winter cold far before winter arrives.

Large-scale climatic teleconnections have an important influence on the weather pattern for a specific region. These teleconnections such as the North Atlantic Oscillation (NAO) and the Pacific North American pattern (PNA) relate large-scale weather patterns across a large distance, consequently having a direct impact on the weather we have experienced this winter.

The PNA is one of the most recognized, influential teleconnection patterns in the Northern Hemisphere. The positive phase of the PNA oscillation tends to be associated with warming over the Pacific and the negative phase tends to be associated with cooling over the Pacific. This warming/cooling is directly correlated to the temperature anomaly in the United States. During a positive PNA there is a ridge in the jet stream over the western U.S. with warm air infiltrating from Mexico resulting in above average temperatures. During this stage there consequently is a trough in the jet stream over the Eastern U.S. with cold air coming down from Canada resulting in cooler than normal temperatures. During a negative PNA, the temperature anomaly is directly opposite to the positive stage with cooler temperatures out west and warmer temperatures in the east. Recently, with the cold air that funneled into the northeastern U.S., the PNA shifted to the positive phase.

Another influential oscillation, the NAO is simply a “blocking” pattern that affects the location and intensity of cold air. The positive phase of the NAO tends to bring above normal temperatures along with relatively wet conditions over the Eastern seaboard. These conditions are associated with a fairly strong upper-level jet stream. During the negative phase, the upper-level jet stream weakens, allowing cold air to filter down along the east coast of the U.S. Additionally; the negative NAO tends to bring conditions drier and cooler than normal. The recent cold blast pushed the NAO into negative territory for only the second time since the beginning of winter.

Distinct changes in temperature and precipitation throughout the winter always correlate to the large-scale meteorological patterns. Understanding how these large-scale teleconnections behave during the winter is extremely important for any meteorologist when making a forecast.

Weather Science

Rainspotting: how to tell the difference between the many forms of winter precipitation.

When you walk outside this upcoming week you might notice something particularly alarming about the weather. It’s cold, real cold. Not the brutal cold Upstate New Yorkers are accustomed to, but cold enough to bring frost and possibly, just possibly a little bit of snow. Although there is a slight chance of a flurry in the forecast, Rochesterians have a better chance of seeing different forms of precipitation such as sleet, freezing rain, or even graupel. These other forms may not bring the beauty fresh white snowflakes bring, but that doesn’t mean they don’t have a significant impact on the surrounding environment.

You might recall one of those depressing winter days where the sun doesn’t seem to exist and you wished you owned a parka. It also just so happens that it is raining and the temperature at the surface is below the freezing point. You might venture outside only to fall right on your bottom due to the ice rink outside the front door.

Yeah, we know.
Photo: Wayne Nalbandian

This precipitation type is known as freezing rain. Warm air just above the surface allows falling snow to melt and fall as rain. A very shallow-layer of temperatures below 32 degrees hugs the surface causing rain to freeze on contact with roads, trees, power lines, and other structures. Light accumulations may cause dangerous travel, while heavier amounts can cause widespread, lengthy power outages.

Now think back to a time where you were driving east on the Thruway, and suddenly falling white pellets ambush your car, restricting your view to a few feet in front of you. This type of winter precipitation is known as sleet or ice pellets.

It’s always much prettier in animated form.

Sleet forms in a somewhat similar environment to freezing rain. Falling snow travels through a warm layer aloft where it melts into rain. After exiting this warmer layer, the raindrops then refreeze into pellets of ice as they fall into a very cold layer of sub-freezing air just above the surface of the earth. The difference between freezing rain and sleet is that with sleet, the cold layer near the surface is quite a bit deeper, allowing the falling rain time to refreeze. Ice pellets essentially take the form of frozen raindrops and even have the ability to accumulate. An important feature to note about sleet is that it is often see-through, solid ice. This can cause problems on any roadway, as sleet tends to act like snow when it accumulates.

Graupel. Because sleet can get more complicated.
Photo: Todd O’Bannon

Sleet is often confused with another form of wintry precipitation that people are not too familiar with known as graupel. Unlike sleet graupel is normally cloudy or white, not transparent. Graupel often forms when water droplets are collected and freeze on a falling snowflake. Since graupel is similar to sleet, it also has the ability to accumulate and cause problems on the roadways.

When you’re traveling around the Rochester area this winter, be sure to look out for these forms of wintry precipitation, as they surely will make a visit to our region.

Weather Science

So, what does the lake effect, exactly? Its not just a winter weather phenomenon:

Envision this setting: mid-day lunch-break on a brisk, bright December afternoon in Rochester. It’s one of those days where you think the blinding sun would warm the air slightly, however the air remains bitter. Now fast-forward an hour, you take a quick glance out your window to find blizzard conditions. This rapid change of conditions that Rochesterians are far too familiar with is lake-effect snow.

Although we likely won’t experience lake-effect snow for another month or so, that doesn’t mean upstate New York won’t be impacted by lake effect precipitation. Lake effect isn’t all about snow, as lake effect rain can occur in September and October.

Generally, cool air temperatures traveling over a much warmer body of water causes lake effect precipitation. Strong, chilly winds blow across a lake, picking up moisture from the water. The weather nerds call that latent heat flux. As the warmer air near the surface rises, it begins to cool and as a result is able to hold less moisture. This drop in temperature and overload of moisture causes the vapor in the air to undergo condensation (to liquid water) or deposition (to ice) forming clouds. When the water droplets or ice crystals grow to a large enough size precipitation falls from the clouds onto the downwind shores.

Upstate New York cities like Rochester are situated in the perfect position for lake effect precipitation. Throughout the year, there is a prevailing wind from the west or northwest over Lakes Ontario and Erie. Since Rochester is downwind, this region is continually pounded with lake effect precipitation from late fall, when the temperatures begin to cool, until March, which is when air temperatures come in line with the temperature of the lakes again.

One might wonder what determines if Rochester receives lake effect rain or lake effect snow. This solely depends on the air temperature. Temperatures greater than the freezing mark during a lake effect event will produce rain, or perhaps sleet; accordingly air temperatures below 32° Fahrenheit will produce snow. As a weather nerd in Upstate New York, lake effect precipitation is one of the most thrilling weather phenomena. Since lake effect precipitation is so localized, predicting where it will hit is almost an art. For example, Boonville, NY averages approximately 220 inches of snow annually, while Utica, NY averages just over 100 inches a year – a difference of 100 inches of snow in just 40 miles that mostly results from lake effect.

These drastic differences in annual snowfall occur near regions of high intensity lake-effect snowfall known as snowbelts. These are regions directly south and east of the body of water, essentially the kill zone for lake effect snow. When passing through these snowbelts traveling on I90, one will often experience snow bands with visibilities at times reduced to near zero. Take some advice from the weather wonk and always expect the unexpected when driving during lake-effect snow season.