Unfavorable Teleconnections for US Winter Weather in 2020? (Credit: Eric Fisher and NOAA Climate Prediction Center)
DISCUSSION: For much of the US east of the Rocky Mountain Front Range, the winter of 2019-2020 has not shown much in the way of persistent chilly Arctic air infiltration. Instead, many areas east of the Rockies have witnessed weather patterns found with large-scale ridging (general poleward motion of air associated with high-pressure systems) while the Intermountain West and Pacific coast have remained fairly cool under the influence of large-scale troughing. So what is a potential cause for the unusually warm winter season? One hypothesized answer lies in the unfavorable combination of teleconnections that are currently being observed. An article previously published here goes into more detail on how these teleconnections work so for brevity, the teleconnection patterns responsible will be mentioned in the light of influencing multi-day/multi-week weather patterns as of late.
Now, all teleconnections are important in some degree since they help modulate, or control, the flow patterns across the world. In other words, a certain pattern that favors milder, more stable weather patterns over one part of the world may be countered by cooler, more progressive (stormier) weather patterns downstream, and several of these large-scale patterns are observed at the same time at different parts of the world. But, these teleconnections can undergo changes throughout the course of a year, more so on an inter-monthly time frame. Therefore, the essence of understanding large-scale patterns is to chain information from multiple teleconnections in order to gain a clearer depiction of what is occurring.
The Arctic Oscillation, or AO for short (pictured above), has remained predominantly positive. Recall that a +AO pattern yields a tendency to retain colder Arctic air north of the midlatitudes while keeping a zonal flow pattern. In essence, there is limited to no buckling in the mean flow that will help displace cold Arctic air equatorward. The North Atlantic Oscillation (NAO), which corresponds to the phenomena located between Icelandic Low and Azores High, shows as being predominantly positive as well. The East Pacific Oscillation (EPO), after being mostly positive through much of winter, is trending towards neutral and negative values. Normally, the EPO can play a key role in modulating the flow pattern across North America such that -EPO leads to large-scale ridging over Alaska and the Pacific Northwest with subsequent troughing over the northern half of the US. However, this is countered by a negative Pacific/North American (PNA), which encourages advection of polar air over the Pacific coast and intermountain west with subsequent large-scale ridging over the eastern two-thirds of the US. Storm tracks are also favored out west due to the strong grip of a typically dominant high-pressure system over the mid-latitude Pacific Ocean. When putting all of the pieces together, the result favors colder, more progressive patterns to the west and milder, subtler conditions over the east. To see all of the teleconnections described in the article, be sure to visit the Climate Prediction Center’s teleconnection page located here.
The science behind the teleconnections and is still a dominant topic in the world of climate science as researchers continue to investigate the impact that different teleconnections have as a unit. There is much to learn about these teleconnections and the interplay between them as certain patterns, such as that of recent, are not always an end-all for distinct weather patterns over the US and the rest of the world. So next time that a weather report indicates longer periods of mild and stable weather or colder and stormier weather patterns, teleconnections may provide clues into the true state of the atmosphere.
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© 2020 Meteorologist Brian Matilla
Flooding is the number one natural disaster in the United States (FEMA Mitigation Directorate), causing more property destruction and financial loss than any other extreme weather event. While these hazards are talked about heavily during the summer months, flooding can cause dangerous impacts throughout the year. This is especially true in areas already prone to flooding such as along rivers or lake shores, and those that routinely deal with significant winter weather. For example, substantial melting of snow and ice in early spring can cause devastating results. Similar outcomes occur when rivers and streams are obstructed by large chunks of melting ice, a phenomenon popularly known as an ice jam. Many meteorological and hydrological processes can result in winter time flooding and it is important to be both aware of and prepared for these circumstances.
One way winter weather can contribute to flooding conditions is simply through the rapid melting of heavy snowpacks. Despite melting on the surface, early spring ground temperatures are often still below freezing. In this instance, water is not actively absorbed by the ground beneath and instead flows along the surface toward nearby rivers and lakes, raising water levels and increasing the chance for significant flooding. Flooding is also common as ice from frozen waterways begins to melt, break apart and flow downstream. If these often large pieces of ice approach either a natural or man-made obstruction (such as a river dam), the rivers flow can be impeded. This ice jam essentially blocks the natural movement of a river or stream and causes significant flooding. When the ice jam finally releases, built up water that was blocked from its usual flow rushes downstream and consequently contributes to serious flooding impacts.
Currently, not much can be done to prevent ice jams from occurring. However, it is essential for anyone living in areas prone to winter flooding to be aware of the danger and what can be done to minimize potential damage. In addition to purchasing flood insurance, the Federal Emergency Management Agency (FEMA) suggests making a flood evacuation plan and keeping important papers in a safe, waterproof place. Taking these precautionary steps can help reduce loss and destruction even during the most obscure or improbable of flooding scenarios.
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©2019 Weather Forecaster Dennis Weaver
Like it or not, winter is officially here. For the next three months or so Midwesterners will endure cold temperatures and, of course, the dreaded “S” word—snow. But did you know that snow is favored in some areas more than others? These areas are called snow belts and lake-effect snow bands amplify these areas. Let’s dive in and take a look.
The official definition of a snow belt is rather simple; it is any area where heavy snowfall is particularly common with the help of lake-effect snow. Also, wind direction helps position these snow belts in the Great Lakes region. As you can see in the image above, most of the snow belts are on the leeward side of the Great Lakes. Cold winds in the winter usually prevail from the northwest. This wind direction produces substantial lake-effect snowfall across the Great Lakes region. The wind direction influences the amount of time cold air is over the lakes, which aids lake-effect snow. Lake-effect snow results from cold air passing over relatively warm waters of lakes. This causes lake water to be evaporated into the air, thus warming it. This warmer, wetter air rises and cools as it moves away from the lake. Once cooled, this causes the moisture to be released in the forms of snow. The greater the temperature difference between the air and water, the greater the potential of a more intense lake-effect snow event.
The Upper Peninsula Snow Belt experiences probably the vastest effect of lake-effect snow in terms of area, with the exception of the Lake Michigan snow belt. Stretching from the Porcupine Mountains (Western U.P.) to Canada, anywhere in this region can experience upwards of 250 inches (20.8 feet) of snowfall per year. For comparison, Duluth, Minnesota which is located on the southwestern tip of Lake Superior, only experiences 78 inches (6.5 feet) of snowfall per year.
Another snow belt in the Great Lakes region that experiences dramatic snow fall is the Lake Ontario and Lake Erie snow belts. These two belts can clearly be seen from the first image above. These two regions see daily snowfall totals that are higher than anywhere in the United States. This is due to intense lake-effect snow bands blasting the region with whiteout conditions. The average snowfall for these regions is roughly around 116 inches (9.6 feet) of snowfall. Due to Lake Erie’s relatively shallow depth, this lake is the only lake that is capable of completely freezing over. If this happens, the moisture source for lake-effect snow bands is cut-off, thus ceasing lake-effect snow events. This is why early in the season lake-effect snow is favored for snow accumulation.
The Lake Michigan and Lake Huron snow belts are similar in terms of intensity. The Lake Michigan side, however, can be rather unique. Under the right conditions, northerly winds can form a single band of lake-effect snow stretching along the Lake Michigan coast. This produces intense and localized snow fall. Lake Huron can experience this same intensity for the Bruce Peninsula and Georgian Bay regions. Lake-effect snow is almost a given during any winter precipitation event. It is only when small bays in this area freeze over that lake-effect snow is cut-off, other than that, localized, heavy snowfall blankets the region.
Are you in a Great Lakes region snow belt? If so, make sure proper plans are in place in case heavy snowfall impacts your area. Even if you are not in a snow belt, the winter months in any area in the Great Lakes region has their fair share of heavy snowfall.
For more winter weather information, click here!
©2019 Weather Forecaster Alec Kownacki
Winter weather can have a major impact on all aspects of life. From school closings to travel delays and cancellations, snow and ice cause a wide range of difficulties during the winter months. Additionally, snowflakes are present in almost all clouds (as most clouds exist at altitudes that experience below freezing temperatures) and as such, it is important to learn about the processes by which they grow. Whether falling to the ground as a melted summer rain or a frozen mid-winter snowstorm, ice crystals play a major role in our weather and the hazards it can sometimes produce.
Once an initial snowflake has formed, there are three primary mechanisms by which it may grow: deposition, accretion and aggregation. In the deposition process, ice crystals grow as a result of the difference in saturation vapor pressure between liquid water and ice at a given temperature. Saturation vapor pressure is the pressure exerted outward by a vapor (in this case water vapor) when the surrounding air is saturated. Since liquid water has a much higher saturation vapor pressure than ice, and molecules in the atmosphere move from high pressure toward low pressure, water vapor travels from liquid droplets toward the lower-pressure crystals. This flow of water vapor aids the growth of ice crystals and development of snowflakes. It is important to note that deposition relies heavily on the presence of both ice and liquid water and is thus temperature dependent. Maximum rates of growth by deposition occur around -15℃.
When ice crystals collide with supercooled water droplets, the crystals grow by a process known as accretion. Supercooled water simply refers to liquid water molecules that exist in liquid form at a temperature below freezing (0℃). As an ice crystal falls through a cloud, any supercooled water it comes in contact with will freeze to the crystal surface, quickly increasing the crystals size. This mechanism is most efficient between 0℃ and -10℃, where supercooled water droplets are commonly present.
Lastly, snowflakes can grow in size by aggregation, a process by which ice crystals collide with one another to form larger ice crystals. The probability that two random snowflakes collide and combine depends strongly on the shape of each crystal and the presence of liquid water on each molecule. This liquid water helps ice crystals bond together molecularly, creating larger and larger ice crystals over time. In fact, snowflakes formed by aggregation can reach 3 to 4 inches in diameter. It is important to keep in mind that all of these mechanisms for snowflake growth refer to the ice crystals within a cloud, not near the surface. However, if the temperature is consistently below freezing between the cloud and surface, these crystals can fall toward the ground and create dangerous winter weather conditions!
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©2019 Weather Forecaster Dennis Weaver
FORECAST DISCUSSION: The start of winter for the United States has already proved to be an impressive one, and despite Hawai’i’s mild weather year-round, it does not disappoint to bring an intriguing forecast to the island chain. The islands have seen unsettled weather patterns in recent days, and it isn’t expected to improve within the immediate forecast period.
Winter often brings slightly cooler temperatures, with a wet pattern, and it has not failed to show itself. West of the island chain an area of low pressure, which is expected to propagate northeastward, will pump in winds from the south. In recent days, high wind advisories have been issued and this forecast period proves no different. Beginning Tuesday afternoon through Wednesday a High Wind Watch is in effect for Kauai Windward-Kauai Mountains-Oahu North Shore-Oahu Koolau-Olomana-Central Oahu-Waianae Mountains. In fact, there is a High Surf Advisory until 6 AM HST Tuesday for Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward Haleakala-South Big Island-Big Island North and East. These winds have the potential to cause damage and power outages of note in downslope mountainous areas of Oahu and Kauai. As the front moves eastward, its strength will decrease significantly towards the Big Island and will generate a more typical trade wind pattern as high pressure moves in behind the front.
In addition to concerns with the immediate weather pattern the National Weather Service (NWS) has issued a special weather statement regarding coastal flooding coupled with strong southerly winds. With high water levels being apparent around the Hawaiian Islands, coastal flooding is possible over the next few days, but chances for coastal flooding reduce in the latter part of the week when the high-pressure system with trade wind pattern are expected to return.
For more on various forecasts and winter weather visit the Global Weather and Climate Center!
© 2019 Meteorologist Jessica Olsen
DISCUSSION: Areas farther north generally see their first measureable snowfall earlier than other places. For example, Montana typically sees it first snowfall in October to November. However, this year parts of Montana received their first snowfall earlier than normal this past weekend in late September. In addition, given Montana’s distance from water and its generally colder temperatures (colder air has a lower capacity to “hold” moisture), individual storms usually don’t produce that much snow in comparison to places near the Great Lakes, for example. However, from 27-29 September, Browning, MT received 48 inches of snow. The 19.3 inches that fell in Great Falls, MT is the second highest 2-day snow total there at any time of year, not just in September, further indicating that large amounts of snow over short periods usually don’t occur in Montana. The picture above is from a webcam in Glacier National Park after the recent snowstorm. Given the topography and latitude of Montana, they are familiar with dealing with snow and winter storms. However, the unusually large amount of snow and early timing of that snow could exasperate the impacts of this particular winter storm (e.g., road closures, power outages, etc.).
The storm was followed by unusually cold temperatures for late September/early October with lows dropping as low 7°F and 9°F on 1 and 2 October, respectively, at Browning, for example. It is important to keep in mind that transient (occurring over a short time period) regional cooling or warming is completely different than global cooling or warming. Global climate change is observed over large space and time scales. Superimposed on this global change are long-term trends in regional warming or cooling. On top of these regional longer-term trends are high-frequency variation. The recent cold spell in the northern U.S. is an example of such high-frequency, regional variation and cannot be used to explain anything related to global climate.
To learn more about other winter weather events from around the world, be sure to click here!
© 2019 Meteorologist Dr. Ken Leppert II
DISCUSSION: When it comes to looking back in history, there is nothing quite like comparing a past historic weather event to a more modern historic weather event. In the case of historic weather events which have occurred thus far in 2019, one of those events which stand out from many others is the March 13th, 2019 Plains blizzard. The most comparable weather event which compared with that event was an event which happened on the same date back in 1993 which was the March 1993 superstorm which is also known as the “Storm of the Century.” This was an unprecedented winter storm which impacted the lives of millions of people across a large portion of the U.S. Plains states. Even though these storms evolved under different circumstances and substantially different environmental factors, they were each impressive due to their collective impacts.
It should be noted that even though these storms happened during completely different eras of atmospheric observations and research capabilities, they still were respectively impressive. Having said that, it still goes without saying that both weather events will go down in history as some of the more impressive early Spring weather events in recorded history. Moreover, in looking back to March 1993, that storm delivered full-throttle blizzard conditions all the way up and down the U.S. East Coast from Alabama to Maine. As we move forward in time by roughly 26 years, you can see how a blizzard of similar ferocity occurred in March 2019 as severe blizzard conditions across parts of the central and northern Plains regions. Thus, this background information just goes to show that even despite these two blizzards being 26 years and approximately 1,500 miles apart, they were impressive nonetheless.
To learn more about other winter weather events from around the world, be sure to click here!
© 2019 Meteorologist Jordan Rabinowitz
These strange-looking images of rolled snow above are called “snow rollers” and yes, they are formed naturally! A rare occurrence, snow rollers only form during limiting conditions.
The National Geographic describes this weather phenomena as the weather equivalent of tumbleweeds. This hints that wind is what pushes the snow into these rolls. But why doesn’t this happen more often? Snow rollers only form when weather conditions have “the right mixture of moisture, snow, wind, and temperature.” The National Geographic further describes these conditions as there having to be a layer of ice with a layer of light snow on top and it must be wet enough for the snow to stick to itself. Also, winds must be around 30 mph.
Not only can snow rollers be formed by wind but, in some cases they form, by rolling downhill. As a snow rollers accumulate snow, it leaves a track behind, showing where they’ve traveled. The formation of a snow roller is very similar to the way snowmen are made. Some have compared snow rollers to the giant snowballs that roll downhill in a cartoon. They have been referred to as “snow donuts” or “snow bails”.
Some snow rollers form with a hollow center and some are packed in a swirl, similar to a cinnamon bun! Here are some pictures of the two different forms.
(A hollow snow roller.)
(A packed snow roller.)
To learn about documented snow roller events, The National Weather Service archives a few articles regarding the phenomenon.These articles include the event of December 2008 in eastern Washington and the event of March 2009 in Idaho. Some pictures the National Weather Service posted of these snow rollers can be seen below.
(Picture from the December 2008 event.)
(Picture from the March 2009 event.)
Snow rollers are just nature's anomaly. They are never dangerous, just a rare but pretty weather phenomenon.
Credit: The National Geographic and The National Weather Service
© 2019 Weather Forecaster Brittany Connelly
With the single digit high temperatures and the below freezing lows now behind us, let’s see how those bone-chilling temperatures helped with ice coverage on the Great Lakes. As shown in the graphic above, the total ice coverage on the Great Lakes is roughly 66.1%. This time last year the total lake ice coverage was about 42.2%. The roughly 24% difference from 2018 to 2019 can be attributed to the above-average temperatures in December and the rapid turn-around, (i.e., 80 degree Fahrenheit temperature change), that was experienced once the polar vortex impacted the Great Lakes region.
Believe it or not, ice-coverage is vital to the Great Lakes watershed and various ecosystems therein. Ice coverage acts as an evaporation shield to help keep the lake water from being evaporated into the atmosphere. This also reduces the amount of moisture in the atmosphere which helps to provide a source for extra low-level moisture which can often help fuel winter storms. With less ice coverage, this will produce more evaporation which, in turn, will cause more precipitation. As previously mentioned, ice on the Great Lakes acts as an “evaporation seal”—diminishing the amount of moisture being brought up to the atmosphere. The availability of more moisture in the atmosphere is what helps build winter storms, which feed off of moisture from the Great Lakes. The growing percentage of ice also helps with the region’s albedo. Albedo is the ratio of incoming radiation that is absorbed and reflected. Ice and snow, which reflects incoming solar radiation, have a high albedo due to their high reflectivity. Whereas black pavement for instance, has a low albedo to due to its lack of reflectivity and high absorption of solar radiation. In addition to albedo, ice on the Great Lakes helps regulate the ecosystem. Plankton, for instance, rely on ice to protect their populations because when ice forms over their habitat, they become more resilient and protected from warmer temperatures. Harmful algal blooms that occur in Lake Erie every year are regulated by ice because of the bacteria that causes the blooms, are killed off.
A year with low Great Lakes sea ice and a year with high Great Lakes sea ice are polar opposite in comparison. This year for instance, as of right now, we are above the 55% long-term average for lake ice. For more ice data and historical ice data visit GLERL’s website here.
To learn more about other winter weather topics, click here
©2019 Weather Forecaster Alec Kownacki
The first thought most people have when they think of winter is one thing: snow. Winters are known mostly for the white, frozen precipitation that occurs when all levels of the atmosphere (and the surface) are at the freezing mark (32 Degrees FahreNheit/0 Degrees Celsius). What happens, though, when not all levels of the troposphere, the lowest region of the atmosphere where all of our weather occurs, are below freezing? Let’s explore that idea.
The troposphere is somewhat complex. It is roughly the first 10km of the atmosphere, and is often measured in terms of pressure (millibars). When precipitation occurs, it falls through various levels of the atmosphere, often travelling through different temperatures that alter the formation of whatever precipitation is falling. When the precipitation is rain that means the temperature is above the freezing mark through at least the first couple hundred millibars of the atmosphere. However, it is often not the case that all levels of the atmosphere are above freezing, especially over much of the continental U.S. In the winter, the atmosphere goes through dramatic changes in temperature; there are different air masses that can occupy different levels. When it comes to sleet, there is likely a shallow layer of warm air in between two layers of sub-freezing temperatures. As ice crystals fall into the shallow warm layer, they melt, but only enough to melt the outer layer of the ice. As they pass through the colder layer nearest to the ground, water droplets freeze on an ice nucleus, the remaining center of the ice crystal, thereby creating sleet.
Freezing rain is a different story. Freezing rain occurs when a much deeper warm, layer is wedged above a shallow cold layer near the ground. The warm layer melts ice crystals to the point where they are rain droplets. When they pass through the shallow cold layer near the ground, they freeze on contact with any sub-freezing surface on the ground. This phenomenon occurs closer to the warm front of a cyclonic low pressure system than sleet, as the warm air is able to penetrate deeper into the atmosphere.
Its freezing rain and sleet, not snow, that often cause the most hazards to the general population. Both types of precipitation are known for the slick effect they have on roadways. They can cause black ice, or icy spots on roadways or sidewalks that are hard to spot due to their clear color. This issue is even more of a problem on elevated surfaces such as bridges and overpasses where both sides are surrounded by open air, allowing surfaces to freeze more rapidly.
The Occupational Safety and Health Administration (OSHA) also advises people to be more careful when icy weather hits. Their website lists two steps people can take to greatly reduce their risk of a life-threatening fall: wear rubber over-shoes with good treads and take slower, smaller steps.
The U.S. Department of Transportation has recorded, based on a ten-year average, over 1,800 deaths annually related to snow and icy roads. Drive safely, minimize travel in the dead of winter, and be smart and we can keep that statistic from growing even more.
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© 2019 Weather Forecaster Jacob Dolinger