 Photo: Satellite image of Lake-Effect Snow over the Great Lakes region on January 5, 2015. (Image courtesy of The Weather channel) Lake-effect snow primarily affects the Great Lakes region of the Midwestern United States but can have an impact on any region near a lake, capable of the ideal temperature differential to form these snows. The difference of 13 degrees Celsius in lake water temperature and air one mile above the surface is the ideal and primary ingredient necessary to form lake-effect snow. While there are other bodies of water that can have this effect (rivers, oceans, etc.), the focus of this discussion will be on lakes. The intensity of these storms can create snowfall rates of 2-3 inches in an hour and have liquid rain to snow equivalent (in inches) ratios anywhere from 1:15 to 1:30. Due to stronger updrafts (faster rising air) from the temperature differential over the lake, this leads to a higher ratio, higher snowfall rates and greater impacts to travel. The Great Lakes is the most common region where this phenomenon occurs, impacting the surrounding coasts based on the wind direction steering the snow bands. Typical lake-effect snow events are steered by a variety of north winds that brings in cold air needed to generate this phenomenon. Other locations with bodies can experience lake-effect snow like the Great Salt Lake in Utah and the finger lakes in New York. Regions can also experience ocean, river or sea-effect snow if the conditions are right. There are multiple types of lake-effect snow bands that can occur based on wind direction and can have various impacts. Short lake axis parallel (SLAP) bands are often multiple small bands that follow the short axis of the lake and impact multiple smaller areas. Long lake axis parallel bands are often single bands following the long axis, usually creating a larger band with impacts over a single greater area. Other types of lake-effect snow are those that make multiple lake interaction, form and remain mainly over the lake, and form a vortex (or a small scale low pressure system, often called a meso-low) that can bring snows onshore. Due to shifting wind directions and weak synoptic (large scale) winds, vorticies can form within the snow band or over the lake. If stronger winds were present, a vortex will generally break up, but with cold air present and weak wind directional changes they can form and bring snow inland.  Photo: Map of Lake Michigan with red line indicating the Short Axis of the lake and the black line indicating the long axis of the lake. Image courtesy of the University of Michigan. Terminology can often be confused when discussing lake induced snow events and how they impact a region. Since there is a difference between lake-effect and lake-enhanced snows, the distinction is necessary to discuss. Lake-effect produces snowfall as an effect of the presence of the lake, thus if the lake were not present, then the temperature differential needed to form the band would not exist. On the other hand, lake enhancement will enhance the snow production of a lake-passing system because of cold air aiding the development of clouds while passing over the warmer waters. Understanding the nature of different types of lake-effect snow will help determine the impacts to a specific region based on the size of the bands. Forecasting is difficult for lake-effect snow because of the mesoscale nature of the event, but there are multiple processes that we can use to better that prediction. Nowcasting becomes necessary to concentrate on the changing structure of the bands and pinpoint the direct impacts to a town. Overall, lake-based snow systems are location dependent on proximity to a lake and how far inland these systems can impact. The best practice during winter storms would be to be aware of and prepared for rapidly changing conditions in real-time to determine the impacts to your specific location. To learn more about other interesting winter weather topics from around the world, click here! © 2018 Meteorologist Jason Maska
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 Of all the media attention the polar vortex has received in recent years, the majority of people just know it as “a system that brings cold air to my city every winter.” Although this is a true statement, let’s discover together what exactly the polar vortex is. After all, the term “polar vortex” has been in use even before the Civil War.
The polar vortex is a low-pressure system that resides over the poles with its strongest extent during the winter months. It is strongest in the winter because of the vast temperature contrast between the Polar regions and the mid-latitude regions. People often associate the polar vortex with cold fronts that produce snow. Although this vortex does bring a punch of cold air, the association of it with snow is simply not the case. The polar vortex primarily occurs in the stratosphere, a layer of the atmosphere about 6-30 miles above the troposphere where most of the weather occurs. Although it is 30 miles above the ground, it still has a profound impact on day-to-day weather. Changes in the stratospheric polar vortex can effect jet-stream patterns by forcing them southward which in turn can influence the location and persistence of cold air masses across North America and Europe. Here’s the kicker: when the polar vortex is strongest, we are less likely to see a deep cold air mass plunge into North America or Europe. It’s actually when the vortex is at its weakest, when we typically will see the cold air punch. An easy analogy to remember this would be when it is strongest (like in the left picture above) with no waves, it keeps the cold air at bay, kind of like a barrier. When it is at its weakest (like in the picture on the right) with multiple waves, the cold air rushes through the barrier and into North America and Europe. Like North America and Europe is currently seeing, the polar vortex occasionally weakens. This is due to energy being brought upward from the lower atmosphere. This happens when the stratosphere suddenly warms, an event referred to as sudden stratospheric warming (SSW). This event raises temperatures to about 50 degrees Fahrenheit, or more, in just a few days in the stratosphere. Although this event sounds rare, it occurs at least once every other winter season. This sudden warming disrupts and weakens the polar vortex which shifts the location of it southward and can physically split the vortex up. When this happens, the weakened vortex forces the jet-stream to become more prone to “blocking.” This blocking usually happens in colder months and can easily break down the “cold air barrier” as stated earlier. This leads to a more persistent cold weather pattern for weeks. A lot goes into fully understanding the polar vortex. Even more goes into forecasting it and knowing exactly when or if it will impact weather patterns across North American and Europe. As we travel through colder weather months, the ever-impending polar vortex may soon make an appearance but before that happens, much more forecasting is needed. For more winter weather topics, click here. ©2018 Weather Forecaster Alec Kownacki    DISCUSSION: An early December winter storm made its way across the southern United States with past week. The storm initially impacted southern California on Dec. 5th, 2018. It brought heavy snowfall to the mountain regions, with some areas recording over a foot of snow. In the lower elevations and closer to the coastline, extreme rainfall lead to flash flooding from Los Angeles to San Diego. According to the NWS San Diego, rainfall totals from the storm were greater than 4.5 inches in some places. John Wayne Airport received 3.5 inches while the San Diego International Airport received 2.6 inches. In addition to causing flash flooding, the extreme rainfall triggered mudslides, resulting in road closures, including the Pacific Coast Highway, and evacuations in areas that recently evacuated due to the November wildfires. However, a silver lining does exist for California, the early season snow has resulted in a higher than average snowpack in the Sierra Nevada Mountains. California relies on the melting of this snowpack in the spring to recharge its reservoirs to support their water demand during their dry season in summer and early fall.
After impacting southern California, the storm traversed across Arizona and New Mexico before moving into Texas, with heavy snow and rainfall impacting the state. In northern portions of Texas heavy snowfall occurred and places like Lubbock saw as much as 10 inches of snow from the storm. Meanwhile, rain was dumped across the Houston region, resulting in major flooding. Some places saw over six inches of rainfall, while College Station broke its daily maximum rainfall record with 4.01 inches of rain on Dec 07, 2018. According to the NWS Houston/Galveston the previous record was 1.70 inches in 1931. Moving out of Texas the storm traversed across southern Louisiana, the Florida Panhandle, and into southern South Carolina. As it tracked eastward, snowfall continued to fall within the northern portion of the system, bringing snow from southern Oklahoma to the Mid-Atlantic Mountains. The heaviest of this snow occurred in the Blue Ridge Mountains in North Carolina and Virginia, with some areas in recording over two feet of snow. One place in North Carolina, the small town of Busick, which is located in the mountains of the state, saw 34 inches of snow. Meanwhile areas not in the mountains saw record breaking snowfall. The Raleigh/Durham international airport North Carolina recorded 7 inches, which is a record for Dec 10. Additionally, the city of Raleigh saw the most snowfall in a single day in the month of December since 1958. Furthermore, this event resulted in the city already exceeding their yearly average snowfall of 7 inches. Another record was set, the second highest single-day snowfall total, at the Richmond International airport in Virginia, which recorded 11.5 inches of snow on Dec 9, 2018. When all was said and done, this early winter storm brought snowfall across the southern US, from CA to VA and heavy rainfall to many locations. It resulted in road and school closures, thousands of cancelled flights and car accidents, hundreds of thousands without power, mudslides, flash floods, and, unfortunately, some deaths. The system is now off the eastern coast, over the Atlantic Ocean but it is still driving freezing temperatures and gusty winds across the south, resulting in freeze warnings extending into Florida. To learn more about other interesting winter weather topics from around the world, click here! © 2018 Meteorologist Sarah Trojniak, Ph.D. How can snow be on Doppler radar but not reach the ground? (Imagery credit: WSI Radar Imagery)12/7/2018   DISCUSSION: Have you ever been impacted by a classic mid-Winter snowstorm and found that there was snow falling over your region according to Doppler radar imagery, but there was nothing falling at the ground? Most people get confused about when this situation unfolds with an approaching snowstorm. However, there is a relatively simple explanation for why this type of situation can be realized in association with any given snowstorm. That simple explanation comes with the meteorological term which is known as virga. Virga is simply snowfall or any other type of precipitation which is unable to reach the ground.
As a classic inland or coastal winter storm is approaching a given region, there is often a particularly cold air mass in place. The exceptionally cold air which happens to be in place across the specific region will typically have the characteristics of being a drier air mass. As a result of there being a drier air mass present over the given region, this will consequently lead to there being a larger dew point depression. A larger dew point depression is simply the measurement which is defined by the difference between the air temperature and the dew point temperature at a given location. The larger the dew point depression happens to be at a given location, the longer it will take snowfall to evaporate the respective layers of the atmosphere from cloud level and on down to the surface of the Earth. As the snowfall continues to evaporate through the depth of the drier low levels of the troposphere, this consequently leads to a decrease in the regional dew point depression values which ultimately leads to an increasingly moister near-surface layer. Thus, as the near-surface to sub-cloud layers become increasingly moister with time, this will gradually allow snowfall to finally reach the ground and accumulate on the ground if the surface is sufficiently cold. Hence, if you ever find yourself looking at a news station’s regional radar imagery or your local National Weather Service forecast office radar imagery and see snowfall over your region (but not quite reaching the surface yet), you will now have more insights as to why this is happening. To learn more about other interesting winter weather topics from around the world, click here! © 2018 Meteorologist Jordan Rabinowitz   DISCUSSION: Even though the month of November just ended, and the month of December is underway, there are still things to remember and learn about Mother Nature. One major thing is that Mother Nature most certainly does not hardly ever follow a set clock. More specifically, most people most often associate the heart of Winter with the occurrence of coastal or inland low-pressure systems which induce regional and/or multi-regional snowstorms. Thus, if someone ever is completely shocked by heavy, accumulating snowfall occurring across the central of eastern United States during late October or throughout any part of November, this should not be nearly as shocking as one would think. That is a result of the fact that under the right combination of larger-scale and smaller-scale atmospheric and environmental conditions, a snowstorm can occur well before the start of December. The upper-most image is taken during the evolution of the third Nor'easter which occurred off the coastline from the northeastern United States in March of 2018.
In looking at the lower graphic attached above (as provided by Meteorologist Tom Niziol from The Weather Channel), the above information is clearly reflected by the corresponding graphical context. To be precise, the context of graphic above reflects the reality that between 1959 and 2000 (i.e., the 41-year period ending at the start of the 21st century) there was a substantial prevalence of blizzard occurrence during the month of November across a good portion of the central and the north-central Plains states. It is important to acknowledge the fact that this graphic came from the journal article published by Schwartz and Schmidlin (2002) which was a research project that involved studying blizzard events across the contiguous United States and was entitled “Climatology of Blizzards in the Conterminous United States, 1959-2000.” As you can see from the lower image above, there have been a solid distribution of November blizzards across the northern tier of the United States with the heart of the November blizzard event concentration being across North Dakota and South Dakota. It just goes to show that as the atmosphere is transitioning from Fall to Winter, the southern shifting of the Polar jet stream likely has a role in the presence of the greater snowstorm frequency across the north-central Plains region of the United States. To learn more about other winter weather topics, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz  DISCUSSION: When it comes to anticipating rounds of cold air intrusions during a given Winter season, there is no doubt that one of the more interesting non-precipitative atmospheric phenomena which occurs is cold air damming. Cold air damming occurs most often during the early to middle parts of Winter when high-pressure systems sink southward out of eastern and/or southeastern Canada and deliver colder air masses to the greater part of the northeastern United States. As this process unfolds, there is a plethora of cold air which will often get “stuck” within valleys or any other regional areas of lower elevation. This phenomenon most often is famous for occurring along the East Coast of the United States in the vicinity of the Appalachian Mountains.
The premiere reason for why cold air damming is so impactful during any part of a Winter season has to do with several different factors which play a key role in association with cold air damming. Cold air damming typically occurs in the mid-latitudes as this part of planet Earth lies within the prevailing westerlies which is an area where frontal intrusions are more common during a typical calendar year. When the Arctic oscillation is in its negative phase and the regional atmospheric pressure is found to be higher over the polar regions, the flow is more meridional (i.e., more curvy flow with respect to the east-west plane), blowing from the direction of the pole towards the equator, which brings colder air into the mid-latitudes. It is worth noting that cold air damming is observed in the Southern Hemisphere just to the east of the Andes Mountains, with cool incursions sometimes observed as far equator-ward as the 10 degrees south. In the Northern Hemisphere, common situations occur along the east side of ranges within the Rocky Mountains system over the western portions of the Great Plains, as well as various other mountain ranges along the West Coast of the United States. The initial is caused by the poleward portion of a split upper level trough, with the damming preceding the arrival of the more equator-ward portion of the approaching upper level trough feature. Furthermore, it is also important to acknowledge that some of the cold air damming events which occur east of the Rockies will go on to continue southward to the east of the Sierra Madre Oriental through the coastal plain of Mexico and down through the Isthmus of Tehuantepec. Further funneling of such cool air occurs within the Isthmus of Tehuantepec, which can occasionally lead to winds of gale and hurricane-force. Other common instances of cold air damming take place on the coastal plain of east-central North America, between the Appalachian Mountains and Atlantic Ocean which are the most common trigger mechanisms for coastal ice storms and heavier snowfall during Winter-time along the East Coast of the United States (and this is also reflected by the idealized graphic which is attached above courtesy of the National Weather Service network). In Europe, areas south of the Alps can also be quite prone to cold air damming which can help to support quite impressive snowfall events in the higher elevation regions spread across the span of the Alps. Lastly, in parts of central and eastern Asia, cold air damming has been documented near Taiwan and the Korean Peninsula which will often have similar consequences to cold air damming events which occur in other parts of the world. To learn more about other high-impact winter weather topics from around the world, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz |
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