The Four Seasons Newsletter

Welcome to the Winter 2024 newsletter in a new format! It will remain a quarterly newsletter, but if you are unfamiliar with the StoryMap format, it is a big change. The StoryMap is a platform for storytelling that can combine interactive maps, multimedia content, and text in a more engaging way than a typical document. To jump to a story of interest, you can navigate at the top. Hope you enjoy!

If you missed it, thanks to a Herculean effort by our local Skywarn coordinator, we now have an NWS Burlington Spotter Training guide in this very Storymap style. It also describes a lot of what we do in the National Weather Service, so if you are curious about what our day to day jobs look like or would like a refresher, take a look. If you are not already a certified spotter and this guide interests you, please reach out via  email  or  phone .

With totality through much of northern Vermont and northern New York, we launched a website with hopes of having a one stop shop for information about the upcoming solar eclipse. It includes emphasis on cold water safety that will be especially important if the weather happens to be nice that day, which is possible but not likely based on climatology. The website will have detailed forecast information for many sites in the path of totality as we approach the event. Following the eclipse, we also plan to share temperature and solar radiation traces from various sites in our region.

On December 18-19, 2023, portions of the Northern Adirondack Mountains and much of Vermont were affected by moderate to major flooding as strong low pressure tracked across the region. For Vermont, it was the second severe flooding event in six months, marking a stark reality that the state is increasingly susceptible to more frequent and extreme flooding events due in part to climate change. While high water was a common theme between this and last July’s severe flood event, the causes were different in December with flooding being primarily driven by non-convective (i.e. no thunderstorms) heavy rainfall and rapid snowmelt. Many watersheds were affected by high water, most notably the Ausable, Lamoille, Otter Creek, Passumpsic and Winooski River basins.

The snowmelt aspect of this most recent event is worthy of discussion and is important in the context of winter/early spring high water events when an extant snowpack lies across the area. Additionally, snowmelt flooding is not an alien concept to those who live in our region, with similar events occurring as recent as January 2018 and April 2019. One of the more important concepts to understand is the process of condensation. Condensation adds heat into a system, which is opposite of evaporation which takes heat away from a system. From a physics perspective, when extremely mild temperatures and dewpoints flow across a snowpack, air in contact with the snow will cool and condense on the pack leading to a process called latent heating. This heating of the snowpack is typically gradual during days which climb marginally above freezing, but under certain conditions, especially when air and dewpoint temperatures are above 45 degrees and rain is falling, this latent heating effect can be extreme. This is exactly what happened during the December event. From our best estimates, much of the existing snowpack below 1,500 feet was rapidly melted off, adding between two and three inches of runoff into area rivers and streams. This runoff, combined with the estimated two to four inches of rain that fell, led to an overall storm event of 4 to 7 inches. 

Another important metric to consider when assessing flood potential is the rapidity in which runoff moves into area watersheds. The faster runoff occurs, the less efficient river and area streams can route the excess water downstream. This concept is critically important to understand during summer when intense convective rainfall from thunderstorms can lead to flash flooding in small watersheds. If an area receives three inches of rain in one hour, it is much more susceptible to flash flooding than if it receives just one inch in that same time frame. While these excessive rates were not observed this past December, the concept still holds true in this case since the majority of the flooding occurred on larger streams and rivers which affect much broader watershed regions.  Additionally, the December excessive rainfall and snowmelt loss generally occurred in just 12 to 18 hours. While this typically wasn’t enough to produce flash flooding, it was more than enough to cause significant to severe riverine flooding, especially considering the time of year and the lack of water uptake/absorption by vegetation.

The main player responsible for this high impact event was rapidly deepening low pressure which arose out of a cluster of intense thunderstorms across the northeastern Gulf of Mexico and portions of the southeast on December 16 and 17. This cluster produced significant flooding across portions of North Florida and South Georgia during this time. The deepening low then tracked steadily north-northeast along the southeast and Mid-Atlantic seaboard by early on the 18th, and then across Vermont later on the 18th into the early morning hours on the 19th. Copious moisture of Gulf of Mexico and western Atlantic origin streamed northward into our region during this time frame, accompanied by extremely mild air by December standards as temperatures climbed into the 40s and 50s along with dewpoints of similar magnitude. While the system produced impressive rainfall totals in its own right, the rapid and near complete melt-off of the existing snowpack in the low to middle elevations was quite remarkable, especially given it largely occurred within 12 to 18 hours. The combined total water into the stream and river system from rainfall and snowmelt averaged between 4 and 7 inches from the Adirondacks eastward, leading to many areas of stream and river flooding, some severe. By later on the 19th the storm system exited quickly northward into Quebec as rainfall tapered off and temperatures cooled. Lingering high water and residual runoff continued into the 20th as flood waters receded, leaving a muddy cleanup to follow.

As with any weather event, impacts were varied across our region and the degree of flooding severity was highly dependent on 1) overall runoff rates, 2) precipitation amounts, 3) relation to the storm track and 4) specific geographic characteristics of individual stream and river basins. Observational data and flood damage reports collected by the United States Geological Survey (USGS) and NWS Burlington highlighted the Ausable, Lamoille and Winooski River basins as being most severely affected and some of those impacts are discussed briefly below. However, many other parts of our region experienced elevated water levels and in some cases varying degrees of flooding.

Over the last fall, NWS Burlington meteorologists Jessica Storm and Robert Haynes have been making rounds across libraries near the Burlington metro to discuss the role of the National Weather Service (NWS) in the community. The aim has been to “pull back the curtain” on how the tools of how we forecast the weather, spread the word, and interact with the public.

So far, they have presented at the Charlotte Public Library, for both an adult and children’s program, as well as Milton Public Library and Colchester’s Burnham Memorial Library. The presentation explained the structure of the NWS and what NWS meteorologists do each day. While the main task of an operational meteorologist at the NWS is forecasting the weather, there’s also plenty of outreach, observations, and information sharing that occurs at a weather station. Attendants of these sessions got up close and personal with radiosondes, which are weather observing instruments, and at the children’s session, the kids got to make their own anemometer out of everyday household items. 

In addition to libraries, the NWS also reaches out to the public through schools. The National Weather Service also has a National School Outreach Team that works, largely virtually, to bring weather and science education to schools across the country–even across the world when they presented virtually to the Shanghai American School at 12:15 AM Eastern Time. Most of the weather presentations the NWS produces for schools include basic weather topics like low and high pressure as well as warm and cold fronts. Then, they bring science into the practical sphere and describe severe weather conditions like tornadoes and flash floods, and how to stay safe when they occur. 

In the coming weeks, we’ll focus on Burlington’s Fletcher Free Library to discuss the upcoming total solar eclipse! This talk will take place on Sunday, March 24 at 1 PM where we’ll be explaining the science behind eclipses. 

Vermont and northern New York are no strangers to windy weather. The combination of the Bernoulli effect and stability-driven processes like downslope or gap wind events results in localized to region-wide damaging wind events due to our topography. Outside of thunderstorm-driven wind gusts, we can experience damaging wind gusts as the result of large cyclones during Fall through Spring due to the greater temperature difference between the subtropics and the poles.

Beginning in the 1990s, the National Weather Service issued Wind Advisories or High Wind Warnings when strong to damaging winds were possible due to large cyclones. For us, this usually occurs as intense cyclones move southwest to northeast from the Great Lakes or rapidly deepening Nor’easters that meander near Cape Cod. On average, we issue our first Wind Headline around October 10th and the last one around April 11th (as of 2022). Since the inception of Wind Advisories and High Wind Warnings, the National Weather Service in Burlington, Vermont began on December 12th (in the year 2000), until 2023! In 2023, it took until December 18th for the first Wind Advisory to take effect. However, that day would be mainly dominated by the significant flooding that occurred across the region. With 251 days between the last advisory issued on April 11th, 2023, it was the 2nd longest gap between the final Wind Headline from the cold season to the next cold season, only bested by the gap between the 2001-2002 and 2002-2003 seasons. How did we go so long without damaging winds?

The synoptic scale pattern on average was not conducive to generating high wind events from September through November 2023. A broad region of anomalous high pressure across Canada, and an elongated zone of below normal heights over the Atlantic prevented large systems from developing over the Great Lakes and kept any coastal systems well offshore. The 850mb winds that we usually examine to determine the likelihood of damaging gusts were below normal across much of the eastern United States (Figure 1). The winds aloft were overall weaker than normal (not shown).

Especially early in Fall and later in Spring, the ability to warm the surface to create a mixed layer is important to developing strong winds. This produces instability that makes it easier for higher winds aloft down to the surface. When a low-level jet is strongest during peak daytime heating, there is less resistance when these winds descend to the surface. Radiational cooling at night or a strong temperature inversion beneath the area of the strongest wind makes it more difficult for higher gusts to reach the surface. Using Burlington as an example, one can see the diurnal trend in strong winds with the frequency of 20 mph sustained winds or higher most likely between 9 AM and 4 PM. Not only had we experienced fewer days than usual for winds above 20 mph, but the instances they did occur were spread out more into the evening and overnight hours. The exception appears to be 8 PM (Figure 2). So when we did have strong winds in place, it was likely harder to achieve higher gusts due to the time of day.

Strong winds are more likely from fall through spring due to the greater temperature contrast. In summer, Florida is likely near 90, but we can also be 90. In the heart of winter, we can be near 0 while Florida is still around 70. Thermal contrasts are a key part of producing wind, and the greater difference in winter makes us more susceptible to high wind gusts. One of climate change’s impacts is a faster rate of warming over time across higher latitudes than over low latitudes ( see maps from NOAA Climate.gov ). As a result,  research shows  that terrestrial stilling may occur, causing an overall decrease in winds over our area as the gap in temperature between our latitude and the subtropics decreases. We will again pick on BTV, mainly for the robustness of its records, but the 30-year running average wind speed from September 1st through December 13th has been observing a decline since the 2000s (Figure 3). While that may be true of the average wind across a season, there will continue to be strong cold fronts and cyclones that will be capable of generating high wind gusts. Thermal conditions may also affect our snowpack, which acts to produce cool air near the surface that often stabilizes conditions. The warming trend may yield an opposite effect during January and February as our average snowpack decreases.

Despite the lack of non-convective high wind events in the region, the damaging winds that occurred on January 9th were notable, resulting in a 69 mph gust at BTV, which is the 4th highest gust in its record books only bested by Hurricane Hazel in 1954, the December 2022 event, and the Great Appalachian Storm of 1950.

The synoptic pattern was a classic setup for damaging southeast downslope winds. An upper low near the Great Lakes and a high amplitude ridge along the Atlantic Coast result in deep, meridional flow (Figure 4). 

Similar to the event on December 23rd, 2022, the event on January 9th-10th, 2024 had a strong area of low pressure tracking north of Lake Ontario and had a low-level jet over 80 knots at 850mb. In this particular instance, the low-level jet had a greater areal extent, which can result in a broader area of high winds (Figure 5).

Analyzing the forecast soundings from the RAP for the two events, you will notice they share many features. The tip of an inversion layer lies in a similar location across the two events, around 5000 feet. The location of this feature near summit levels results in favorable conditions for winds that crest over the mountain to accelerate down western slopes. The green line representing the dew point indicates dry air compared to the layer above it, and beneath the tip of the inversion are very strong winds. In the case of the January 9th-10th event, the maximum wind speeds were not quite the 100-105 knots experienced on December 23rd, 2022 (Figure 6).

While models likely underestimated low-level instability, the event on January 9th-10th had more snow and was cooler compared to December 23rd, 2022. Eastern Vermont received 7-12 inches of snow just 2 days prior. The system on December 23rd also continued to strengthen and produced additional strong winds over northern New York and created blizzard conditions, while the January 10th system quickly lifted north and only marginally strengthened (Not Shown). This resulted in a smaller extent of the most damaging wind gusts.

We had a late start to the windy season this Fall and Winter. A combination of an unfavorable pattern, stronger winds outside daylight hours, and potential shifts from climate change resulted in fewer damaging winds to start the season. However, a classic case of damaging southeasterly downslope winds occurred January 9th-10th that particularly affected Chittenden County in Vermont and resulted in BTV’s 4th highest wind gust. In quick succession, another high wind event occurred on January 13th. We will continue to have impactful wind events across the region. The scale may shift as climate change impacts the seasonal snowpack, preferred storm tracks, and synoptic pattern, but we will continue to study and understand these events.

Another general concern of Airport Operations is when runway temperatures get very near freezing. This can cause melting and refreezing and the potential development of ice, which is much more difficult to remove than snow. Also, precipitation type transitions are difficult for BTV Airport Operations, especially when it involves freezing rain. Thankfully, runway temperatures were in the 20s during the familiarization visit so there were no ice concerns.

This familiarization visit was coordinated by the BTV Aviation Team and was part of an effort to deepen the collaboration between WFO Burlington and BTV Airport operations. NWS Burlington is the only WFO in the country situated inside of an airport terminal. The firsthand experience of observing airport operations, particularly on the runways and taxiways, provided our forecasters with an invaluable perspective that helps in providing the decision support services needed to keep the airport running efficiently during periods of snow and ice.