Winter's here! What kind of weather can we expect?

December 1st marked the first day of Meteorological Winter. The months of December, January, and February comprise this time frame. Meteorological seasons are easier to track and compare for record-keeping, unlike the astronomical seasons, which vary by a day or two and don't fall neatly into calendar months. So, with the start of this season, let's explore the types of weather we might encounter, and learn some fun facts along the way. 

Snow blankets the Hintz Alumni Center on the campus of Penn State University. (my own photo)

Snow: Snow is synonymous with winter, but we don't always see a lot in a Tri-State winter. That's because it has to be cold enough for snow from the point of the snowflake's origin, the cloud, all the way down to the surface. While it can snow when the surface temperature is above freezing, it has to be below freezing right above ground level, 100 feet or so, in order for the snowflakes to remain frozen on impact. This combo is not easy to accomplish in a coastal area like ours; more on that later.

Did you know that snow's texture makes it hard to predict how much will fall in a storm? Typically, if an inch of rain were to fall as snow, it would pile up to around a foot, but the ratio varies widely. A forecast of 3"-6" of snow, for example, is equivalent to forecasting rainfall to within a couple tenths of an inch. Snow totals are also impacted by variations of temperature and humidity within the cloud, which produce different types of snowflakes (see figure below). These nuances are not fully resolved by the computer forecast models that meteorologists rely on. 

Types of snowflakes/crystals; several of these can be present in the same storm.
Credit: SnowCrystals.com

The next two types of wintry precip we'll discuss, freezing rain and sleet, occur in a process called overrunning. Warm air is less dense than cold air, allowing a melting layer to sneak into many winter storms (see diagram below). This is most commonly seen in the warm front section of midlatitude cyclones. It's also a feature of a stable atmosphere, one that is not prone to much vertical motion. 

This NOAA graphic shows varying types of precipitation that can fall when precipitation starts as snow at cloud level.

Sleet pellets are generally only a few millimeters in diameter. Photo from Britannica.com

Sleet: Sometimes mistaken for small hail, but formed in stable atmosphere, whereas hail only forms when an unstable, convective atmosphere is present. Sleet results when raindrops freeze into individual ice pellets before reaching the ground. This means there has to be a sufficiently thick layer of warm (above freezing) air in between the cloud and the ground, and there has to be a sufficiently thick layer of cold (below freezing) air between that melting layer and the surface. 

Diagram of overrunning, and the types of wintry weather it can produce, from NOAA.

It sounds like this combination should be rare, but it's actually quite common around here. In the winter, ocean temperatures are frequently milder than air temperatures in the Tri-State. An onshore flow can inject mild air above the ground while it remains below freezing close to the ground (remember that cold air is more dense than warm air). And it doesn't take a lot of mild air to melt the tiny ice crystals in snowflakes. Sleet frequently mixes in during snowstorms, which has the impact of lowering snow totals since sleet pellets are more compact than snowflakes. 

Image of the Great Ice Storm of 1998 from NOAA Vlab. Up to 3 inches of ice accretion was recorded

in some parts of New England.

Freezing rain: The layer of cold air needs to be even more shallow to produce freezing rain. Temperatures aloft have to be too warm for the raindrops to freeze before reaching the ground. Freezing rain is likely to happen when it's in the 20s at the surface, with temperatures spiking into the 40s a couple hundred feet or so above the surface. 

The "skew T diagram above is used by meteorologists to understand the vertical temperature profile of the atmosphere at one location. The image above (courtesy of NOAA) is typical for freezing rain.

Freezing rain events are frequently referred to simply as ice storms, and they can cause a lot of damage. Ice gets heavy quickly; while a cubic foot of dry snow can weigh as little as a few pounds, a cubic foot of ice weighs about 57 pounds.  And when ice coats every individual twig or leaf of a tree, the collective weight can easily take down large tree limbs and power lines, and cause roofs to cave in. 

A layer of graupel on top of snow. (Image from NOAA)

Graupel (or rimed ice): 10 years ago, many of us had never even heard of this winter phenomenon, but as more people get interested in the weather, more weather terms are getting recognized (remember when everyone suddenly knew what the Polar Vortex is?). Graupel forms in a process similar to hail, with unstable air and strong vertical motion. This is very different than the stable setup described earlier, which creates sleet and freezing rain. Graupel particles are seeded by a snowflake or tiny ice crystal. An updraft then carries the ice crystal into supercooled water vapor, which condenses onto the snowflake or ice crystal. Graupel often looks like small balls of styrofoam because the dynamic process that forms each pellet smooths out the rough surfaces. 

The image below looks like graupel, but these are snow pellets, which are snowflakes that partially melted and then re-froze while falling. It's not possible to tell the difference from a still image, but if you know whether the atmosphere creating the precipitation is stable or unstable, you can probably make a correct guess. 

Snow pellets, which look similar to graupel. (my own photo)

For more on winter weather safety, check out the resources from NOAA's Weather-Ready Nation page. And for a quick dopamine hit, I've included an unnecessary (but adorable) picture of my dog in the snow. Enjoy! 

Gratuitous picture alert: my dog, Trace, experiencing his first big snowstorm in 2020. (my own photo)

Does the late arrival of winter bug you out? Here's why

Wednesday was the coldest day in New York City since March 14th of last winter; with a high temperature in the upper 30s, it certainly feels like December is right around the corner. This past Saturday, on November 25th, we reached a low temperature of 30°, officially recording the first freeze of the season. The average is on November 14th, and it has happened much later; the latest first frost on record in NYC was on December 22nd, 1998! But overall, the first freeze in the Tri-State is trending later and later. The ten latest first frosts in Central Park have primarily happened in the 21st Century, and only two happened before 1950: 

Data from the National Weather Service office in Upton, NY

For those of us who don't love winter's chill, this is not necessarily a bad thing! And it certainly helps with the heating bills as well. But there are negative consequences to a late freeze. 

For example, bugs LOVE it. Deer ticks, which are vectors of Lyme Disease, do not die off until temperatures dip to the teens or single digits. Many deer ticks survive the winter by burrowing underground or finding other protection from harsh temperatures, like piles of leaves. A deep freeze is more likely to kill the eggs of these disease vectors, so a milder winter could increase your chances of encountering ticks carrying Lyme Disease when the weather turns warmer. 

Graphic courtesy of Climate Central

Deer ticks can be active if the temperature is above 34°, so it's important to check for ticks after any outdoor adventure, especially in the woods or tall grass. 

Another warm weather pest, mosquitoes, thrive in hot and humid weather; they're most comfortable with temperatures in the 80s. However, some mosquito eggs (especially Northern species) can survive in areas that later end up under ice or in frozen mud, and adults can hibernate during the colder months. 

Graphic courtesy of Climate Central

The later start to winter allows mosquitoes to stay active longer, and it can contribute to larger populations as well. It takes a prolonged deep freeze to kill hibernating adult mosquitoes and their eggs, assuming that they found a safe spot. And that's just not happening as often in the Tri-State as it used to. The table below shows the number of days below 10° in Central Park since records started in 1869: 

Data from the National Weather Service office in Upton, NY

The most recent year on the list was during the Great Depression. Of course, we are no stranger to very cold winter days, but when it happens, the cold snap is brief and anomalous. This trend is reflected in more recent decades by the rise in average temperatures in New York: 

Winter is the fastest-warming season in NYC. The average temperature is computed for

December, January, and February in this graphic from Climate Central. 

4 degrees may not sound like much, but in the winter, it can be enough to turn a pond into a skating rink. According to an article in Olmsted Now, "Central Park welcomed its first official visitors, the ice skaters" on December 11th, 1858! The idea of ice skating on a natural pond in Central Park is kind of unthinkable today, but it was a popular winter activity for 19th Century New Yorkers.

Image courtesy of New York Public Library

So as we try to shake off winter's first chill in the Tri-State this week, just remember: it used to be a lot colder, for a lot longer, during a typical New York winter. 
This article references reports and materials from Climate Central. Thank you! 

Will the return of El Niño mean the return of snow?

Last winter was a disappointment for snow-lovers in the Tri-State area. Though it snowed several times, including a bunch of trace amounts, we ended up with a grand total of 2.3"of snow in Central Park. Last year at this time, we were in La Niña, or the cold phase of ENSO. Now that the pattern has flipped to El Niño, or the warm phase, the outlook for our upcoming winter is quite different. Whether it results in a larger snow total remains to be seen. 

If you use the Farmer's Almanac to help plan your winter, I have bad news for you: It's already wrong about this year. The Almanac's winter forecast says, "snow will arrive beginning in November" in the Tri-State area. You're probably aware that we have yet to see a single flake of snow. This is true not only in NYC, but throughout the entire region. Whoops! 

Only time will tell if the Farmer's Almanac forecast for "oodles of powder" pans out 

for the Northeast and Mid-Atlantic. 

As Yogi Berra famously said, "It's hard to make predictions, especially about the future". Boy is this true when it comes to seasonal snowfall forecasting. Keep in mind that the snow-to-rain ratio is usually about 12:1; that means foot of snow melts down to about an inch of rain. Add up an entire season of snowstorms, and the margin for error is as wide as the Hudson. But, there are insights we can gain from our current pattern- insights that could be more valuable than a prediction for the number of inches of snow we'll see this year. 

In the Northeast, La Niña winters tend to be drier, whereas El Niño usually brings more precipitation. But that doesn't mean all El Niño winters are snowy in the Tri-State area!

Above is a list of the snowiest seasons ever recorded in NYC on the left, with the least snowy on the right. The snowiest winter of 75.6" occurred during El Niño. The third-snowiest, at 61.9", was during a strong La Niña. And #6 on the least snowy list was also a Super El Niño year! While knowing the ENSO phase doesn't help us figure out how many inches of snow we'll get, it can offer valuable insight to the winter ahead. 

La Niña winter conditions are more favorable for two types of snowmakers: Alberta Clippers and Four Corner Low pressure systems (often called "Colorado Lows"). Even if you're not familiar with the term Alberta Clipper, you probably know what they feel like: a little bit of powdery snow followed by an Arctic blast. Clippers usually bring 3" of snow or less to the Tri-State. 

Source: NOAA's Weather-Ready Nation Infographics Page

A Four Corners Low is so named because it originates in the Rockies, near the "Four Corners" region where Colorado, New Mexico, Arizona, and Utah meet. After exiting the Rockies, their trajectory often leaves the snowy side of the storm to our north, leaving us with slush or rain- but not always. 

Source: NOAA's Weather-Ready Nation Infographics Page

In an El Niño winter, coastal systems like Nor'Easters are more common and more frequent than the Clippers and Colorado Lows described above. These coastal storms draw more water from the Atlantic Ocean than their continental cousins, bringing heavier precipitation to the Northeast and Mid-Atlantic. And if it's cold enough when the Nor'Easter arrives, we could have a major snowstorm on our hands. 

Source: NOAA's Weather-Ready Nation Infographics Page

Such was the case during the record-breaking snowstorm of January 2016. We were in a "Super El Niño", with temperatures way above average in the equatorial Pacific. After a record-shattering warm December, it finally got cold in the Tri-State by the second half of January. The environment was primed and ready to turn a disturbance in the Gulf Coast into a raging blizzard. 

Graphic (above) and map (below from NWS Upton, NY)

When making a forecast for an entire winter season, meteorologists look for analogs, or similar atmospheric setups. ENSO is one factor, but other patterns such as the North Atlantic Oscillation play a role. So does the supply of cold air upstream from the Tri-State, even as far away as Siberia. 

So here's what I'm thinking: 

• In a year with a strong El Niño like this one, it's more likely that the entire season's snow will fall in a couple of big storms, rather than us having to deal with several "nuisance" snowfalls. 
• In most NYC winters, we don't need to break out the shovel until after the New Year. But this winter, a plowable snowfall is likely before Christmas- maybe even before the final night of Hanukkah!
• This winter's pattern is NOT conducive to a big, damaging ice storm. Hooray!
• This pattern is also NOT favorable for a nasty March (remember in 2017 when we dealt with four consecutive snowstorms?) Double Hooray! 

Here are some of the resources I used to put this outlook together. 

Climate Prediction Center's ENSO Page & North Atlantic Oscillation Page 

Rutgers Global Snow Lab Monthly Snow Cover Maps

Dr. Judah Cohen's Arctic Oscillation discussion

National Weather Service in Upton, NY's Climatology/Historical Data Page

Not again! The pattern of soggy weekend weather continues

Since the Fall season unofficially began after Labor Day, it has rained on at least one day of every single weekend. The only exception was the weekend of September 30th through October 1st; however, you'll probably remember that on the Friday preceding that weekend, former Tropical Storm Ophelia helped to fuel record-breaking rainfall in spots like Park Slope, Brooklyn (9.80"), while most of the Tri-State saw at least 5 inches of rain, more than a month's worth, in just one day. The incredible deluge led to public transit shutdowns and absolutely nightmarish scenes in the Five Boroughs and beyond. 

The calendars below show September and October (so far); the days marked in blue indicate when more than two-tenths of an inch of rain fell during that day. The second half of September was awfully soggy, and October has been drier, but the common thread is weekend rain. 

The blue boxes indicate days in September when NYC's Central Park recorded at least one tenth of an inch of rain. 

(Data from the National Weather Service office in Upton, NY)

The blue boxes indicate days in October when NYC's Central Park recorded at least one tenth of an inch of rain. 

(Data from the National Weather Service office in Upton, NY)

And the forecast for this weekend calls for more rain on both Friday and Saturday. What the heck is going on?!?

The forecast for Friday through Sunday from the National Weather Service office in Upton, NY on 

Thursday afternoon, October 19th. 

This may seem like an unfortunate coincidence, to have seven consecutive weekends with rain (or flooding-related cleanup), but it's not as flukey as it seems. A series of storm systems at the synoptic scale (don't worry, I'll explain) have moved across the United States at a fairly consistent speed, and it's going to take a break in this pattern to bring us a completely dry weekend. 

A typical planetary wave pattern from a vantage point above the North Pole.

Image courtesy of NOAA

Synoptic-scale systems comprise the largest size category in weather. A synoptic system can easily span over 1,000 miles. Weathermakers such as Nor'Easters are synoptic in scale, as are dry weather features like a dome of high pressure, such as the Bermuda High we're familiar with in the summertime. These systems move with the jet stream; surface low pressure systems (again, like Nor'Easters) are found in troughs, whereas high pressure areas are usually centered in ridges. The image above shows a typical trough-and-ridge pattern from the vantage point above the North Pole; you'll recognize the continental United States under trough #2. 

Notice that the pattern was similar last weekend, when we got nearly an inch of rain in Central Park on Saturday, October 14th. One of the dips in the wave, or troughs, had moved through the Tri-State, carrying a large storm system with it. 

Along with their size, synoptic-scale weather systems are relatively slow-moving. They last for days to weeks; in comparison to weather features like a single thunderstorm, which has a time scale spanning minutes to hours, synoptic systems last a long time. Imagine synoptic systems are like the elephants of the weather world; powerful and large, but not very nimble. In this analogy, thunderstorms are like jackrabbits. They're fast and can change direction quickly, but they cut a small path. 

Forecast surface analysis for Friday, October 20th from the Weather Prediction Center. 

Notice that the area of green has a long barbed blue line (cold front) 

draped through it; the length of this line is an indication of a synoptic-scale system.

Why is this relevant to our persistent pattern of wet weekends? Because, along with being unable to change speed or direction quickly, these large storm systems consequently tend to develop a regular cadence for the observer at a single spot- in this case, that spot is the Tri-State area. It will take a shift in this cadence, which could occur in the form of the development of a blocking pattern, to create a new rhythm in the atmosphere. Eventually, the pattern will change, even if it's because of the change in seasons, a lowering sun angle, and a natural shift of the polar jet stream southward. But if the texts I've been getting from my mother are any indication, my guess is that many people in the Tri-State are hoping for an earlier end to this pesky pattern. The good news is that it has been drier so far in October than it was last month, and the Climate Prediction Center believes that trend will continue.  

This weekend's partial eclipse will be a total miss for Tri-State viewers

Are you ready for Saturday's annular eclipse, which stargazers in the Tri-State have been waiting for since the summer of 2017? The eclipse, which will be visible on the East Coast around 1pm EDT, is also referred to as a "ring of fire" eclipse because of the effect produced by the moon blocking nearly all of the sun's rays, save for a narrow ring in the path of annularity. 

"Annular", by the way, means ring, so this is a fitting name! The path of annularity is narrow, and for this particular eclipse, it will trek across a less populated part of the continental United States. So, most of the people who will witness the full spectacle of this eclipse firsthand will have traveled some distance to get there. This interactive eclipse viewer from NASA is the origin of the image below, which shows the path for best viewing in gray:

NASA's interactive tool shows the path of annularity for Saturday's eclipse.

And as you can see, it's nowhere near the Tri-State area. (The total eclipse taking place next April will produce a better view for us... more on that later.)

A close-up view of the Northeast and Mid-Atlantic from the NASA interactive viewer shows that people in the NYC area will have a 20%-25% blocked sun at the peak of the eclipse from our perspective. 

A close-up from NASA's interactive tool shows that we will have a very limited 

view of Saturday's eclipse.

Saturday's annular solar eclipse will look similar to this one for viewers in the Tri-State- except that, since this eclipse is happening around mid-day Eastern time, the sun will be high in the sky.
Photo courtesy of NASA

Not all solar eclipses are alike. The shadows they cast vary widely, with every location on Earth experiencing at least one eclipse at some point in history. And, some solar eclipses are total, with darkness instead of the "ring of fire" effect that an annular eclipse creates. These variations are made possible by the Moon's irregular orbit, and the fact that Earth's orbit around the Sun is not in sync with the Moon's orbit around Earth. 

Types of solar eclipses; NYC will experience the partial eclipse on October 14th, while viewers in locations like 

Texas's Hill Country and Medford, OR will be treated to a sight similar to the middle image. 
Graphic from NASA

When the Moon's orbit is closer to Earth and its path aligns with the Sun, a total solar eclipse happens. That's what will happen on April 8th of next year, when a path of totality will be created. 

Another close-up from NASA's interactive tool shows the path of totality for the eclipse on April 8th, 2024.

The irregular orbit of the Moon is also responsible for the phenomenon dubbed the "Supermoon", which is simply a full moon that occurs when the Moon's path is at perigee, or closest to Earth. Supermoons are more common than solar eclipses because the Sun is so much bigger; its diameter is about 401 times that of the Moon. The Sun is also about 389 times farther away from Earth than the Moon, which is why the two celestial objects appear to be the same size from our perspective!

This depiction of the setup for Saturday's annular eclipse is not to scale because it would be impossible to depict in a legible image. The distance from Earth to the Sun is much greater, and the Sun is much larger than Earth, too.

Graphic courtesy of Smithsonian Air and Space Museum

You can easily demonstrate the effect of the Moon's irregular path by moving your thumb closer and farther away from your eyes to block an object in front of you. When the Moon's path is farther from Earth, it does not block as much of the Sun's rays, causing a partial or annular eclipse like the one happening Friday. 

Unfortunately, the weather forecast is very bad news for anyone in the Northeast or Mid-Atlantic who's hoping for a crescent-shaped view of the sun. A storm system that moved onshore in the Pacific Northwest will trek all the way across the country, brining us clouds and rain by Saturday. Bummer! If you're hoping for a change to that forecast, keep tabs with the National Weather Service's NYC-area office... and maybe petition them for a sunnier outlook? In the meantime, we can hope for better weather for the total solar eclipse on April 8th, 2024; that eclipse's path will be much closer to the Tri-State, anyway. 

If you want to learn more about both upcoming eclipses from a real-life NASA heliophysicist (try saying that three times fast!), register for this free webinar organized by the AMS Weather Band, happening on Wednesday, October 11th. 

Why isn't this early October warmth a "Second Summer"?

The month of October has gotten off to a very warm start in the Tri-State. Through the first 5 days of the month, the average temperature is 70.4°, a whopping 7.6° above the 30-year normal. On the warmest day, October 4th, we reached a high temperature of 83° in Central Park, equal to the average high for a day in mid-August. It was like a second summer in the Tri-State... or was it?

In my youth, the term "Indian Summer" was commonly used to describe a late-season warmup. While the antiquated term is no longer recognized by the American Meteorological Society, its scientific definition and criteria are the same as for the term "Second Summer". The AMS Glossary defines Second Summer in the following way:

"A period of abnormally warm weather that occurs in mid- to late autumn and after the first frost.
The comparable period in Europe is termed the Old Wives' summer and, poetically, may be referred to as halcyon days. In England, dependent upon dates of occurrence, such a period may be called St. Martin's summer, St. Luke's summer, and formerly All-hallown summer."

Unlike other meteorological terms, like heat wave, Second Summer does not have a minimum number of days ascribed to it, nor does it have a threshold temperature or temperature departure from average. The only firm stat in the definition is a key phrase: "...after the first frost". And therein lies the reason this early-October warmup is not a Second Summer. We haven't even come close to our first frost of the season in the city. In fact, the lowest temperature we've had so far this season was a 50° low temperature on September 27th. The map below shows the average first fall frost in the Northeast and Mid-Atlantic:

Average first frost map, using the current 30-year average

from the Northeast Regional Climate Center

The Tri-State area stands out as one of the latest spots on the map to experience a killing frost. The average first frost in NYC doesn't happen until November 20th, and the earliest first frost happened at the Central Park observation site all the way back on October 15th, 1876.

Even though they haven't been bitten by frost, a lot of the Tri-State's summer plant life is starting to wither. Our vegetable gardens have largely stopped producing anything new to eat. Despite the August-like temperatures we had this week, the October sun angle is preventing new growth from taking off. Just compare the image below of the sun angle's path through Central Park in October (first image) to its angle through the Park in June (second image):

NYC is solidly in the "midlatitudes", almost exactly halfway from the Equator to the North Pole, and this leads to a big variation in seasonal daylight. On the Summer Solstice, there's just over 15 hours between sunrise and sunset, but on October 6th, we're down to 11 hours and 33 minutes of daylight. With most veggies needing more than 6 hours of full, direct sun to flourish, the October sun is just not enough. It's no coincidence that the most recent full moon we had in September is referred to as the Harvest Moon!

One final thought for the gardeners reading this post: The amount of sunlight we have today is the same as it is on March 7th (in a non-Leap Year). This is why it's super helpful to start your plants inside, before the last frost of the Spring, which typically happens in April in the Tri-State. There's usually enough sunlight in your garden several weeks before the temperatures are high enough for plants to grow. Once the danger of frost has passed, you can permanently move everything outside and let them take advantage of all that sun! 

A new way to find trusted weather experts

In the 20 years or so since its inception, social media has produced a mixed bag of results for meteorologists. On the plus side, social media makes it easy for people who specialize in forecast communication to interact with their audience, sharing information quickly without the need for a TV timeslot. It also gives weather professionals access to real-time information from viewers, such as photos and snowfall amounts, during high-impact weather events. On the flip side, social media provides those same tools to bad actors, untrained or inexperienced forecasters, and even trolls. Today, every big weather event is forecasted and reported in a seemingly endless sea of websites, blogs, and apps- and unfortunately, some of the information is complete junk. But how can you know which sites to click, and which are just clickbait?

On Monday, the American Meteorological Society officially launched a program that aims to help credentialed meteorologists stand out in the digital space. It's called the Certified Digital Meteorologist Seal, and as the name implies, it was designed for meteorologists who are not on television and work primarily in digital media. When the CDM logo appears next to an article byline or a social media handle, it means that person attained a 4-year degree in atmospheric science, and that they passed a rigorous exam to assess their knowledge of meteorology and earth science. CDMs also submit a broad scope of work samples for review by a panel of experts in digital meteorology, appointed by the AMS. 

The snazzy new logo for the Certified Digital Meteorologist Seal, 

developed by the American Meteorological Society

I worked with staff and volunteers for the American Meteorological Society to develop this new Seal. Here was the process:

1. It started by talking to meteorologists working in the digital space- forecasters for newspaper websites (aka "digital dailies"), meteorologists for independent forecasting and consulting services, professional stormchasers, and the like. These folks gave me advice and feedback for the framework I would need for the next step.
2. Once I felt confident that we had a workable idea, I submitted a proposal for a new Seal (anyone can do this). The Professional Affairs Commission approved the proposal and recommended the formation of an Ad Hoc Committee to work out the details. 
3. Some of the folks from Step 1 served on the Ad Hoc Committee, and I reached out to other AMS Sealholders, both from the Broadcast Meteorology program and the Certified Consulting Meteorologist program, to provide additional support. About a third of the meteorologists who served on this Committee are bilingual. All Committee members are listed at the end of this post.
4. Over the next year, the Ad Hoc Committee worked with AMS Staff to determine the eligibility requirements for prospective Sealholders, including the type and quantity of work samples that would be required for assessment.
5. Then, the Committee developed a grading rubric and scoring guidance to reduce ambiguity and increase consistency in scoring. 
6. Finally, the Ad Hoc was dissolved and replaced with the AMS Board of Digital Meteorologists. Many of these inaugural Board members also served on the Ad Hoc. AMS Staff also drafted organizational procedures for this new Board.

If you'd like to apply for the CDM, find more details about the eligibility requirements and acceptable work samples here. To prep for this process, make sure you request a copy of your college transcript, start studying for the multiple choice exam, and save some of your best work samples published within 60 days before and/or after your application is received. Here's the application link: https://apply.ametsoc.org/prog/certified_digital_meteorologist_cdm_program/

Members of the CDM Ad Hoc:
Amy Aaronson, WBFF (Fox Baltimore)
Vanessa Alonso, Local Now/The Weather Channel
Steve DiMartino, NYNJPA Weather
Emily Gracey, The National Weather Desk/Sinclair
Joe Martucci, The Press of Atlantic City
Matt Lanza, Space City Weather
Jason Samenow, Capital Weather Gang/The Washington Post
Sean Sublette, Richmond Times-Dispatch
Mike Ventrice, PhD, DRW Holdings

Harvest Moon and high tide bring coastal flooding to the Tri-State

Flood alerts are in effect in parts of the Tri-State into the weekend, but it's not because of the risk for heavy rainfall. Instead, the main culprit is the impending Full Moon, called the Harvest Moon because of its early Fall timing. As a result, the flood water will mostly be coming up from the ocean, not down from the sky. This type of coastal flooding is sometimes called sunny-day flooding, and it's becoming more common.

Sunny-day flooding can happen when an event other than rainfall causes water levels to rise. It's most common when the Moon's phase is full, as it will be early on Friday morning. You're probably familiar with the tide cycles and their alignment with the Moon's orbit around Earth. The Moon's gravitational pull is strong enough to pull ocean water toward it, so if you're at the beach and the tide is coming in, it's likely that the Moon is rising at your location. 

This graphic from NASA Science shows an exaggerated view of the 

Moon's gravitational pull on our oceans.

And during Full Moon, the Sun is in alignment with Moon's orbit, which further amplified the difference at high tide. But high tides at Full Moon didn't used to cause "nuisance flooding" on Tri-State streets on a regular basis. An updated report from Climate Central, released this week, shows that coastal flood days are on the rise as the sea level inches upward. The graphic below shows the average rise in sea level at gauges in the US since its lowest recorded point in 1925, and the corresponding rise in days with minor coastal flooding. 

In the US, sea levels were lowest nearly 100 years ago, and have been on a fairly consistent rise ever since. Climate Central used information from the National Ocean Service to develop this report.

But, the sea level has not risen evenly at every coastal city in the US. For example, the increase in Charleston, SC is about double what we have seen at New York's Battery Park: 

Graphics and information provided by Climate Central

The rise in sea level varies for two main reasons: local topography, and local latitude. Yes, you read that right: local latitude is a factor- and the science behind it is fascinating. 

The two main contributors to sea level rise are a process called thermal expansion and the melting of polar ice sheets. Sea ice, while frequently reported on as a signal of climate change, is not a long-term contributing factor to global sea level rise. This is because sea ice is cyclical; it melts during polar summer, but at least some of it re-freezes during polar winter. The graphics below show the latest sea ice maximum and minimum from this past (Northern Hemisphere) summer: 

Graphics and information from NOAA

But the loss of multi-year ice, which is found on glaciers and ice sheets, does contribute to sea level rise. The ice sheets at both poles are massive- literally miles thick at their highest points. This makes them big enough to have their own gravitational pull, and ocean water is gravitationally attracted to the poles. 

Graphic depicting the impact of ice sheet loss on the sheet's gravitational pull 

from the Virginia Institute of Marine Science

But as the ice sheets melt, they lose some of this attractive power, and the ocean's water gets more evenly distributed at higher and lower latitudes. This, in turn, contributes to bigger sea level increases near the Equator, and in polar regions, the sea level may actually be lower than a century ago. This article has a fantastic, well-researched description of this phenomenon. 

This year, rain will fall under the Harvest Moon. But if you live along the coast, don't assume that the water on the street is from rain! Get the weekend forecast from the NYC National Weather Service office here. 

Weather alerts from KOKX on the morning of Thursday, September 28th

Rules of the (tropical) road: how hurricanes chart their course

The Atlantic hurricane season is now past its climatological peak. As such, we should start to see fewer new disturbances in the Atlantic turning into tropical storms and hurricanes. Historically speaking, by October 1st, the number of named storms is about half what it was at the peak just a few short weeks prior. 

Hurricane and tropical storm climatology, from the National Hurricane Center

In early September, Hurricane Lee was forecast to make a beeline toward the coast of the Carolinas, then turn north without making landfall in the US. A couple of my relatives asked me at our football tailgate (Go Penn State!) why the storm would change course like that. It seems like the kind of question that would lend itself to this blog! So, without further ado, here are the peculiar paths that tropical storms follow- and the rules governing their movement.

Rule #1: Tropical systems follow the path of least resistance. When it comes to tropical storms and hurricanes, this is the path with the warmest water and the least wind. You can see this when looking at historical storm track maps in comparison to persistent ocean currents. For example, Hurricane Idalia's movement closely followed the Loop Current in the Gulf of Mexico.

Above: the wind history of Hurricane Idalia, August 26- Sept 2,  2023. 

Below: satellite imagery with color coded temperature data in the Gulf of Mexico, showing the Loop Current.

Rule #2: Tropical systems go where the (trade)winds take them. Without the Earth's rotation, there wouldn't be hurricanes. As thunderstorms form in the tropics, the Earth's rotation imparts the Coriolis effect on these clusters, allowing them to organize and spin around each other. Since tropical storms form over the ocean, there's nothing below them to steer their direction. 

This map from the National Hurricane Center shows the point of origin (red dot) and track (grey line) of tropical storms 

that have formed in late August. Notice that the lines emanating from the dots on the right side of the map, 

near the west coast of Africa, primarily move due East.

This is why almost every tropical storm that forms off the Cape Verde islands initially moves almost due east. Storms that form off the western coast of Africa move from east to west with the trade winds, unless or until another weather feature takes over. Speaking of which: 

Rule #3: Large-scale weather systems easily steer tropical storms. It may be surprising, since the average hurricane expends more energy than that in the entire nuclear arsenal on Earth, but these storms are pretty easy to move. If you've ever seen the sport of curling, it may be helpful to envision the curling stone- which weighs over 40lbs!- sliding effortlessly on the low-friction surface of ice. Tropical storms have a similarly frictionless surface below them, which means even "low energy" weather features like a dome of high pressure will change a hurricane's path. When a Bermuda High is strong, for example, you can almost guarantee that the small island nation will not take a direct hit from a tropical storm; instead, the storm track will curve near the East coast, sometimes making landfall in a spot that juts out into the ocean, such as the Outer Banks of North Carolina. 

Bermuda highs often steer tropical storms nearly parallel to the Eastern seaboard of the United States. 

Full animation available at NASA Scientific Visualization Studio.

Rule #4: Tropical systems can get caught up in each other. When two storms get close to each other over the ocean, they can rotate around each other, creating what's known as the Fujiwhara effect. Eventually, the dueling vortices will go their separate ways, or if one storm is smaller, it can get absorbed into the bigger one. 

GIF of Hurricane Irwin and Hurricane Hilary interacting to create the Fujiwhara effect.
Source: National Weather Service "News Around NOAA" site

For more on tropical storms and hurricanes, check out this post about Hurricane Idalia from August, and my thoughts back in May about the start of the tropical season in the Atlantic Basin.