Hurricane Outlook and Hurricane Myths

Hello! It’s been a while. March disappeared before our very eyes and we are now healthily within meteorological Spring (which runs from March through May).

This is the beginning of an irregular series that addresses common misconceptions about weather and climate. I’m beginning with hurricanes because last week Colorado State University (CSU) issued their famous seasonal hurricane outlook for this year, so let’s take a look. (For the record, they are also instrumental in a fascinating group forecast website that aggregates hurricane season forecasts from other institutions. I highly recommend taking a look over the next couple of months as it populates for the 2021 hurricane season.)

The take home point from the outlook is that they are forecasting a more active than normal hurricane season in the Atlantic, by about 25-40% (this number varies depending on which of their forecast parameters you use). Interestingly, the core component of their forecast is the expected state of ENSO, which we’ve discussed a couple of times in this newsletter.

Hurricanes are low pressure systems that form over warm water in the ocean. There is an exchange in energy between the ocean and atmosphere, which functions much like the engine in your car. In order for there to be enough energy exchange between the ocean and atmosphere to generate a hurricane, the ocean temperature must be at least 26C (79F) and the winds at the top and bottom of the atmosphere must be, more or less, the same. If the winds at the top of the atmosphere are significantly stronger than those at the bottom (something we call wind shear), they will blow the top of the hurricane off the bottom and disrupt the delicate exchange of energy.

The end of La Niña, and transition towards neutral conditions, most likely mean that wind shear over the Atlantic will be slightly below normal to near normal, and ocean temperatures will be, at least, normal. A perfect breeding ground for hurricane activity. Such a situation may favor enhanced hurricane activity in and around the Gulf of Mexico during the middle-end of hurricane season (August - October), so I wouldn’t be surprised if we saw more activity than usual in that area. See page 32 of CSU’s hurricane forecast for probabilities near you.

Now that we’re talking about hurricanes, let’s take a look at some common hurricane myths to round things out!


Myths

The category of the hurricane is directly related to the storm’s potential danger.

False. Hurricanes are classified using the Saffir-Simpson scale which is based entirely on the storm’s sustained wind speed. Hurricanes are classified as major when their sustained wind speed is at least 111 mph, which corresponds to a Category 3 on the Saffir-Simpson scale. It is true that faster winds in a hurricane will exponentially cause more damage, but wind damage is not the primary concern from a hurricane.

Most hurricane damage is caused by flooding, not by winds. The Saffir-Simpson scale does not account for flooding potential. Flooding potential is dictated by the direction of the winds and level of the tide (collectively called storm surge), and the amount of rain that the hurricane produces. These are nearly always much more important than the storm’s wind speed. It is important to look at storm surge forecasts for landfalling hurricanes in addition to their forecast category.

Hurricanes and typhoons are different.

False. The umbrella term for all hurricanes and typhoons is tropical cyclone. Tropical cyclones are low pressure systems (similar to what we experience as winter storms) that are found over water instead of land. In the Atlantic and East Pacific, these storms are called hurricanes. In the West Pacific they are called Typhoons, and in the South Pacific (around Australia) and Indian Ocean, they are simply called Cyclones. There is nothing meteorologically different among hurricanes, typhoons, and cyclones. They are simply different regional names for the same phenomenon.

You should open your windows before a hurricane hits to equalize the air pressure inside your house.

False. This applies to tornadoes also. Opening your windows before a hurricane hits is a bad idea. Your house isn’t air tight, so you aren’t going to allow the pressure inside your house to equalize much quicker with the windows open. Also, an open window will let in water and other debris from outside that a closed window can more easily stop.

You weatherfolk can’t predict the weather next week so I shouldn’t believe your forecast for the next few months!

Kind of false. We will explore long range weather forecasting in the next newsletter and relate it back to the beginning of wine season. Stay tuned!

Kyle

It turns out that La Niña wine isn't really La Niña wine

Hello!

There are some good indications that this winter’s La Niña is coming to an end. Most of the weather forecast models predict that we’ll be out of La Niña conditions by the summer. The million dollar question from there is whether it will stay gone for summer, fall, and winter. Interestingly, this winter’s La Niña didn’t really have a standard effect on weather in the U.S. (remember that meteorological winter goes from December through February in the Northern Hemisphere).

Based on the previous post about La Niña (image below), it was reasonable to expect that we’d have dry weather throughout the southern U.S. and wet weather throughout the Pacific Northwest and Midwest.

Turns out, this isn’t what happened in the winter of 2020-21. Take a look at the maps below (temperature on left, precipitation on right).

Almost nothing from the winter of 2020-21 matches the typical La Niña conditions. Texas was cold instead of warm. The Midwest was dry instead of wet. The northern Plains were warm instead of cold. This teaches us an important lesson: not all La Niña winters are created equally and it is not safe to assume that the weather during a La Niña (or El Niño) winter will match our historical expectations. Other weather phenomena, such as the polar vortex that we discussed in the last post, can occur at the same time as La Niña and easily overpower La Niña’s effects.

This is an important lesson to remember as we discuss this week’s wines, which are both from La Niña years. Sometimes it just doesn’t matter!


This week’s two delicious La Niña wines are from the Southern Hemisphere. Let’s stand on our heads and get to work! (Fun fact: weather systems spin opposite ways in each hemisphere but toilets and sinks do not.)

The first is Penfold’s Bin 28 Shiraz from South Australia, Australia. The second is a Catena Malbec from Mendoza, Argentina. Although these regions are thousands of miles apart, they are nearly at the same latitude of ~33°S and 35°S. For comparison, the Ancient Peaks vineyard from the first newsletter is around 35°N, nearly the same distance north of the equator that this week’s wines are south of the equator. Interesting, right?

We should also remember that seasons in the Southern Hemisphere are reversed, so the growing season is closer to October-April instead of April-October like we had in California previously.

South Australia, Australia

For the first wine, we’ll travel just northeast of Adelaide, the capitol city of South Australia. There lies the Barossa Valley.

This wine is the 2018 vintage of Penfold’s Bin 28 Shiraz. I’ll level with you - I’ve been a sucker for Australian shiraz ever since I spent nearly two weeks there for a conference during my grad school days and drank a tremendous amount of it. So it should come as no surprise that I loved this wine - deep red, strong jam on the nose, low tannin but high alcohol mouthfeel. Absolutely perfect. 

This was a 2018 vintage, so the growing season would have begun in October of 2017, around the same time as La Niña. To first order, it’s not hard to imagine what the weather is like during La Niña events over Australia, since the water around Indonesia is, by definition warm, which results in increased rainfall.

As expected, our area of interest was on average warmer than normal (left map) during the spring and summer. However, the rainfall was slightly less than normal (right map). As we mentioned earlier, past La Niña conditions do not guarantee future conditions. Although it led to reduced yields compared to 2017’s season (which was quite a bit wetter), the grapes didn’t bloat up with extra water, which typically dilutes their sugar content and makes for worse wine.

Mendoza, Argentina

The 2018 Catena Malbec is actually a blend of Malbec grapes from 3 different vineyards around the Mendoza region.

La Niña summers in Argentina aren’t quite as predictable as in other places since La Niña has its strongest remote impacts in the winter hemisphere. The north/south temperature gradient is strongest during the winter, which provides a mechanism for the atmosphere to “pluck” the jet stream.

Summers tend to be slightly warmer around Mendoza during La Niña and there isn’t a strong rainfall relationship to speak of. The 2017-2018 growing season was pretty normal throughout Argentina. There wasn’t too much rain and temperatures were cool and consistent. In fact, this season was heralded as one of the best growing seasons in years, largely because of these typical conditions.

In the next newsletter we’ll attack common meteorological misconceptions. Stay tuned!

Kyle

The Polar Vortex strikes again!

Hello everyone,

It seemed appropriate to take a break from wine and discuss what’s been happening throughout the Plains. For those who may have missed it - it’s been cold. Really, really cold.

The map below shows red dots at places that broke or tied their lowest temperatures ever recorded. These record breakers were observed between February 11-17, 2021. Many of the records in Oklahoma, Texas, and Kansas were broken by more than 8°F. Eight of the records were previously set over 100 years ago and most were set before 1990.

The overnight low temperatures were absolutely bitter. Of the 90 records that were set this week, 84 were below 0°F. Owen, WI is about 140 miles due east of Minneapolis and had an overnight low temperature of -45°F. Bottineau, ND, just south of the Canada border, had the lowest temperature I could find at -54°F. Oklahoma City hit -14°F on February 17.

Why was it so cold?

I often see the term polar vortex thrown around in the media during cold outbreaks like this and this year was no exception. At its simplest, the term polar vortex refers to a permanent circulation of air that extends from the troposphere (the part of the atmosphere that we live in) to the stratosphere (about 30 miles up) and spins counterclockwise (like the earth) above the Arctic. The polar vortex spins faster during the winter than it does during the summer, but there is always some kind of rotation in the stratosphere over the Arctic. I recommend watching this quick video from the UK’s Met Office for a helpful animation before reading on.

Although the term polar vortex has become more popular over the past decade, the phenomenon is neither new nor particularly scientifically interesting. We’ve known about the polar vortex for a long time and the science behind it is actually quite simple.

Remember in the La Niña newsletter when I wrote that the secret to meteorology is temperature gradients (temperature difference between two areas)? Turns out we can use them to explain why there’s a polar vortex. Typically, air is warmest near the Equator and coldest near the North Pole. This temperature difference is stronger in the winter than the summer, but it exists in all seasons. Such a temperature gradient results in fast winds above the surface of the earth that move from west to east, often called the jet stream. In general, the jet stream is strongest when the north/south temperature gradient is strongest, which is usually in winter.

Of course, it’s not always as simple as cold in the north and warm in the south. The jet stream will rise and dip to follow these changing temperature gradients. In the image on the right above, you’ll see that cold air often comes down form the Arctic when the jet stream dips south and vice-versa. It’s these dips that lead to cold air outbreaks like the one we’ve seen during the past week.

The image below shows the jet stream (left) and cold air (right) during the past week. Notice how the jet stream dipped south of Texas into Mexico and the cold air filled in the dip.

Dips in the jet stream are common during winter, but a dip that reaches as far south as the Mexico border is rare, as you’d expect from all the cold records that were set this week.

What caused this dip?

It’s hard to say for certain and I’m sure plenty of research will be published over the next couple of years attempting to answer this question. That said, I’ll give it a try.

The jet stream usually has ridges and troughs because atmospheric waves propagate along it, just like you’d expect waves to move along a string that was plucked. The atmosphere’s equivalent to plucking the jet stream can occur when strong storms hit somewhere along the jet stream, which forces it to rise in one area and dip in another.

It appears that a couple of especially strong low pressure systems that moved off of East Asia got picked up by the jet stream and pushed north in early February. The jet stream then bent north around the storms, forcing it to rise over the North Pacific and dip over the central United States.

Is the cold over? What’s next?

The coldest temperatures are likely over. The latest forecasts suggest that the cold will subside during the next week and we’ll end February with much more “normal” temperatures.

This event was so rare that the electrical and plumbing infrastructure in Texas downright failed. Of course, this turned into a political blame game, but the failure highlighted how easily cold weather can hurt people and cause property damage. Even worse, the science indicates that cold outbreaks like this will become more common (although most will probably be a bit milder). Regardless of how cold a future cold outbreak actually is, it’s important for everyone in vulnerable areas to be as prepared as possible.

Take care,

Kyle

Laying the foundation: El Niño and La Niña

One of the biggest secrets of meteorology is that nearly everything related to weather can be explained by temperature variations around the globe. We refer to these temperature variations as gradients and they exist everywhere on earth in every direction: north, east, south, west, up, and down. Some of the most important temperature gradients exist along coastlines (why the beach is breezy) and mountain ranges. You can also find temperature gradients in the oceans, especially the Pacific.

Under normal conditions, the warmest part of the Pacific Ocean is around Indonesia, a place often referred to as the Pacific Warm Pool. The temperature gradient extends east and the coldest waters are found off the west coasts of Ecuador and Peru. During summer, the temperature difference, on average, is around 18°F (10°C). I’ve annotated a figure below showing this gradient.

Like most things in the atmosphere, the temperature gradient isn’t constant. It changes in an oscillating fashion called the El Niño Southern Oscillation (ENSO); it’s most often measured in monthly increments.

La Niña occurs when the cold water in the east Pacific gets colder (and sometimes the warm water around Indonesia gets warmer) for at least 3 consecutive months. This strengthens the temperature gradient. An El Niño exists when the cold water in the east Pacific gets warmer for at least 3 consecutive months, which weakens the temperature gradient. At a high level, that’s all ENSO is. It’s deceptively simple, but globally significant.

The graph above shows El Niño events (red) and La Niña events (blue) since 1980. You’ll notice that ENSO oscillates irregularly, so each El Niño and La Niña begins at different times, lasts for different lengths of time, and ends at different times. Sometimes you can even get two in a row. The irregularity makes things especially difficult to predict.

So, why do we care about El Niño and La Niña in this newsletter? Well, it turns out that as the temperature gradient in the ocean changes, so does the flow in the atmosphere. In fact, changes to the water temperature create atmospheric patterns that emanate out of the tropical Pacific and affect the entire world.

Above is an image from climate.gov showing the typical impacts of La Niña on the U.S. during winter. Since each La Niña is different, these impacts don’t always happen, but most La Niña winters are similar to this.

You might think that this looks familiar. That’s because this year is a La Niña year. La Niña conditions began in fall of 2020 and are forecast to continue at least through spring 2021. If you’re wondering what the best forecast for the rest of the winter is… the image above is a great place to start!

In the next newsletter we’ll tie ENSO to wine and explore two wines from two very different parts of the globe that were impacted by ENSO. We’ll discuss a Shiraz from South Australia and a Malbec from Mendoza, Argentina. Exploring wines from previous La Niña years might give us an idea of what to expect from the 2021 vintages. Stay tuned!

Did the record heat in 2017 affect this California wine?

Probably in a good way.

The summer of 2017 was difficult for California. The meteorological summer (Jun-Aug) was the warmest on record in much of the state. The hot summer resulted in a strong wildfire season with more than 9,000 individual fires that burned more than 1.5 million acres. We’ll talk more about the kinds of weather that lead to wildfires in a future post, but suffice it to say that they are often fueled by hot, dry conditions mixed with some kind of warm wind event (regional names vary but Santa Ana and Diablo are often used to describe these winds).

Fortunately for California’s wine country, most wildfires held off until mid-October by which time most of the grapes had been harvested. There are a number of articles detailing the fascinating resilience that the wine industry has in these difficult times, an important trait for the future when such wildfires are forecast to become more common. Since the wildfires don’t appear to be a major player in this week’s wine, I’d like to focus instead on the extreme heat that California endured.

This week’s wine is a 2017 Zinfandel from Ancient Peaks, which is based in San Luis Obispo County, California. Its vineyard is the only one in the Santa Margarita Ranch AVA (American Viticulture Area). The vineyard is slightly under 100 miles northwest of Santa Barbara, located in a valley in the southern part of the county. The wine itself is delicious - the plum and pepper notes stood out to me the most, with much less of a tannic mouthfeel than I expected. Bold yet very smooth. How did the warm summer affect this wine?

Agriculture can be difficult to study meteorologically because the density of weather observations isn’t very high. Most official weather observations are kept at airports throughout the country. This is great for large-scale weather events, but weather that occurs in between the airports is lost, which includes most terrain induced phenomena. The map below shows the official observational stations in California. This map doesn’t actually have every single location, but it should give you a reasonable idea of how spread out the stations can be.

The biggest problem for me is that I can’t find any observations near the vineyards. The closest observational areas that I was able to find were the Paso Robles Airport (KPRB) and the San Luis Obispo County Airport (KSBP). The Paso Robles airport is about 40 miles due north of the County Airport and neither is within the Santa Margarita valley. The County Airport is considerably closer to the coast. Nevertheless, this is the best freely available data (see footnote for more information about the data and analysis) that I could find, so we’ll have to make do.

Above is a graph showing the long term average temperatures at each airport (solid lines) compared to the 2017 temperatures (dotted lines). There are considerable regional differences. The northern airport, Paso Robles, is warmer than the County Airport during the summer and cooler during the winter. Why is that?

The simplest explanation is that the Paso Robles airport is further inland than the County Airport, shielded from the immediate effects of the ocean. Since water has a significantly higher heat capacity than air, it is much more resistant to changes in temperature, resulting in very little seasonal variability. This is the main reason that so much of the West Coast has a temperate climate.

As expected, 2017 was warmer than the long term climatology at both airports. One popular way to measure the amount of heat during a particular season is to calculate growing degree days (GDDs). GDDs are defined as the difference between the average daily temperature and some threshold, usually 50°F (10°C). So, if the average temperature for today were 65°F, then today would have 15 GDDs. We use these and similar units (e.g. heating and cooling degree days) all the time in meteorology throughout several industries (especially agriculture and energy).

The graph above shows the total number of growing degree days during the core growing season of April - October for every year between 1999 and 2020. As we might expect, there are considerably more GDDs at Paso Robles every year than at the County Airport, which matches the summer temperature comparisons quite well. There are several interesting questions that we could pursue from this graph. Why are the stations so similar in 2009? Why does 2012-2020 appear to have more GDDs than 1999-2011? Is the answer as simple as global warming? What happened between 2015-2017? Hopefully we will tackle these questions as we explore weather and climate in future newsletters.

So, 2017 was hot and had among the highest GDDs at both stations. I think it’s reasonable to conclude that the vineyard also shared these characteristics during 2017 (with actual values closer to KSBP than KPRB). The last important metric to consider is the diurnal cycle, which is simply the difference between the daily maximum and minimum temperatures. Larger diurnal cycles imply hotter days and colder nights than smaller diurnal cycles. You’ll notice that the coastal station has a much smaller diurnal cycle than the inland station, an effect of the ocean’s moderating abilities. There isn’t a significant difference between the long term average and 2017, but an argument could be made that temperature swings in the summer were slightly above average while swings the fall were below average.

So how did the warmth affect the wine? Interestingly, Vivino’s ratings highlight the 2017 vintage as having the highest rating compared to any other year for this particular wine. Zinfandel grapes require a large diurnal cycle, which was especially pronounced during the summer of 2017.

So where does this leave us? First, 2017 was among the hottest years in California’s history… but that didn’t seem to adversely affect this wine. In fact, this vintage was the best rated! Santa Margarita, like many wine regions, is well insulated from the annual weather variability. The 2017 temperature and GDDs throughout the region were only slightly higher than average, despite the entire state having a record warm year. This year-to-year consistency is extremely important to vineyards around the world and creates a clear distinction between conventional wine regions and those in more variable climates, like upstate New York or Virginia.

Next week we’ll explore a more general theme: how does El Niño affect global wine growing regions? Stay tuned!


Data availability statement: All data and analyses in each newsletter will be available on my GitHub page for anyone that wants to wade through my Python code to see where the graphs and maps come from. This week’s data was downloaded from NOAA.

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