Your Tri-State forecast: fire and ice

 If you keep a keen eye on the daily forecast in the Tri-State area, you probably saw at least one of the alerts today, which are depicted in the colorful map below: 

Watches, Warnings, and Advisories valid 12pm ET on May 17th, 2023.

Image from the National Weather Service

Did the top two alerts in the column on the right catch your attention? A Freeze Warning (dark purple) has been issued north of the city for tonight, and a Red Flag Warning (in pink) is in effect for most of the state of Connecticut. In the city, the "Special Weather Statement", shaded in beige, gives information about the potential for fire weather through Wednesday evening. This begs the question: why do we have both fire and ice in the forecast?

First, let's look at the weather setup for today. The image below from NOAA's Weather Prediction Center shows the main weather features in place at 11am ET on Wednesday. The cold front (blue line with triangles) swept through yesterday, bringing barely a drop of rain, is now offshore. In its place, high pressure is nudging in from Quebec, just north of the Great Lakes. 

Surface analysis, valid 15Z on May 17th, 2023.
Image from the WPC

Cold fronts don't always bring cold air; instead, they are marked by a drop in dewpoint after the frontal passage. In simple terms, the air behind a cold front is less humid than the air preceding it. Yesterday's dewpoints were in the upper 40s to low 50s. This afternoon, they are falling into the 20s. The relative humidity has dropped below 30% across much of the Tri-State. Also notice on the map above the isobars, or lines of constant pressure, draped from north to south across New England into the Tri-State. Those lines indicate winds from the north, bringing in drier continental air from Canada. The lines are also fairly close together in relation to the other isobars on the map. This reflects the gusty breezes we've seen today, which have topped 30mph at times. The National Weather Service issued a Red Flag Warning in Connecticut because of this combination of dry air and gusty winds. 

Fire weather is not as common in May as it is earlier in the spring because as new growth develops on our trees and shrubs, it introduces moisture into the air (the process is called transpiration). In mid-May, this process is usually robust enough to keep humidity a little higher at ground level. But it hasn't rained in the Tri-State in about 10 days, and surface water is in short supply. You may have even noticed that there wasn't much dew on the grass this morning; dewy grass is usually the most obvious sign of transpiration. The newly-sprouted leaves also provide a canopy to shade the forest floor from the strongest rays of the sun. If any wildfires do happen today, they will most likely happen in spots that are not shaded.

Wildfires start more readily when a canopy of trees is not present.

Image courtesy of Drought.gov

As it turns out, the same weather setup that produces today's elevated fire danger is also responsible for the Freeze Warning in tonight's forecast. Temperatures could drop into the 20s north and northwest of the city, despite high temperatures in the 60s across most of the area. Dry air heats and cools more readily than humid air, so once the sun sets this evening, the heat that accumulated during the day will quickly escape into the upper atmosphere. Windy conditions can help impede the cooling process by keeping the atmosphere "mixed"; that is, not allowing for a sharp vertical temperature difference. However, our winds are expected to settle down after sunset as the center of high pressure slides closer to the Tri-State, allowing temperatures to drop as much as 30° or greater overnight. The cooldown will be especially pronounced inland, away from the tempering influence of the Atlantic Ocean, and at higher elevations, where the air is less dense and even more susceptible to nighttime heat loss. 

USDA plant zone hardiness map. Farmers and gardeners use their hardiness zone as guidance for planting and harvesting.
Image available for download at usda.gov

As you may know, Frost and Freeze alerts are only issued during the growing season; that's why we don't see these warnings in the winter. For more information on your growing zone, and how it has shifted in recent decades, check out this research by Climate Central. 

Thanks for nothing, El Niño! Drought relief in the southern Plains not likely

NOAA issued its weekly drought report today, and the news continues to be bad for the southern Plains, especially west Texas, Oklahoma, Kansas, and Nebraska. On the map below, the areas shaded in deep red are suffering the worst drought conditions, which are referred to as "exceptional":

Drought monitor from May 9th, 2023.
Produced by the National Drought Mitigation Center and University of Nebraska, Lincoln

Unfortunately, the Climate Prediction Center is holding out little hope that the situation will improve in the coming weeks and months. Below is the seasonal drought outlook until the end of July. Notice that roughly the same areas are expected to remain at least in severe drought: 

 

US Seasonal Drought Outlook, valid May 1 - July 31, 2023.

Courtesy NOAA Climate Prediction Center

The CPC forecast for precipitation reinforces the notion that it's going to remain dry in the region. 

Seasonal Precipitation Outlook for the CONUS & Alaska, valid May - July, 2023.

Courtesy NOAA Climate Prediction Center

The forecasted return of El Niño has its fingerprints on this forecast. As mentioned here, ENSO's impacts are much more significant in the winter than in the summer, especially in the continental US. But, there's still something to be taken from a forecast of ENSO-positive conditions taking hold this summer. The main takeaway is the strength and position of the storm track. 

Depiction of the Pacific Jet Stream during ENSO phases.
Courtesy of NOAA National Weather Service

As you can see in the image above, El Niño conditions generally strengthen the southern branch of the jet stream and make it flatter (yellow line), with less pronounced dips and ridges. During the summer months, the jet is usually at its weakest, because jet stream winds are derived from a temperature contrast between the equator and the poles. As you might expect, this gradient gets weaker in the summer, when the sun's direct rays are in the Northern Hemisphere. But El Niño enhances the southern jet, leading to a more active storm track and more chances for precipitation. Here's the precip outlook map again so you don't have to scroll up:

This could be one of the reasons the CPC anticipates more precipitation (green blob) from the Ohio River valley to Florida. But what's behind the brown shading from Arizona to the Texas panhandle? Isn't this region also the southern US? Alas, the double-edged sword of El Niño may be to blame. At the same time El Niño strengthens the southern jet, it also makes the conditions for the southwestern monsoon less likely to develop. And since the monsoon is the #1 source for precipitation in the desert southwest, a return of El Niño likely means less rain through the first half of the summer. 

Let's add a silver lining to this outlook, shall we? For that, we need to go back to today's drought monitor. Did you notice that nearly the entire state of California is excluded from the drought? 

It's one heck of a difference from where we were at the end of 2022:  

Drought monitor from December 27th, 2022.
Produced by the National Drought Mitigation Center and University of Nebraska, Lincoln

The incredible influx of moisture from multiple atmospheric river events at the beginning of the year caused catastrophic flooding, but the good news is that all the rain and snow essentially ended the drought. And with El Niño conditions expected to return, it's likely that the spigot will stay pointed toward the notoriously drought-stricken state again this winter. 

At long last, is El Nino coming?

Hey there El Niño, it's been a minute! Your sister La Niña had been dominating the Pacific for the past three years. Is it finally time for the warm phase of the El Niño Southern Oscillation, or ENSO, to return? Scientists at the Climate Prediction Center seem to think so. An El Niño Watch has been declared, with the CPC reporting a 62% chance of ENSO-positive conditions developing by the summer.

From January through March, sea surface temperatures in the Niño regions (more on that later) were just slightly below average, signaling a shift into the ENSO Neutral phase. This is the first time since the Fall of 2020 that the La Niña threshold of -0.5°C was not met for three consecutive months. And forecast models are nearly unanimous that the upward temperature trend toward El Niño will continue: 

Forecast model projections for sea surface temperature (SST) anomaly in the Niño 3.4 region.

(Chart from Columbia Climate School IRI for Climate and Society)

Before we dive into the possible impacts of an El Niño summer on the Tri-State, let's define the phenomenon. The onset of El Niño is declared when the surface water in the equatorial Pacific is warmer than average for at least three consecutive months. The map below shows the Niño regions, or specific areas that are closely monitored for ENSO: 

Map of Nino regions from Climate.gov

El Niño is triggered by a weakening of trade winds that normally push warm water away from the coast of equatorial South America. When these winds are weaker, warm water gets trapped along the coastline. The subsequent impacts of this seemingly subtle change can be felt worldwide. 

In the Tri-State area, the impacts of El Niño in the summertime can be subtle. 

Map of El Niño impacts for June through August.
 (Image from Climate.gov)

According to the map above, NOAA does not attribute El Niño to any meaningful change in summertime temperatures or precipitation in the continental USA. However, we can look for clues in previous ENSO-positive summers in the Tri-State. Data from the local National Weather Service office shows that, in this century, summer temperatures were below average in almost every El Niño year. The only exception was during the "Super El Niño" summer of 2015, when temperatures along the equatorial Pacific were nearly 2°C above average, far exceeding the +0.5° threshold. There's not a strong precipitation signal coming from recent El Niño summers, either. Only one stands out, 2009, which was a summer with marginal El Niño conditions. 2009 is still the 4th-wettest summer on record at Central Park. In particular, June was drenched by over ten inches of precipitation, with rain falling on 18 separate days in the month. (Incidentally, this was the same year as "The Summer That Isn't" in New York. Remember that?) 

El Niño conditions tend to have a noticeable impact on Tri-State winters, usually leading to above-average temperatures and rainfall. But, that doesn't mean an El Niño winter is snow-free, as this phase of ENSO tends to bring us more coastal storms like Nor'Easters. In fact, the record-breaking blizzard in January 2016 occurred during the same "Super El Niño" mentioned above! 

One thing that's well documented is the impact of ENSO on hurricane seasons. The tropics tend to be less active during an El Niño, and it has to do with wind shear. In the ENSO-positive phase, the unusually warm water off the west coast of South America repositions the southern jet stream over the Caribbean and subtropical Atlantic, introducing higher wind speeds to the upper atmosphere in this region. Stronger upper-level winds are disruptive to tropical storms, which rely on light wind speeds through the entire vertical column of the atmosphere to strengthen. Learn more about hurricane development here. 

El Niño events typically last for several months to over a year. The La Niña event that ended in March was a bit long for the cool phase of ENSO, but it could be a sign of an overall trend toward a semipermanent La Niña phase. 

When warm becomes the norm: April nearly sets a new bar for heat

 It may already feel like a distant memory with this early-May chill in the air, but April brought some serious heat to the Tri-State. Temperatures reached into the 90s on 2 days of the month, April 13th and 14th. Overall, we finished with an average temperature of 57.6°, making April 2023 the 2nd-warmest on record in NYC. The chart below from New York's local National Weather Service office will need some updating:

Top 10 warmest and coldest Aprils on record.
Data and chart from NWS Upton, NY

With last month added to the list of warmest Aprils, the list will be further stacked with records from this century. By contrast, May has gotten off to a cool start. Both Monday and Tuesday's high temperatures were well below the average high of 68°, and the average temperature for the month is 7° below average so far. However, it's quite unlikely that this month will make it onto the top 10 coldest list below:

Top 10 warmest and coldest Mays on record.
Data and chart from NWS Upton, NY

The rationale is simple, even without knowledge of the upcoming forecast. It's unlikely that this month will be one of the coldest Mays on record because it's been 56 years since May temperatures have been chilly enough to make the list!

 

The near-record warmth in April is just a small part of an ongoing trend, which is easier to see when looking at Central Park's complete records dating all the way back to 1869. 154 years is long enough to have seen lots of ups and downs. But what we've seen instead is an increase in warm temperature records, while cold temperature records have almost disappeared. The chart below shows the number of months on the top 10 record warm versus cold lists through the decades (tied records are included). Again, all data is from NWS New York. 

The trend of record-breaking months may be a better indication of our warming world than the often-cited daily temperature records. While a daily record high can be caused by a brief spike in temperatures or by an unusual weather pattern, the monthly average temperature takes it all into account- highs and lows- through the course of an entire month. 

Despite this early May cooldown, 2023 has been a warm year so far. January set a new record in Central Park; the average temperature of 43.5° was nearly 10 degrees above normal. And February was not far behind. With an average temp of 41.1°, February 2023 will take the #3 spot on the all-time list. A looming El Niño increases the chance for a hot summer in the Tri-State. More on that coming later this week. Stay tuned! 

Summer preview: Last weekend's rain was more than an "April Shower"

We closed out the month of April with a deluge of rain in the Tri-State area. Many spots in and around the city recorded more than 5 inches of rain between Saturday and Sunday's one-two punch of a storm system. Widespread street flooding led to several vehicles stranded on inundated roadways in New York and New Jersey. 

Storms cause flash flooding in the northwest Bronx on April 30th. 
Video courtesy of Melba Kurman/Twitter

Rainfall rates exceeded .5"/hour across the area on Sunday evening, making this late April downpour reminiscent of a July thunderstorm. And as the weekend came to a close, Central Park tallied 5.1" of rain, which is over an inch more than the Park normally records in the entire month of April. 

Rainfall totals from April 28th-30th, 2023.
Graphic created by NWS Upton, NY

A new report by Climate Central shows that these these rainfall events are becoming more common. Across most of the US, even in the desert Southwest, heavy downpours are getting heavier. 

This means that the rainfall rate is increasing, even in places where the average annual precipitation has decreased in recent decades. Why does rainfall rate matter? When rain falls in short intense bursts rather than slowly and steadily, flash flooding is more likely. Flash floods can make roadways impassable, as we experienced firsthand over the weekend. Even more importantly, people can get trapped in floodwaters, as they're more likely to be unprepared for the sudden rise in water levels. It's also bad news for agriculture and maintaining a reliable water supply, with less rainwater having a chance to penetrate into aquifers.  

The Northeast region as a whole has seen a 13% increase in hourly rainfall intensity. NYC's report, indicated by a brownish dot in the graphic above, is the only location in the region with a slight downward trend since 1970.

The trend line indicates that rainfall intensity has dropped by about 0.005"/hour since 1970. The argument could be made that this isn't a statistically significant change, as any rainfall amount under 0.01" is recorded as a trace (T) by the National Weather Service. With all other surveyed sites in the Northeast seeing an increase in heavy downpours, NYC's slight decrease is an anomaly. And with summer being the wettest season in the Tri-State, more heavy storms are likely lurking in the months to come.

                                                                        

The polar vortex and its connection to climate change

The bitterly cold Arctic air that spilled into the Tri-State last week, and again this morning, runs counter to the message we've been hearing from meteorologists and climate scientists around the world: 2016 is on track to become the hottest year on record. While most people understand fully that it can occasionally get cold, despite an overall warming pattern, it still is a curiosity. If the Earth is getting warmer, where is the unusually cold air coming from?

The answer lies with the Polar Vortex. This weather phenomenon is certainly nothing new-- it has existed as long as Earth has had an atmosphere-- but has only gained widespread recognition outside meteorology circles in recent years. This term is used for the frigid air that semi-permanently resides over both the North and South Pole. The Polar Jet Stream keeps this cold, dense air contained to the high latitudes through most of the year. The strong winds of the Polar Jet are created in part by the contrast in temperatures between the high latitudes and mid-latitudes. When the temperature gradient from north to south is weakened, the jet-level winds respond by slowing down. Slower winds are a weaker barrier and are easier for a polar airmass to penetrate (see video).

In the Northern Hemisphere, the Polar Vortex sits over the Arctic Ocean, which is a much less stable surface than at the South Pole, where the landmass of Antarctica encompasses nearly the entire Antarctic Circle.

The Arctic Ocean has shown its susceptibility to climate change, especially in recent decades. Sea ice, which typically covered the ocean's surface year-round during the first half of the 20th Century, has been melting more thoroughly than in decades past. The thinning sea ice is not just a sign of milder ocean temperatures beneath; it also exacerbates the warming process because snow and thick ice reflect the sun's rays, whereas thin ice and ocean water absorb much more solar radiation, making the Arctic ocean and surrounding air that much milder. Though the entire  Earth has seen a rise in average temperature since the beginning of the 20th century, the change is starkest north of the Arctic Circle, as seen in the picture below.

Global average temperature anomaly in 2015. Red and orange areas have the highest positive temperature anomaly. (Courtesy of NASA)

This has profound effects on the Polar Jet's strength. The strength of the jet stream is directly proportional to the temperature gradient, or change in temperature over a given space. So when the Arctic gets disproportionately warmer than the rest of the Northern Hemisphere, the gradient weakens, and so does the Polar Jet. Weaker winds make it easier for Arctic air to head south. So, a weak Polar Vortex can bring a strong Arctic outbreak to the continental United States. 

If global temperatures continue to rise, or even if they remain steady, we can expect that the Polar Vortex will become more unstable, since Arctic sea ice extent continues to shrink to new record lows.  http://svs.gsfc.nasa.gov/4494 This could result in even more frequent and dramatic temperature swings than the 33-degree drop in temperature we endured between Sunday afternoon and Monday morning. 

Snowiest March in decades! Is winter over yet?

An image from Front Royal, VA
during the March 17th storm.

An image from Spotsylvania, VA
during the March 3rd storm. 

The St. Patrick's Day snowstorm we just endured will go down in the record books as the biggest storm on that date in DC history. The previous St. Patrick's Day record was only 1.9" of snow. While this storm easily broke the record with 3.9" on the holiday itself, the real wallop was with the storm total. We got 7.2" of snow at Reagan National from Sunday night into Monday morning! This storm, plus the early-March snow and ice, bring us to a grand total of 11" for March 2014. This is the snowiest March on record since... can you believe it... 1960?!?! It's been half a century since DC has had this much snow, this late in the season!

"SnowPatricksDay", as it was dubbed, seems to be the last straw for many Washingtonians, even those who normally love the snow. Our seasonal snowfall total is now almost exactly double the 30-year average, at 30.3" (The average is 15.4"). However, this number is still not high enough to crack the top 10 snowiest winters in DC. So now the question is, will we get one or two more snowstorms that push us over the edge, and into the top 10?

While it is possible, of course, that we could get another big snow storm or two, it's highly unlikely. As of today, we are exactly nine inches short of creating a tie for the 10th-snowiest winter in DC (39.3" in 1910-1911). A snowfall of nine inches or more in DC has only happened twice this late in the season in recorded history (in 1942 and in 1891). And these storms were extreme outliers; on average, after March 17th, we only get 0.2" of additional snowfall for the entire season.

April is notoriously sparse for snow-lovers. The most recent April snow at Reagan National happened in 2007, but it was a paltry 0.4" of accumulation. Since 1888, there have only been nine April snowfalls of 1" or more, with the most recent one happening in 1924! Yep, that was quite awhile ago. We can only expect accumulating snowfall (more than a trace) at Reagan National about once every ten years. Add in trace snowfall events, and the number only increases to two out of every ten years. That means, most of the time, we do not see any snowflakes in the month of April in Washington. By stark contrast, DC gets accumulating snow nine out of every ten Marches, and we have never had a March in recorded history without at least a trace of snowfall in the month. So, the frequency of snow events rapidly declines from March to April, and so does the amount of snow in each event!

One last thing... a snowy March doesn't increase the odds of getting snow in April. Since 1888, there have been 24 Marches with at least 6" of snow. Only three of those Marches were followed by snow in April of more than a trace. Statistically, it means that there is no correlation. This winter has been surprising at every turn, though, so maybe we'll get one last surprise to finish out this sneakily snowy season!