Matt Lanza

Snowstorms like the one on Tuesday offer some of the most difficult challenges for consistently accurate weather forecasting

One sentence summary

Snow forecasting is hard, and we’ll use yesterday’s storm in the Eastern U.S. to show off some of the inherent challenges that exist.

Yesterday’s storm exposed the limitations of making consistently good snow forecasts

As a native of New Jersey and having spent the first 5 winters of my career in the lake effect snow belt, I know how tricky snow forecasting can be. There are endless amounts of complications with snow: Elevation, air temperature and snow ratio, dry slots, ground temperatures, sun angle, mixing, vertical velocity and relationship to the snow growth zone in the atmosphere, and on and on. It remains one of the more elusive forecast problems. My own opinion on this is that our ability to forecast snow has hit a bit of a plateau. I will admit that I don’t know if statistics actually bear that out. Hurricanes continue to show improvement, temperatures show improvement, but snow? Precipitation in general? I don’t know how much better it is today than 5 or 10 years ago. Feel free to blast me in the comments if that’s not accurate.

But, look, even if forecasts have improved considerably in the last 5 to 10 years, the inherent challenges with snow forecasting will always remain. Here’s an example.

An impressive snowfall gradient in northeast Pennsylvania from yesterday’s frustrating snowstorm. (NOAA)

Yesterday morning, in northeast Pennsylvania, east of Scranton there was a pretty interesting scenario. In Shohola, on the Delaware River border with New York, 15 inches of snow was reported at 10 AM. About an hour later, roughly 10 miles west of Shohola, PA a total of 3.5 inches was reported just northeast of Lake Wallenpaupack. Doing some quick math, that comes out to near a foot of difference over 10 miles, or an average of over 1 inch of change in snow accumulation per mile.

Now, does this prove anything? No. We don’t know whether the measurement in Shohola was perfectly reliable. We don’t know what elevation it was taken from. I suspect the 3.5 inch report was from about 1,200 feet in elevation, and I don’t suspect the Shohola report was from much higher an elevation. But, you never know. Other reports nearby seem to corroborate it enough. But even if it’s 12 inches instead of 15, the point is still clear here: Snowfall gradients like these are common in Eastern U.S. snowstorms. They are the rule, not the exception.

Radar image from northeast Pennsylvania around 6:30 AM ET on Tuesday. (RadarScope)

If you look at radar from yesterday morning in Pennsylvania, one of the key hurdles in snowfall forecasts shows up. We call this mesoscale banding. Weather modeling has come a long way in the last 10 to 15 years, but one of the trouble spots can occasionally be these mesoscale bands. You’ll get widespread, large scale snowstorms, like the one yesterday in the East. But embedded within the overall precipitation will be these narrow, sometimes stationary bands of snow. For those of you on the Gulf Coast, think of what happens when you get training lines of thunderstorms that cause flash flooding. In storms like the one yesterday, by a different process, you get a similar outcome but with snow. On the map above, you can see the roughly 10 mile separation between Shohola and Kimbles (where the 3.5 inch report came near). Over that distance at 6:30 AM ET, the radar shows heavy snow near or just southeast of Shohola, diminishing to light to moderate snow 10 miles west of there. The difference between the heavier snowband in yellow and the generic light to moderate snow can easily be 1 or 2 inches an hour. So, if that band parks there for 3 hours, you could easily see 5 to 7 inches of snow under it with 1 to 3 inches just west. Extrapolate that over a couple more hours or increase the snow rates in the heavier mesoscale band and, well, you get the idea.

Let’s look at another example. This one comes from Connecticut yesterday.

Radar loop from Tuesday morning in Connecticut. Note the heavy, nearly stationary band of snow just northwest of Hartford. (RadarScope)

A report of 15.5 inches of snow occurred near Farmington, CT, which was placed precariously under that heavy band of snow you can see northwest of Hartford and north of I-84 if you look at the radar loop above. Notice how the area northwest of that snowband into Litchfield County, CT sees almost none of that. Well, that area to the northwest ended up with plenty of snow, about 4 to 8 inches worth in many spots.

Snow totals in northern and western Connecticut (a few of these are incomplete) shows 4 to 8 inches in northwest Connecticut with a very well defined corridor of 10-15 inches just north of I-84 and northwest of Hartford. (NOAA)

But you come just southeast of there, in the area from near Danbury through Waterbury through Bristol and Farmington out toward Tolland, and you see 10 to 15 inches of snow. Over short distances, you could have gone from 7 or 8 inches to 14 or 15 inches. Granted, both cause impacts, but nearly doubling up snow at those amounts can have severe ramifications on travel or forecast perception.

A rough estimate of snow totals yesterday based on available reports. (Tomer Burg)

So why am I devoting a whole post to this topic? Well, for one, the East has seen a bit of a snow drought the last couple years, and it’s nice to revisit the idea that this stuff happens. But also, I saw a number of posts about threats, real or exaggerated, received by meteorologists because the forecast of this event busted in spots. Just a couple days ago, it looked like a broad, major New England storm. Some accused meteorologists of lying or just not being honest.

Here’s the truth: Snow forecasting is inherently difficult. You see the images above. Imagine now if that mesoscale band is forecast to be over Long Island and Central and Southern New Jersey in modeling ahead of the storm. Instead, when the storm happens, the band shifts northwest by about 25-35 miles and ends up over Philadelphia and New York City instead. Not to discount the southern half of Jersey (my homeland) or Long Island, but you’re talking about a shift where millions of people see considerably more snow than perhaps they expected. Or, even worse for perception, it ends up out to sea by 25 miles and now a few million fish get heavy snow instead of a few million people? Now, imagine if the models just simply give you little to no indication that the mesoscale snowband is going to setup at all? In these events, everything is connected. Trying to pin down storm track, trying to predict if and where banding will occur, trying to factor in temperatures in the air and on the ground (like yesterday where many places saw a lot of falling snow but not a lot of accumulating snow), or forecasting things like sleet or rain mixing in: It all matters. And the reality is, we’re decent at it all, but we’re not consistently great at it, and neither are models.

So it’s important to recognize that meteorologists try very hard to do good work during these events. Sometimes things change. Sometimes it’s at the last minute too. And that can lead to a significant forecast bust in some places. Or less a bust and more of an over or underestimate. It will happen again, and because of the geography of the Northeastern Megalopolis, there will be times where millions of people get outcomes that are somewhat unexpected. Just remember: No one is misleading you or lying to you. The atmosphere likes to challenge us, and some days we are left with egg on our face.

As California looks to a stretch of quieter weather, some signs that more rain is on the horizon

One-sentence summary

After an impressive rainstorm in California, quieter weather will settle in for a few days before we watch the potential for another rain event in the region beginning around next weekend.

California cleaning up

After a wild sequence of rain in California, the time for clean up has begun.

Click to enlarge a map of the last week’s worth of rain totals in the LA Basin and surrounding areas. (NOAA)

The amount of rain that fell in some places is staggering. California is always a place of extremes, and with these extremes seemingly happening more often and at greater amplitudes, it’s making for a lot of whiplash.

In quite a change, Sierra snowpack now ranges from 68 percent of normal in the south to 78 percent of normal in the north. Those values were 36 and 60 percent respectively a week ago. While these storms come with damage and a lot of headaches, on net this is probably good news for California. The Sierra needed snowpack replenishment desperately. A year ago, after another sequence of ferocious storms, the amount of water in the snowpack ranged from 170 to 240 percent of normal.

Sierra snowpack has made major gains over the last week, but it’s still below average for this time of year and severely lagging last year’s high-end pace. (California DWR)

For the rainy season so far, San Diego has seen 7.88″ so far, about two and a half inches above normal. Downtown Los Angeles near the USC campus has recorded 15.43″, which is over seven inches above normal. Almost 90 other years back through the 1870s did not even see that much rain during the entirety of the rainy season in Los Angeles. Downtown San Francisco has seen over 16 inches this rainy season, which is about 3 inches more than usual to this point. As is typical for El Nino, the anomalies are strongest in SoCal.

What’s next? A break, but then potentially more rain

Things really do calm down for California over the next few days with one more round of rain, then some showers, then a dry stretch.

Additional rainfall expected over the next couple days in southern California. (Pivotal Weather)

Rainfall over the next week or so should be manageable, though some residual issues due to mudslides and high water will continue. But new problems should be minimal.

Beyond early next week, things start to change again, and it’s possible we have another round of significant weather aiming for California beginning around next weekend. Why? If we look up at the jet stream level on today’s run of the European model, we can see what’s happening. Initially, California is in a calm bubble of sorts. This is what will help restore fair weather to the region.

The jet stream relaxes into next week, but another surge of Pacific moisture may be on the horizon, aiming for Mexico or California. (Tropical Tidbits)

If you look out over the Pacific Ocean, you’ll see the jet stream winds again stretching and extending toward the U.S. West Coast around late next week or weekend. This shows up on most modeling and has for a few days now. Thus, it seems reasonable to think that another round of storminess may approach California around that time. In fact, the Climate Prediction Center has again highlighted California and the West for another round of heavy rain and snow risk beginning around February 17th.

The Western U.S. will again be in line for the risk of another round of heavy precipitation beginning around next weekend. (NOAA)

Modeling backs this up, with the GFS Ensemble from earlier today showing anomalies of about 1 to 2 inches on average across California from February 16th through the 23rd. Ensemble means tend to “wash out” extremes by design. In other words, because you’re looking at an average of a 30 member ensemble, you will lose some of those extremes in favor of a dominant signal. What we know? It will likely get wet again in California. What we don’t know? How wet.

GFS Ensemble mean forecast precipitation anomaly for February 16-23, showing above normal precipitation in California, Arizona, and Texas. (Tropical Tidbits)

All that said, this is looking like another potentially robust multi-day, multi-inch rain event next weekend into the following week in California. A good idea to stay tuned.

El Niño and California rain: The complex relationship and how the current storm fits the overarching narrative

One-sentence summary

With a powerful storm hammering California today, we take a look at why California is targeted so often during El Niño winters and a quick look at what’s to come after this week.

California Streaming

One of the biggest storms in years is impacting portions of California. Rain totals are expected to be in the double digits in a few places, with upwards of 5 more inches of water coming to LA and 1 to 4 inches or more up and down the coast from the Bay Area through San Diego.

Additional rainfall as forecast by the National Weather Service in the southern half of California over the next 2 days. (Pivotal Weather)

This has led to two days worth of high risks from the Weather Prediction Center for excessive rainfall. High risks are uncommon, and they are historically very well correlated to flood damage. Over 80 percent of flood-related damage and nearly 40 percent of flood-related fatalities occur on high risk days.

Click to enlarge maps of Weather Prediction Center excessive rainfall outlooks today and tomorrow. High risks are posted for California, level 4 out of 4, indicating a significant risk of life-threatening and/or damaging flooding. This includes the Los Angeles metro area. (NOAA)

Seeing back to back high risks in California is certainly impressive. Translation: Take all warnings seriously.

In addition, powerful winds are impacting the Central Coast, Bay Area, portions of SoCal, and in the mountains. Wind gusts are expected to top 60 mph, and in fact the NWS actually issued a hurricane-force wind warning offshore of California for the first time in this region. The wind should begin to ease up a bit heading into tomorrow.

Wind gusts will be powerful today and tonight on the coast of California and inland as well. (Pivotal Weather)

Lest we forget, the winter weather impacts from this storm will also be big. The Sierra will see upwards of 1 to 3 feet of snow, wind, and risk of avalanches.

A late Sunday morning view of I-80 at Donner Summit. (CalTrans)

Additional moisture will continue to move through California and the West into midweek, but it should be a less impactful than what is currently ongoing.

Why California?

When you think of El Niño, many of you will think of Chris Farley or weak hurricane seasons or warm Pacific Ocean water. All three, while nuanced are correct thoughts to have. You may also think of rain in California. If you’re a student of the 90s (raises hand), you may remember the 1997-98 El Niño, which at that time was one of the strongest on record. So why does California tend to be stormy during stronger El Niño events, such as the one we have ongoing in 2024?

If you look at a comparison of the 30-year average of jet stream zonal winds side by side with just El Niño years from December through March in the higher end El Niño events since the 1960s, you get a few interesting things that stick out.

A comparison of jet stream level winds between December and March in a “normal” year (left) and a strong El Niño year (right). (NOAA)

In the area I’ve labeled “A,” you can see that the map on the right has stronger winds farther east than in the map on the left. What does this mean? We usually see a stronger jet stream extended across the Pacific during El Niño winters, which points it more often at the southern half of California or northern Mexico. For the area labeled “B,” you can see that in the map on the left, the peak of that portion of the jet stream is centered more over the Mid-Atlantic. The map on the right shows that more over Florida or the Southeast. Basically, the jet stream aims south more often. And historically, you’ve ended up with a wetter southern tier of the United States from California to Texas to Florida.

Rainfall patterns during a typical El Niño event around the world. (Columbia University IRI)

Essentially what happens is that as the water of the Pacific Ocean warms in the Equatorial region, you force typical features to displace relative to normal. This shakes up the landscape, including that shift and intensification of the jet stream. Not every El Niño is created equal, even the strong ones. The 2015-16 El Niño was a bust in SoCal in terms of rainfall, but the following year was pretty wild, especially in NorCal.

You can see a pretty good positive correlation between rainfall in Southern California and “maximum ONI,” which is the Oceanic Nino Index: As El Niño peaks stronger, precipitation tends to increase. Or so it was through the 2000s. (NWS Los Angeles)

But in general, the relationship does hold up historically. A strong El Niño usually produces a wetter SoCal. Things are a little more muddled in the northern part of California. And notably, when you consider flood damage, things get more confusing, with only four of the top 13 most damaging winters in terms of flooding in all of California occurring during El Niño years according to Jan Null of Golden Gate Weather Services.

So all in all, while this storm is particularly intense, it basically fits the overarching narrative of an El Niño winter in California.

The storm train should calm down some in California after this week, with the jet stream aiming a bit farther south more into Mexico and deep south Texas and Florida.

The 8 to 14 day outlook shows that California rain may ease up some after this week, but wetter than normal conditions are still favored broadly across the southern tier of the U.S. (NOAA CPC)

It’s not unheard of for a rebound of storminess to occur in California into March or even April in years like this, so I doubt we’re done yet. But it will be good to hopefully have some time to dry out a bit between storms there.

Winter prepares to take a break, so we’ll talk about crawfish, recent research, and the 2024 Atlantic hurricane season

One-sentence summary

Today’s post will discuss the return of mild weather, Louisiana crawfish, rain in California, some recent hurricane research, and some too early thoughts on the 2024 hurricane season.

Winter hiatus

After a week where we’ll see heavy rain in the south and wintry precipitation scattered across remnant cold air to the north, the bigger weather story may be what’s coming next week.

Significant expected rainfall is both welcome and concerning, as Flood Watches are posted from north of Houston into northern Alabama. (Pivotal Weather)

First, we have some flooding concerns in spots in the South to get through, although in some areas, like Texas and Louisiana and Mississippi, this rain is quite welcome. Some of those areas remain in extreme drought. While this is good news in general, it may be too late for crawfish season in Louisiana. Between high costs, the recent cold, and the long-term drought plaguing the region, it’s going to be a tough harvest this year. Mark Shirley of the LSU AgCenter shared thoughts on this season recently, and they were not positive. Not great news for those who bank on this season.

Rains should wind down later this week, setting up a drier week in the South next week. In terms of temperatures, where won’t it be cold for this time of year? Virtually everywhere.

If you like winter weather, the 8 to 14 day outlook is a tough one to look at. (NOAA)

The most recent 8 to 14 day outlook from NOAA’s Climate Prediction Center looks bleak for winter lovers. Weather model data strongly backs this up, and it appears that winter will go on hold for a time as we close January and open February. Though this map stops at the border, it’s expected that warm air is going to flood most of Canada as well, particularly central Canada. The Maritimes and West Coast will be much more moderate than this.

If there is some good news, it comes in California and the West, which is expected to see above average precipitation continuing into next week as well.

California looks to be targeted by a pretty wet stretch of weather next week. This should hopefully also help the snowpack some in the Rockies. (NOAA)

The combination of warm temperatures and wet weather is not always the greatest duo to see out West in winter, but hopefully this can at least continue the snow building process in the higher terrain. Whatever the case, there is high confidence in a fairly wet pattern in that region.

Recent hurricane research: More storms with more rainfall

Shifting gears now, as we advertise ourselves as primarily a hurricane-focused site! I wanted to highlight a couple batches of recent hurricane research that came out in late 2023. Last June, we published an article that summarized a bunch of research on the Gulf of Mexico and rapid intensification. We want to try to continue to keep you informed on general interest research on the tropics, particularly as it relates to the Atlantic Basin and North America.

A preprint being reviewed for publication in Nature Portfolio authored by a group from both the Pacific Northwest National Laboratory and Lawrence Berkeley National Laboratory was posted in early November discussing climate change, flooding, and social inequality from Hurricane Ida back in 2021. The general gist of the paper is like this: We know that climate change will tend to exacerbate social inequality. Disasters tend to hit more vulnerable populations both worse and more often. The paper finds that the climate change impact on flooding is non-linear, which is to say that as the planet warms, flooding should worsen at a faster rate. So as social vulnerability increases, the risk of suffering from deep flooding also increases. “In other words, for deep flood exposure, both past and future warming increase social inequality in the direction that exacerbates relative flood risk for more vulnerable people.”

The study essentially confirms that the flood impacts from Ida’s remnants were worsened by increasing precipitation due to climate change and the most vulnerable segment of the population saw a worse outcome than they otherwise would have because of this. You can see the paper to learn more about their methods and some of the specific numbers they used, which are in line with prior studies done after Hurricane Harvey. While the rainfall element of hurricanes has always been a bad one, we’ve seen several recent examples where it seems as though it’s getting worse, and research in recent years continues to basically confirm that feeling.

Another interesting paper published in November in the European Geophysical Union’s Journal of Atmospheric Chemistry and Physics touches on how aerosols from anthropogenic (human) sources impact the destructiveness of hurricanes. What stood out about this article was their finding that an increasing concentration of aerosols near the immediate coast (say, as storms approach the petrochemical complexes of the Gulf Coast) can lead to a weaker but larger storm with an 11 to 22 percent increase in precipitation within 100km of the center. The study used Hurricane Katrina as a model storm, and they worked to utilize very sophisticated modeling to solve for this.

“For the first time, a 3-D atmosphere–ocean fully coupled regional model (WRF–ROMS) at the cloud-resolving scale was used to simulate Hurricane Katrina in order to investigate the aerosol–TC system with the inclusion of air–sea interaction,” the authors write.

What I think our readers should take away from this research is that it adds to the growing body of evidence that suggests hurricanes are becoming more moisture-laden as they approach land, which is resulting in more rain, more flooding, more damage, and more impacts to more people.

2024 hurricane season early vibes check

There have been some items published in recent weeks about the upcoming hurricane season. I’ve seen everything from people expecting an active season to a very active season to the “hurricane season from hell.” Hyperbole aside, what does the very early data actually show about the hurricane season that begins in just over 4 months?

The first question we’d ask is whether El Niño will continue into this summer. The answer to that question is still an uncertain one. Modeling is aggressively weakening the El Niño event by late spring, as is often typically the case with strong events like this one. The European model below, for example, has us close to “ENSO Neutral,” or “La Nada” by mid-summer, with extrapolation pushing us toward weak La Niña perhaps by the peak of hurricane season.

The ECMWF long range forecast suggests an aggressive weakening of El Niño this summer, but still some question as to whether or not we end up in La Niña. (ECMWF)

Why does this matter? La Niña events are much more favorable for active hurricane seasons than El Niño events. So if we were to hypothetically tip back into La Niña this summer, it would likely aid an active hurricane season.

Historically, since 1950 in hurricane seasons immediately following a higher-end El Niño, we average 6 hurricanes (with as many as 10 in 1998). A normal hurricane season has about 7 hurricanes, so one could argue that the season proceeding after stronger El Niños may not necessarily skew dramatically higher. From that end, I don’t know that we can currently read too much into El Niño vs. La Niña chances this summer and how they may influence the season.

That being said, one thing we can absolutely read into are sea-surface temperatures (SSTs). The globe’s oceans are undergoing a heat wave of sorts. In fact, water temperatures in the main development region of the Atlantic basin are currently in line with what you would normally expect to see in July. That’s not a typo.

So I do not think it’s a stretch to say that we would expect SSTs to be warmer, if not much warmer than normal again this summer across much of the Atlantic basin. A look at one model in particular, the NMME, which is an ensemble of several different climate models shows an extremely warm look to the Gulf, Caribbean, and Atlantic this summer.

The NMME model forecast for SSTs this summer is ugly looking in the Atlantic with warm water everywhere again. (NOAA)

Now, what I might caution about here is that the NMME also has a dramatic La Niña signature in the Pacific that might be a little (or a lot) overdone. Just because these maps are scary looking does not mean that they will verify as shown. But, there’s more than enough evidence right now to suggest that SSTs will skew warm this hurricane season.

So sitting here on January 23rd, I would be willing to say this right now: The combination of warm water and a weakening El Niño probably suggests an active hurricane season ahead in 2024, but there is a very long way to go yet. No reason to worry about the sky falling right now, but I would not be surprised to see some fairly aggressive seasonal forecasts in the months ahead.