February 17, 2024

How an incredible band of snow developed over the Northeast on Saturday

One of the most impressive bands of snow we've seen in a long time developed over a narrow swath of the Northeast overnight Friday into early Saturday, creating an intense snowfall gradient over very short distances.

A fast-moving low-pressure system tracked out of the Midwest toward the East Coast through the day Friday, producing heavy snow along its path from St. Louis to New York City.

A radar snapshot from around 1:30 a.m on February 17, 2024 (RadarOmega)

This wasn't an ordinary clipper system, though. While weak systems like this tend drop a few inches of snow before moving on their way, we saw a dramatic band of snow develop on the northern side of the low.

The band left behind enhanced snowfall totals from Indiana to New York, reaching its peak intensity over portions of Pennsylvania and New Jersey. 

A tremendous gradient sliced through Allentown, Pennsylvania, with folks on the south side of town witnessing 13" of snow while neighborhoods north of town saw just 3" of snow. Farther east, observers measured 10" of snow in Brooklyn, while only 2" of snow fell in Central Park just a dozen miles to the north.

Even more impressive is that parts of New Jersey saw snowfall rates of 5" per hour at the peak of the snowfall. This kind of convective banding is something more like what you'd see off the Great Lakes rather than eastern Pennsylvania and northern New Jersey. Instead of lake-warmed air fueling intense bands of snow, we saw intense frontogenesis along the northern side of the low.

Frontogenesis in the mid-levels of the atmosphere early Saturday morning. (NOAA/SPC)

Frontogenesis occurs when an airmass on the move collides with another airmass nearby. This interaction creates a stretching motion through the atmosphere. Winds slowing down and fanning out leaves a 'void' in the atmosphere that air has to rush upward to fill, a rising motion that creates a convective band of very heavy snowfall.

We often see this process during classic nor'easters when a shield of very heavy snow develops on the northwestern side of the storm. But it's not limited to nor'easters—conditions were just right for this relatively weak system to generate the intense dynamics needed to drop a tremendous amount of snow across a narrow stretch of real estate.

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February 11, 2024

From 60s to Snow: The Northeast's Nor'easter's Set To Hit Hard Tuesday

It's been a long wait, but we've finally got a classic nor'easter brewing for the eastern half of the United States.

A foot of snow is likely to fall on Tuesday where temperatures climbed up into 60°F on Saturday. While this one isn't going to bring the blockbuster I-95 snows that weather enthusiasts gush about for the rest of their lives, this will be an impactful storm for a full one-third of the country.

We've got one heck of an upper-level low scooping through the southern half of the U.S. on Sunday and Monday. This feature, along with a ripping-fast jet stream, will help develop and strengthen a robust low-pressure system at the surface over the next few days. 

A model image depicting the upper-level pattern early Monday morning. (PolarWx/Tomer Burg)

Ample moisture streaming into the storm from the Gulf of Mexico will provide plenty of juice to fuel thunderstorms, heavy rains, and eventually heavy snowfall once the storm rolls into the Northeast late Monday into early Tuesday.

Severe weather risk

We're already feeling the start of this robust storm across the south where heavy rain and severe thunderstorms are ongoing from Texas to Mississippi.

The Storm Prediction Center issued an enhanced risk for severe weather on Sunday—a 3 out of 5 on the scale—stretching from the Houston metro area east into central Mississippi, with a broader risk for severe weather radiating out to cover the northern Gulf Coast up toward Birmingham and Atlanta.

The main risk we'll see with any thunderstorms that develop on Sunday will be damaging wind gusts of 60+ mph and hail the size of quarters or larger. A few tornadoes are possible, as well, especially in and around the enhanced risk area.

A risk for tornadoes on Super Bowl Sunday is a pretty nerve-wracking prospect for broadcast meteorologists in the region. It's likely that some CBS affiliates will have to cut into game-day programming for live coverage of tornado warnings.

Local television news stations are required to cover tornado warnings. It's a public service they're compelled to fulfill in exchange for holding a federal broadcast license. Getting the word out that deadly severe weather is imminent is more important than any show or sporting event, angry viewers and hateful comments be damned.

The risk for severe weather will push east into the day Monday, with a threat for damaging wind gusts spreading over most of the southeast as our low-pressure system continues strengthening as it tracks into the Mid-Atlantic.

Sharp cutoff in heavy snowfall totals

After the severe weather wanes, attention turns to the threat for disruptive snows across the Northeast.

This is going to be an all-or-nothing ordeal for many folks from northern Pennsylvania east toward coastal New England. The Weather Prediction Center's Winter Storm Severity Index (WSSI) shows "major impacts" are likely across eastern Massachusetts, centered around the Boston area, as a result of the very heavy snowfall expected on Tuesday.

Heavy snow will spread over much of Pennsylvania early Tuesday morning as our storm pushes into the region. We'll see snow steadily push east through the morning hours Tuesday as the low-pressure system intensifies and tracks toward the coast. 

This will be a daytime storm for the region, with very heavy snowfall rates and gusty winds plastering northern Pennsylvania through southern Maine into the evening commute before the snow eventually tapers off through the evening and overnight hours.

A very sharp cutoff in precipitation on the north side of the storm, combined with a sharp changeover to rain near the center of the low, will produce a swath of heavy snowfall that rapidly tapers to a dusting to the north and south.

Double-digit snowfall totals are likely for portions of northeastern Pennsylvania, upstate New York, northern Connecticut, and just about all of Massachusetts. This is likely going to cause traffic jams across the area, especially if people head to work and school in the morning and then try to head home after the snow begins. 

Slight changes in the track of the storm can have huge implications on snowfall totals with a cutoff this sharp. A nudge of five or ten miles to the north or south will drag those hefty snowfall totals right along with it.

If the current forecasts hold up, this is likely going to be Boston's largest snowstorm in two years. The city's last big-time snowstorm was 23.9" on January 29, 2022. The city's only measured 21.6" of snow in total across all of the storms they've seen in the past two winters combined.

[Satellite image via NOAA]

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February 3, 2024

Flooding risk as atmospheric river fuels rainy, windy storm sweeping into California

A series of systems rolling through the eastern Pacific will wash ashore in California over the next couple of days, fueling a spell of foul weather across the state. 

Heavy rains are likely up and down the coast, with southern California facing the greatest risk for flooding and landslides. Prolific mountain snows are likely across higher elevations throughout the Sierra. Gusty winds are likely across the state early into the workweek, which may lead to tree damage and power outages.

A quick look at the pattern across the country this weekend almost looks like an image you'd see in the El Niño chapter of a meteorology textbook. A strong jet stream draped across the entire southern half of the U.S. is exactly what you'd expect to see during an El Niño winter.

A model image of the jet stream on Saturday afternoon. (Tropical Tidbits)

An upper-level trough digging into the West Coast will drive the active weather over the next couple of days. The most impactful conditions will arrive courtesy of a low-pressure system hitting the coast late Saturday through Sunday, though heavy rains will continue Monday and Tuesday across the southern half of the state.

Powerful southwesterly winds hauling tail into the California coast is dragging a plume of tropical moisture straight from Hawaii. This ribbon of elevated moisture—called an atmospheric river—will fuel the drenching rains we'll see across California over the next few days. Atmospheric rivers provide a vast reserve of moisture for storms to tap into to create excessive rainfall rates.

A model animation showing the atmospheric river moving into California through Tuesday, Feb. 6. (PolarWx/Tomer Burg)

Rain is already starting to come ashore in California on Saturday afternoon, and we'll see the rain increase in coverage and intensity through the overnight hours and into the day Sunday.

We'll see the rain slacken a bit for the northern half of the state as this initial system moves east. However, the heart of the atmospheric river will continue streaming over southern California through at least Tuesday, providing ample opportunity for bouts of heavy rain to continue throughout the area.

The Weather Prediction Center's latest forecast calls for 3-5 inches of rain for just about the entire California coast from San Diego all the way up to near Eureka. It's going to be a soggy couple of days for San Diego, Los Angeles, and San Francisco.

Higher terrain will enhance rainfall totals up and down the coast—especially along the Transverse Ranges, where 5-7 inches of rain is in the forecast with the potential for some areas to see locally higher rainfall totals.

Folks in the Central Valley will benefit (or suffer, depending on your point of view) from the rain shadow effect, with less than an inch of rain over the next week for communities like Bakersfield and Fresno.

As usual, a glut of torrential rain heading for California is a recipe for widespread flash flooding and landslides. The greatest risk for flooding exists across southern California.

The usual safety spiel is extra important this time around: Never attempt to drive across a flooded roadway. It's impossible to tell how deep the water is until it's too late, and California's rugged terrain makes it more likely that the road is washed-out beneath the water. 

Precipitation will fall as heavy snow across the Sierra above 3,000 feet. Freezing levels will rise to 4,000 to 6,000 feet as mild air flows inland with the surge of subtropical moisture. Several feet of snow and whiteout conditions are likely in areas expecting all snow, which will force some road closures through the mountains as the heavy, blowing snow will be extremely difficult or downright impossible.

Widespread gusty winds are likely as this initial storm rolls across the state Saturday night through Sunday. High wind warnings and wind advisories are in place for most of California. Gusts of 40+ mph are possible in areas under a wind advisory, while gusts of 60+ mph are expected for communities under high wind warnings.

The combination of strong winds and heavy rainfall will lead to a risk for tree damage and power outages. Avoid rooms where trees or tree limbs may fall through the walls or roof during high winds, and try not to park beneath trees if you can help it.

[Satellite image courtesy of NOAA]

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