December 31, 2024

Reality check: What we do and don't know about the looming cold snap


A surge of cold air looming over the eastern half of the United States has social media buzzing with the possibility of frigid temperatures and wintry precipitation.

We're really going to endure a spell of colder-than-normal temperatures through at least the first half of January. There’s also a real chance for widespread snow and ice during this period. Here’s a look at how much of the chatter is just that—chatter—and how much is legit. 

The Setup

The upper-level pattern for the past couple of weeks has featured a ridge over the eastern half of North America with a trough over the western half of the continent.

Ridges foster sinking air, which warms up and dries out as it descends toward the ground. Troughs allow for colder temperatures and more unsettled conditions as they force air to rise.

We’re going to see this pattern flip around heading into the first week of January, with broad troughing developing over the eastern halves of the U.S. and Canada. This will send eastern temperatures dipping below normal as we experience a phenomenon known as cross-polar flow—which is exactly what the name implies. 

A model image of the upper atmosphere on January 7, 2025. (Tropical Tidbits)

In the model image above, you can see a broad trough over the east while a sharp ridge builds over the West Coast. Most of the lines are parallel from far Northern Canada all the way to the southeastern U.S., opening the floodgates for Arctic air to pour southward by the second week of January.

As a result, the Climate Prediction Center (CPC) is extremely confident that we’ll have below-normal temperatures through the first half of January. The agency gives 80-90+ percent odds of below-normal readings between January 7th and January 13th. 

Reality Check

What does this mean for you? Here’s a look at average high and low temperatures for any given January 10th for a few eastern cities:


If we get temperatures coming in 10 degrees below normal, we’re looking at days hovering around the freezing mark in New York and D.C., while it’s a brisk winter’s day for places like Memphis and Atlanta.

There are hints in the models that we’re going to get temperatures much colder than 10 degrees below seasonal. It’s still too soon to place exact numbers on daytime highs and nighttime lows, but it’s possible that we could have the opportunity for single-digit or even subzero readings to dip pretty far south for a day or two.

And then there’s the elephant in the room: snow. Any chance for snow is a big deal south of the Mason-Dixon line, especially in places like North Carolina where it’s been two years since our last shovelable snowfall.

This type of pattern is favorable for one or more low-pressure systems to develop in the southern states and move toward the East Coast. Snow and ice are a decent possibility with cold air already in place.

We’re still more than a week from this pattern settling into place. It’s far too early to say who could see snow, sleet, or freezing rain, let alone detailing any accumulations

The heart of winter is on its way. Prepare for an extended period of chilly conditions with opportunities for wintry precipitation thrown in the mix. Keep up with the latest forecasts and make sure you’re prepared to stay warm—and possibly even shovel some snow.


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December 22, 2024

Cloud streets billow over the Atlantic behind a powerful cold front


There's always something cool bubbling in our skies above. It's worth taking a peek at satellite imagery every day to see what nifty features you can spot floating around.

A powerful cold front brought frigid temperatures to much of the eastern U.S. and Canada this weekend. New York City dropped to 15°F on Sunday morning, while Toronto made it all the way down to 0°F—the city's coldest morning in nearly two years.

All that frigid air spilled over the relatively warm Atlantic Ocean. Air in contact with the surface of the ocean warmed up and rose through the Arctic airmass above, producing countless cloud streets tracing the winds as they flowed across the ocean. It's the same principle that gives rise to bands of lake-effect snow off the Great Lakes. You can even see thicker bands of clouds flowing off Chesapeake Bay and Delaware Bay. 

Here are some links if you want to add them to your bookmarks:

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December 16, 2024

How are the Appalachians ten degrees warmer than lower elevations today?


There's a fascinating temperature profile across the Appalachian Mountains today thanks to winds interacting with the region's rolling terrain.

It's a raw and chilly day across the Piedmont today as daytime high temperatures struggled to break above the 50-degree mark from central Virginia all the way down into northern Georgia. 

Meanwhile, it's a comfortable afternoon high in the Appalachian Mountains, where temperatures are easily 10+ degrees warmer. Asheville was 61°F at 2:00 p.m. while Greensboro was shivering at 51°F. 

A model image showing winds a few thousand feet above the surface on Monday afternoon. (IMAGE: Tropical Tidbits)

What's the deal?

Strong southwesterly winds are blowing through the interior southeast and the Ohio Valley this afternoon as a low-pressure system tracks through the Great Lakes region. Meanwhile, we have a strong center of high pressure over the Northeast that's blowing cold air in from the north.


These chilly northerly winds are slamming into the side of the Appalachian Mountains and forcing that cold air to pool up at lower elevations. This is the infamous cold air damming we see so often this time of year.

The cold air is just shallow enough that it's sitting below the mountain ridges, allowing those warm southwesterly winds to rise over the cold air and blow over the peaks of the Appalachians.

Eventually, we'll see the cold air damming erode through Tuesday morning, allowing daytime temperatures to soar into the mid-60s at lower elevations. 


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December 9, 2024

Southern California's fire threat Monday night is a 'particularly dangerous situation'


The fire danger growing across parts of southern California is a "particularly dangerous situation," forecasters warned on Monday. 

Santa Ana winds expected to roar over the region will intensify Monday night into Tuesday, potentially fostering "extreme and life-threatening fire behavior" throughout the region.

A strong air pressure gradient building over southern California will spark a classic Santa Ana wind event, which features strong winds rushing over the coastal mountains toward the Pacific Ocean. These winds force air to warm up and dry out significantly as it rushes toward lower elevations.

Strong winds and low humidity are a perfect combination for the rapid ignition and spread of wildfires throughout the affected areas.


"Due to the expected long duration of dangerous fire weather conditions and critically dry fuels, there is the increasing potential for any new ignitions to have very rapid fire spread and extreme fire behavior (including long range spotting)," the NWS office in Los Angeles said on Monday.

The agency added: "As a result, there is the increasing threat for fire weather conditions to rival other historical fires in recent times including the Mountain and Thomas Fires."

Forecasters expect wind gusts of 50-80 mph to build across the region, with the greatest fire threat existing between 10:00 p.m. Monday through 2:00 p.m. Tuesday. 

The high-risk nature of this event prompted forecasters to add the wording "particularly dangerous situation" to the red flag warning draped across the region. This phrase, which is more commonly seen during severe thunderstorm outbreaks, emphasizes the threat to life and property posed by any fires that spark during these favorable conditions.

Please remain alert for rapidly changing conditions over the next couple of days if you're in an area prone to wildfires. Take extra care not to engage in any activities that could accidentally spark an out-of-control blaze, including stuff like flicking cigarettes on the ground and parking your vehicle on dry grass.


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December 4, 2024

Greensboro, N.C., breaks second-longest snowless streak on record


Success! 

A brief burst of snow across central North Carolina on Monday night dropped 0.4" of accumulation at Greensboro's airport, breaking the observation station's 1,038-day snowless streak.

This was the airport's second-longest stretch of consecutive days without any measurable snowfall. Previously, the last time it snowed enough in Greensboro to take a measurement was on January 29, 2022.


A large portion of the region saw a nice dusting of snow as this upper-level disturbance swung through the region. The snow fell vigorously enough to overcome the dry air, giving some communities between Greensboro and Charlotte as much as 1-2 inches of snow by the end of the night.

The snow didn't manage to reach the Raleigh-Durham area, and Charlotte's airport only recorded a trace of snow. A trace of snow occurs when snowfall is observed, but it doesn't accumulate on the ground. The streak of consecutive days without measurable snowfall continues at these airports. 

This was likely our last opportunity to see snow for at least the next two weeks. A ridging pattern will build back over the eastern U.S. through next week, forcing temperatures to return to normal (50s during the day) for a while. 


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December 2, 2024

Will North Carolina break its three-year snowless streak tonight?


A disturbance crossing the Appalachians on Monday night could bring parts of North Carolina their first measurable snowfall in nearly three years. There's a lot riding on the word "could," though, and it's just as likely that little to no snow falls during this brief window of opportunity.

There could be enough lift in the atmosphere to support the development of a batch of light snow across central North Carolina between about 9:00 p.m. Monday and 3:00 a.m. Tuesday.

Monday afternoon's forecast from the National Weather Service called for less than an inch of snow across south-central portions of the state, including Charlotte and Asheboro. 


One potential hiccup, as always, is the risk for dry air. It's going to be quite cold tonight. Temperatures were already in the mid-30s before sunset, and we're likely going to dip into the middle to lower 20s through the overnight hours.

Very cold and dry air is no good if you're hoping for snow, especially when you're expecting light snow rather than a solid thumping. It's very likely that we'll see snow on radar, but it'll be virga—evaporating long before it ever reaches the ground.

Even so, just the potential for a scattered dusting of snow is big news around these parts.


It's been a warm couple of winters across North Carolina. As of Sunday, December 1, it's been 1,037 days since January 29, 2022—the last time the airports in Greensboro, Raleigh, and Charlotte recorded more than a trace of snow.

Measurable snow counts as 0.1 inches or greater. A trace of snow occurs when snowflakes melt when they hit the ground, leaving behind no accumulation. 

This is the longest snowless streak on record in Charlotte (records back to 1939), the second-longest in Greensboro (since 1928), and the third-longest streak without measurable snow in Raleigh (since 1944). 

We'll see if anything happens tonight. If not, those snowless streaks will keep on rolling for a while longer still.


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November 27, 2024

Unseasonable chill to sweep U.S. to start December; lake-effect snow to finally begin


It's been a long while since we've started the month of December with any sort of widespread cold air sweeping the United States—but that's what we're in for this week as a surge of Arctic air floods south out of Canada. 

A powerful cold front will send temperatures plummeting over the next couple of days. Subfreezing daytime highs will envelop the Upper Midwest before the chill eventually makes its way down into the Deep South by this weekend.


Looking ahead to next Tuesday, December 4, high temperatures will struggle to make it into the lower 40s along the I-95 megalopolis, with temperatures remaining in the 40s as far south as Birmingham and Little Rock. Even Miami will "only" reach 74°F next Tuesday, the poor things.

It's about time we've had a widespread chill. Temperatures have been running a fever this fall. 


We just have to take a look at the Great Lakes to see how much of an effect that warmth has had so far this season.

Water temperatures across all five lakes are running several degrees Celsius above normal for the final week of November. Parts of Lake Erie are still hovering around 65°F thanks to the relative lack of cold air of late.

Frigid air pushing over very warm lakes will be a recipe for ample lake-effect snow beginning Thursday on Lake Superior and continuing for all the lakes through the weekend and well into next week.


West-northwesterly winds will create a long fetch across Lakes Superior, Erie, Huron, and Ontario, creating narrow corridors of very heavy snowfall on the downwind shores. Some areas will see more than a foot of snow by Saturday morning, and these totals are likely going to soar higher as the lake-effect snow event continues into next week.

Happy Thanksgiving! Winter is here (...for now, anyway).


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November 19, 2024

Phenomenal bomb cyclone develops off the Pacific Northwest


An exceptionally powerful low-pressure system developing off the coast of the Pacific Northwest this afternoon is a sight to behold on satellite imagery.

The storm is undergoing bombogenesis—hence the "bomb cyclone" moniker. Bombogenesis occurs when a low-pressure system rapidly intensifies at a rate of about 24 millibars in 24 hours.

Our low-pressure system has more than doubled that rate of intensification. At the time of this post, its minimum central pressure had dropped from 981 mb to 955 mb in just six hours between 1200 UTC and 1800 UTC. It's likely going to strengthen further heading into Tuesday evening.

Such a remarkable rate of intensification is a sheer display of the atmosphere's power. 

A broad upper-level trough over the northern Pacific Ocean coincided with two jet streaks, or regions of stronger winds within the larger jet stream. Winds collide together and fan out as they blow around troughs and as they enter and exit these jet streaks. 

A model image of the jet stream on Tuesday morning. Source: Tropical Tidbits

When winds fan out—or diverge—it leaves a void in the upper levels of the atmosphere that air from the surface has to rush upward to fill. This upward motion leaves less air, and lower air pressure, at the surface. This is how most of our everyday low-pressure systems develop and sustain themselves.

Multiple sources of divergence working together can force a massive amount of air to rise into the upper atmosphere, very quickly creating a powerful center of low pressure at the surface. That trough combined forces with the two jet streaks to rapidly intensify this storm as it swirls off the coast of the Pacific Northwest.


While the center of the storm will remain far offshore, folks on land are certainly feeling its effects.

Widespread high winds will likely lead to tree damage and power outages from Oregon to British Columbia. Flooding rains are also expected as the storm pushes a surge of tropical moisture ashore. Rainfall totals of 7-10+ inches are expected across sections of northern California through the weekend.


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