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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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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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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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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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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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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Soon-to-be Tropical Storm Sara brewing in the Caribbean

A robust tropical disturbance in the western Caribbean Sea is on the cusp of growing into a tropical storm. This system, which would earn the name Sara, is forecast to bring extensive flooding to Honduras before potentially entering the eastern Gulf of Mexico next week.

The National Hurricane Center began issuing forecasts for 'Potential Tropical Cyclone Nineteen' on Wednesday. This mouthful of a designation allows the NHC to issue watches and warnings for a disturbance before it formally becomes a tropical system, giving folks on land extra time to prepare for dangerous conditions.

Tropical storm and hurricane watches are in effect for the northern coast of Honduras as this system approaches the region.

Soon-to-be Sara is currently a cluster of disorganized thunderstorms over the western Caribbean. The disturbance is in a favorable environment to develop and strengthen over the next couple of days.

The Caribbean is home to the warmest waters in the entire Atlantic basin, with sea surface temperatures hovering in the upper 80s. As a result, the storm could be close to hurricane strength as it approaches the northern coast of Honduras this weekend.

A large ridge of high pressure over the southeastern United States will cut off the storm's ability to immediately turn north, forcing it to linger around Honduras for several days. This is why the below forecast track looks like such a hot mess—the storm will remain in the same general vicinity for about 48 hours.

Forecasters noted that this forecast is tough right now because 1) the storm has yet to form, and 2) it's going to be very close to land, which introduces its own uncertainty in terms of the storm's ultimate strength.

Regardless of its intensity, the stalling storm will produce flooding rains across Honduras' highly vulnerable terrain. Widespread rainfall totals of 10 to 20+ inches are expected throughout northern Honduras, which could lead to "life-threatening and potentially catastrophic flash flooding and mudslides," according to the NHC.

The remaining question: where will it go from here?

Most weather models show the ridge breaking down by late this weekend as a strong trough digs across the United States, allowing the storm to finally begin pulling away from Honduras and toward Mexico's Yucatan Peninsula. This track would pull the system (or its eventual remnants) into the Gulf of Mexico by early next week.

A northeasterly turn toward Florida is likely given the potent trough that'll swing into the region early next week. It's way too early to know what kind of impacts this storm or its remnants could have on the state, but it's something to keep in mind heading into the weekend.

Make sure your hurricane preparedness kits and plans are still ready to go in case this storm threatens hazardous conditions next week. 

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A new tropical system may develop in the Caribbean this week

The Atlantic hurricane season isn't over just yet. 

Forecasters are watching yet another disturbance in the Caribbean Sea for potential tropical development later this week. The system has a medium (50 percent) chance of development, according to Monday afternoon's tropical weather outlook from the National Hurricane Center.

The disturbance, which is currently south of Hispaniola, will gradually move into the western Caribbean over the next couple of days.

It'll find an environment with relatively low wind shear, decent humidity, and plenty of warm waters to fuel ample thunderstorm development. We could have a tropical depression in the region by the end of the week.

If this system manages to become a tropical storm, it would earn the name Sara as the eighteenth storm of the 2024 Atlantic hurricane season.

This is right where you would expect to see tropical development this late in the year.

High wind shear and puffs of cold, dry air blowing off North America make for an increasingly hostile environment across the Atlantic basin. The favorable conditions of the Caribbean remain one final refuge for late-season storms to try pulling their act together.

Make sure your hurricane preparedness kits and plans are still ready to go in case anything threatens land over the next few weeks. As we've seen several times this year, you don't have to live near the coast for flooding, power outages, and tornadoes to affect communities hundreds of miles inland.

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