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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New York City witnesses its driest month since records began in 1869

It's official: October 2024 was New York City's driest month in recorded history.

Any record in New York City is exceptional given that Central Park has one of the longest periods of record in the United States, with regular observations stretching all the way back to January 1869.

The city's observation station only picked up a measly 0.01" of rain during a light shower on October 29, which was the only measurable precipitation recorded there during the entire month. Given that scant amount of rainfall, the city has never witnessed a completely dry month devoid of any measurable precipitation.

A short drive down I-95 finds the longest dry streak on record in Philadelphia, where records began back in 1871. The city has gone 34 days without measurable precipitation as of November 1, and they're likely going to pad that streak heading into next week.

October 2024 was also Philadelphia's first month on record without any measurable precipitation. Previously, the city's driest-ever month was October 1963, during which they saw only 0.09" of rain.

This isn't just a regional lack of precipitation. As I outlined last week, most of the United States was unusually dry through the month of October save for a few select locations.

The latest update of the United States Drought Monitor found that 87.16% of the country was abnormally dry or mired in some level of drought.

This is the greatest area of aridity the USDM has ever measured since they started keeping track in January 2000.

A pattern shift heading deeper into November should reverse the trend a bit, especially in the Midwest and across the southern Plains. However, it's likely that ridging will continue to dominate the Mid-Atlantic and Northeast, bringing above-seasonal temperatures and drier-than-normal conditions to the region for a few more weeks.

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