Source: Strange Sounds

A widespread and intense heatwave is roasting large portions of the U.S. and Canada, shattering daily and monthly temperature records.

Why it matters: Winter is the fastest-warming season across the U.S., and the lingering warmth is shortening the snow season in places like Colorado and Montana, where mountain snowpack is a critical source of water during the summer months.

The big picture: On Wednesday, British Columbia saw its highest temperature ever recorded during the month of December, with a high of 72.5°F recorded in Penticton, which is 250 miles east of Vancouver. This tied the country’s all-time record high for the month.

Temperatures across the Plains, portions of the Rockies and Central states are running as much as 35°F above average for this time of year.

Four states have tied or broken records for the hottest temperatures seen during December: Washington, Montana, Wyoming and North Dakota.

In Denver, Colorado, which often picks up heavy snow at the start and end of winter, it has not yet snowed, setting a record for the latest measurable snowfall. The forecast high temperature on Thursday is in the mid-70s, about 30°F above average for this time of year.

Records are expected to fall Thursday in Denver, Omaha, Oklahoma City, and Kansas City, Missouri among other locations. According to Herrera, Hettinger, North Dakota, located just north of the South Dakota border, hit 71°F on Wednesday, setting a monthly record for the state.

In Cheyenne, Wyo., the temperature reached 70°F on Thursday, setting a state record for the month of December. A monthly record was also set in Nebraska.

The warmth, combined with powerful atmospheric river events rolling in off the Pacific Ocean has eaten away at the snowpack in the Pacific Northwest and the Canadian Rockies, contributing to flooding at lower elevations.

While continental US records extremely mild temperatures, strong blizzards are in for Alaska and Hawaii!

https://twitter.com/extremetemps/status/1467678932065394691?ref_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E1467678932065394691%7Ctwgr%5E%7Ctwcon%5Es1_&ref_url=https%3A%2F%2Fstrangesounds.org%2F2021%2F12%2Fearly-winter-heat-dome-temperature-records-usa-canada.html

The proximate cause of the warmth is a sprawling area of high pressure, or heat dome, parked across the West, with the clockwise flow of air around it pumping mild air across the region.

This weather feature has been deflecting storminess far to the north and prevented colder air — record cold currently lurks in Alaska, for example — from spilling south.

Long-term trends also favor more frequent and intense extreme heat events. This ongoing event is likely to make the already lopsided balance between warm and cold records in the U.S. for 2021 even more in favor of the warm milestones, possibly by more than a 2 to 1 margin.

The National Weather Service’s Climate Prediction Center heavily favors continued milder than average conditions across much of the Lower 48 states during the next two weeks, after a temporary cool down this weekend.

Beyond that point, however, there may be a pattern change in the works, thanks in part to a typhoon currently spinning across the northwestern Pacific Ocean.

But what exactly is a heat dome and how do they form?

A heat dome is created when an area of high pressure stays over the same area for days or even weeks, trapping very warm air underneath – rather like a lid on a pot.

At our latitudes, pressure systems usually move from west to east, but sometimes they can get blocked, often when the jet stream weakens and buckles. The jet stream is a core of strong winds high above the Earth’s surface that helps to develop and steer areas of low pressure around.

Stuck in a rut

The position and strength of the jet stream can lead to blocking situations, such as an omega block where high pressure ends up sandwiched between two low-pressure systems, forming a shape like the Greek letter Ω.

The high pressure stops weather fronts from moving through, so they either skirt around the edges of the high or grind to a halt. Your position under the omega block will determine the weather you get. Under the high, the weather is usually dry and settled, whereas low pressure brings wet and windy conditions. Weather blocks can persist for days, weeks, or even months, like the summer of 1976.

Under the dome

The problem with a stubborn area of high pressure is that already warm or hot air trapped under the high will become hotter and hotter, creating a heat dome. Hot air will rise into the atmosphere, but high pressure acts as a lid and causes the air to subside or sink. As the air sinks, it warms by compression and the heat builds. The ground also warms, losing moisture and making it easier to heat even more.

Until the pressure pattern changes, the high will continue to exacerbate the hot conditions, bringing a risk of wildfires, drought and heat-health issues.

These are cycles. In a bit more than two weeks, we are probably going to talk about a unprecedent Arctic blast. Be ready and prepared! [Axios]

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