Atmospheric river meets bomb cyclone: ​​the result is like a fire hose out of control

When these two phenomena meet, the weather becomes difficult to predict, which meteorologist Chad Hecht of Center for Western Weather and Water Extremes at the University of California, San Diego explains.

What happens when an atmospheric river meets a bomb cyclone?

An atmospheric river is exactly what it sounds like – it’s a long, narrow river of water vapor in the lower atmosphere. These rivers in the sky transport moisture from the subtropics to the mid-latitudes.

As an atmospheric river flows up the west coast of North America, the mountains and complex topography force the air to rise, cool, and the moisture to condense and precipitate. This could mean feet of snow at high altitudes and precipitation elsewhere.

That’s not always a bad thing. Weaker atmospheric rivers help replenish reservoirs essential for supplying water during the dry season. California relies on atmospheric rivers for up to half of its annual rainfall and flow.

However, this storm is expected to be stronger and more unpredictable than usual due to bomb cyclone.

Atmospheric rivers explained.

Atmospheric rivers tend to form ahead of cold fronts that are associated with low pressure systems known as extratropical cyclones. These are air mass boundaries that circulate around the area of ​​low pressure. ONE bomb cyclone is a rapidly intensifying extratropical cyclone.

These two weather phenomena go hand in hand. A strong atmospheric river will carry moisture into the low pressure system, fueling the cyclone. The stronger the low pressure system becomes, the stronger the atmospheric river becomes.

What will this combination mean for the West Coast?

This atmospheric river is impressive for its duration – it is expected to continue to hit the coast with moisture for several days and may spread precipitation as far south as southern California. Its integrated vapor transport – a measure of how much moisture is moving in the atmosphere – suggests that we will see a lot of rain and snowfall, with over a foot of rainfall expected in some areas.

On top of that, the bomb cyclone – one of the strongest we have seen along the coast – brings strong winds.

As the atmospheric river hits the coast, the bomb cyclone will sit over the ocean off the Pacific Northwest and spin. As it spins, it transmits small frontal waves through the atmosphere there pushing the atmospheric river inland. This creates a lot of uncertainty for the forecasts.

If you imagine the atmospheric river as a firehose pointing toward the shore, these frontal waves are essentially the fireman taking his hands off the firehose and letting it go all wavy. It can move north and then back south. The question is how far it will go before it turns and how quickly it will turn back.

A forecast of the storm’s vertically integrated vapor transport shows the direction and size of the water flow, measured in kilograms per meter per second.
Center for Western Weather and Water Extremes

So how long specific parts of the coast will see rainfall and how intense the rainfall will be are the big questions.

The harder and longer it rains, the more influences you are likely to see.

The saving grace of this storm is that it is early in the season, so the ground is relatively dry. This means that they will be able to absorb more of the moisture. If a storm like this hit in the winter after the ground was already saturated, more water would run off, causing more widespread flooding than an early season storm.

Are areas recently burned by wildfires in trouble?

With a storm this intense, the main concern in areas recently burned by wildfires is high-intensity rainfall.

When earth burns, the surface may become impermeable – almost hydrophobic. So when that surface is hit by high-intensity rainfall, the water runs off much faster than if the vegetation or soil were able to absorb the water. Wildfire burns scars tend to be in hilly places so it can lead to debris flows.

The outlook for the atmospheric river hitting the west coast covers the 20th-27th. November 2024, shows a strong storm. Much of Northern California is expected to experience an atmospheric river of Category 4 strength on a scale of 1 to 5.
Center for Western Weather and Water Extremes

It is really a matter of chance whether the burn scars get the intense precipitation. A strong cold front may form narrow cold-frontal rain bands that bring bursts of high-intensity rain. These small but powerful rainbands tend to form in dynamically robust storms. Meteorologists are seeing some signatures that suggest these features may be forming.

The best advice if you are in one of these areas is to pay attention to the National Weather Service’s watches and warnings.

Does this storm hint at a wet winter ahead?

Unfortunately, a really impressive storm doesn’t tell you anything about what lies ahead.

We have seen seasons of heavy early season storms and then not much else. October 2021 is an example: A really strong storm hit that month, with record 24-hour rainfall. But then the faucet turned off, and California ended up with below-normal precipitation for the rest of the year.

An animation of satellite data shows how the atmospheric river draws moisture from the subtropics and is pushed towards the coast by the bomb cyclone.
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin–Madison

There are tools that give an indication of whether the season will be above or below normal, but they are not 100% accurate.

For example La Niña weather pattern that are close to forming in the Pacific typically indicate drier than normal conditions in California. But if we look at the last 10 years, we’ve had some really wet La Niñas – 2017 to be a grandfather of them all with record rainfall in the Northern Sierra.

Meteorologists typically have a good sense of what’s coming in the short term, about 10 days out. But what the rest of the season will look like is really an educated guess at this point.

Chad Hechtresearch and operational meteorologist, Center for Western Weather and Water Extremes, University of California, San Diego

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