The West Coast’s wet season has arrived in power, as an atmospheric river carrying moisture from the tropics joins a bomb cyclone off the Pacific Northwest coast. Heavy, moist snow started falling within the mountains on Nov. 19, 2024, and bursts of rain have been blasting the Oregon and Northern California coasts.
These storms are forecast to final for days, hitting up and down the West Coast. Elements of Washington have seen greater than 70 mph winds from the bomb cyclone.
When these two phenomena get collectively, the climate will get onerous to foretell, as meteorologist Chad Hecht of the Heart for Western Climate and Water Extremes on the College of California, San Diego explains.
What occurs when an atmospheric river meets a bomb cyclone?
An atmospheric river is strictly what it appears like – it is a lengthy, slim river of water vapor within the decrease ambiance. These rivers within the sky transport moisture from the subtropics to the mid-latitudes.
When an atmospheric river runs up in opposition to North America’s West Coast, the mountains and complicated topography power the air to rise, cool, and the moisture to condense and precipitate. That may imply toes of snow at excessive elevations and rainfall elsewhere.
That is not at all times a foul factor. Weaker atmospheric rivers assist replenish reservoirs which are important for supplying water in the course of the dry season. California depends on atmospheric rivers for as much as half of its yearly precipitation and streamflow.
This storm, nonetheless, is predicted to be stronger and extra unpredictable than ordinary due to the bomb cyclone.
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Atmospheric rivers tend to form out ahead of cold fronts that are associated with low-pressure systems known as extratropical cyclones. These are air mass boundaries circulating around the area of low pressure. A bomb cyclone is a quickly intensifying extratropical cyclone.
These two climate phenomena go hand in hand. A robust atmospheric river will feed moisture into the low-pressure system, offering gasoline for the cyclone. The stronger the low-pressure system turns into, the stronger the atmospheric river turns into.
What is going to this combo imply for the West Coast?
This atmospheric river is spectacular for its period – it’s forecast to proceed hitting the coast with moisture for a number of days and will unfold precipitation as far south as Southern California. Its built-in vapor transport – a measure of how a lot moisture is transferring within the ambiance – means that we’ll see lots of rain and snowfall, with over a foot of precipitation anticipated in some areas.
On prime of that, the bomb cyclone – one of many strongest we have seen alongside the coast – is bringing highly effective winds.
Whereas the atmospheric river is hitting the coast, the bomb cyclone will sit over the ocean off the Pacific Northwest and spin. Because it spins, it sends small frontal waves by way of the ambiance that push the atmospheric river inland. That creates lots of uncertainty for forecasts.
In case you image the atmospheric river as a hearth hose pointed on the coast, these frontal waves are primarily the fireman taking his arms off the hearth hose and letting it go all wavy. It may well transfer northward, after which again southward. The query is how far it should go earlier than pivoting, then how shortly it should pivot again.
So, how lengthy particular elements of the coast will see rainfall and the way intense that rainfall can be are the massive questions.
The more durable and longer it rains, the extra impacts you are more likely to see.
The saving grace with this storm is that it is early within the season, so the soils are comparatively dry. Meaning they will be capable to take in extra of the moisture. If a storm like this hit in winter, after the soils have been already saturated, extra water would run off, inflicting extra widespread flooding than an early-season storm.
Are areas just lately burned by wildfires in bother?
With a storm this intense, the primary concern in areas just lately burned by wildfires is high-intensity precipitation.
When land burns, the floor can develop into impervious – virtually hydrophobic. So, when that floor will get hit with high-intensity precipitation, the water runs off quite a bit sooner than if vegetation or soil was capable of take up the water. Wildfire burn scars are typically in hilly locations, so that may result in particles flows.
It is actually a matter of likelihood whether or not the burn scars get that intense precipitation. A robust chilly entrance can type slim cold-frontal rain bands that carry bursts of high-intensity rain. These small however highly effective rain bands are likely to type in dynamically sturdy storms. Meteorologists are seeing some signatures suggesting that these options might type.
The very best recommendation in case you’re in a kind of areas is to concentrate to the Nationwide Climate Service’s watches and warnings.
Does this storm counsel a moist winter forward?
Sadly, one actually spectacular storm does not inform us something about what’s forward.
We have seen seasons with highly effective early-season storms after which not way more. October 2021 is an instance: A very sturdy storm hit that month, with record-breaking, 24-hour precipitation accumulations. However then the faucet shut off, and California ended up with below-normal rainfall for the remainder of the 12 months.
There are instruments that provide some indication of whether or not the season can be above or beneath regular, however these aren’t 100% correct.
For instance, the La Niña climate sample that is near forming within the Pacific usually signifies drier than regular circumstances throughout California. However wanting on the previous 10 years, we have had some actually moist La Niñas – 2017 being the granddaddy of all of them, with record-breaking precipitation within the northern Sierra.
Meteorologists usually have an excellent sense of what is coming within the brief time period, about 10 days out. However what the remainder of the season will appear like is basically an informed guess at this level.
Chad Hecht, Analysis and Operations Meteorologist, Heart for Western Climate and Water Extremes, College of California, San Diego
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