Saturday, January 16, 2016

"Fire Hose" Aimed At Northern California

A representative map of an atmospheric rier.
That orange band coming across the Pacific Ocean
to the West Coast is a narrow band of intense moisture
that would produce, in the example given here,
torrential rain and flooding to central California.
Get ready people in northern California and parts of coastal Oregon.

The Pacific Ocean fire hose is ready to soak you down but good.  Expect inches upon inches of rain over the next few days.

Sometimes in the winter, a fairly narrow zone of fast moving storms aims at the West Coast. It looks like northern California and Oregon are in for it over the next week.

Some of the computer models are forecasting over a foot of rain over the next seven days. Some areas are likely to get two to six inches of rain just on Sunday in northern California.

I'm calling it a fire hose, as the weather pattern is basically a narrow, intense stream of deep moisture hittng parts of the West Coast. Meteorologists, though, call this an "Atmospheric River."

And that's pretty much what it is, a river. Except it's a narrow band of deep moisture in the atmosphere, not a literal river. Still, the amount of moisture transported in an atmospheric river hitting the West Coast can be ten times the flow of liquid water in the Mississippi River as it it approaches the Gulf Coast.

These atmospheric rivers hit the West Coast most winters. They're more likely when there's an El Nino, and of course we've got a strong one under way now. Which means I'm totally not surprised there is one expected in the next few days.

Atmospheric Rivers can come in various intensities, of course. They are sometimes good, as they are on average respondible for a third to a half of the needed winter precipitation along the West Coast.

They are sometimes bad, as you can imagine, because they can easily bring flooding and mudslides to  the West Coast.

The atmospheric river about to hit northern California and Oregon seems to be moderately intense, and is both good and bad.

Good, because it's putting another substantial dent in the California drought, especially for the northern half of the state.

Bad, because this one will put some rivers above flood stage and swamp urban streets whose storm drains can't handle the downpours.

Most dangerously, it can cause sudden and life threatening mudslides and debris flows, espeically in areas that had wildfires last year that stripped vegetation from the landscape. Note there were a LOT of wild fires in the region being hit by this atmospheric river, so, Not Good.

Though it's not going to happen this time, there is fear that one day, there will be a Super Atmospheric River that would overwhelm the levees protecting very flood prone Sacramento and surrounding areas.  If that ever happened, it could be a catastrophe on the scale of Hurricane Katrina in New Orleans.

Again, we're not expecting anything nearly that bad this time.

But you will see news and images of flooding and mudslides in the northern half of California and in Oregon over the next week.

Atmospheric rivers don't continue on across the United States. The mountains in the West tend to break them up. But the remnants of these almost always stir up new storm systems that cross the country eventually.

One such atmospheric river-spawned storm is likely to cross the southern United States next week, but that particular storm at this point does not look like it will be particularly strong or dangerous.

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