Yet ANOTHER Huge Flood: Atmospheric River Drowning Part Of California

Severe flooding in Guerneville, California today.

It seems we have one record flood after another here in the United States.

We now likely have another bad one

The Russian River in northern California is forecast to reach one of its highest crests on record later this evening. That level would trash, among many other things, the popular resort town of Guerneville.

Even before tonight's expected crest, Gueneville was badly flooded. Live images early this afternoon via ABC 7 in San Francisco shows vast residential and business areas of the town already deep under water.  The video showed the tops of cars barely poking above the muddy water, and water entering many dozens of homes.

The water level at Guerneville this afternoon was well over 40 feet. The expected crest tonight would be 46.1 feet. The record height of the river there is 49.7 feet in 1955. The current flood would be easily the worst in more than two decades.

By this afternoon, Guerneville was an island, with no way to drive in or out of the small city. Most of the area has or will soon be evacuated. The nearby town of Monte Rio is also under water.

Flood warnings are up for most of northern California north of a San Francisco - Yosemite line, at least in the lower and middle elevations.

An atmospheric river focused most of its fire hose intense rain from the Pacific Ocean going inland just north of San Francisco and into north-central and northeastern California.  According to NOAA, atmospheric rivers are long, narrow regions in the atmosphere that resemble rivers in the sky. They transport a lot of water vapor, often from the tropics to coastal areas. These rivers can also continue well inland.

Another view of flooding today in the Guerneville, California area.

When atmospheric rivers hit land, they often unleash torrential rains in a relatively narrow strip. That's just the what happened here. Most of this atmospheric river focused on an area between California's border with Oregon and the San Francisco Bay area.

Guerneville is about 75 miles north of San Francisco and lies near the epicenter of this latest atmospheric river. Reports of eight to 12 inches of rain in two days are common in the area around Guerneville. The town of Venado, in the Russian River basin, got more than 20 inches of rain in those two days.

The rain is lighter now, but the damage is done with such a huge amount of water coming out of the sky.

Flood warnings and mud slide alerts remain in effect for much of northern California, including all areas north of San Francisco, the Sacramento metro area and points north. This includes what's left of the town of Paradise, destroyed by a wildfire late last year.

In the central and northern Sierra Nevada mountains, two to five feet of snow fell with the atmospheric river, on top of already an already very deep snow cover. While lower elevation rains won't be as heavy in the coming days as they were recently, heavy snowfall will persist in the Sierra Nevada.

A 70-mile portion of Interstate 80 through the Sierra Nevada mountains is closed because of the snow, and there's no word on when it will reopen.

The California flooding comes while the south central United States is battling serious flooding from torrential rains last week. Some rivers had record high flood levels. Major flooding is ongoing along the lower Ohio River. Large stretches of the Mississippi River are also flooding.

Obviously, I can't draw a direct line between climate change and all these floods, but the trend is consistent with global warming, most scientists who look at the problem say. Flood are increasing in the United States, partly due to climate change.

Other factors are at work, too. More people are in the way of potential floods. Plus, development which puts concrete, asphalt and other impermeable surfaces over the ground don't allow water to soak in. Instead, the water runs off and makes floods worse than they otherwise would be.

Here's some video. This is what Guerneville looked like from the air today:

Dangerous "Atmospheric River" To Cause Mid-Atlantic, Northeast Flooding

Three to seven inches of rain, and locally higher amounts, are
likely, especially in the red areas of this map during the next seven days.

An atmospheric river is set to dump tremendous amounts of rain on a broad strip running north and  northwestward off the Atlantic Ocean through areas around North Carolina and Virginia, northward through Pennsylvania and central New York. 

That means these areas need to be really, really on guard for serious flooding over the next few days.

First, we need to define for us all what an atmospheric river is. I'll let NOAA do it:

"Atmospheric rivers are relatively long, narrow regions in the atmosphere - like rivers in the sky - that transport most of the water vapor outside the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.

When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow."

The problem is, atmospheric rivers often release extreme amounts of rain, causing extreme floods.

Atmospheric rivers are to blame for extreme floods around Nashville, Tennessee in 2010; and another extreme, deadly flood around South Carolina in October, 2015 that dumped up to two feet of rain in that state.

Not all atmospheric rivers are bad. They're common in California in the winter, and help replenish reservoirs and add to the snowpack in the Sierra Nevada mountains. Melting snow in the summer then keeps reservoirs and drinking water supplies at reasonable levels.

Like anything in weather, atmospheric rivers vary in intensity. Some aren't too tough to take, dropping a few inches of rain on a drought-stricken area, for example.

But often, they are terrible. And they can unload their tremendous rainfall well inland from wherer they come ashore -- like in the 2010 Tennessee flood I just mentioned.

The atmospheric river slated to hit parts of the East Coast this week is being created by a really wonky weather pattern now in place.

Usually this time of year, the jet stream, which controls the paths of storms and fair weather systems, sputters in a general west to east course near the United States/Canada border. In July, there's rarely much in the way of big southward dips or huge northward bulges in the jet stream.

There's an exception going on now and the weather pattern is really off the rails.  There's a big dip in the jet stream centered over the Great Lakes. This dip is causing unusually chilly weather for this time of year near the Great Lakes, and strangely severe storms in the South. Those storms created hail as big as tennis balls in Georgia and some other southern states.

Meanwhile, the Bermuda high is pushing a big heat ridge northward in the western Atlantic Ocean. Part of the Bermuda High has pushed as far north as Nova Scotia.

This is setting up the atmospheric river flowing northward from the tropics between the big Bermuda High to the east and the dip in the jet stream to the west.

This pattern is kind of stuck until later Thursday and Friday, when things will start moving. So the heavy rains will keep going over the same places from North Carolina  to New York through then. This could extend into southwestern Quebec as well.  Remember, this is tropical moisture, so any showers and thunderstorms can create one to two inch per hour rainfall rates.

This can cause life-threatening flash flooding. There was already destructive flooding and water rescues in Pennsylvania this morning.  That flooding will likely worsen this afternoon in parts of Maryland, Pennsylvania and the southern tier of New York.

We here in Vermont are on the eastern edge of this atmospheric river pattern. That means we are subject to the torrential downpours, but not the constant fire hose of rain. I noticed during a batch of rain that moved northward through Vermont this morning, an inch of rain fell on Rutland in just an hour.

We desperately need the rain here in Vermont, so precipitation this week is a good thing. We still have to watch out, especially Wednesday night and Thursday, when the axis of heaviest rain will probably shift to near or over the Green Mountain State.

Despite the dry conditions, an excessive downpour, or a series of downpours going repeatedly over the same spot, can cause dangerous flash flooding. It's impossible at this point to determine if, or even whether Vermont will see any flash flooding from this system.

The National Weather Service is eyeing the possibility of that happening, so stay tuned.

Hawaii Flood Disaster Might Have Set New U.S. Rain Record

A destroyed house floats down a Hawaii creek during epic
flash floods earlier this month. Photo via Twitter
@TheRobertHayes

This was little noticed in the media, and I didn't cover it, but there was a terrible flash flood on the island of Kauai about a week and a half ago.

This storm is now reported to perhaps have broken the United States record for the most rain within 24 hours.

The National Weather Service office in Honolulu, Hawaii says a remote rain gauge in Waipa, near the north shore of Kauai recorded an incredible 49.69 inches of rain within 24 hours on April 15. That would break the United States 24 hour record, which has been 43 inches in Alvin, Texas on July 25-26, 1979.

Some of you might have thought the epic rains of Hurricane Harvey in Texas last August beat these records as there were media reports of 50 inches of rain in that disaster.

I checked, and that 50 inches was a storm total from Harvey, not rainfall in just 24 hours. Some places in Texas got 30 inches of rain within 24 hours, which is incredible and extreme, yes, but not the record.

The new Hawaii record is preliminary, not official. There's an outfit called the National Climatic Extremes Committee that will convene, review the data, and determine its validity. If this committee decides everything checks out, then this will become the new 24 hour United States rainfall record.

No word yet on when the committee will come up with a decision.

Whether or not that 49 plus inches of rain is real, there's no doubt the flash flooding on Kauai was extreme.

Military troops and helicopters airlifted at least 220 people stranded by flooding and massive landslides. Many tourists were also stranded. Dozens of homes were destroyed, and many others were demaged.

One woman said she was forced to climb to the roof of her house, and was eventually rescued by someone on a Jet Ski. 

Tourism, certainly a lifeblood of the region, is taking a big hit. The northern shore of Kauai is a mess, but the southern half of the island is fine, and open for business. And so is the rest of the Hawaii island chain.

Some of the moisture associated with the Kauai disaster was picked up and turned into an "atmospheric river" or "pineapple express" that deluged the Pacific Northwest a few days after the Hawaii disaster, notes the Cliff Mass Weather And Climate Blog. 

The atmospheric river produced some relatively minor flooding in Washington State, but nothing like what was experienced on Kauai.

VERMONT, NORTHEAST WATER

In case you haven't noticed, it's wet out there this morning. At least it's not snowing. Just note that there is some high water and minor flooding around.  This water has nothing to do with the above mentioned Kauai flood disaster. However, you should stay away from any local floods and not drive through inundated streets.

Widespread flooding is not expected around here with this rain. Even with snow melting off the mountains, there's just not enough precipitation to cause any serious, widespread floods.

That said, the Ausable River in the Andirondacks was under a flood warning, but that has since been canceled. I wouldn't be surprised if some Vermont rivers approach  minor flood stage today.  Still, current forecasts indicate no real flooding from Vermont rivers today, which is good.

Further north and east, flood watches are up for most of Maine.

It has been a relatively wet April in Vermont. As of midnight last night, the month was running an inch ahead of normal for precipitation this month. It kept raining this morning, showers will continue much of today, and additional showers will come in Friday afternoon or night.

"Atmospheric River" To Flood Pacific Northwest In Coming Days

This map shows the atmospheric river potetial
later this weekend in the Pacific Northwest. The blue
and green hues hitting the region in this map
Sunday represent a corridor of moisture from the
tropics headed to British Columbia and Washington

It's inevitable that this would happen, as it occurs almost every winter along the West Coast.

An "atmospheric river" is taking aim at Washington State and neighboring coastal British Columbia later this weekend and maybe on into early next week. That almost always means at least some flooding.

I'll let NOAA define what an atmospheric river is for us:

"Atmospheric rivers are relatively long, narrow regions in the atmosphere - like rivers in the sky - that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow."

As you can imagine, there's an absolute huge shitload of water that comes through the lower Mississippi River, so you can imagine that these atmospheric rivers can unleash an incredible amount of rain or snow.

The upcoming atmospheric river in Washington and British Columbia won't be at all extreme, certainly in comparison to some epic ones in the past.

Some of the nation's worst floods both in historic times and in recent years have been caused by atmospheric rivers.

In the winter of 1861-62, probably the worst and most persistent atmospheric rivers struck California. The firehose of water shifted a bit during the winter from southern to northern California and back again, but it just wouldn't quit.

Los Angeles got 66 inches of rain, four times its annual normal. Sacrameto remained submerged in ten feet or more of water for months. Telegraph poles in some areas were completely under water.  Nowadays, California has nearly 40 million people, so you can imagine what a mega-disaster we'd see if 1862 happened again.

Atmospheric rivers don't just hit the West Coast. They've caused immense flooding in Great Britain, for example. The Gulf of Mexico, Caribbean and Atlantic Ocean can trigger mean atmospheric rivers. An extreme Tennessee flood in 2010 was caused mostly by such a set up, for example. A catastrophic South Carolina flood in 2015 was in large part caused largely by at atmospheric river

I don't mean to be alarmist. Most atmospheric rivers aren't as destructive as the ones I just outlined. Not all atmospheric rivers are historic, and the one about to hit Washington State and British Columbia won't be the Worst Ever.

One saving grace is the atmospheric river will move, first hitting British Columbia then sinking south into Washington, so any one place won't have days of torrents.

Still, flooding is very likely in the western half of Washington State. The atmospheric rivers that come into the West Coast do have their origins in the tropics, as most of them do, so snow levels in the Cascades will rise quite a bit.

That means rain will fall at high elevations. There won't be that much snow to capture the water, and the atmospheric river will melt some of the snow in the high elevations. The combination of heavy rains and snowmelt will make the risk of flooding that much worse.

Sometimes, atmospheric rivers, once they're done with the West Coast, help spin up pretty intense storms in the central and eastern United States. This time, the atmospheric river in the Pacific Northwest might end up being one ingredient that could spin up a fairly strong storm in the eastern United States in about a week.

That scenario, though, is still quite uncertain.

West Coast Nightmare: Floods, Ice, Snow, Wind

Some people in California are comparing the current
storm to the great flood of 1997, pictured  here.
I doubt things will get nearly as bad as 1997 in the current
storms, but serious flooding is still a good bet
over the next few days in California and Nevada.

Now that the East Coast storm has gone out to sea, we turn our attention this busy weather month to the West Coast, which, as expected, is really getting hit hard by a variety of bad, and in some cases extreme weather.

This includes the flooding in California, which has been anticipated for days, a very serious ice storm in a heavily populated area of Oregon, strong winds, and incredible mountain snows.

If you look at the National Weather Service home page map, the western United States is a kaleidoscope of colors, which is never a good thing. That means there's a wide variety of weather warnings in effect.

OREGON ICE:

The big problem in Oregon is freezing rain. Lots of it. An ice storm warning was in effect this morning for most of the Willamette Valley, which includes bigger cities like Portland and Eugene.

Some areas are expected to see ice accumulate to three quarters of an inch. Combined with winds gusting to 35 mph, that's more than enough to bring down plenty of trees and power lines.

The freezing rain will probably go over to plain rain in the western parts of the Portland metro area by early afternoon, but eastern areas of the metro area closer to the Columbia Gorge might stay as freezing rain into tonight, which isn't good

Power failures are likely, and not surprisingly, people are being told not to bother driving.

Eugene, Oregon and surrounding areas suffered a terrible ice storm around three weeks ago and hasn't fully recovered from that. Now they're getting another one. Today's ice storm might not be quite as bad as the one in December, but it will cause more trouble for sure.

In southwestern Oregon a flood watch is up as they will be on the northern fringe of the California atmospheric river, which I'll get to in a moment. In this part of Oregon, relatively heavy rain will combine with snow and ice melt to raise river and stream levels.

On top of that, many creeks are clogged with broken branches and trees from recent ice storms. That debris can form dams that can back up water and cause flooding, then these "dams" can break loose and cause downstream flash flooding.

Kind of a mess if you ask me.

CALIFORNIA/NEVADA

The atmospheric river of subtropical air and heavy rains has moved into California as expected.

Flood watches have been upgraded to flood warnings in a huge swath of California's central valley and the western slopes of the Sierra Nevada.  Many areas here have already seen three to seven inches of rain in the 24 hours ending early this morning, and it's still raining hard.

Flooding is also a problem in western Nevada, including the Reno area. This is a very warm pattern, so it's raining at elevations as high as 9,000 feet. The rain and warmth is melting Sierra snows, which is creating additional runoff and just making the situation worse.

There's even a chance that downtown Reno could become flooded with this.  Carson City, Nevada could also see some serious flood damage.

A state of emergency has been declared in Nevada and people are being warned to stay away from the rising Truckee River

It's not good near the California coast, either. Flash flood warnings were already up Sunday morning north of San Francisco, including the Sonoma area. Making things even worse, many areas near the coast, especially in the hills, are also under a high wind warning.

The Weather Channel says there have been numerous water rescues in California as of midday Sunday as waters rise across roads and freeways. There is an increasing number of mudslides, too as heavy rains continue.

Flooding will be a worry in the west for the rest of the week as several more storms are due to come in off the Pacific. Snow levels in the Sierra will drop some, so areas the highest elevations that are getting unwelcome rain will switch over to more seasonable snow.

VERMONT IMPACTS

The California storms, and the weather pattern that's bringing them, isn't good for Vermont if you're into winter sports and if you want the winter tourism industry to thrive.

As has been the case since mid-December, this pattern favors storms crossing the country and then moving northeastward through the Great Lakes, putting Vermont in the warm, rainy side of these systems.

Such is the case with the storms that will form in the middle of the nation this week, using the California storms as seeds for their birth. Two storms, one Tuesday night, the other Thursday, will go by west and north of New England.

Mixed precipitation Tuesday night will change to rain. The next storm Thursday will also come in the form of rain.

The storms after that are question marks, since they're so far down the road. There's always a chance these subsequent storms could go further south and give us snow, but we'll have to wait and see.

In any event, despite the cold today, expect a big January thaw this coming week in Vermont.

Will California Dream Rains Become A Nightmare Soon?

Incredible amouts of precipitation predicted over
the next seven days in northern and central California.
A foot or more near the coast. The 20+ inches
in the Sierra Nevada translates to more than 20 feet
of snow in the next week. 

It's been a gloriously wet winter in California so far this year, with storm after storm coming in off the Pacific to dent the state's punishing six-year drought.

The drought is not over and southern and parts of central California in particular need more precipitation.

And they're going to get it, big time.

I alluded to this the other day: A moderate strength "atmospheric river" just hit central California over the past day or two dumping up to five inches of rain in some lowlands and a three to five  feet of snow in the Sierra Nevada.

It's looking increasingly likely that an even stronger atmospheric river is about ot hit California this weekend.

Atmospheric rivers are relatively narrow bands of very wet tropical or subtropical air that sometimes come ahore on the West Coast.

They're basically like giant fire hoses directing a hard stream of water, in the form of downpours or heavy mountain snow that hit a particular area for hours or days on end.

Like a fireman wielding a fire hose's blast of water from one flaming window to the next on a burning building,  the atmospheric river, when it sets up, can move up and down the West Coast.

Atmospheric rivers are relatively common in California, though they certainly have been lacking in the latest drought years.

Early indications are the upcoming atmospheric river this weekend could be relatively strong, which means the dreamy drought-denting rains and snows that have been drenching California over the past month or so could turn nightmarish.

A foot of rain could come down on parts of central and northern California over the next week. I also wouldn't be surprise if more than 15 feet of snow accumulates in parts of the Sierra Nevada, which have already gotten two to five feet in recent days.

That much precipitation in a few days could really set off some nasty flooding, mudslide, debris flows and urban flooding from overwhelmed drains.

This could be some of the worst flooding parts of California has seen in a decade or more.

Of course, this isn't totally set in stone yet, and we're not sure where the heaviest rain will set up. Still, the National Weather Service office in San Francisco is already starting to sound the alarm.

In a hydrological statement today, they said: "It is very likely that many flood-prone creeks and rivers of the Bay Area and Central Coast will rise above their banks."

 It does appear most of it will miss southern California, which is too bad, because they need the rain more than the north. But they will get some rain, on top of some that's already fallen over the past five weeks, so it's not as dire as it once was there.

Fifteen or 20 feet of snow in the Sierra sounds dire, too, but it's really not. The more snow that piles up the Sierra in the winter means better water supplies for California in the summer. The state relies in large part on meltwater from the Sierra for their warm season water supplies.

Despite above normal precipitation, the snow pack in the Sierra is a little below average right now, believe it or not. It's been so warm that a lot of the snow up there has been melting prematurely.

Another problem with the upcoming atmospheric river is it's a warm pattern. Snow will be limited to the highest elevations. Plus, rain will fall on some  upper elevations that just got snow. Melting snow combined with the rain could make flooding worse.

I'm sure all weather eyes will be on California for the next couple of weeks.

DOWNSTREAM IMPLICATIONS:

Once these atmospheric river storms hit California, they have to go somewhere. True, a lot of their moisture gets wrung out over the Golden State, but the storms move east from California, sometimes picking up new moisture from the Gulf of Mexico or Atlantic Ocean.

The first of these impulses is already starting to cross the Rocky Mountains, prompting widepread winter storm warnings in that region, all the way from Oregon to Colorado.

This initial storm will cross the central and southern United States just south of a strong push of Arctic air.

Once it gets to the southeastern United States, it could unleash some ice and snow in that region.

Despite the fake news and hype issued by some internet weather trolls, there's NOT going to be a Blizzard To End All Blizzards in North Carolina.

That's just people trying to get click views. Instead, there's the possibility of ice or snow in the Southeast Friday and Saturday. It only takes a little bit of wintry weather to really screw up things in that part of the country, so you ought to be careful if you're heading down there this weekend.

After that, more storms will parade across the country, but it's too soon to tell whether they will be big or small, or where exactly they'll hit.

"Mayan Express" Poses Big Weather Risks In. U.S. South This Week

Torrential rains, with up to a foot expected in Louiiana,
are expected in a broad area of the lower Mississippi
Valley this week due to the "Maya Express" Another
zone of heavy, flooding rains are likely from norther
California to Washington. Red, orange, and yellow
zones are most at risk

A lot of us have heard of the "Pineapple Express," a stream of rich tropical moisture from near Hawaii that sometimes crashes into the West Coast, causing huge rains and floods.  

Ever heard of the "Maya Express?" It causes a similar type of problem, only in the southern and eastern United States. The reason I bring it up now because some areas in the South, particularly Louisiana, Arkansas and surrounding states, are under the gun for this so called Maya Express this week.

I'll set the whole thing up: The well-publicized, and welcome rain storms have arrived in California, where flash flood watches are up for many areas today, and several feet of snow are coming down in the Sierra Nevada.

Up to a foot of rain is forecast for parts of northern California over the next seven days. While this isn't technically the Pineapple Express, its still boatloads of rain, for sure. One person has already died in the California flooding, and there have been reports of dangerous mudslides and debris flows. The stormy California weather is expected to continue for the next few days.

One of these California storms is expected to move southeast into northern and central Mexico. Storms don't usually do that, but such a setup is a bit more likely in years with a strong El Nino, like this one.

This storm will set up a rich flow of very wet air from off the east coast of Mexico, from the western Caribbean and push it toward the western Gulf Coast and lower Mississippi Valley. Pieces of that Mexican storm will also move north into Texas, encouraging that south to north flow of wet air northward.

In a Forbes article this week, Dr Marshall Shepherd, Director of the Atmospheric Sciences Program at the Univeristy of Georgia, and host of the Weather Channel's "Weather Geeks" program, says the upcoming weather pattern this week is an example of the "Maya Express"

Shepherd, in Forbes, cites the work of researchers Paul Dirmeyer and Jim Kinter, who examined immense floods in the Midwest in 1993 and 2008. Although numerous factors went into those floods, a primary cause was a rather narrow but intense band of atmospheric moisture moving north from the western Caribbean into the United States.

The researchers dubbed this type of weather pattern the "Maya Express," based on its source region.

The Pineapple Express and the Maya Express are examples of atmospheric rivers, narrow channels of intense moisture that flow like a river, except in the atmosphere. These can obvious cause extreme rainfall events and floods.

Another disaster, the Great Memphis Flood of 2010, was also largely due to the "Maya Express."

So you can see why meteorologists are nervous about places like Louisiana and Arkansas this week. The flooding will probably extend into much of Missouri and on into the southern half of Illinois as well.

Thry've had a lot of wet weather this winter, so rivers are already running high there. Up to a foot of rain is likely later this week from the "Maya Express," so this will only cause a dangerous situation, both with flash floods and large, surging floods in the bigger rivers.

Some of these places that are in the path of the "Maya Express"  had a record wet 2015 and a soggy first month of 2016. They don't need the rain.

A secondary, but real risk from this weather pattern is severe weather and tornadoes. The greatest risk of that happening is in Texas and Oklahoma Monday, eastern Texas and western Louisiana and southwestern Arkansas Tuesday, and in and around Louisiana Wednesday.

Bottom line: Anybody living in the central and southern Plains, and the southern and central Mississippi Valley had better keep an ear toward the forecasts and weather warnings all week. It could get dangerous.

"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.