Heavy Rain, Feet of Snow Target California as "Miracle March" Atmospheric River Weather Pattern Arrives

Chris Dolce
Published: March 4,2016
California is about to see a parade of powerful Pacific storms that will unleash bouts of heavy rain, mountain snow, and gusty winds at times through at least next week. This much wetter weather pattern will be accompanied by a so-called atmospheric river, or "Pineapple Express", at times.
With the typical dry season quickly approaching as spring nears, this could be the last hurrah for the rainy season after a very dry February in the Golden State. However, this "miracle March" heavy rainfall will also be too much of a good thing in some areas, resulting in flooding, mudslides and rockslides.
(MORE: Timing Running Out For California)
Below we look at what we can expect and also present some facts about atmospheric rivers.

Forecast: Stormy Weather Returns This Weekend

The first moisture-laden weather system will arrive this Saturday, hauling the first major slug of heavy rain, snow, and gusty winds. A second storm system will then arrive late Sunday into Monday.
As the graphic below shows, the ribbon of moisture, known as an atmospheric river, associated with this first storm will stretch from California to near and west of Hawaii. This will be ushered in by a strong jet stream with winds topping 200 mph in the high altitudes over the Pacific Ocean late this week.

Moisture Plume Expected This Weekend

Here's what to expect through this weekend:

• Most lower elevations will likely pick up an inch or more of rain in northern and central California this weekend. Foothill and coastal range locations will see several inches of rain.
• The heavy rain could result in some flooding, mudslides and rockslides even after seeing dry conditions the last month or so. Flood watches are already in effect for burn areas.
• Up to 3 feet of total snowfall is expected in California's Sierra Nevada above 7,000 feet. Snow levels may start at 7,500-8,000 feet Saturday evening before falling to valley floors Sunday morning, which would then affect travel through passes.
• Strong winds are also likely in the Sierra on Saturday, with gusts topping out at 100+ mph over ridges, according to the National Weather Service. The foothills could see gusts to 80 mph while valley wind gusts may hit 60 mph. Power outages and downed trees are possible.
• Rain and mountain snow will also impact Washington, Oregon, and much of the Intermountain West this weekend as the frontal system slides inland.
• Southern California will also see some rain in two rounds the next several days. First from the storm Sunday and then from a separate low-pressure system that is expected to impact the area Monday.
• Large, possibly damaging surf is expected this weekend into Tuesday. The National Weather Service in Oxnard, California, is mentioning the potential of moderate coastal flooding and damage to coastal structures. The threat of rip currents will be high, as well.

(MORE: The Water Rise of Folsom Lake From Space)

Pacific Satellite

Beyond this weekend, various computer forecast models are depicting that several more storm systems will affect the West Coast through next week and beyond. However, it's too early to provide specifics.
That said, we can provide a broad overview of the impacts we can expect in California during the extended forecast:

• It's safe to say we'll be measuring snowfall totals in feet in California's Sierra Nevada from this weekend onward.
• Depending on how wet the storms are and how quickly they follow after each other, flooding could become widespread. Significant rises on rivers and streams is also likely.
• Mudslides and rockslides are probable, and may become numerous, particularly near burn areas.
• The lion's share of the rain and mountain snow may impact the northern half of California rather than the south.

Our forecast rain and snow map through next Friday (March 11) illustrates the expectation of several inches of rain in the lower elevations and feet of snow in the mountains. The heaviest amounts are focused from northern and central California into the Northwest.
(MAPS: 7-Day Forecast)

Rain and Snow Forecast Next Week

When looking at the Sierra Nevada in detail, there is the potential for 6 feet or more of snow at the highest elevations during the next 14 days. Keep in mind, however, that this is a very long range outlook so there is uncertainty, but it does provide and idea of the amount of snow we could see over the course of time.

Snowfall Potential Next 14 Days

What is an Atmospheric River?

Meteorologists use the term "atmospheric river" to describe a long, narrow plume piping deep moisture from the tropics into the mid-latitudes. One type of atmospheric river you may have heard of is the "Pineapple Express," a pronounced plume tapping moisture from the Hawaiian Islands to the U.S. West Coast. This is a type of weather pattern we look for from late fall into winter on the West Coast.
Here are some more facts about atmospheric rivers:

• According to NOAA's Earth System Research Laboratory (ESRL), a strong atmospheric river can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River.
• If an atmospheric river stalls over a particular area, significant flooding can be the result.  In fact, a study by Ralph et al. (2006) found atmospheric rivers responsible for every flood of northern California's Russian River in a seven-year period.
• They're also important for western water supply considerations. According to NOAA/ESRL, 30 to 50 percent of the average annual precipitation in the West Coast states typically occurs in just a few atmospheric river events.

MORE: Western Storms Slam California - January 2016