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The West Coast will be bracing for a prolonged period of heavy precipitation this week from a phenomena known as the “Pineapple Express”. Flooding rain along the coast and hills as well as heavy mountain snow will threaten some communities, especially in California.

Pineapple Express in weather refers to a river of abundant moisture that originates in the tropical Central Pacific and moves up into the west coast of the United States.

What to expect:

Steady precipitation looks to arrive in California on Tuesday and then become heavier and more widespread Wednesday and Thursday. Drier weather will move into southern California on Friday, but unsettled weather will remain farther north.

The mountains of southern California as well as the Sierras look to receive the heaviest precipitation through the end of the week. 4-8 inches (100-200 mm) of rain is possible for elevations between 500-5,000 feet (~150-1,500 meters).

The major cities along the I-5 corridor will also be susceptible to bouts of heavy rain. Los Angeles looks to take the brunt of the heaviest rain with 2-4 inches expected over a 3-day period.

Flash flooding and mudslides will be a concern, particularly in the hills and across recent burn areas from wildfires.

Across the Sierras above 5,000 feet, 2-4 feet (60-120 cm) of snow is likely to fall. Snow levels across southern California will be closer to 9,000 feet (~2,700 meters), which is above most mountain peaks.

Drought Relief:

Much of California remain in some form of a drought, with severe drought conditions across portions of southern California. The expected rainfall will be good news for the drought relief, despite the flooding risk mentioned above.

 

Beijing, China received its first measurable precipitation in 145 days on Saturday morning when snow dusted much of the city. At least 0.1 mm of precipitation was recorded at the city’s international airport.

The last time measurable precipitation was recorded at Beijing International airport was on October 22nd, 2017 when 0.1 mm of rain fell. According to a Chinese-based news outlet, the dry stretch of weather is the city’s longest streak in 47 years.

Snow fell in Beijing Saturday morning, breaking the city’s longest dry spell in at least 47 years. pic.twitter.com/xJa2VMOzqo

— China News 中国新闻网 (@Echinanews) March 17, 2018

A fast moving disturbance that crossed north-central China early Saturday generated just enough moisture for the snow. Weather models depicted this precipitation event in advance rather well.

Why So Dry

Much of western and northern China reside in the Gobi Desert. The desert lays west of Beijing, however, the cold season pattern often brings a west to northwest flow which steers the dry air from the desert into the city.

Even during the drier winter months the city still averages about 3-9 mm of precipitation a month.

A major contributor to the recent stretch of dry conditions is a storm track that has been consistently to the south of north-central China.

As the cold season transitions to the warm season, moisture from the Pacific and Indian Oceans penetrate farther northward in China. Eventually that moisture reaches Beijing with increased chances for precipitation from April to October.

March has already brought three intense nor’easters to the East Coast, and now a fourth storm may be brewing.

Low pressure will emerge from the Colorado Rockies on Sunday and track into the Tennessee Valley Monday. Beyond Monday, low pressure will redevelop off of the North Carolina coast and threaten the Mid-Atlantic and Northeast.

Along and south of the storm track, an area of rain and thunderstorms will break out. There is a risk for isolated severe weather across the south-central Plains Sunday afternoon. The threat for some gustier thunderstorms will spread across portions of Mississippi, Alabama, and Georgia on Monday afternoon.

Meanwhile, to the north of the low track a cold rain will could mix with snow at times.

2 Scenario Next Week

As low pressure approaches the southern Appalachians on Monday, a second area of low pressure will develop near the North Carolina coast. The location and movement of the second low are crucial to potential impacts across the Mid-Atlantic and Northeast.

The first scenario would feature a storm track to the east-northeast out to sea. In this scenario, a wintry mix of rain and snow could impact interior Virginia. However, much of the Northeast would remain dry.

The second scenario would feature a slightly stronger low pressure system that would take a more northeasterly track. A track closer to the Northeast coast would bring a greater risk of precipitation. Cold air is expected to be in place, so much of the precipitation would likely fall as Snow or a rain/snow mix.

A favored scenario in the track of low pressure is expected to emerge by the end of this weekend.