Atmospheric River Heads for California as a Massive Field Study Gears Up

Relief is on the way for drought-stricken parts of the U.S. West later this week, as an intense atmospheric river (AR) takes aim. This ribbon of deep moisture and strong wind will move onshore by Thursday night, kicking off a multiday series of downpours from roughly the northern half of California into the Pacific Northwest. The impending AR will likely be the most intensely observed in weather history, thanks to an armada of instruments deployed across California and the Northeast Pacific for a two-month interagency study called CalWater 2015. The project’s goals are to improve prediction of ARs affecting California and to assess the importance of aerosols (airborne particles) in shaping rainfall patterns across the region.


Figure 1. NOAA’s Ronald H. Brown research vessel is among the research platforms now being deployed to study atmospheric rivers in the CalWater 2015 project. Image credit: NOAA.

ARs are well worth getting a handle on, since they play an outsized role in both drought relief and flooding across California. It’s been estimated that 25 - 50% of California’s water supply is derived from AR events. As research intensified on ARs in the 1990s, scientists recognized that the rain-bearing moisture channel often called the “pineapple express”--extending from Hawaii to California--was just one example of a global phenomenon. More than 90% of all the water vapor that flows into midlatitudes from the tropics is channeled by ARs, which span just 10% of the latitudinal band at 35°N. When we look at a global loop of satellite-derived moisture content, ARs show up as bright filaments, stretching poleward like taffy being pulled from the perennial band of rich moisture that encircles the tropics. When a midlatitude storm draws in moisture from an emerging AR, the moisture becomes further concentrated as winds converge just ahead of the associated cold front. ARs can extend more than 1000 miles long but average only about 200 - 400 miles wide. Along with the the classic “pineapple express” pattern, ARs can take on a variety of other shapes and trajectories, which adds complexity to predicting how they’ll behave.

Computer forecast models can now spot many ARs five or more days before they threaten the U.S. West Coast, but there are often big uncertainties among models in the strength, position, and timing of each AR event. Many of the question marks arise simply because of the limited data available from the Pacific Ocean. Passive microwave sensors can estimate the total amount of water vapor located over the Pacific, but upper-level winds are more difficult to gauge by satellite. The first CalWater study in 2010 shed light on some of these uncertainties, as well as the importance of aerosols in either stimulating or suppressing rain and snow at various altitudes. A separate project called HIPPO showed that a surprisingly large amount of aerosols can reach the North Pacific from Asia, which implies they might be able to influence the evolution of some ARs. This year, CalWater is expanding its sights with a beefier set of observing platforms. They include three aircraft (P-3 and Gulfstream IV “hurricane hunters” from NOAA and a Gulfstream I from the U.S. Department of Energy) along with NOAA’s Ronald H. Brown research vessel, which will launch weather balloons and employ its newly upgraded dual-polarization radar. Specialized instruments aboard the aircraft will measure cloud microphysics and aerosol properties.



Figure 2. Cumulative 168-hour (7-day) precipitation from the GFS model run starting at 1200 GMT Tuesday, February 3. Image credit: NOAA.

The outlook for this week’s AR
Light rain is already pushing into western Oregon and Washington today and Wednesday ahead of the core AR event. The newly upgraded GFS model, whose resolution was sharpened to 13 kilometers last month, is the most bullish on heavy rain for the upcoming AR. The 12Z Tuesday output (Figure 2) delivers more than 5” of rain to the north part of the Bay Area through the weekend, with the potential for 5” – 10” or more and very strong winds in and near the coastal range through far northern California and southwest Oregon. More than 10” of liquid equivalent could also over mountainous parts of Washington. Hefty precipitation totals are also expected along the central and northern Sierra Nevada in California, with available water vapor close to record-high amounts for February. Unfortunately for the snowpack, temperatures through the weekend will be warm enough to keep most of the bounty in the form of rain below 7000 - 8000 feet, with a more general lowering of snow levels only toward the end of the multiday bout of storminess. The European and NAM models produce substantially lower precipitation amounts for this system, so it’ll be worth watching to see if the upgraded GFS comes through with the best overall precipitation forecast, as it did with last week’s Blizzard of 2015 (Juno) in the Northeast. Model resolution plays a key role in predicting heavy precipitation, especially in mountainous areas where the local topography can be better resolved.

Allen White, a research meteorologist with NOAA and a mission scientist for CalWater 2015, reports that the NOAA G-IV will be transecting the atmospheric river offshore on Wednesday, perhaps followed by the P-3 on Thursday with possible flight paths above the Ron Brown and over the coast near Bodega Bay. Flights on Friday would likely be over California, where the aircraft data would supplement an array of vertically pointing precipitation profilers and enhanced rain gauges. Meanwhile, the G-1 will focus on aerosol-related missions, including the impacts of long-range dust transport. “Having all these observing assets available is a huge bonus,” White told me. “We can now observe the AR and the precipitation it generates from above, below, and throughout. This requires a huge forecasting and coordination effort and highlights a very successful interagency collaboration among federal, state, and local agencies, as well as academia.”

A 2014 open access paper in Frontiers in Earth Science provides a good overview of AR research for readers comfortable with some relatively technical content.



Figure 3. Based on GFS model output from 1200 GMT Tuesday, February 3, NOAA’s automated AR detection tool indicates that a plume of enhanced moisture will be headed squarely for northern California at 1200 GMT Friday, February 6. Colors show integrated vapor transport in units of kilograms per meter per second, with arrows indicating the direction and strength of transport. The model indicates that the core of the AR will carry more than 1000 kilograms (about 2200 pounds) of water vapor every second across every meter of ocean that’s oriented perpendicular to the flow. Atmospheric rivers are generally defined by values of at least 250 kg/m/s. Image credit: NOAA Earth System Laboratory.

Digging out in the Midwest and Northeast--for now, at least
The sprawling winter storm dubbed Linus turned out to be an overachiever along much of its west-to-east course from the northern Great Plains to New England. With blizzard warnings posted on Sunday, Chicago ended up with its fifth largest snowstorm on record (19.3”) and the most snow for any calendar day in February (16.2”). Detroit racked up its third largest storm on record with 16.7”, the most observed in any event since 1974. On Monday, New Yorkers again found themselves on the margins of a winter storm, this time laboring through a sloppy sequence of snow, sleet, rain, freezing rain, and snow that left patches of treacherous ice on Monday night. Meanwhile, much of eastern New England is reeling from the one-two punch of heavy snows last week and again this week, with an intervening lighter storm only adding insult to injury. From January 24 through February 2, Boston racked up 47.9” of snow; according to the Weather Channel’s Nick Wiltgen, this is the largest total for any ten-day period on record in Boston. The town of Lunenburg, about 45 miles west of Boston, did even better (or worse) as it notched 36.6” and 19.7” from Juno and Linus, respectively, for a grand total of 56.3” of snowfall. Bangor, Maine, reported 45" of snow on the ground Tuesday morning, the most since a record 53" in February-March 1969. A weak clipper will bring more light snow to the Midwest and Northeast later this week, followed by a shot of bitter cold. Looking further out, winter-weary and wary New Englanders could get pummeled by yet another significant snow by the weekend, although model guidance so far has been inconsistent.

Bob Henson


Figure 3. An evocative depiction of snow without end, taken Monday evening, February 2, at Kennebunk, Maine: “It was a very dark and gloomy day, without big fluffy flakes or brightness in the sky. Temps have remained in the single digits, so the snow has remained small and granular as well. Thought I'd try one more shot while there was a bit of daylight left.” Photo credit: wunderphotographer Kennebunker.