|Above: Kevin Kimo Laughlin, a maintenance worker for Aramark, wades back to dry ground as the Merced River rises due to rain in Yosemite National Park's Yosemite Valley on Saturday, April 7, 2018. Northern California was drenched by a powerful Pineapple Express storm, but no major problems were reported after the heaviest rain from the atmospheric river of subtropical moisture moved through late Friday. Image credit: Eric Paul Zamora/The Fresno Bee via AP.
If you love atmospheric extremes, but you hate to see people in harm’s way, you couldn’t ask for a more pleasing event than the phenomenal infusion of moisture into the western U.S. from Friday into Saturday. No deaths or serious injuries were reported from the weekend rains, although Yosemite National Park was closed over the weekend as valley roads were inundated by up to 4 feet of water. The storm brought rain to unusually high altitudes for early spring, which led to a slight net loss in snowpack across the Sierra, but the rains also helped push the major reservoirs of central and northern California even closer toward full capacity.
|Figure 1. The narrow tendril from Hawaii to the U.S. West Coast shown here in GOES water vapor imagery at 9 am PDT Saturday, April 7, 2018, is actually a Pineapple Express atmospheric river that carried a record-setting amount of moisture into the western U.S. Image credit: NWS/Spokane, WA.
The most striking aspect of this storm is something that’s much less obvious: the amount of water vapor that streamed from the central tropical Pacific well into the western United States. Simply put, this storm brought more moisture into parts of the West—by far—than any other winter-type storm on record, going back to when radiosonde launches began measuring conditions through the depth of the atmosphere in the late 1940s.
This event was a weird mash-up of winter and summer elements. One way to see this is through precipitable water (PW), or the depth of water that could be squeezed out of an imaginary column of air directly above a point. The table below shows six stations that have just recorded their highest values of PW ever observed in the six-month period from November to April. Some of these records were broken by huge margins on multiple dates, as evident in Figure 2.
|Figure 2. The amounts of precipitable water measured above Oakland, California, at 12Z Friday, April 5, and 0Z and 12Z Saturday, August 6, were each far above anything observed in the months of November through April since upper-air sampling by weather balloons began in 1948. Image credit: Base graphic courtesy NOAA/NWS Storm Prediction Center.
An atmosphere loaded with moisture but starved for dynamics
Most of the six locations in the table above get more rain in winter than in summer, mainly because of the dynamics provided by the active winter jet stream and its ability to pull in moisture from the tropical or subtropical Pacific.
Ironically, though, the highest PW values at each spot tend to occur in the summer, when the atmosphere is generally more stable and tranquil and moisture has a chance to build up or to filter in from Mexico via the North American Monsoon.
In the case of this past weekend, we had a Pineapple Express laden with richer moisture than pretty much any other on record from California to Nevada—but lacking the intense winter-type dynamics that could best squeeze that moisture out of the atmosphere over large areas. The result was a dose of heavy spring rain and high-mountain snow that was more beneficial than troublesome, although it certainly earned a place in the record books. In downtown San Francisco, the two-day rain total (Fri.-Sat.) of 3.29” was its wettest for any April since before the Civil War—topped only by the 3.59” on April 16-17, 1853. In Sacramento, the two-day total of 2.42” was its third heaviest for April in records extending back to 1941.
Tracing this strange Pineapple Express to its roots
At some point, we’re bound to see at least one in-depth study of this stupendous atmospheric anomaly, but a quick look gives us a few early clues. Satellite imagery and model output made it clear this was indeed a Pineapple Express, as the moisture streamed into California after passing across Hawaii.
At the jet stream level, it’s easy to see what put this Pineapple Express on track. Strong flow at 500 millibars (about 19,000 feet up) extended from well west of Hawaii all the way to the U.S. West Coast, powered by unusually cold upper-level air northwest of Hawaii and unusually warm air to its southeast. Maps of model-calculated precipitable water show that the channel of air streaming toward the west coast was exceptionally rich in moisture.
|Figure 3. Height of the 500-millibar surface on 00Z Friday, April 6, 2018,(black contours, in tens of meters) and the departure of that height from the seasonal average, in standard deviations (colors, with reds denoting a positive anomaly and blues a negative anomaly). The strong contrast between lower-than-average heights to the north and higher-than-average heights to the south helped propel the Pineapple Express from the central tropical Pacific toward California. Image credit: tropicaltidbits.com.
|Figure 4. Amount of precipitable water on 00Z Friday, April 6, 2018, expressed as a departure from the seasonal norm in standard deviations (colors, with blues denoting a positive anomaly and browns a negative anomaly). Also shown is sea-level pressure (black contours, in millibars) and winds at 850 millibars (wind flags, in knots). Rich moisture extends from California southwestward to Hawaii and further west to the frontal zone flanked by the remnants of Super Typhoon Jelawat (left). Image credit: tropicaltidbits.com.
These maps also point to another likely culprit at work: the remnants of Typhoon Jelawat, which was assessed as a super typhoon with 150-mph peak winds by the Joint Typhoon Warning Center. After Jelawat played out near the Mariana Islands with only minor impacts and lost its identity as a tropical cyclone on April 2, its remains scooted eastward along the frontal zone that extended into the atmospheric river. It appears that the moist core associated with Jelawat’s remnants flowed into the western end of the moisture channel that extended into California.
At the base of the Pineapple Express, sea surface temperatures have been running more than 1°C (1.8°F) above the seasonal norm from Hawaii all the way to southern California. This means that air flowing from the surface and converging into the Pineapple Express would have a chance to be carrying greater-than-usual amounts of low-level water vapor, thus intensifying the moisture channel.
|Figure 5. Departures from the seasonal average of sea-surface temperature (SST) for the northeast Pacific as of Sunday, April 8, 2018. Image credit: NOAA/NWS/NCEP/EMC.
As the richest slug of moisture moved through Hawaii on Wednesday, April 4, it helped keep the night air at record-setting warm levels for April. Honolulu had its mildest April night in records going back to 1890, as the low temperature dipped only to 77°F. Likewise, Hilo beat its monthly warm-minimum record with 73°F on Wednesday—then beat it again with 74°F on Friday! Records in Hilo began in 1949. Wednesday also saw the warmest April minimum at Kaluhui (75°F), where records date back to 1954. Most of these records were broken by a mere 1°F, but in a climate where temperatures vary so little over many years, even 1°F is significant.
Another swing and a miss for Los Angeles
It’s sadly ironic that a winter with a catastrophic flash flood (the Montecito disaster of early January 2018, which killed at least 21 people) ended up being such a paltry rain producer for the Los Angeles area. The city gets close to half of its water from central California, where supplies should be close to average this summer, but the SoCal landscape could use a solid natural watering, and the season for getting it is running out fast.
For the water year beginning on October 1, 2017, through Saturday, April 7, downtown L.A. has recorded only 4.61”, roughly a third of the long-term average. In data going back to 1877, only two other water years have ended up drier: 2006-07 (3.73”) and 2001-02 (4.42”). It’s quite likely this water year will rank among the twelve driest on record for L.A.—which would mean that 6 of the 12 driest water years since 1877 would have occurred since the year 2000.