Spring Outlook for U.S. Drought and Flood: Dry to the Southwest, Wet to the Southeast

An unexpectedly dry Southwest has put a twist on this spring’s prospects for drought evolution and flood risk, according to dual outlooks issued by NOAA on Thursday (see Figures 1 and 2). Mild weather has limited the winter snowpack over the Midwest, but saturated soils and near- to above-average streamflows will heighten the risk of moderate flooding this spring over the middle and lower Mississippi Valley, as well as the far Southeast and recently hard-hit east Texas and north Louisiana (Figure 1). Meanwhile, drought conditions are projected to improve near the intersection of California, Oregon, and Nevada, while holding steady over southern California and southwest Nevada and developing over most of Arizona and southwest New Mexico (Figure 2).


Figure 1. NOAA’s spring flood outlook for 2016, issued on Thursday, March 17. Image credit: NOAA/NWS.


Figure 2. NOAA’s outlook for drought risk through June, issued on Thursday, March 17, 2016. Image credit: NOAA/NWS/CPC.


The trickster El Niño of 2015-16
Normally during a strong El Niño, winters tend to be wetter than average from California across the southern Rockies to the Gulf Coast. One of the strongest El Niños on record has been in play this winter. Every El Niño has its quirks, but this one has gone against the grain in several ways, most notably in U.S. precipitation (Figures 3 and 4). Instead of slathering the southern tier of the U.S. with moisture (Figure 4), this El Niño has aimed its firehose in two distinct paths, as shown in Figure 3. One extends from central California north to Washington, and the other stretches from Texas and the Gulf Coast north and east into the Midwest and Southeast (plus south Florida). At times, these swaths have featured atmospheric rivers often referred to as the Pineapple Express (flowing from the central tropical Pacific to the West Coast) and the Maya Express (streaming from the Gulf of Mexico into the eastern U.S.).


Figure 3. Observed precipitation as a percentage of normal for the 90 days ending at 8:00 am EST Thursday, March 17, 2016. Image credit: NOAA/NWS/AHPS.


Figure 4 (at right). Enhancements in the risk of wet extremes (green) and dry extremes (red) when El Niño is present in the January-to-March period. These estimates are based on more than 120 years of U.S. climate data and the Multivariate ENSO Index, which has been extended back to 1871. Image credit: NOAA/ESRL/PSD.


A puzzling drought threatens to expand

Seasonal prediction is not for the faint of heart. Over much of the country, the rainfall patterns of the past three months (Figure 3) have been directly counter to what’s most likely during El Niño (Figure 4). The unexpectedly soggy Midwest and parched Southwest are especially striking. No major storms are in the immediate forecast for the Southwest, and Pacific storms become much less frequent from late March onward across southern California into Arizona and New Mexico.

At the NOAA/NWS Climate Prediction Center, climate scientist Michelle L’Heureux put together a comparison of anomalies (departures from average) in the 500-mb wind (about four miles above sea level) for the Dec-Jan-Feb period (see Figure 5, below). “What is pretty clear to me is that the Pacific jet is shifted north of its normal position,” L’Heureux told me. “The typical wintertime Aleutian low is weaker than it is normally during an El Nino event. The El Niño wave train is there; it is just not *exactly* where it is located typically. But as we’ve been saying for the past year, no single year perfectly matches the ‘typical’ pattern. These sort of shifts are not unexpected to us, which is why our forecasts are probabilistic.  A strong El Nino doesn't negate the fact there is uncertainty and it is intrinsic to the climate system.”


Figure 5. A comparison of 500-mb height and wind anomalies for the December-January-February period in a typical El Niño (left) and during 2015-16 (right). Blue colors indicate lower-than-average heights (corresponding to upper-level troughiness); red colors indicate above-average heights. Image credit: Courtesy Michelle L’Heureux, NOAA/NWS/CPC.


It sometimes rains in Southern California
I asked Alex Tardy, warning and coordination meteorologist with the National Weather Service in San Diego, to weigh in on how things look from where he sits. “We never told anyone there would be flooding or 100-year storms (though they told us that!),” said Tardy, “but with high confidence, we expected at least average precipitation, with the more likely scenario of 125% of average in our region. While most people expected flooding and torrential rains, we only had small doses of it in early January. In fact, we've had more impact in our region from squall lines and high wind (Jan. 5-6, Jan. 31, Mar. 7, and Mar. 11).”

Mountain snowpack in the Sierra Nevada is running near the seasonal average (Figure 6). By itself, that’s not enough to fully alleviate the impact of four-plus years of drought, but it should help keep the tap flowing at least modestly this year over southern California, which imports a good share of its water from the Sierra. Additional water comes into SoCal from the Colorado River basin, where the snowpack has been reasonably close to average, although powerhouse early storms over this region segued into a largely dry late winter. Meanwhile, the landscape of Southern California has received only a few moistening storms this winter. In its weekly U.S. Drought Monitor report issued Thursday, the National Drought Mitigation Center kept 35% of California in exceptional long-term drought (the most dire category). This area now extends roughly south and west of a line from San Francisco to Reno to Los Angeles.


Figure 6. The amount of water held in snowpack (snow water equivalent) across the western U.S. on Wednesday, March 16, 2016, as a percentage of the median value for the date. Much of the mountain West has near- or above-normal snow water equivalent, but values below 25% are widespread across New Mexico, Arizona, and southern Utah. Image credit: USDA/NRCS.

“Most of our region [far southern California] is going to end up with from 40% to 70% of normal precipitation for the water year, unless we get a couple of April or May anomalies,” Tardy told me. “Given the significant green-up that we saw from the winter rains we did receive, there definitely is a lot more small fuel available for wildfires.” Although San Diego and Los Angeles haven’t gotten the El Niño onslaught they expected, they may get more sultriness than usual again this summer, according to Tardy, as sea-surface temperatures remain unusually warm west of Baja California for the third straight year. Hot temperatures may add to the discomfort: San Diego just notched the warmest February in its 142-year climate record, only the latest in a series of heat records set over the last two years.

What the year 1992 tells us about today
The weird effects of this El Niño on U.S. precipitation don’t resemble the other two “super” events in recent times (1982-83 and 1997-98). However, there is something of an analog, according to Michael Ventrice (The Weather Company). Ventrice has analyzed the location and strength of the semi-permanent equatorial trough, or standing wave, in the Pacific that corresponds to each strong El Niño of recent decades. In a prototypical El Niño, the most unusually warm water and most concentrated convection (showers and thunderstorms) are in the far eastern tropical Pacific, which tends to bring the subtropical jet stream directly into the California coast. This winter, the most anomalous warm water and convection has been in the central Pacific, close to the Date Line. The associated subtropical jet has occasionally punched into the Southwest but more often headed toward northern California, Oregon, and Washington. This was also the case in April 1992, toward the tail end of the strong El Niño event of 1991-92. Then, as now, the focus of El Niño’s oceanic warming was near the Date Line. “The enhanced precipitation signal in April 1992 looked to have set up further north than what you'd expect in El Nino base states,” Ventrice told me. “The Northwest and northern California saw the bulk of enhanced rainfall. Southern California was fairly dry.”

Ventrice added: “We probably won’t see a repeat of April 1992 for the western U.S. pattern in April, as other external forcing mechanisms [including a recent strong split in the stratospheric polar vortex] are expected to overpower the El Nino base state and drive the pattern across the Northern Hemisphere during April. Long-range sub-seasonal models are indicating highly anomalous warmth and dry weather across the entire western U.S. during the first two weeks of April.”

A parting swipe from winter across the Northeast
A nor’easter developing off the East Coast late this weekend could bring a strip of moderate to heavy snow over or near the major coastal cities from Washington, D.C., to Boston, although much uncertainty remains. Total snowfall could end up as high as 3-6” in D.C. and 10” or more in Boston. There’s been plenty of conflict in the model guidance on the timing, strength, and positioning of this storm, which will make a big difference in snowfall potential. Given the lack of intensely cold air, a nighttime snowfall would be more likely to produce accumulations in the D.C. area, mostly on grassy surfaces. Capital Weather Gang will be tracking the storm closely. Steve Gregory examined the prospects of significant snow, and the longer-range U.S. outlook, in a post on Thursday.


Figure 7. Infrared image of Cyclone Emeraude at peak strength (Category 4, with 145 mph winds), collected by the VIIRS instrument aboard the NOAA/NASA Suomi NPP satellite. Image credit: NOAA.

Emeraude hits Category 4 strength in South Indian Ocean
Tropical Cyclone Emeraude has been putting on quite a show this week across the remote waters of the South Indian Ocean. Emeraude quickly spun up to Category 4 strength, rocketing from peak sustained winds of 65 mph at 06Z Wednesday to 145 mph at 06Z Thursday, according to the Joint Typhoon Warning Center. Emeraude underwent an eyewall replacement cycle later on Thursday, bringing down its intensity. Weak steering currents have allowed Emeraude to linger and pull up cooler water with reduced oceanic heat content, hastening its decline. At 06Z Friday (2:00 am EDT), Emeraude’s peak winds had dropped back to 100 mph. The cyclone could get a final shot at intensification late this weekend or early next week as it begins accelerating toward the southwest. Emeraude is not posing a threat to any land areas.

We’ll be back with our next post on Monday. Have a great weekend, everyone!

Bob Henson