Water Year 2024 Summary

Scanning through hydrological and meteorological data often feels like a study in averages. Events are constantly compared to one another, or more commonly to "normal". Normal itself is a bit of a misnomer given that climatic normals are updated frequently and are generally limited to the most 30-year period of record. In the case of snowpack the norm is to compare data to percent of normal. That in itself poses challenges given that the normal or average value almost never occurs in reality, especially in the West.

Water Year (WY) 2024 in the California-Nevada River Forecast Center's domain was and was not a departure from that last statement. On the heels of the incredible WY 2023 and with an outlook for a strong El Niño, many expected another banner year of precipitation. What happened instead was fairly... average.

The climate across the CNRFC's forecast domain varies wildly, from extremely hot and dry in the south and east to exceptionally wet in the far north and west. See the  Water Year 2023 Summary  for more details.

Despite another exceptionally hot July-September, soil moisture in most places remained high party due to the unusually heavy precipitation brought by Tropical Storm Hilary. Only very small portions of the Klamath and Pit River basins entered the year in any sort of drought status. By contrast, the entire area had been in drought one year earlier. Large portions had reached the two most extreme classifications on the US Drought Monitor Scale.

On the east side of the Sierra Nevada,  Natural Resource Conservation Service  (NRCS) sensors recorded record to near-record soil moisture at sites across the Carson, Humboldt, and Truckee River basins. Meanwhile on the west side of the Sierra Crest, high-altitude basins carried over snow into the new Water Year (WY).

Seasonal forecasts at the beginning of October offered some excitement but not much clarity on what to expect for the winter. The  Climate Prediction Center 's projections offered equal chances of above or below average precipitation for the early winter. Alongside these came an advisory for a El Niño event with potential to be historically strong and persist through the spring, sparking widespread speculation of another big snow year.

It is important to understand what the El Niño-Southern Oscillation (ENSO) is and its effect on the western hydroclimate. This can be broken into two parts. El Niño is the irregular cyclical anomaly in Pacific Ocean Sea Surface Temperatures (SSTs), while the Southern Oscillation is an inverse correlation in atmospheric pressure over the tropical South Pacific. El Niño itself can occur in three phases: warm SSTs (El Niño), cool SSTs (La Niña), and Neutral. Visit Climate.gov for a  deeper dive on ENSO .

ENSO's effects on the US West Coast are felt primarily during the fall and winter. La Niña generally brings cooler temperatures to northern latitudes and funnels precipitation to the Pacific Northwest. Blocking high pressure often accompanies La Niña, leading to below mean precipitation in much of California. Conversely, El Niño brings warmer temperatures and can enhance precipitation over the southern portions of California.

Or does it? Across the west no climatic variable is as important to water supply as winter snowpack. This is especially true in California where almost no precipitation falls in the low elevations between April and September. Nevada's statewide average over the same months is only slightly greater. Spring snowmelt and multi-year reservoir storage sustain the states through dry periods. The following images compare ENSO to each state's mean April 1 Snowpack and precipitation.

Several things must be noted regarding the following analysis. Nevada is much drier during the winter months due to less snowfall crossing over the Sierra Nevada. Additionally, the two states' snowpacks cannot be compared equally. California data comes from the  California Department of Water Resources (DWR) , who use a weighted average to compute statewide Snow Water Equivalent (SWE). That value is different for the years 1950-1994 than for 1995-2024. Nevada SWE data are much more sparse and obtained from NRCS. One mean value is used for the full period 1979-2024. Precipitation data for both states are obtained from the National Centers for Environmental Information's (NCEI)  Climate at a Glance . Categories are defined as Wet (> 110% of Mean), Average (90 to 110% of Mean), and Dry (< 90% of Mean). Both states' annual precipitation are split into periods of 1950-1994 and 1995-2024 for ease of viewing. El Niño years are displayed in red, La Niña in blue, and Neutral in gray.

Taken as a statewide average, the El Niño (warm) phase of ENSO coincides with above average precipitation winters only slightly more often than dry winters in both California and Nevada. However, over a period of record spanning 3/4 of a century, snow is equally likely to be above or below average during El Niño. Precipitation tends to be below average during La Niña winters across both states, with below average snowpack in California. Oddly, those same winters tend to coincide with above average snowpack in Nevada. Neutral years have a strong tendency to be dry regardless of the precipitation phase or location. In the extreme case of California snowpack, ENSO Neutral and below average conditions are about twice as likely as wet and average years combined.

Does El Niño lead to above average precipitation? On average. La Niña tends to coincide with above average snow in Nevada, but below average precipitation. Broken down to the regional level, there is a north-south split in which precipitation tends to be higher in the north during La Niña and in the south during El Niño. Neutral years tend to be exceptionally dry everywhere. Snow is more difficult to describe due to data scarcity and geographical constrictions, but again, the Neutral years are commonly the dry ones. More to the point, these are averages. Perhaps the more important part is that, as we saw in the plots at the beginning of this section, individual years vary wildly. For example, over the period of record, El Niño is only 5% more likely to coincide with above average California precipitation than it is to coincide with dry. ENSO phases do not guarantee anything regarding precipitation.

As discussed in the Water Year 23 Summary, the biggest contributors regional precipitation are storms known as  Atmospheric Rivers  (ARs). The two greatest differences between the winters of WY23 and WY24 regard the number and strength of such storms. In WY23 a total of 32 ARs passed over some part of California and/or Nevada. Nearly the same number had made landfall by the end of January 2024. It may then be tempting to expect even heavier precipitation, however the strength of the WY24 ARs was on average much less. The following slides will focus on periods of heavier precipitation across the CNRFC domain. In many cases what might be described as several distinct ARs are grouped together due to their close proximity in time.

All precipitation totals shown are liquid water equivalents. Data sources for the following slides are:   PRISM Climate Group   (Precipitation),   National Operational Hydrologic Remote Sensing Center   (NOHRSC) (snow), and   National Centers for Environmental Information   (NCEI) (records). Flood stages only consider CNRFC Official Forecast Points which had been established prior to WY24.

April often means a change in focus from precipitation to temperature. While not as dramatic as the concern over widespread flooding in WY23, the spring of 2024 did not stray from this trend. Winter conditions hung on as the calendar turned to April. During three different 10-day periods in April and May temperatures fell to -30°F to -20° below monthly averages before rebounding quickly to +15°F +20°F above. Despite the warm-cool oscillation over those two months, day time maximum temperatures averaged over each of April and May were warmer than normal except for coastal Southern California, portions of the Central and Southern Sierra (both months), the Central Coast, and Northeastern Nevada (May). Isolated portions of other areas did record average or cooler temps. However the warming trend which began at the end of May carried into June, and then July. Small areas along the coast did hold below average temperatures through June but the vast majority of the region posted monthly averages +5°F to +10°F above normal. July, typically the hottest month for most of California and Nevada, proved even hotter. Almost nowhere saw below average monthly highs. Mountain areas across California and in Northwest Nevada retreated to below normal in August, when departures were much smaller. September highs showed greater increases in most areas. Below average temperatures were limited to the northern and southern extremes of the Sierra and patchy areas along the coast.

Daily minimum temperatures followed a similar pattern. April nights tended to be cooler in the south and warmer in the north. In May, much of Northern Nevada, the Southern Cascades, most of the Coast and Central Valley all averaged much cooler than normal minimum temperatures. Notably the Sierra had already begun to trend warmer. Once June arrived things warmed substantially. Most mountain areas averaged at least +5°F warmer than normal at night. Over the course of the month the heat broke nearly 650 records for highest minimum temperature across California and Nevada. In July that number rose to 720 in California alone. Nevada's July count fell just over 150 from 240. Wide swaths of mountain areas departed +10°F or more from the 30-year average. August lows dropped much closer to normal however many areas though night time departures were much greater than those observed during the day. That heat lingered into September, especially in mountain areas. Many coastal and valley areas did trend cooler than average at night over both months.

Before examining WY24's April-September precipitation it is especially important to discuss regional normals. Much of the desert southeast receives little rain until about July. At the same time nearly everywhere west of the Sierra and south of the Eel River Basin become completely dry. Even tiny amounts of rain have the potential to wildly skew percent departures from normal.

Water Year 2024 exhibited such deviations beginning in April, when more than 150% of normal rain fell from about Point Conception to just west of Modesto. Western Mojave Desert areas also saw well above average totals. Drying began for most areas in May. Except for small areas above average in the Central Valley, along the North Coast, and from Monterey to the Bay Area, most areas received 70% already of already small totals. Most of the desert southeast recorded no rain. June's heat coincided with a massive portion of the CNRFC region staying completely dry. Central Nevada and the Eastern Sierra fared only slightly better while a very small portion of the North Coast stayed above average. On the Channel Islands and northwest of Blythe, tiny amounts of rain produced the effect described in the preceding paragraph. This happened again in July at several points in the Central Valley. Parts of the Central, Southern, and Eastern Sierra, and much of north-central Nevada saw exceptionally wet months. No month produced a skew as dramatic as August. Two to six inches of rain along the North Coast, around an inch in the southernmost Cascades and Northern Sierra, and less than a tenth of an inch southeast of Bakersfield exceed 300% of monthly average. In most areas south of I-80 no rain fell. September brought heavier totals to much of Northwest Nevada and the Salinas River Valley but again extreme dry to many areas across both states.

Viewing the Water Year as a whole shows a much different picture than splitting it in two. The statewide NCEI averages put WY24 as the 4th warmest mean temperature across both California and Nevada. More striking is the difference between day and night. The yearly average maximum temperature stands as California's 8th warmest and Nevada's 9th. However, the yearly average minimum was the 2nd warmest for both. Viewing gridded mean minimum temperatures shows just how widespread warm nights were. Large portions of the CNRFC averaged at least 2°F warmer across the entire year. Mountain areas were often more than 4°F warmer. Only isolated areas stayed below climatic averages.

The Oceanic Niño Index (ONI) measures a 3-month running average of SSTs. Averaging the 3-month periods from October-December through February-April returns an average ONI of +1.66°F over the fall/winter of WY24. This ranks as the 4th strongest El Niño event on record since 1950. SSTs peaked at +2.0°F from November-January then steadily weakened. By March-May SSTs had decreased to +0.4°F and stayed in the range of ENSO Neutral throughout the end of the WY.

Recalling the discussion earlier in this StoryMap, on average ENSO-warm links to above average precipitation across most of the CNRFC forecast domain, especially in the south, while ENSO neutral ties to below average. However both ENSO's effects and precipitation arrive primarily in winter. T

This WY and El Niño nicely portray that no pattern is guaranteed. Much of Southern California, the North Coast and the western San Joaquin Valley north to Modesto received 110% or more of average precipitation, as did smaller portions of the Humboldt and Walker River Basins. Death Valley found itself unusually wet at about 150% of normal. Most inland mountain areas landed under 90% of that mark. Big portions of the Central and Southern Sierra, southern Nevada, and California's far southeast received less than 75%. The northern and southeastern Central Valley and Northern Nevada stayed about average. NCEI data show California's statewide average at 102% of mean and Nevada 87%.

Perhaps more striking is how concentrated precipitation was to just a few months. The southern areas which did come in above average on the year often saw more than 40% of the year's precipitation in February alone. January through March accounted for 50% or more of the year's total for all but 5.2% of the region--the 5.2% being primarily in Northern and Central Nevada and the southern Klamath headwaters. December was the only month in which a large area received more than 10% of the year's precipitation. Conversely October and each of May through September often account for less than 2.5% each.

Surprisingly all of this had little effect on drought in the region. While some portion of the CNRFC area found itself abnormally dry for every month the total remained fairly small until August when the summer's sustained heat and lack of precipitation finally elevated most of Northern Nevada and Northwest California into abnormally dry. Even as the end of the WY progressed moderate drought stayed limited to small sections in the far north and southeast.

In a sense, the most average thing about WY 2024 was the extremes--extremely warm nights throughout the summer, extremely heavy precipitation in just a few months, and extremely dry periods outside thereof. The warm ENSO cycle largely did not deliver the hoped for heavy precipitation except in isolated pockets of Southern California. By percentages, 40.9% of the region received below average precipitation (< 90% of 1991-2020 mean), 40.4% near average (90% to 110%), and only 18.7% above average. The fact that drought conditions did not greatly worsen is a testament to the importance of snowpack, which, in a departure from the "average" El Niño year, settled above average in the CNRFC's northern basins and near average in the southern basins.