How much precipitation is needed to end the drought in western states?

precipitation end drought 3 months

The three-year drought in the western United States and especially in California became more obvious this year as wildfires were influenced by low moisture in live vegetation, and in some areas once-healthy trees began to show drought-induced stress.

The current El Niño is expected to influence weather patterns during the coming winter and forecasters predict higher than normal precipitation across the southern portions of the United States, including southern California.

The map above illustrates how much precipitation is needed over a three-month period to end or ameliorate the current drought. Most of northern California will need from 6 to 12 inches according to NOAA.

Drought Monitor 10-27-2015


NOAA’s disclaimer about the map at the top of the article:

This [map] only tells you how much precipitation a location needs to get the Palmer Hydrological Drought Index (PHDI) to a certain value based on the model’s equations. It does not tell you how much precipitation is needed to refill a reservoir, restore groundwater to normal, or bring an ecosystem back to normality. It also does not incorporate snowpack into its calculations, and mountain snowpack is a crucial part of hydrology in the U.S. West.

What to expect from El Niño

El Niño forecast

The question of IF there will be a strong El Niño weather pattern in the contiguous United States this winter is now settled. NOAA reports that there is a 95 percent probability that El Niño will continue through the 2015-2016 Northern Hemisphere winter. In an indication of the strength to expect, the June-August average of sea surface temperatures in the Niño3.4 region was 1.22° C above normal. This is the third-highest June-August value since records started in 1950.

El Niño isn’t a storm that will hit a specific area at a specific time. Instead, the warmer tropical Pacific waters cause changes to the global atmospheric circulation, resulting in a wide range of changes to global weather.

The map above is a composite of how precipitation varied from average during the strong El Niños of 1957–1958, 1965–1966, 1972–1973, 1982–1983, 1991–1992, and 1997–1998. There is a large variability in those six events that makes it difficult to predict the effects at any specific location. The map below is a composite of temperatures for the same periods. Again there is much variability, and you will notice that it is very different from the actual forecast, farther down, for this winter.

El Nino temp composite

The impacts of El Niño are typically largest in the U.S. during the cool months from October through May. During the winter season, the southern half of the country — from California to the Southern Plains, as well as along the East Coast — typically receives above-average precipitation. Below-average temperatures also often accompany this above-average precipitation in these regions. Across the northern half of the country, the winter season tends to be warmer and drier than average, particularly in the Northwest, Northern Plains, and Ohio Valley.

Below are NOAA’s outlooks for temperature and precipitation for December 2015 through February 2016.

precipitation outlook temperature outlook

Effects of El Niño will vary by regions

El Nino
The image above shows the typical pattern in the winter during El Niño events. The polar jet stream tends to stay to the north of the Midwest region, while the Pacific jet stream remains across the southern U.S. With the Midwest positioned between the storm tracks, warmer and possibly drier conditions can develop during El Niño events. NOAA image.

From NOAA:

El Niño conditions have continued this summer and forecasts indicate that this El Niño will strengthen, with an 84% chance of it peaking as a strong event in late fall or early winter. In terms of how long the event may last, the Climate Prediction Center (CPC) says there is a 95% chance that these conditions will last through the winter, gradually weakening through spring 2016. Research has shown that strong El Niños are often followed by La Niñas, so conditions should continue to be monitored closely, especially if the El Niño weakens next spring, as predicted.

El Niño in Winter

An El Niño develops when sea surface temperatures are warmer than average in the equatorial Pacific for an extended period of time. This is important to North America because El Niño has an impact on our weather patterns, most predominantly in the winter.

Although each El Niño is different, there are some general patterns that are predictable. For instance, the polar jet stream is typically farther north than usual, while the Pacific jet stream remains across the southern United States (see figure above).

This pattern brings above-normal temperatures to much of the Midwest region, particularly across the northern states. This does not mean that cold weather will not happen this winter but typical extreme cold weather may be milder and less frequent. In addition, this pattern may bring drier conditions to eastern portions of the Midwest.