Summary for California, Nevada, Washington and Oregon

NOAA El Niño Advisory: There is a greater than 90% chance that El Niño will continue through Northern Hemisphere winter 2015-16, and around an 80% chance it will last into early spring 2016. During June, sea surface temperatures (SST) anomalies exceeded +1.0oC across the central and eastern equatorial Pacific Ocean. The largest SST anomaly increases occurred in the Niño-3 and Niño-3.4 regions, while the Niño-4 and Niño-1+2 indices remained more constant through the month. Positive subsurface temperature anomalies weakened due to the eastward shift of an upwelling oceanic Kelvin wave, which reduced above-average temperatures at depth in the central and east-central equatorial Pacific. In many respects, the atmospheric anomalies remained firmly coupled to the oceanic warming. Significant westerly winds were apparent in the western equatorial Pacific and anomalous upper-level easterly winds continued. The traditional and equatorial Southern Oscillation Index (SOI) were both negative, which are consistent with enhanced convection over the central and eastern equatorial Pacific and suppressed convection over Indonesia. Collectively, these atmospheric and oceanic features reflect an ongoing and strengthening El Niño. Nearly all models predict El Niño to continue into the Northern Hemisphere winter 2015-16, with many multi-model averages predicting a strong event at its peak strength (3-month values of the Niño-3.4 index of +1.5oC or greater). At this time, the forecaster consensus is in favor of a significant El Niño in excess of +1.5oC in the Niño-3.4 region. Overall, there is a greater than 90% chance that El Niño will continue through Northern Hemisphere winter 2015-16, and around an 80% chance it will last into early spring 2016. Across the contiguous United States, temperature and precipitation impacts associated with El Niño are expected to remain minimal during the Northern Hemisphere summer and increase into the late fall and winter.

California, Nevada, Washington and Oregon

Temperature

August

Only California and western Nevada remains above normal for temperature with most of the other three states being slightly to moderately below normal.

September

Temperatures for much of the four state area are slightly to moderately above normal. Much of eastern Washington and Oregon should be near normal to slightly cooler than normal.

October

Warmer than normal across the entire four state region with temperatures in the mountainous areas at close to 1 degree C (1.8 degree F) above normal.

November

Slightly warmer than normal across Washington and Oregon and near normal for California and Nevada.

December

Slightly warmer than normal across California and Nevada and central Oregon and Washington and near normal for the rest of Oregon and Washington.

January

Slightly to moderately cooler than normal across much of the four state area with the exception of central Oregon and Washington and southernmost California.

Precipitation

August

Much of California and the southern half of Nevada should be wetter than normal however long term normal precipitation for the month is very low for the Central Valley. Mountainous areas have slightly greater rainfall than the month of July so the positive anomaly is a welcome outlook. Much of the coastal ranges of Oregon and Washington remain considerably drier than normal with much of the rest of both states being near normal to only slightly drier than normal.

September

Much of California and the southern half of Nevada should be wetter than normal however long term normal precipitation for the month is very low for the Central Valley. Mountainous areas have slightly greater rainfall than the month of August so the positive anomaly is a welcome outlook. Much of the coastal ranges of Oregon and Washington remain considerably drier than normal with much of the rest of both states being near normal to only slightly drier than normal.

October

Near normal throughout the region but slightly wetter than normal for nearly all of California and across southern Nevada while much of the rest of the state is should be close to normal for this historical month for the onset of the wetter season. In Washington and Oregon near normal to slightly drier than normal conditions should prevail except in the ranges of northwest Washington where precipitation should be slightly greater than normal.

November

Most of the California and Nevada should be near normal to slightly drier than normal in the north but only slightly and this is a rainy season month. The southern half of California and Nevada in contrast could be only slightly wetter than the long-term average although in the southern-most part of California this is a very dry month historically. Washington and Oregon should be slightly to moderately drier than normal while north central Washington should be near normal.

December

The northern quarter of California and Nevada could be slightly to moderately drier than normal while the rest of both states should be close to normal and just with the southern third of California slightly wetter than normal. In Washington and Oregon slightly to moderately drier than normal conditions are forecast.

January

In California, the Central Valley and the southern half of the state should be slightly wetter than normal along with southern Nevada. The northeast coastal ranges and northern Sierra should be slightly drier than normal while much of the northern half of Nevada should be slightly to moderately drier than normal. The coastal ranges of Oregon should be slightly drier than normal along with eastern Oregon while central Oregon should be near normal to slightly wetter than normal. Most of Washington State should be near normal to only slightly wetter than normal in the central regions.

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