January 20, 2015

U.S. Long Term Soil Moisture Outlook

• Precipitation has generally improved the soil moisture conditions in some of the driest areas, though long term deficits persist.
• Changes reflect changes in ocean temperatures in both oceans.
• The forecast ocean temperatures for the late winter and spring favor more bursts of precipitation for California and a wet start to the growing season in the central U.S.

Soil Moisture Overview

The drought monitor reflects the long term drought in California and the central and southern Plains.

Current soil moisture reflects the continued deficits in California and the central states.

The three month change shows how it has improved in California and Texas.

The past 120 days have seen more rains into northern California and western and southern Texas. Dryness is evident in the Mid-South and northern and central Plains.

California

The December rains brought precipitation this water year (starting October 1) to 23 inches, not reached until April 1 in 2014.

In California, the areal percent of severe to exceptional drought has improved 20% in the last 3 months, but as you can see much more is needed this wet season. Given the nature of the conditions in the Pacific, we have believed that the rains would come in bursts and that has been true to form so far. There are some indications that another burst will occur in the next month.

Texas & Plains

In Texas, close to 14% of the state is no longer considered to be in drought.

Farther north in the High Plains, the area abnormally dry area increased, though the area in drought diminished.

Technical Discussion

Drought in the United States relates to the Pacific and Atlantic Ocean temperature configurations. The PDO (or Pacific Decadal Oscillation) manifests as a horseshoe of warmth or cold around the opposite. The warm horseshoe (positive PDO) favors more, stronger and longer El Niños, whereas the cold (negative PDO) favors stronger, multi-year and more frequent La Niñas.

Currently, the Pacific Ocean is in a strong positive state.

This is occurring despite the fact that we have been in the cold decadal mode. Similar temporary changes occurred in the late 1950s and the early 2000s.

In the Atlantic, the AMO (Atlantic Multi-decadal Oscillation) has been in its warm mode since 1995. It has weakened significantly over the last year.

Back in 2004, McCabe and Betancort found the combination of the AMO and PDO affected the probability of drought in the United States. In general the warm AMO favored drought and the PDO state determined where the probability is greatest.

The bottom right map shows where we were from 2010 to 2013. Indeed, a drought in Texas back to California followed. This past half year we have transitioned to the top left configuration, which helps explain the good growing season and improvement in soil moisture, especially in Texas.

The forecast is for the warm horseshoe to gradually fade by the late spring and summer, reversing to its expected long term, negative state. The Atlantic is likewise confused.

Spring Rainfall Outlook

The analog forecast for precipitation during the important April to June period reflects this and is biased towards wet in all but the Rockies.