Current Conditions and Impacts
La Niña conditions continue across the Equatorial Pacific Ocean and sea surface temperature (SST) anomalies in Figure 1 indicate that below normal temperature anomalies extend from South American to Indonesia. During the past 30 days the extreme eastern Equatorial Pacific has begun to show the development of patch above normal SST anomalies off of the coast of Peru.
Consistent with La Niña climatology, the weakening of La Niña conditions normally occur during the December-January time frame. The Climate Prediction Center (CPC) indicates in their latest El Nino Southern Oscillation (ENSO) update issued January 14, points toward a continued weakening through the spring months of 2021. The outlook contained in Figure 2 projects that the Equatorial Pacific will return to neutral conditions (SST anomalies between -0.5 and +0.5 C) in the March-May or April-June time frame, depending on which model averages are used.
There continues to be an expansive area of below normal anomalies below the eastern half of the Equatorial Pacific and the above normal SST anomalies at the surface only extend 25-50 meters below the surface. However below this surface anomaly there is a pool of below normal temperatures that extends 150-300 meters below the surface. It is this deep cold pool that will take time to warm and return the region into neutral ENSO territory.
The central Equatorial region of the Pacific is being reinforced by a cold channel at the surface originating from the Antarctic region, partially as a result of summer ice melt. The Antarctic has reached its climatological peak of summer, so ice melt will begin to wane rapidly over the next two months as fall approaches. If this La Niña event is to end as predicted, this reinforcing channel of below normal SST’s will need to dissipate over the next two months.
The cumulative impacts of the current La Niña event has followed some of the climatological trends we have seen with previous events across Nebraska. The dry fall pattern with above normal temperatures is the strongest La Niña signal across the central Plains, with below normal winter temperatures the second strongest La Niña signal. The dry/warm signal typical during the fall season of a La Niña year has occurred, but so far winter temperatures have been above normal.
The first half of this winter has averaged 4-10°F above normal with the warmest deviations located across north central Nebraska. Anomalies decrease as one moves toward the Kansas border. Much of the warming has been due to very warm nighttime temperatures from heavy cloud cover. However, this has resulted in little if any frost in the ground which makes it easier for moisture events to be absorbed during and after precipitation events.
Figure 3 indicates that only a small area of northeast Nebraska has seen four inches of moisture since October, while much of the western half of the state has failed to receive over 1.75 inches of moisture. More importantly, precipitation deficits since the beginning of October range from 1.50-3.00 inches across the western half and southern 1/3 of the state as depicted in Figure 4.
In order for the soil moisture recharge period (October 1 – present) to return to normal by spring planting, precipitation events across the western half of the state will need to be substantial over the next 3.5 months. To make up these deficits by the end of winter would require nearly 400% of normal moisture through the end of February across south central Nebraska. This same area would require 150-175% of normal through the end of April to make up precipitation deficits incurred since October.
2021 Weather/Climate Outlook
There has been an increase in winter storm activity since the third week of December across the central United States. Three powerful storms crossed the state on December 22-23, December 28-29, and January 14-15. Blizzard conditions occurred with the pre-Christmas and January storm, while the post-Christmas storm brought 3-8 inches of snow to eastern Nebraska. Unfortunately, most of the western ½ of the state missed out on these precipitation events.
Even though western Nebraska has missed out on the moisture from these storms, the recent trend of strong storms offers a glimmer of hope that this active pattern will continue into the early spring period. Storms that have kept precipitation south of the region have been providing moisture to western and central Texas, which should translate into more surface moisture moving northward as temperatures warm into the spring.
If these storms continue on the strong side into the spring, I expect that we will see several intense severe weather outbreaks across northern Texas through southern Kansas. It is usually these types of events that occur when a strong upper air trough pushes out of the western United States. During March through early April, these types of pattern lead to heavy snow events across western Nebraska. During late April through May, these same events can lead to both severe thunderstorm outbreaks and long lived steady rain events.
If the current drought situation across the High Plains is to break down this year, we will need to see the development of this type of pattern to make up for the lack of fall moisture. Soils across the state have little moisture available for plant growth below the top foot of the profile. Without an aggressive moisture pattern developing prior to the period of maximum crop water use (July-August), deep subsoil moisture reserves will not be able to support extended dry periods.
Therefore, the decay of La Nina conditions going forward will play an important role in regards to the precipitation patterns across the corn belt this growing season. The latest monthly and seasonal temperature outlooks issued by CPC on January 21 (Figures 5-8).
CPC indicates that the southeastern 2/3 of the United States should experience above normal temperatures, with southeastern Nebraska on the western periphery of this expansive area. The remainder of the state has equal chances for above normal, normal, or below normal temperatures. Precipitation is expected to be above normal across the northern 1/3 of the United States, along with most of the Ohio River valley. Nebraska lies in the region depicted as having equal odds of receiving above normal, normal, or below normal precipitation.
CPC’s rolling 3-month temperature outlooks point to above normal temperatures for the United States for most of the period from now through the end of the growing season. A dry precipitation forecast is projected for the southern through central Plains into the spring months, before returning the region into climatologically equal chances of receiving above normal, normal, or above normal moisture.
The most significant wetness trends during the upcoming growing season are assigned to the region extending from eastern Nebraska into the Ohio River valley. CPC has an above normal moisture anomaly during the spring season across the central and eastern corn belt, shrinking this area to the eastern corn belt and the eastern half of the central corn belt. If this materializes then it is likely than corn and soybean production will rebound from last season’s drought impacts.
In my opinion, La Nina conditions are likely to hold through most of this upcoming spring. Although surface temperature anomalies in the eastern Equatorial Pacific are moving to the warm side, the central Equatorial region has a cold pool nearly 300 meters in depth. Even if ENSO conditions return to neutral, the central Equatorial region will likely have an anomalous cold pool into the first half of this summer.
Under this scenario, the Pacific Equatorial region could develop a dipole configuration (warm east, cool central). This would allow more water vapor than normal to be lifted north into the eastern United States, while the cold pool would support atmospheric ridging across the southern Rockies during the summer. The central and eastern corn belt would be the primary recipients of this increased water vapor and spring rainfall activity may lead to planting delay issues if the strength of recent storms continue.
By the end of last summer, the western United States upper air ridge extended northward into southern Canada. With a weakening La Nina, atmospheric support for this ridge should wane by allowing more energy to move through the northern and central Rockies during the spring season and support normal to above normal snowpack values. This usually translates to good warm season precipitation east of the central and northern Rockies.
So I am cautiously optimistic that a more aggressive precipitation pattern will develop across the High Plains region as we progress into the late spring to early summer period if current CPC La Nina forecasts verify. However, if this La Nina event only weakens into the early summer and then begins to gain strength during the second half of the summer, I would expect a repeat of last growing season’s dryness issues during the critical grain fill period.
Special Note
The January agricultural climate update will be issued the first week of February and will look at current conditions across the state and a short-medium term outlook for the month of February.
Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office