Current Conditions
The Climate Prediction Center (CPC) released its latest El Niño Southern Oscillation (ENSO) update on the current strength of the ongoing La Niña event in the Equatorial Pacific on November 10th. CPC indicates that the current strength of this event is just below the threshold to be considered a strong event. The area in the Equatorial Pacific that is used to determine the strength of a La Niña or El Niño event currently sits at -1.3 C and will need to reach -1.5 C to be considered a strong event.
La Niña conditions increase the westerly winds above the ocean surface, moving water westerly at a faster rate than normal. Below normal SST anomalies appear because the surface water has less time to absorb solar radiation as it moves toward the western Pacific. In addition, upwelling waters replace the water drawn westward by surface currents and are reinforced as long as the sub-surface heat content is below normal. Figure 1 shows the extent of the La Niña signal (determined by pressure patterns above the Equatorial Pacific) through sea surface temperature (SST) anomalies. Below normal SST’s extend westward to 4/5 of the way to Australia.
According to the CPC, the consensus forecast by global climate models project that this La Niña event has a 95% chance of lasting through the winter and a 50% chance that these conditions will last through the 2021 spring. Last month the projections were a 65% probability of La Niña lasting through the spring season. Figure 6 shows the various ENSO forecast projections and the overwhelming majority show a warming trend into the summer months reaching neutral conditions (+0.5 C to -0.5 C) from March into June, depending on the model used.
Based upon the sub-surface heat content cold anomaly that is currently working its way to the surface in the eastern Equatorial Pacific, it appears to me that this La Niña event will reach peak intensity sometime within the first two weeks in January. This would be slightly later than the normal peak intensity that occurs during the second half of December. In order for this event to decline in strength going forward, the supply of sub-surface heat content needs to warm and pressure patterns above the Equatorial Pacific need to weaken,
Sub-surface heat content underneath the western Equatorial Pacific continues to warm. This water is drawn westward by normal sub-surface currents. Over the last month, this expanding area of above normal heat content has not been able to move any further eastward than the central Pacific. Until signs appear that this warm pool has move into the eastern Equatorial Pacific, this current La Niña event will be slow to weaken. It is my opinion that we will be dealing with La Niña impacts through the entirety of the spring as global model ENSO projection have developed and ended events much quicker than has actually occurred during the past four years. In fact, almost all of these models failed to predict the development of this event and/or its current strength.
Climate Impacts for Nebraska
Typical fall weather during La Niña events across Nebraska tilt toward above normal temperatures and below normal precipitation. Temperatures this fall were normal to 2°F above normal for the September-November period, while precipitation was well below normal for most of the state. Drought that developed during the early summer across western Nebraska spread eastward during the late summer and intensified with the warm and dry conditions experienced this fall. Figure 3 depicts the current drought status across Nebraska as of December 8. Widespread D3 (extreme drought) covers much of the Panhandle and southwest Nebraska, with D2 (severe) and D1 (moderate) conditions over the remainder of the state.
The intensification of drought conditions this fall can be directly to the lack of fall moisture events necessary to build up soil moisture supplies for the 2021 growing season. Figure 4 shows the accumulated moisture from October 1 to present across the state. Only a small area of east central and northeast Nebraska have seen more than 3 inches of moisture since the beginning of October, which is considered to be the beginning of the soil moisture recharge period because agricultural crops are usually mature and no longer require water for grain production.
Dry conditions in August and September depleted sub-surface soil moisture by growing crops and any soil moisture recharge that has occurred is contained in the upper 2 feet of the soil profile. For the western ¾ of the state, precipitation since the beginning of October has been insufficient to bring the top one foot of the soil profile to field capacity. Research that the High Plains Regional Climate Center conducted in the early 1990’s found an average fall-spring recharge rate of 70% of the moisture received. For the western half of the state, there has been less than an inch of soil moisture recharge since the beginning of October.
Figure 5 shows the precipitation deficits that have occurred since the beginning of October. A substantial portion of the state has accumulated deficits of 1.50-2.50 inches of moisture. In order to make up these deficits before the end of winter, we would need 200-250% of normal moisture through the end of February. If this moisture were to materialize, it is likely that a substantial portion of it would run off due to frozen soil surfaces. Therefore, it is highly unlikely that accumulated soil moisture recharge deficits will improve substantially until next spring.
In order to feel comfortable that a respectable chance for an active spring weather pattern will develop, I believe we will need to see some semblance of normal precipitation patterns across the central Plains over the next two months. Even as dry as conditions were this fall, seasonal snowfall accumulations are running normal to slightly above normal across eastern Nebraska and slightly below normal for western Nebraska. The problem is that they are coming in big events separated by long intervals of precipitation free weather. Thus, sustained snow cover has not developed and coupled with below normal soil moisture recharge, the door is open for deep frost penetration and winterkill issues if an Arctic air outbreak should occur that is not accompanied by protective snow cover.
Short Term Weather Outlook
Since the late October snow event, the upper air pattern has favored troughs moving into the Pacific Northwest with ridging over the eastern U.S. Troughs that were able to move into the central Rockies were not strong enough to develop widespread precipitation until they moved into the eastern High Plains. That keep the majority of precipitation events south and east of the state, while temperatures remained above normal most of the time as Arctic air remained bottled up in northern and central Canada.
The storm system that moved through the region December 11-12 may be a sign that the upper atmosphere is beginning to transition to favoring the development of an upper air trough pattern across eastern North America and a ridge pattern across the west. Short term models project that a series of short waves will move southeast along the periphery of the broad upper air trough over central and eastern Canada through Christmas. This pattern would lead to quick warm ups and cool downs with little moisture since systems would be originating to our north and not have access to moisture streaming northward from the Gulf of Mexico.
From Christmas to the end of the year, the GFS model indicates that low pressure in the Gulf of Alaska will move toward the Pacific Northwest in response to a high pressure system building in from the west. This high pressure system is projected to build into eastern Alaska, while the surface low gets pushed southward. At the same time, an expansive high pressure ridge is forecast to develop across the North Atlantic, which deepens the Hudson Bay low and leads to a deeper upper air trough over the eastern United States. During this period, the GFS model indicates a period of above normal temperatures for Nebraska as the jet stream lies to our north across the Canada – United States border region.
The GFS model projects that the Pacific Northwest low that originated in the eastern Gulf of Alaska moves eastward and merges its energy into the backside of the eastern North America upper air trough at the end of December. This would allow for the formation of a surface low across eastern Montana that would move south along the front range of the Rockies (similar to the October snow event) and pull Artic air southward into the High Plains region. If this pattern develops as currently forecast, then snow and Arctic air will move into the state at the end of December.
The length of time that Arctic air continues to influence Nebraska will be determined by how long the North Atlantic upper air blocking ridge remains in place. If the block is temporary, the Arctic air intrusion will last less than a week, but if the block is more long lived then expect Arctic air to dominate the first half of January and expect occasional Alberta clippers moving south out of Canada to reinforce the Arctic air mass east of the Rocky Mountains. Winter conditions may be finally arriving after a warm start in December. Figure 1 shows the extent of the La Niña signal (determined by pressure patterns above the Equatorial Pacific) through sea surface temperature (SST) anomalies.
The next La Niña update is scheduled for release January 18-22, 2021.
Al Dutcher, Agricultural Extension Climatologist - Nebraska State Climate Office