Current Conditions
Sea Surface Temperature (SST) anomalies in the Equatorial Pacific Ocean continue to slowly warm and the cold anomalous pool is shrinking in areal extent and depth under the surface. Figure 1 shows widespread above normal SST anomalies in the eastern quarter of the basin, with cold anomalies concentrated in the central basin. This cold pool is being supported at the surface from anomalous cold water emanating out of the Gulf of Alaska. The cold pool in the southern Pacific Ocean emanating from Antarctica has weakened significantly over the past 30 days and is becoming a minor contributor to surface cooling in the central Equatorial Pacific.
More importantly, below normal sub-surface temperature anomalies has shrunk from 300 meters of depth to near 100 meters of depth in the past two months. Underneath of this anomaly, over 300 meters of abnormally warm water has now reached 2/3 of the way under the Equatorial Pacific and continues to build eastward.
As this warm pool has built eastward the past 30 days, it has forced the cold pool above it to the surface while reducing its depth by 50%. If this continues (likely), the primary contributor to the surface cold pool will be the Gulf of Alaska below normal SST anomalies. The Climate Prediction Center (CPC) indicates a 60% likelihood that this La Niña event will end before the beginning of summer (Figure 2).
Compared to last month’s forecast, the only significant change in the La Niña outlook is to back away from La Niña conditions redeveloping during the second half of the summer. That being said, if the warm anomalous pool underneath of the Equatorial Pacific continues to build eastward, we may be seeing the onset of an El Niño event late this year into the first half of 2022.
The Gulf of Alaska will determine whether El Niño conditions develop or whether neutral conditions dominate the second half of this summer through the fall harvest. If the eastern Gulf of Alaska remains under the influence of low pressure aloft this growing season, it would continue to support surface cooling in the central Equatorial Pacific. However, if high pressure aloft builds into this region, it would support surface warming, weaken the cold surface anomalies and allow the eastern Equatorial Pacific warm SST anomalies to build toward the central regions of the basin.
An eastern Gulf of Alaska upper air low would be supportive of active weather across the northern half of the western United States, with dryness concern shifted toward southern Canada, along with the southeastern United States. This scenario would bring welcome moisture to most of the western corn belt during the growing season.
If High pressure dominates, then drier than normal conditions are likely across the western 1/3 of the United States and wetness anomalies would shift into the eastern corn belt and the Southeast. Dryness across the central and eastern half of the western corn belt would be determined by how far westward the backside of the eastern U.S. upper air trough extends. It is the backside position of this trough that would be underneath a northwest flow aloft and be susceptible to convection forming from energy moving southeastward out of the northern Plains (like Alberta Clippers in the winter).
Official Climate Outlooks for 2021-22
For the second consecutive month, CPC has altered their seasonal precipitation outlooks dramatically from the previous month’s release, but continues to indicate that above normal temperatures are likely for most of the nation through the end of the growing season. Figure 3 shows the rolling 3-month temperature outlooks and the major change from last month’s release is to push the warmer than normal outlooks into the northern Plains region.
CPC’s precipitation forecasts for this upcoming growing season are concerning if they verify (Figure 4). Even with the exceptional precipitation event experienced during the middle of March, most of the western 1/3 of the High Plains region from Texas to North Dakota still has significant short and long term drought concerns. Below normal moisture during any of the 3 month periods between now and the end of August would lead to rapid drought intensification due to the lack of deep sub-soil moisture.
My opinion from last month in regards to CPC’s precipitation outlooks have not changed. I firmly believe that CPC is not taking into consideration how frequently southern stream storms are moving from the southern Rockies into the central Plains and the amounts of moisture that are being delivered by these systems. As I have pointed out before, the current pattern we are experiencing began to ramp up significantly when the eastern Equatorial Pacific began to develop above normal surface temperatures.
At the end of February, CPC increased their dryness forecast for the central and southern Rockies eastward in the western half of the High Plains. What happened? Two cutoff upper air lows pushed out of the southern Rockies and slowly moved northeast resulting in widespread heavy precipitation. Most of Nebraska received enough moisture in March that it exceed what would be expected from October through March.
If this same pattern materializes in April, May, or June, then widespread heavy precipitation and severe weather would be expected for those who fall under the cutoff upper lows storm track. More importantly, the frequency of these systems has been fairly regular since December. In general, every 14-20 days, we have switched from troughs ejecting out of the southwest to an upper air trough building over the eastern United States.
If I am correct with this pattern, look for a drier conditions to develop during the first half of April as an upper air trough builds over the eastern U.S. The switch back to upper air troughs moving energy from the eastern Gulf of Alaska into the southern Rockies should return the second half of April into early May. The central and southern Rocky Mountain snowpack should remain within normal range if this scenario unfolds.
The next cycle in this pattern would bring drier weather to the state from early to mid-May, before upper air trough rebuilds into the western U.S. in late May and early June. If this two month pattern fails, significant snowpack reductions would be expected for the southern half of the Rocky Mountains due to lack of storm this activity and early melting. Drought risk for the western corn belt would increase dramatically as heat and dryness would rapidly build northward like we experienced last summer across western Nebraska.
The type of weather pattern that develops this summer will be dictated by how aggressive the precipitation patterns becomes from now through the end of May, especially across western Nebraska. Because we are moving through a decaying La Niña event instead of building to one like last summer, patterns locking into place for extended periods of time are less likely.
Right now the southern and central Rockies snowpack east of the Continental Divide is very close to normal. The southern Rockies have almost double the snowpack of last year at this time. If a couple of additional storms move through the region in April, then I would expect the snowpack to support front range convection through mid-June.
These thunderstorms would provide moisture to the Texas Panhandle northward into west central Kansas and reduce the likelihood of heat and dry conditions building into western Nebraska before front range convection east of the central Rockies kicks into gear during late June and July. If the eastern Equatorial Pacific continues to warm, it would support increased moisture into the southwestern Desert region during the second half of the summer and more humid and wetter conditions across the southern and central Plains region.
If the snowpack falls apart the next 45 days due to the lack of storm activity, then odds lean to a rapidly expanding dry and hot pattern like we experienced last summer. Since subsoil moisture levels across the western High Plains remain below normal, drought conditions will have a much easier time expanding from its current extent.
In short, I remain optimistic given current weather patterns that the worst of the drought conditions are behind us. However, it will not take much to slip backwards if dry conditions take over through the end of May. With all of the moisture received the past few months, expect high relative humidity levels once vegetation starts growing are going to be high and will remain high if regular precipitation events continue through the growing season.
Producers across western Nebraska should remain defensive in regards to drought impacts until we see long term precipitation deficits (6 month to 18 months) disappear. This is not likely to occur completely before the end of the summer, even with favorable moisture patterns. East of this region, March precipitation has moved the soil moisture recharge period (Oct-present) into surplus territory. Barring a shutoff of precipitation going forward, available soil moisture in the top 5 foot of the profile should be at or above normal to begin the growing season.
Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office