La Nina Weakens - Storm Activity Increases

SST Anomalies
SST Anomalies

Sea Surface Temperature (SST) Anomalies for February 21, 2021 Source :NOAA Office of Satellite and Product Operations

SST Anomalies

ENSO Forecast
ENSO Forecast

Climate Model Projections for La Nina Conditions Issued February 11, 2021 Source: NOAA Climate Prediction Center

ENSO Forecast

Current Conditions

Sea Surface Temperature (SST) anomalies in the Equatorial Pacific Ocean used to track the strength of La Nina and El Nino events are beginning to show clear signs of warming the past two months. The eastern half of this region has now warmed to near normal SST’s at the surface, with pockets of above normal temperature anomalies. In the central Equatorial Pacific, cold anomalies have weakened to the strong side of a weak event (0.5 to 1.0 C below normal). In Figure 1 you will also notice two main channels of below normal SST’s originate out of the Gulf of Alaska and just north of the Antarctic region.

With fall quickly approaching Antarctica, surface melt will weaken and the anomalous cold pool should weaken further, as it has done for the last 30 days. The Gulf of Alaska cold pool has strengthened as the winter has progressed and is compensating for the decreased feed of anomalously cold Antarctica surface water. The Climate Prediction Center (CPC) issued their La Nina update on February 11th (Figure 2) indicates that consensus of Global models used to predict La Nina and El Nino events is for the La Nina event to weaken into neutral range by early summer, but begin to cool by late summer. Thus a second year La Nina event is a distinct possibility.

Based upon sub-surface heat content that is moved west to east from Australia to South America, anomalous warmth has been unable to make much eastward progress the past 3 months. However, there is some signs over the past two weeks that the weakening anomalous cold surface conditions in the southeastern southern Pacific Ocean is beginning to allow the warm pool to build eastward from the western Pacific. If this trend continues, it should help support further warming in the eastern Equatorial Pacific and limit the strengthening of La Nina conditions this late summer through early winter. This would also help reduce the likelihood of a strong blocking ridge developing across the entirety of the western 1/3 of the United States, which occurred as La Nina conditions strengthened to just shy of a strong event.

Official Climate Outlooks for 2021

The latest 3-month long lead outlooks issued by CPC on February 18th shows a radical shift in their thoughts regarding precipitation tendencies during 2021, Prior to this release, CPC has been consistent with forecasting above normal moisture across the central and eastern corn belt during the spring, then extending that area westward toward eastern Nebraska during the summer.. At the following link:

You will notice that CPC dramatically shrinks the above normal moisture to the Ohio River valley, while increasing dryness tendencies across the Rocky Mountain region as the year progresses. CPC indicates a strong tendency toward below normal moisture across the southern Plains, southern Rockies, and western central Plains from March through May. This area is shifted into the central Rockies during the May – July period, then to the northern Rockies in the June-August and July-September periods.

If CPC’s forecasts verify, drought conditions currently in place across the western High Plains from Texas to Oklahoma would be negatively impacted by the loss of convective potential along the front range of the Rocky Mountains. It would also argue that below normal precipitation forecast currently indicated would like encompass areas up to 150 miles east. This forecast would also increase the likelihood of above normal temperatures with drier surface conditions. In fact, CPC has increased their odds of above normal temperatures and nudged the area northward into the Dakota’s a full month earlier than January’s release of the seasonal forecasts.

My Take on the Upcoming Year

To be perfectly honest, the radical shift in precipitation tendencies offered by CPC with their latest outlooks took me by complete surprise considering the weakening of our current La Nina event has been handled by global models fairly well. The most prominent reason for this shift that I can come up with is that CPC feels that La Nina conditions will begin to redevelop by the end of this summer. In Figure 2 only 4 models make it to the bare minimum for La Nina classification, which is -0.5 C or colder Equatorial Pacific SST anomaly averages. Past history also indicates that La Nina events that develop during the late summer and early fall are usually weak and short lived.

If La Nina redevelops during the early summer, like last year, then there is a high probability that drought conditions will continue, if not strengthen and expand eastward from the western High Plains. This is due in large part to the expansion of the southwestern upper air ridge toward the southern Canadian Rocky Mountains. The controlling influence on slowing this expansion northward is the amount of Rocky Mountain snowpack in place prior to the spring melt season. The snowpack status across the Rockies, which was released by the Natural Resource and Conservation Service (NRCS) shows near normal conditions across the southern, central, and northern Rockies.

Prior to the brutal Arctic air of the past two weeks, the central Rockies snowpack was below 80% of normal. The Arctic surges led to several rounds of heavy snowfall in the mountains and helped push this region into normal range, although slightly below normal. The better news is that the southern Rockies snowpack is near normal and almost twice the snowpack as last year at this time. If this trend continues through early April, I would expect that it will support much moister conditions during the spring months from east central Colorado southward. If regular moisture events return to this region, it would help mitigate the impacts of heat building earlier than normal due to dry surface conditions.

It is my opinion that La Nina like conditions will last through this spring, but will not be as dominant of a force as experienced during the development of the 2020 drought across Nebraska. With the eastern half of the Equatorial Pacific SST’s exhibiting normal conditions and small above normal SST pockets, I would favor the development of a dipole pattern. A dipole pattern is one where two polar opposite trends butt against each other (ie warm east – cold central Pacific). Under this type of pattern, the atmosphere can alternate between La Nina and El Nino like tendencies. With this in mind, here is my seasonal outlooks based upon conditions through the third week of February.

Spring: Heavy snow activity across the southern Plains and the eastern half of central Plains (KS, NE) the past two months has been sufficient in erasing precipitation deficits that occurred during November and December. At the same time, western Nebraska and northwestern Kansas have been stuck in a persistently dry pattern and have missed most of these storms systems as they have bypassed to the north and south of the region. The recent recovery in the central Rockies snowpack is a positive going into the two largest contributing months to seasonal snow totals. Barring a complete collapse of snow activity over the next 8 weeks, southern and central Rocky snowpack values should be close to normal by the end of April.

If you have noticed, systems moving through the continental United States since last fall have been aggressively strong when they develop. In addition, the frequency of events has increased the past 30 days, with numerical models underplaying their strength on a consistent basis. I see no reason that this pattern will not continue through the spring months. In fact, we should begin to see eastern U.S. Arctic air intrusions wane during March as the eastern U.S. upper air trough weakens and the mean trough positions shifts to the western U.S. This would direct surface lows out of the central and southern Rockies, which usually have higher moisture contents than systems moving southward out of the Canadian Prairie Province region.

It is these storms that also increase severe thunderstorm outbreaks across the central and southern Plains, while bringing wrap around snowfall to the western half of Nebraska and the Dakota’s. There is strong evidence from the Storm Prediction Center that La Nina spring’s bring a significant uptick in tornadoes and severe weather during April and May, shifting northward into southern Nebraska during the May and June period. If this pattern unfolds, then western Nebraska will likely see late season snowfall that may impact calving and spring planting, but will provide valuable moisture for spring growth. However, with soils bone dry, it will take continuous moisture events throughout the growing season to undo the drought damage inflicted on the region for the past 9 months.

Across eastern Nebraska, warmer conditions are likely to build into the region during April as upper air troughing switches to the western United States. It is not uncommon for the severe weather outbreaks across the eastern half of the southern Plains to rob moisture being transported northward toward Nebraska. As the mean jet position shifts northward during the late spring, severe weather outbreaks also shift northward and it is not uncommon to see this pattern bring above normal moisture to eastern Nebraska during May and June. I expect this to pattern to unfold, which will further help build up depleted soil moisture reserves prior to summer cropping demands. A failure of this pattern to develop would likely leave soil moisture recharge below normal and increase the odds that extended dry spells would negatively impact yields.

Summer: This period will be dependent on two overriding factors, the status of the Rocky Mountain snowpack and the evolution of the current La Nina event. If normal to above normal snowfall occurs over the next two months, then there should be sufficient moisture available for front range thunderstorm development during June and July. This was lacking last summer due to rapid melt out conditions during May and June across the southern Rockies. I expect western Nebraska will see more normal precipitation patterns, but drought recovery will likely be slow, unless it turns aggressively wet like we did in 2013 following the strong 2012 drought. Even though I am inclined to lean toward better precipitation patterns, the will likely be insufficient to make up the lack of deep subsoil moisture recharge and irrigators are likely for face above normal applications this summer.

Soil moisture reserves should be in much better shape across eastern Nebraska if recent storm activity continues through this spring. The concern I see for this region would be severe weather outbreaks during the first half of the summer and the potential for La Nina conditions to emerge again during the late summer and shutting off precipitation like we experienced last summer. A weaker event would make this scenario less likely as it would be less supportive of a strong upper air ridge developing across the entirety of the Rocky Mountains and pushing low pressure systems far north of the central Plains. However, since we are following a major drought year, the lack of deep subsoil moisture so far could produce sub-optimal yields if short dry periods develop between mid-July and the end of August.

With the warming that has occurred during the past 60 days across the eastern Equatorial Pacific, tropical activity off the western coast of Mexico should be more robust this year. Strong winds aloft and below normal SST’s along the coast inhibited tropical formation and led to a below normal hurricane season. If tropical activity increases this year, then the chances for a normal to above normal monsoon season in the desert southwest also increases. This moisture also gets directed toward the southern Rockies and helps to increase precipitation events across the western half of the southern Plains. Surface moisture recovery in this region usually results in an increase in precipitation output for western Nebraska also.

Fall: This is a very difficult forecast this far in advance while also dealing with a decaying La Nina event that could reappear this fall. If La Nina conditions do develop, fall conditions tend to be drier and warmer than normal across Nebraska, just like we experienced in 2020. However, with a weaker event, the strength of the western U.S. upper air ridge should not be as extensive and suppressed toward the southern half of the Rockies. In addition, tropical system activity in the Pacific Ocean should also be stronger than last year and late season storms have a greater tendency to move up the Baja Peninsula and into the southwestern desert region. This would increase the likelihood of more widespread southern High Plains precipitation events, some which would likely move northward into the central High Plains.

If the monsoon season is more active, then more frequent precipitation events should be expected across the state. At this point, there is no expectation of significant harvest delays due to a wet fall, just a more normal harvest season. As we move into the late fall, watch for renewed surges of Arctic air. They may not be long lived, but would be favored into the very early winter before the Equatorial Pacific attempts to move into El Nino like conditions during 2022. This would favor a return to a more active southern jet stream and increased precipitation activity across the southern third of the U.S. and warmer and drier conditions across the northern Plains during the heart of the winter season.

Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office

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