There has been a rapid deterioration of the Equatorial Pacific anomalous cold pool that has supported our current La Nina event. Above normal Sea Surface Temperature (SST) anomalies continue to build in the eastern half of the basin and cold sub-surface temperature anomaly trends support further erosion, if not elimination of the La Niña signal before the end of May.
Figure 1 depicts current global SST anomalies as of April 13th. The eastern Equatorial Pacific has developed a fairly substantial area of above normal SST’s, while the central Equatorial Pacific cold anomalies are being primarily supported from cold currents originating in the eastern Gulf of Alaska. The Antarctic connection continues to weaken in response to the continent entering fall which terminates glacial melt water contributions.
The Climate Prediction Center (CPC) issued their latest La Niña watch on April 8th and increased their probability that the event will end in the May-July time frame to 85 percent, compared to 65 percent last month. Warm anomalies below the surface have been progressing eastward since January and this pool has pushed the last of the sub-surface cold pool to the surface as of March 29th as depicted in Figure 2.
Over the last 14 days, the sub-surface warm pool has pushed toward the surface in the eastern Equatorial Pacific and the depth of the anomalous cold surface pool has shrunk to less than 100 meters. In addition, the areal extent of this cold pocket reaching the surface has shrunk by 50%. Given that there is no colder than normal upwelling current in the eastern Equatorial Pacific and a warm anomaly that extends westward to Australia, there is a high probability that this La Niña event will be over before the end of May.
Going forward through the summer, the strength of the central Equatorial Pacific cold anomaly will depend on whether the colder than normal fetch of surface water from the eastern Gulf of Alaska can continue to offset the above normal SST’s in the eastern part of the basin working westward. It is normal for the Gulf of Alaska low to weaken into the summer as the temperature differential between land and water is at a minimum compared to the winter.
The further the warm pool shifts westward toward the central Equatorial Pacific, the more likely that the southern jet stream will increase in activity. Although an El Niño is not expected to develop at this point in time, the entire basin may move into the warm side of neutral conditions. This would support more moisture transport into the southern 1/3 of the United States, which would increase the probability for an active monsoon season in the southwestern U.S. desert region.
The absence of this monsoon moisture last summer helped fuel the rapid expansion of drought conditions in the central corn belt from the western High Plains. An active monsoon season would support better August moisture across the southern High Plains, which increases the likelihood that surface evaporation would pull moisture northward into the central High Plain. This would support more widespread precipitation events from cold fronts moving eastward across the northern United States.
Official CPC Long Lead Outlooks – April Release
The Climate Prediction Center (CPC) updated their long lead seasonal forecasts this morning. The temperature outlooks can be seen in Figure 3, while the precipitation outlooks appear in Figure 4. CPC continues to maintain a forecast of above normal temperatures through the entire growing season across the major crop growing regions of the United States. The weakest temperature probabilities continue to be assigned to the upper Plains and central corn belt.
Just like the past three months, CPC has altered their precipitation outlooks for dry weather across the western half of the United States. Last month CPC predicted that dryness in southern/central Rockies would shift north, then eastward into the northern half of the High Plains during the June-August period. With this morning’s release, CPC has shifted dryness into the northern half of the central Rockies northwestward toward the Pacific Northwest. During the second half of the summer, this dryness is shifted eastward to encompass the northern and central Plains.
Figures 5 and 6 show the differences in CPC’s summer forecast between this and last month’s seasonal outlooks for precipitation. CPC has eliminated/reduced the forecasted summer dryness in the March release (Figure 5) for the southern Plains and shifted forecasted dryness for the northern/central Plains toward the northwest. Regardless, CPC continues to forecast summer dryness for Nebraska during the June-August period, the only difference being that they reduced the probabilities that the outlook will occur.
It should also be noted that CPC now pushes the maximum dry signal for the northern/central Plains into the second half of the summer and early fall. At the same time, the southern Plains is depicted as having equal odds of above, below, or normal precipitation. This suggests to me that CPC is expecting a normal monsoon season.
The northern/central Plains is the real wild card in CPC’s precipitation outlook considering that the last three months have seen a trend towards cutoff upper air lows slowly migrating through the central United States. Considering the jet stream will gradually shift northward to the northern Plains by mid-summer, any lows cutting over during the warm summer months will lead to significant moisture.
If CPC’s dry summer forecast is to verify, we will need to see the Rocky Mountain snowpack rapidly disappear over the next 45 days. The most recent Natural Resources Conservation Service (NRCS) snowpack report indicated basins east of the Continental Divide had slipped below 90% of normal for the southern and central Rockies. The storm crossing the region now through the first half of this week is expected to return these levels to above 90% by next week’s NRCS release.
The cold stretch that we have been in for the past week is expected to continue through the 25th, before warmer conditions build eastward from the western United States. There is one potential weather event the 26-27th advertised for the central Plains. Depending on which model run you look at, this event will either be a major storm or a minor inconvenience to Nebraska. If it does develop into a strong storm, the central and southern Rocky snow pack will end April within normal range.
If a normal snow pack can be maintained through the remainder of the snow season, then should provide the moisture necessary for front range thunderstorm development in May and June. This is a normal climate pattern for western Nebraska and these events usually cluster and move eastward during the overnight hours across eastern areas of the state.
Thus, the snow pack and spring storm tracks will determine drought risk across Nebraska. If the snow pack is normal entering May, then a wetter forecast for the state is likely compared to CPC’s current thinking. If the warm eastern Equatorial Pacific warm pool continues to expand westward, then the southern jet stream will be a much bigger player for precipitation events across the southern and central Plains, further weakening the odds of the drought strengthening and expanding eastward across the state as the summer progresses.
Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office