By Al Dutcher, NSCO
This past winter was certainly a much easier ride than what Nebraska experienced last year, particularly during the February through mid-March period. Although we experienced occasional spells of below-normal temperatures, they were relatively short-lived as the brunt of Arctic air remained bottled up across northern Canada and Alaska. This lack of sustained cold was the primary reason we were unable to maintain snow cover for most of the winter period.
With milder weather this winter, maximum frost depths at our Mesonet locations never exceed 18 inches across the northern third of Nebraska, 12-18 inches across the central third of Nebraska, 8-12 inches across the southern third of the state. At the end of February 2019, frost depths across the northern half of the state ranged from 20 to 40 inches under a snowpack that ranged from 6 to 24 inches.
Four major storm events affected the state from late last fall through the end of February. The first storm moved down the front range of the Rocky Mountains at the tail end of October bring heavy snowfall from eastern Montana southward through the Texas Panhandle, including western Nebraska. Major winter storms also crossed across the state during the Thanksgiving Holiday weekend, between Christmas and the beginning of 2020, as well as mid-January.
The October event occurred under the influence of a strong northern jet, which dominated the month of October into the middle of November. The pattern change in November resulted in a more active pattern for the southern jet stream, while the northern jet stream weakened. Thus systems coming out of the southwest were able to move northeast into the central Plains. The mid-January event ushered in a pattern change favoring a more active northern jet stream that lasted through the end of February.
The pattern change unfolding toward the end of February appears to be favoring a more active southern jet, so it is likely that an increase in heavier precipitation events is likely across the southern and central Plains over the next six to eight weeks. The degree of wetness for Nebraska will be highly dependent on the number of storms that can eject out of the southwestern U.S. and move northeast toward Nebraska.
If the northern jet is active enough to create a mean trough across the northern Plains, then many of the southern stream storms will move eastward toward the southeastern U.S. and continue to bring heavy rain and flooding to the lower Ohio and Mississippi River valleys. With soils across most of Nebraska at full field capacity, our flood risk will be high through spring planting and an active storm pattern with lows ejecting out of the southwestern U.S. would only increase our spring flood risk.
The pattern changes mentioned previously are a normal part of our climate across the continental U.S. Typically, the position of the mean upper air trough across the eastern U.S. will last six to eight weeks, before the trough switches to the western U.S. If this scenario holds, then expect more storm activity across the western half of the U.S. Some of this energy will likely be picked up by the southern jet stream and be pushed east or northeast each time another storm system approaches the west coast of North America.
If the pattern change occurs as expected during the second half of April, then a return to an active northern jet stream is likely. This would increase chances that temperatures will be close to normal to below normal across the northern Plains, including the northern half of Nebraska. Precipitation events will be dependent on the northern jet stream position. If the backside of the trough favors eastern Nebraska, a drier-than-normal pattern may be in store for the western half of the state. If the western extent of this trough lies along the front range of the Rocky Mountains, then cool and wet conditions will likely affect the entire state from early May into the second half of June.
Briefly looking beyond the next three months, there are increasing signs that the Equatorial Pacific is beginning to transition into a potential La Nina event. The consensus of global atmospheric models that predict El Nino and La Nina events is that the Equatorial Pacific will shift to the cool side of normal sometime this summer. In January, the projection was for this crossover to occur in the July-September time frame, and it was moved up into May-July period with the February forecast.
A new model consensus updated forecast will be released by the Climate Prediction Center by mid-March. If it shows cooling occurring earlier than the February forecast, then La Nina conditions will likely develop by the end of the summer. Typically, the earlier a La Nina event develops, the better the odds that warmer and drier conditions will develop across the southern and central High Plains region.
At this point in time, the strength and length of this event is uncertain, but we are long overdue for substantially drier and warmer pattern. In reality, we have been in an aggressively wet pattern since the 2012 drought, so a movement toward a drier pattern is just a matter of time. If this pattern does evolve, the areas from I-80 southward through the northern half of Texas would have the greatest odds for dryness impacts to crop production during the second half of this summer.