November 8, 2013

Early October-November Storms Bolster Soil Moisture Reserves

The first storm of November brought significant moisture to most of the state. Although not as intense as the early October blizzard and severe weather, this system dumped a month’s worth of moisture in pockets across eastern Nebraska. Rainfall of  1-2 inches was common from south central through east central Nebraska. Much of western Nebraska received 0.25 to 0.75 inches of moisture, mostly in the form of wet snow.

This is the second time in four weeks that a storm dropped as much moisture as you would typically see for the whole month in a single event. While a headach for producers trying to complete fall harvest, these impressive precipitation totals are building soil moisture reserves for 2014. Second, continual replacement of surface moisture lost to evaporation is helping to constrain the excessive warmth that was common during the 2011 and 2012 falls due to the lack of precipitation.

Except for southwest and south central Nebraska, exceptionally wet conditions during October have led to impressive soil moisture gains in the upper two feet of the profile. Southwest and south central Nebraska were the only areas not to receive normal moisture during the month. Eastern Nebraska received 3 to 7 inches, which translates into 200% to 400% of normal moisture.

October hasn't been this wet since 2009, which marked the third consecutive October with above normal moisture based on statewide totals. Even more impressive is the fact that only one of the High Plains Regional Climate Center’s soil moisture monitoring sites had lower available soil moisture in the profile at the end of October than was measured at the beginning of March 2013.

The primary reason that soils across the state were virtually devoid of stored soil moisture last fall was the lack of fall storm activity in 2012. This was subsequently followed by a lack of significant snow activity through late February. If the state receives normal moisture from November through February, I have a high degree of confidence that will we enter the 2014 production season with some of the best available soil moisture that we have seen since spring 2010.

Late Fall and Winter Forecast

One thing is for certain, if this pattern of strong low pressure systems crossing the state continues into the winter, a lot of snow shoveling can be expected. The ultimate factor affecting this will be the mean jet stream position during the next four months. Right now, strong upper air lows entering the northwestern U.S. have been able to deepen as they push southeastward into the central Rockies. This pattern has resulted in large amounts of moisture being drawn northward out of the Gulf of Mexico.

In the short-term, it appears that we will get a brief lull from heavy precipitation as the storm track shifts slightly north of the state. An upper level trough is expected to dig southeastward out of the northern Rockies early next week, but its quick movement will limit the feed of moisture from the Gulf of Mexico until it passes east of the state. Precipitation forecasts for this system are for less than 0.20 inch of moisture for the western two-thirds of the state, with upwards of 0.25 inch for eastern Nebraska.

The next significant weather event is slated to arrive around Nov. 15. The storm track is unclear in the models, but it appears that the southern half of the state has a decent chance of widespread moisture. Current models indicate it will come in the form of rain, but snowfall is not out of the question if enough cold air can be tapped by the surface low.

The early fall jet stream pattern has featured a strong ridge and trough pattern across the continental U.S. This means that a strong ridge over the western U.S. leads to an amplified trough over the eastern half of the country and vice versa. Three characteristics can be associated with this type of atmospheric pattern:

• large temperature swings,
• intense winds, and
• concentrated areas of significant moisture.

If we get an early and sustained period of Artic air before an insulating blanket of snow, I expect frost depths will easily reach several feet into the profile, given the abundance of soil moisture. The last two winters have been devoid of significant surface moisture and sustained periods of frozen soils were hard to come by.

In order for temperatures to average above normal this winter, the atmosphere will need to lock into place. This means that a ridging pattern will need to set up across the central U.S. and block the movement of upper air lows into the region. If the past two months are any indication, the odds of developing a blocking pattern remain very low.

In a typical winter, the jet stream will continue its southern drift through mid-January, then reverse and begin its slow migration toward its mean summer position along the northern border of the United States. With these late fall and early winter storms being so intense, we will need to monitor how far south the jet stream dives during the heart of the winter.

A usual southward displacement would keep the storm track across the southern Plains and place Nebraska firmly into the cold sector of the air mass being pulled south from Canada whenever a surface low develops across the southern Plains. As the jet stream moves northward and closer to the central Plains in February and March, strong winter storm activity would be expected.

Considering all this, I believe the first half of the winter will likely be characterized by strong storm activity, with distinct periods of below and above normal temperatures. Temperatures may tend to be below normal, but I do see continued agricultural and hydrological drought improvement.

Mid-winter to early spring conditions will depend on the jet stream position. If the jet stream is displaced well south of its typical January position, expect strong Arctic air intrusions for Nebraska into February, with a return to moderate or strong snowstorm activity during February and March. If the jet fails to displace this far south, frequent snows are likely most of the winter season.

Al Dutcher
State Climatologist, Nebraska State Climate Office