November 2, 2012
New Weather Systems Likely to Bring Moisture — Just Not Enough
As fall harvest edges closer to completion, attention is turning to whether there will be substantial drought relief prior to the 2013 production season.
In October southeast Nebraska saw slight improvements in its drought situation, but unfortunately, most of the state saw long-term moisture deficits increase by 0.50 to 1.00 inches.
October temperatures were below normal from the front range of the Rocky Mountains east to the western slopes of the Appalachians. In Nebraska temperatures across the eastern half averaged 2-4 degrees below normal while those in the western half averaged 1-3 degrees F below normal. In October 2011 temperatures were 1-4 degrees F above normal.
Since the third full week of August, the number of below normal temperature days is outnumbering above normal temperature days by a 3:2 ratio. During the first seven months of 2012, above normal temperature days outnumbered below normal temperature days by a 3:1 ratio. In short, the jet stream pattern has moved from a ridge dominated pattern over the central U.S. to a trough dominated pattern.
Why is this trough pattern significant, even though precipitation has not yet responded appropriately across Nebraska? Simply stated, it has supported the development of powerful upper air low pressure systems across the northern Midwest and Great Lakes whenever energy ejects out of the western U.S. As we progress toward the Winter Solstice, the position of these upper air lows should sag southward and increase the likelihood of precipitation.
During the past 20 days, these explosive lows brought two snow events and a significant windstorm that caused widespread blowing dust from the Dakota’s south through northern Texas. So far, the most significant moisture improvements have been from the southern Mississippi River valley northeast into the northern Ohio River valley. There also have been pockets of above normal moisture across Kansas, Oklahoma, and Texas.
I expect that this general atmospheric pattern will continue with significant swings between below normal and above normal temperatures. If these upper air lows continue to intensify over the northern U.S., significant drought improvement would be expected across the central and eastern Corn Belt. I do expect to see a return to more favorable moisture patterns for Nebraska over the next few months, but it will likely be insufficient to reduce Nebraska’s drought depiction by more than one drought category.
It should be noted that this forecast is in disagreement with the most recent release of the Climate Prediction Center’s (CPC) forecast for November and this winter. The CPC issues two-week lead forecasts for the following month and then revises that forecast at month’s end. The past two month-end forecasts were polar opposites of the original, mid-month forecast.
These changes indicate a high degree of uncertainty regarding U.S. climate this winter. The primary driver for CPC seasonal forecasts is the status of ocean temperatures in the Equatorial Pacific. During the past two months, the Equatorial Pacific has oscillated between slightly above to slightly below normal temperatures as the expected El Nino event has failed to materialize. Thus, the CPC forecasts are likely keying on these oscillations, even though they may not be the key driving force in regard to the jet stream pattern over North America.
There is a positive aspect to these changing forecasts. The lack of forecast consistency suggests that the CPC models have moved away from a persistent drought signal, indicating a lack of overwhelming evidence that the drought will remain at its current strength once winter is over.
Now that the remnants of Sandy have lifted into the Canadian Maritime region, a robust upper air trough approaching the Pacific Northwest appears to be poised to generate a significant weather event across the central and northern Plains region as early as next weekend. If this happens, the region could see major precipitation, with heavy snowfall probable from eastern Colorado into western South Dakota.
There is some evidence that another powerful system will develop about 7-10 days after next weekend. I would be inclined to believe that this storm pattern will continue through December. If the current upper air pattern continues into early spring, it is likely to be a long, hard winter with several opportunities for significant snows and Arctic intrusions across Nebraska.
Extension State Climatologist