For much of the fall season, the jet stream hung out near the U.S. – Canadian border. This was linked to very warm weather across the U.S. and rather cold weather in many parts of Canada.
Recently, the southern branch of the jet stream has become more active, spawning a series of storms that brought snowfall quite far south and led to an extensive U.S. snow pack. The storms also brought severe weather and tornado outbreaks, including the most active tornado outbreak on Christmas Day in recent recorded times.
In the past few days, the active “southern branch” of the jet stream has been accompanied by what meteorologist’s refer to as a “split flow” (Fig. 1). In this situation, the Pacific jet stream splits into two parts, a northern and southern branch. The northern branch acts to help keep arctic air bottled up in Canada; the southern branch allows for storminess, heavy rainfall and possible severe storms across parts of the south.
The split is analogous to a river of water that encounters an obstacle. The water is forced around the obstacle on two sides. For the river, the obstacle could be a rock, an island, a fallen tree, or perhaps a bridge piling. In the atmosphere, the obstacle is often the formation of a closed upper level high-pressure system or ridge that acts like a “block” to the airflow.
Within this split flow pattern, the strong southern jet stream was able to start interfacing with a meteorologically ripe (i.e., warm and humid) Gulf of Mexico air mass. The two started to interact across Texas and have begun to move to the east. The result has already been much-needed, widespread, locally heavy rainfall. In some places, especially west and north of Austin, two to four inches fell during the two-day period ending on the morning of Jan. 10, 2013 (Fig. 2). Five to eight inches fell in parts of southern Louisiana. An additional one to three inches of rain is expected to fall across much of Louisiana during the next two days (Fig. 3). Early this Thursday morning, flash flood watches were still in place across many places in Louisiana (Fig. 4).
Even though a jet stream and high moisture values are present, other parameters needed for severe weather are not very favorable. Hence, there is only a slight risk of severe weather today, mostly for the lower Mississippi Valley.
Before this storm event, Texas, much of the central U.S. and a large part of the southeast remained in the grip of a major, long-term drought; some areas in the central U.S. are in the exceptional drought category (Fig. 5).
Impacts to farming have been widespread. However, low water flows on rivers, like the Missouri, have also made the news. Low water flows on downstream rivers, like the Mississippi have led to major river transportation impacts.
This storm system has helped out the Texas drought situation and should help lessen drought conditions in parts of the Mississippi River Valley and the southeast U.S. At least an inch of rainfall is on tap for the next few days across at least eastern sections of the main central U.S. drought region.
However, by the end of the weekend, a new arctic air mass will slide into the western and central U.S. The upper level wind pattern that evolves (Fig. 6) will not let the leading edge of this air mass (the arctic cold front) make it to the East Coast. Instead, the front will stall out, parallel to the upper winds, from northeast Ohio to southeast Texas by Sunday morning (Fig. 7); then the front will make only a snail’s pace migration toward the East Coast during the next three days (Fig. 8). This will allow for a prolonged period of heavy rainfall to develop across the Tennessee, Ohio and lower Mississippi River Valleys. Places like Nashville, TN and Atlanta, GA could see cloudy, rainy skies for a whole week! This could allow for one to six inches of rain to fall during the five-day period ending Jan. 15 (Fig. 9).
Overall, this a great forecast for water. Mostly needed rainfall for agriculture, human water needs and water-based transportation should occur, mostly without serious flash flooding and severe weather concerns.
Longer-term, the water machine turns off as colder and drier air dominates the U.S. into the Martin Luther King – Presidential Inauguration weekend and beyond. Both events fall on Jan. 21 this year because the official swearing in of the President is not held on Sunday.
© 2013 H. Michael Mogil
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