Ducks, jet streams and rainy weather

Published 7:00 am Tuesday, March 10, 2015

If you are a duck owner, you are going to have some happy birds this week. Those of us without webbed feet may not enjoy the next several days quite as much. We could get three to five inches of rain Tuesday through Thursday.

The upper level winds over North America have shifted into a much different pattern than we have seen over the last few months. This new pattern will persist at least through Thursday, and with it will come occasional periods of rain.

This pattern change is associated with changes in the jet stream. The NWS’s Online School for Weather describes a jet stream as, “A narrow band of strong winds in the atmosphere that controls the movement of high and low pressure systems and associated fronts….Wind speeds can reach 200 mph or higher…. They are usually found at 30,000 to 40,000 feet above the earth’s surface. They owe their existence to the large temperature contrast between the polar and equatorial regions. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet streams.”

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During the non-summer months, the southern half of the United States is affected by both the polar jet stream and the subtropical jet stream, both of which more or less encircle the globe. While they flow generally west to east, they meander north and south around wave-like troughs of lower pressure and ridges of higher pressure, some of which are large enough to cover the entire U.S.

This winter, even more than usual, the polar jet has dipped far south from Canada into the southeastern U.S. carving out a deep upper trough and pushing cold surface air into the central and eastern U.S.

As indicated by its name, the subtropical jet stream meanders around closer to the tropics. Its average position is about 30 degrees north latitude, which happens to coincide with the southern United States.

The subtropical jet does not influence surface temperatures as directly as the polar jet. However, when the subtropical jet becomes active over the southern U.S., it has the dual effect of bringing us high-altitude moisture from the Pacific and providing upper level conditions favorable for the development of surface low pressure systems.

Through Thursday the subtropical jet will be strong over the southern U.S. while the polar jet retreats north. Winds from the surface and upward many miles into the atmosphere will be from the southwest, south, and southeast, transporting massive amounts of water in both its gaseous and liquid forms—in other words, high humidity and a lot of clouds—over the Gulf South.

The pattern is predicted to become less favorable for rain on Friday and into the weekend, but to quote the Slidell NWS’s online discussion, “Confidence is lower than average for the extended period with such a complicated pattern of several troughs and ridges in play.”

Meanwhile, local ducks and other fans of the subtropical jet stream will have plenty to cheer about.

By Skip Rigney.