A Cool, Wet End to Summer

The end of summer brings with it classwork, colored leaves, and cooler weather. The lattermost item on that list is of particular note as the summer of 2009 draws to a close, as much of Oklahoma is in the midst of what is shaping up to be a cool, rainy September. In this month's article, we'll take a look at the current rainy spell, and delve into the realm of synoptic meteorology and climate to explain why we're seeing the cool end to summer that we are.

Though scattered convection was not uncommon during the first week of September over Oklahoma, the current rainy pattern began in earnest during the second week of the month, as a cold front moved through the region during the afternoon and evening hours of Sept. 9, bringing with it showers and thunderstorms. The rain continued into the weekend, as an upper-level low pressure system developed over northern Texas, bringing more moisture and clouds to the region.

Much of the behavior and movement of upper-level low pressure systems is determined by the large-scale flow patterns aloft; for example, the presence (or absence) of a strong westerly component in the upper level winds to expedite the passage of the low. Such large-scale patterns are, in turn, affected by seasonal changes in the weather pattern, such as the jet stream strengthening and occurring farther southward in the Northern Hemisphere during the winter months. When strong large-scale upper-level flow is present (such as in the case of a strong winter jet) a low pressure system may only bring a single day, or in extreme cases merely a single evening, of rainy weather, followed by cold, dry conditions and brisk northerly winds as the strong flow aloft ushers in high pressure behind the low.

At other times, however, weak upper-level flow may be present over a large region. In the southern plains, this oftentimes occurs during the summer months, when the jet stream tends to be weaker and be situated farther north. Under such conditions, when weak and variable upper-level flow is present aloft, a trough which enters the region or a low pressure system that forms locally can result in a low-pressure area that is “cut off” from the primary westerly flow far to the north. Without upper-level westerlies to steer them, such cut-off lows move notoriously slowly, meandering for days and bringing cool, cloudy weather to the area beneath them, until they finally reconnect with the westerlies to the north, move slowly away, or dissipate.

The current low pressure system affecting Oklahoma is such a cut-off low, and current forecasts by numerical models and the national weather service predict that by the time it exits the region, this single low pressure system will have brought daily clouds and rain to central Oklahoma for well over a week. However, though the gray skies, cool weather, and daily rain may seem dreary, they have a positive side as well. Due in part to a very dry June and July, central Oklahoma was upwards of five inches below normal for rainfall during 2009. Thanks to the showers and storms of August and now September, that deficit has been nearly erased.


To keep up to date on the latest weather conditions (be they good or bad), tune in to your local NOAA weather radio station (see http://www.weather.gov/nwr/ for more information), or check the weather service website at http://www.weather.gov. And of course, if it's weather data you're looking for, there's no better place than right here on HOOT to find it!



Story is © Nate Snook, 2009