WRAL WeatherCenter Blog

Cut-Off Departs...

Posted October 9, 2006

The weekend certainly brought an interesting weather pattern to bear across much of the southeastern U.S., one that we saw coming a good ways out but that nonetheless is a type that's often a challenge for both human forecasters and computer models. A mid-level low pressure center became "cut-off," meaning it formed into a closed circulation that became detached from the broader west to east mid and upper level circulation across the country. Once this happens, the systems tend to move very slowly for a few days until changes in the pattern upstream begin to impact the system and cause it to move along, usually with an associated weakening trend as the strong horizontal temperature gradients that helped to instigate it in the first place fade.

The system that came in this weekend left many of us with lots of clouds, periods of drizzle and rain, and temperatures that ran well below normal, although there was a tendency for temperatures to run substantially warmer over the eastern coastal plain and near the coast compared to parts of the Piedmont, where highs in the low to mid 60s were common. Today, we've finally started to see some eastward motion with the system, and computer models are suggesting that it will continue that trend over the next day or so, allowing substantial drying, more sunshine and the warming that goes with that, all before another frontal system pushes through the region Thursday or Thursday night, with a strong rush of chilly dry air into the area for the start of the State Fair on Friday.

Here's a look at the upper level low on a water vapor channel satellite image, it's center clearly indicated by the circular flow over east-central Georgia around 11 am on Sunday. At around the same time on Saturday, this feature had been drifting southward across the central NC/SC border.

By this morning (below), the center of the cutoff low was moving east-northeast, and had slipped offshore over the Atlantic southeast of Charleston, with a moist circulation around its northern perifphery still producing cloudiness and some spotty showers and storms over eastern NC.

The computer model output below, from the Global Forecast System (GFS), shows nicely the closed circulation indicated by lines of equal height (isohypses) of the 500 mb pressure surface, located southeast of Charleston early today in line with the satellite image above. As is often the case with long-lived cutoff lows, the area of greatest rotation, or vorticity, is very near the center of the low, marked here by an X. Another common feature of mature cutoff lows is that surface lows are often stacked almost vertically below the upper level feature, as seen in the second image below, which shows the surface pressure pattern and model-estimated precipitation coverage, indicative of the chance of rain over eastern NC. The surface low in this case is not indicated by a closed circulation, but by an inverted trough that extends northward to a point immediately below the 500-mb low. If we were to zoom in and analyze the surface trough with a higher resolution (showing a contour of equal sea level pressure for every 1-mb difference instead of the 4-mb used here, for example), we'd likely see a small closed low in that position.


A significant change is indicated in the next two images, which are forecasts for 2 pm Tuesday afternoon at the same levels shown above. The cutoff low at 500 mb is seen to move farther northeast and to be weakening into an "open wave," while at the surface a low pressure area has also moved northeast from the former position of the inverted trough, and is now well at sea east of Cape Hatteras. The combination of weak high pressure ridging at 500 mb and drier air filtering in behind the low pressure areas leads the model to indicate all remaining precipitation at that time to be just east of our state, and it's likely we'll see more sun than clouds for Tuesday, along with temperatures at or a little above normal for the date.


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