Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 10:10 PM GMT on October 10, 2013
The current 500-mb pattern across the contiguous states provides a learning opportunity so that readers have a better sense for cut-off lows. Although the standard contour interval for 500-mb heights is 60 meters (see today's 18Z Rapid Refresh model analysis of 500-mb heights), I generated the 18Z Rapid Refresh model analysis of 500-mb heights (below) using 30 meters as a contour interval to better indicate the cut-off low over the Middle Atlantic States.
The 18Z Rapid Refresh model analysis of 500-mb heights on October 10, 2013. Contour interval is 30 meters instead of the standard 60 meters. This non-standard contour interval allows us to better identify the closed low over the Middle Atlantic States. Courtesy of Penn State.
I summarize, on the image below, the important distinction between a generic closed low (such as the closed low centered over northern Arizona on the 500-mb chart above) and a generic cut-off low like the one of the Middle Atlantic States today.
Courtesy of, and copyright by, Penn State's online certificate program in weather forecasting.
No, the 500-mb low over northern Arizona today (revisit the 18Z RR 500-mb analysis) is not cut-off from the westerlies. But it is a closed low. That's as far as you can go there.
True cut-off lows like the one over the Middle Atlantic States today are typically quite lethargic. One thing's for sure...there's very low confidence in the details of the quantitative precipitation forecasts (QPF) associated with this cut-off low. When I was a young forecaster (a long time ago), one of our mantra's was "Upper-level low, weatherman's woes."
The 1455Z mosaic of composite reflectivity on Friday, October 11, 2013. Note the mesoscale bands of convectively enhanced rain (mostly in yellow). Larger image. Courtesy of Penn State.
Update: Indeed, it's impossible to predict, except for very short-range forecasts, where and when mesoscale bands of convectively enhanced rain will form. To see what I mean, check out the 1455Z mosaic of composite reflectivity (above; larger image) on Friday, October 11, and note the mesoscale bands of convectively enhanced rain (mostly in yellow).
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