Sunday, December 04, 2011

Two Recent Comments

Chuck Doswell commented: These are not "cut-off" lows - they're closed lows embedded in the southern branch of the split flow. Cut-offs are not embedded in any flow and so move slowly. All cut-offs are closed lows, but not all closed lows are cut-offs. I like to call them "bowling balls" ...

It's somewhat ironic that I get caught using inaccurate terminology, since I sometimes rant about meteorological gibberish I come across in NWS Forecast Discussions. So it goes. Chuck is, of course, right, and I've been way too loose in using the term "Cutoff."

Above 500 mb chart shows an example of a cutoff low off the California coast. Whereas, the chart below
shows a 500 mb closed low within the southern branch of the flow, which splits over Montana.

Stephen B. commented on your post "Big Changes Have Arrived":  The models depicted the arrival of the system within +/- 12 hours. However, the models were off on strength of backside jet, timing of frontal passage in CA and AZ, and depth of the cold pocket associated with it. This made for some highly uncertain forecasts even 24-48 hours out.

I would just add the following: The models have improved dramatically during the past decade, and I am frequently amazed at how well large-scale evolution can be predicted. But, as Stephen says, the model forecasts for significant areas can be lacking wrt the details, even at quite short time frames. It's of course the details that lead to important, sensible weather - like strong winds or heavy precipitation. The modeling centers tend to emphasize how forecast model verification statistics have improved for the 500 mb level - but very few people live at 500 mb.

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