Localized heavy snow can surprise forecasters
“Norlun trough.” No, this is not a diagnosis from your doctor after complaining about lower back pain. It’s a pair of words in the weather dictionary that even the avid weather watcher may not have ever heard before. Whether you are a weather enthusiast, or someone travelling anywhere in upstate or downstate New York, including New York City, Pennsylvania, Connecticut, Massachusetts and New England, you need to be aware of this feature. In fact, heavy snow could be a reality in the northeast as soon as Friday afternoon and Friday night!
Without getting overly technical, I will attempt to cut through a lot of the intellectual “weather-speak” and cut to the chase of what a Norlun trough can do and how it works. You see, the “Norlun trough” acts very much like lake effect snow because a narrow corridor of intense heavy snow occurs with this feature, while other areas get very little snow. It generates rapid snow fall rates of 2 to 6 inches per hour in a very narrow band. In addition, thundersnow is something else that can occur with both lake effect snow and snow produced by Norlun troughs. The name is quirky because it was named for two meteorologists who did research on the topic after 2 “surprise-type” snow storms dumped heavy snow in intense thin bands in New England. One occurred on March 21st in 1992 across Maine where 1 to 2 feet of snow occurred and caused some portions of New England to receive 4 to 6 inches of snow in 1 hour. The other storm occurred on February 19th in 1993 when Cape Cod received 30 inches of snow after only 3 inches was predicted. The 2 meteorologists are named Steve NOgueiRa and Weir LUNdstedt (Nor-Lun). If you really want to get into the “nuts and bolts” of their research click here. The problem with Norlun troughs is that not only can they be tough to predict like lake effect snow, but a small shift in the position of these bands will create massive different forecasts and therefore results for cities across the northeast (or anywhere else in the country).
A quick overview of their research may lose you but here is a very brief breakdown. Like lake effect snow, the “Norlun trough” is really an inverted trough that has upper level instability to concentrate intense snows. I may have already lost you. Basically, it is like a weather front that acts very much like lake effect snow because there is so much instability due to other atmospheric sources aloft where airplanes fly. The lake effect snow derives its instability because the Great Lakes are much warmer than the very cold air aloft flow in unison. The Norlun trough makes air rise dramatically into rapid instability due to vorticity advection which causes air to rise even more rapidly. There, I am done. I don’t want to completely lose my web viewers. If you want more information on this, there are great web articles here and here.
I have also provided some pictures with telestration on one computer model’s interpretation of how this system may hit sections of the northeast. According to this once computer model, sections of central and eastern New York state receive 5-10” of snow just by Saturday morning (Figure 2). The heavier snow east of Lake Ontario is partially due to lake effect snow. The surface Norlun trough can be seen on all 3 figures, but especially on Figure 1. The upper level instability is created by the vorticity shown on Figure 3. A secondary “Norlun trough” coupled with lake effect snow will reinvigorate this snow across much of central and eastern New York and New England Saturday and Saturday night.
Stay tuned for updates!
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