It was a cold and quiet day across Long Island. But just to our south, heavy snow fell across southern New Jersey along what forecasters call a Norlun Trough.
What is a NorlunTrough you ask? Well first, a trough is a weak disturbance in the atmosphere – in this case, near the surface – marked by lower surface barometric pressure that results from converging wind direction and speed. Surface convergence of wind results in rising air, and rising air produces clouds and precipitation. Because they are near the surface, troughs can often cause locally heavier bands of precipitation.
Now a Norlun Trough is a type of inverted trough, meaning that it extends northwest from a surface low. It was noted by operational forecasters Steve Nogueira and Weir Lundstedt (Nor-Lun) that when troughs are inverted, and are also beneath a middle or upper level pool of cold air, localized bands of heavy snow could result. On occasion, these intense, narrow bands can produce in excess of one foot of snow, and are largely missed by computer guidance.
While our computer weather models have improved significantly since Steve and Weir’s initial study over 25 years ago, the placement and intensity of these complex troughs continues to be a challenge. While this morning’s model runs indicated 1-3″, but some spots picked up 3-6″! While Long Island has seen many of these in the past, fortunately today’s occurrence just missed. –Geoff