What is a Norlun Trough?

While it was a rather chilly December day across much of the Northeast on December 5, 2018, most areas were basking in the sun without a hint of any snow.  That was a different story over parts of the New Jersey shore and extreme southeast PA.  Yes, locations between Philadelphia, Cape May, and Toms River had their own private snowstorm that day. In fact, Ocean County, New Jersey saw localized amounts upwards of half a foot of snow, with Brigantine the biggest winner with 7.5 inches!

 

 

So, how exactly did this happen? There definitely weren't any Nor'Easters in the forecast, nor was there anyone besides South Jersey who saw this snow. The answer is a fairly rare phenomenon called a NORLUN trough. These set-ups have been known to bring localized swaths of heavy snow to parts of the Northeast, not too different from how lake effect snow across the Great Lakes works. These troughs most commonly affect parts of Maine, New Hampshire, and Massachusetts, but have been known to occur as far south as New Jersey before. Typically with these systems, amounts upwards of 6"+ are commonplace but localized. 

 

 

This set of ingredients allows for narrower, slow-moving bands of heavy snow that can drop significant amounts at a time. This was the case on December 5th, with a developing low pressure off the mid-Atlantic coast (as is shown in the surface analysis above). Note the brown lines off New Jersey and how they push back toward the coastline, which depicts a surface pressure trough (marked out in an orange dashed line). The upper-level weather pattern from December 5th was also conducive to NORLUN trough development over New Jersey.  This archived model forecast from the NAM, depicted below, shows a closed upper-level low pressure near Pittsburgh (indicative of cold air aloft), with the darker orange/red colors over southern New Jersey indicative of a source of upward motion. 

 

 

                 (Surface analysis from the Weather Prediction Center valid 1:00 PM 12/5/2018)

 

 

             (NAM depiction of Upper-level Pattern Wed - Courtesey of Tropical Tidbits)

When you combine upper level and lower level dynamics, you end up with what occurred Wednesday across southern New Jersey. As we mentioned this is a fairly rare event, though it is not unprecedented.  The last big Norlun trough across NJ was back in January of 2011. During that year there were actually two events that occurred in a matter of two days! One trough left plow-able snow across parts of South Jersey, while another brought up to 10" to parts of Connecticut.