From your boy Matt Noyes
First, what is a "Norlun Instability Trough?" 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 air results in rising air (afterall, air converging at the surface must go either up or down, and clearly can't go into the ground), and rising air produces clouds and precipitation. Hence, troughs can focus locally heavier bands of precipitation. It was noted by operational forecasters Steve NOgueiRa and Weir LUNdstedt (Nor-Lun) that these troughs, when inverted (extending northwest from a surface low) and beneath a middle and upper level atmospheric cold pool, could result in very strong, localized bands of precipitation. On occasion, these narrow but intense bands of snow could produce in excess of one foot of snow, and largely were missed by computer guidance. Mr. Norgueira and Lundstedt both worked to find methods of forecasting these events, even when computer modeling missed them. Since their initial study in 1992, computer modeling has improved significantly, and these events often are projected in advance, though placement and intensity continues to be a challenge, especially in more complex setups.