The following is a post that appears on my Weather Analysis Page.

Many folks have asked me what a "Norlun Trough" is, and if I can explain more about this phenomenon.  Yesterday, I linked to a previous post in which I'd examined the Norlun Trough, but I'd love to take a moment to expand upon what's coming together in this particular circumstance.

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.

Where does the "mid-level" and "upper-level" instability come from?  Instability in the atmosphere results, most often, from cold air above warm.  Warm air rises, so warmer air parcels beneath a colder air layer become buoyant, or "unstable," meaning they exhibit a tendency to rise.  Cold air aloft accompanies storms in the upper levels of the atmosphere, and the cold pool with this particular event can be seen in this projected temperature forecast at the 500 mb pressure level (approximately 22,000 feet in altitude).  Note temperatures as cold as -38 Celsius at the core of the cold pool.  As this cold pool drifts east across the Northeast United States, this will increase the "instability" by cooling the layer above relatively warm near-surface air.

How do the instability and surface trough interact?  Putting the upper level and near-surface philosophy together, we generate a scenario in which the surface trough of converging air results in rising air, and because of the cold pool aloft, this rising air is unstable, meaning it will continue rising, and at a swift rate.  This results in a corridor of fast upward vertical motion along the trough, meaning healthy cloud development and therefore enhanced precipitation.

Where do Norlun Troughs usually set up?  Inverted troughs with instability most often setup along the coast of Maine - especially the Mid-Coast of Maine.  The farther southwest from that location one is, the less likely a significant Norlun Trough event usually is, unless topography (terrain) helps to enhance precipitation rates, as hills/mountains can assist in forcing air upward.  For instance, inverted troughs draped across Southeast New York State often can produce little fanfare in New York City, but several inches in the Catskills of New York.

Why is Norlun Trough position sometimes difficult to forecast?  The advancement of atmospheric modeling technology in the 2000s has vastly improved the ability to forecast location of Norlun Troughs.  There are a few issues that still are significant obstacles to the operational forecaster, however, most noteably total precipitation amount (QPF forecast) and placement/development of Norlun troughs when more than one inverted trough is involved.

In this instance, two inverted troughs develop - one in Northern NJ that drifts into Connecticut Thursday night to Friday evening, and another off the Eastern MA coast that extends into the Mid-Coast of Maine Friday night into Saturday (see surface pressure/temperature/wind forecast below, and see if you can find the well defined trough near CT, and developing trough off the coast Saturday morning).  Even highest-resolution guidance, which is the strongest tool to predict localized events, are strikingly varied in their solutions, from a very heavy snow event in Central New Jersey, to a weak event at best in Connecticut.  Interestingly, there is excellent agreement on the most significant event occurring right where it should - the coast of Maine, with the second trough development.

I'm skeptical of forecasts for a substantial event in New York City to New Jersey, simply because we so often see such forecasts end up incorrect, with only the Catskills and areas northwest of Poughkeepsie, NY, seeing substantial accumulation in New York, though the hills of Connecticut can often enhance snowfall amounts, as well.  One wildcard is if a small-scale "mesolow" storm center can develop along the inverted trough near Long Island Sound, which would increase Connecticut snowfall, and is the reason I've highlighted Connecticut as one area of locally maximized precipitation, particularly for New Haven, Middlesex and New London Counties, northwest to Hartford County.

The more significant event along the coast of Maine with the second trough would occur Friday night through Saturday, focused just northeast of Casco Bay.  In the transition from one event to the other, the upper level cold pool mentioned earlier, will drift over New England, and this will mean increased instability, resulting in snowbursts migrating from southwest to northeast Friday and Friday night, producing varied snowfall amounts from a dusting to three inches.  Amounts should gradually increase beyond those values through far Northeast MA, Southeast NH and Maine, until reaching the axis of heaviest precipitation, which could be several inches.  Keep in mind that part of the reasoning for a broader accumulation is the presence of a northeast wind Friday into Friday night for many areas, which blows off the ocean and should provide at least some snow for many areas, regardless of Norlun Trough placement.

Hopefully this helps to shed some light on the Norlun Trough, as well as this forecast scenario in particular.

Can't get enough?  Read the original research paper from National Weather Service forecaster Weir Lundstedt by clicking here. Download NORLUN

-Matt