Climatology of the NORLUN Instability Trough: Initial Development and Use in Improving New England Snowstorm Forecasts
Gregory A. Zielinski, Daniel K. Cobb and Hendricus J. Lulofs (National Weather Service, Caribou, Maine)
Almost every winter in New England, there is at least one occurrence of a snowstorm that was not forecasted or an “under forecasted” snowstorm that produced snowfall totals above 10 inches when the original forecast was only for 1-2 inches. In such cases, NWS warnings are often issued in the middle of the event, when heavy snow is already occurring, which limits the effectiveness of the warning. The most common cause for these problematic forecasts, particularly from the perspective of the general public, is the presence of a NORLUN instability trough (NORLUN), a trough that has the ability to focus and lift moisture to form intense and persistent snow squalls (Lundstedt, 1993). Identification of the trough as the driving force in such scenarios occurred in the early 1990s following two separate snowstorms that hit the Portland, ME, area (21 March 1992) and Cape Cod (19 February 1993) with 3-4 inch snowfall rates and total accumulations of 20-30 inches. Unfortunately, numerical models do not appear to predict consistently development of the NORLUN (QPF) nor do they appear to predict reliably snowfall amounts (QPE). Consequently, we are in the process of developing a detailed climatology of the NORLUN as a means to identify the processes involved in NORLUN formation and the parameters essential for recognizing, and thus predicting the presence and impact of the trough. An ultimate goal is the construction of a conceptual model, which would facilitate consistent prediction and provide adequate warning lead times for such events.
In addition to a re-evaluation of the storms originally assessed by Lundstedt, we initially evaluated the characteristics of two NORLUN events during the 2001-2002 winter (21 December 2001 and 16 January 2002). From these analyses we suggest that there are at least two NORLUN types. A Type I NORLUN appears to occur most frequently as a 25 to 50 mile wide band of heavy snowfall amounts with the axis of heaviest snowfall oriented NW-SE extending from the coast into the hills of interior New England. Synoptically, this type appears to be characterized by a weakening surface low moving into the St Lawrence River valley with a developing ocean storm too far to the east to affect New England directly. This situation is often associated with onshore flow of potentially buoyant air from 500 to 1500 meters above the surface ahead of a trough axis that connects the two low centers. A Type II NORLUN produces a snowfall axis that tends to mirror coastal topography in areas south of Mid-Coast Maine. In this case, there is only one low (i.e., an ocean storm passing too far to the east to directly affect New England) with a significant inverted trough extending to the northwest of the developing low center. The inverted trough appears to be the result of an advancing shortwave aloft with heavy snow along and ahead of the trough axis. However, the orientation of the snow bands tends to be parallel to the coastline and not necessarily parallel to the trough indicating that a secondary mesoscale response, such as coastal convergence induced by frictional differences between sea and land, may be more directly responsible for the maintenance of the 2 to 4 inch per hour snow rates. Consequently, geography of the coastline may be critical to development of a Type II event. We expect that these characteristics, as well as others discovered in the evaluation of additional NORLUN events, will be components of our conceptual model for the NORLUN instability trough.
WHAT IS AN INVERTED TROUGH?
METEOROLOGIST JEFF HABY
In the mid-latitudes of the Northern Hemisphere a trough is usually seen as a southerly bulge in the height contours. The lowest heights are generally located to the north of the trough. In an inverted trough situation, the height contours bulge to the north. This is more common in the tropical regions where regions of low pressure ride south of a mid-latitude high pressure but can happen in the mid-latitudes when low pressure is south of high pressure. An inverted trough bulges to the north. At first it may look like a ridge, but on further inspection it is a trough. Both a trough and an inverted trough have a cyclonic (counterclockwise) flow pattern. A trough will tend to have more westerly winds associated with it while an inverted trough will tend to have more easterly winds associated with it. If an inverted trough is actually a ridge then the winds will be flowing with the height contours in the opposite direction (anti-cyclonic direction). The direction of windflow through the feature is how a ridge is discerned from an inverted trough.
Tropical waves will show up as inverted troughs because they are generally south of mid-latitude high pressure and have an easterly wind associated with them. The image below shows how a typical trough in the mid-latitudes looks and that of the inverted trough.
