Sunday, November 4, 2012

November 5, 2012: Strong Nor’easter Expected in Wednesday-Thursday Timeframe

In the wake of Superstorm Sandy, the prospect of another major East Coast storm is most unwelcome.   Yet, these intense coastal low pressure systems, called Nor’easters, are a staple of our region’s cool season.  Before we get into specifics about the storm expected here mid-week, let’s review a bit about Nor’easters.
What Is A Nor’Easter?
Like hurricanes, Nor’easters are powerful cyclonic vortices, producing high wind, damaging ocean waves and heavy precipitation.   Aside from similar sensible weather and oceanic impacts, there are big differences between these two types of storm.  I’ll point out these differences below.  Also referred to as coastal lows, Nor'easters can produce hurricane-category wind and water damage along the immediate coastline.
Nor’easters get their name from the direction of strong winds, blowing from the northeast, inward toward the core of low pressure.   Since these storms track along the coastline, the northeasterly winds sweep inland, pushing large amounts of water against the shoreline.   It’s also true that these storms track towards the northeast.  This odd opposition between wind direction and storm movement was first explicated by Ben Franklin in the 1700’s.
How and When Do Nor’easters Form?
Nor’easters are a cool-season phenomenon, impacting the Mid Atlantic and New England coastal regions between October-April, with a peak in the December-March timeframe.   During the coldest months of winter, these are the big “snowmaker” weather systems that occasionally bury East Coast cities under one or more feet of snow.  
Our region experiences a few of these coastal storms each year, but some years are more active than others.   For instance, the winter of 2011-2012 experienced very few of these storms, while the previous winter (Snowmagaddon) was visited by numerous, crippling Nor'easters.   A typical year may average a half dozen or so in our region;  in any given year, we experience more Nor’easter than tropical-type systems in the Mid Atlantic.
Nor’easters develop during the cool season because they require strong contrasts in air mass temperature to form and intensify.  This occurs when cold, Canadian air masses sweep over the relatively warm waters of the western Atlantic. The East Coast, particularly along the warm oceanic Gulf Stream off Cape Hatteras, is a preferred breeding ground.
Nor’easters are also creatures of the jet stream.  Their formation requires an intense trough of low pressure in the upper atmosphere.  The most intense Nor’easters develop when two separate troughs or “waves” collide and merge (phase) along the coastline.  Jet stream troughs are most numerous and intense during the winter months.
What Are The Differences Between Hurricanes and Nor’easters?
Recent hybrid-type storms, such as Superstorm Sandy, have blurred the distinction somewhat – but Sandy was highly atypical.   Hurricanes develop and intensify in the deep tropics during late summer and fall, while Nor’easters develop in the middle and high latitudes during late fall, winter and early spring. 
Nor’easters draw their energy principally from jet streams and fronts, while hurricanes are fueled by rings of intense tropical thunderstorms.  These thunderstorms are sustained by ocean waters that exceed 80 F.   However, note that the Gulf Stream's warm waters boost the energy level of a Nor’easter by about 20%-30%.
How Is A Nor’easter’s Impact Measured?
Many are familiar with the Safir-Simpson hurricane intensity scale, which rates hurricanes according to five categories of wind speed.   Nor’easters also have a rating system, called the Dolan-Davis scale, developed at the University of Virginia.  The scale ranks Nor’easters according to five categories of coastal damage, including storm surge and beach erosion.   If a Nor’easter produces heavy snowfall, the impact is assessed separately by the five-category NESIS (Northeastern Snowfall Impact Scale).
What Are The Weather Impacts of Nor’easters?
In coastal areas, a combination of high seas, freshwater flooding and strong winds can be as devastating as a single hurricane.  What’s worse, if several Nor’easters visit a region in a given season, their effects are cumulative.
The magnitude of impacts, particularly storm surge and winds, depends on several factors, including (1) storm intensity (measured by minimum central pressure), (2) pressure gradient, (3) speed of movement, and (4) track.   All of these factors are highly variable from storm to storm.   In terms of storm surge and wave inundation, the phase of the lunar tidal cycle is also critical.  Since these storms tend to be larger in size than hurricanes, their effects often linger across multiple high-tide cycles.
Strong high pressure to the north of the Nor’easter – often situated over Newfoundland and Greenland – plays an important role in creating damage.  For one, high pressure blocks the northeastward progress of the storm, slowing it down or stalling it offshore.  This prolongs the action of wind, rain and high seas.  Secondly, high pressure intensifies the pressure gradient along the north along the north side, increasing the strength of onshore winds.  Some of the most infamous and devastating Nor’easters – including the 1962 Ash Wednesday storm, 1950 Great Appalachian Storm, 1991 Perfect Storm, and even the 2012 Superstorm, all featured a strong, blocking high to the north.
So What’s Expected With This Week’s Nor’Easter?
The various forecast models have not yet come into agreement regarding intensity and track, as is typical for the 3-5 day time range.
- For the Baltimore region, in short, the period from Wednesday to Thursday will be raw, but nothing nearly as intense as Superstorm Sandy.
- Those along the coast will experience a 24-36 hour period of strong winds (gusts to perhaps 40-50 mph), several inches of rain and battering waves.
- Those of us lying inland (including Baltimore and D.C.) can expect 12-18 hours of moderate to heavy rain and gusty winds (perhaps up to 40 mph, approaching Wind Advisory criteria).  And it will be chilly – temps will probably not break 40 F.
- For the mountains to our west and north, there will be 12-18 hours of precipitation, which may include a period of heavy, wet snowfall…and gusty winds (to 40 mph).

- Factors that argue AGAINST measurable snowfall over Baltimore:  (1) the ocean waters are warm,  and the storm will draw significant amounts of warm air off the ocean, warming the lower layers of the atmosphere to temperatures above freezing;  (2) there is not a major source of deep, cold air inland associated with high pressure to our north.
In all, these are NOT hurricane-force impacts.   The Nor'easter's effects will be more of an inconvenience (likely including some local travel disruptions) but not a major, multi-day disruption.
Unfortunately, our concern and attention should shift further to the north and east, along the Jersey and New York coastlines, where much more significant impacts will be felt.

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