Unique beasts known for punishing weather, nor’easters serve an
important purpose for our globe despite their infamous reputation.
A type of extra-tropical cyclone (ETC), nor’easters get their
colloquial name from unique localized characteristics, mainly the coastal
northeast flow that occurs before the onset of the storm, which often portends
snowfall, coastal high surf, and high winds.
The factors influencing the paths of these beautiful and destructive
systems are similar. Serving a critical purpose, ETCs and nor’easters
redistribute heat energy from the tropics to the poles.
The earth, engineered to seek equilibrium, uses ETCs as a
synoptic-scale (massive) temperature and moisture regulating mechanism.
Irregular heating and our spinning sphere prevent total equilibrium, yet the
earth accomplishes its need to redistribute energy.
1: General circulation of air from equator to poles and poles to equator.
COMET® Program graphic.
As seen in Figure 1, heat rises from the equator and lifts north. The
airmass, as it reaches the poles, cools and then sinks to the surface, heading
south to start the process over again. As heat rises and lifts from the equator,
and the other air masses cool, descend, and travel south, they eventually meet
in the mid-latitudes.
At 44.17°N, Mount Washington’s latitude is less than 1° away from the
exact middle point between the North Pole and equator. The position of the
Presidential Range relative to the Atlantic Ocean, which is a considerable
distance, and the elevation of the peaks, put the White Mountain summits in
prime position to experience some intense winds and snowfall rates from ETCs.
When combined with the orographic (how mountains alter weather), wedge
set-up of the Presidential Range, ETCs and nor’easters have produced some of
the most extreme weather events experienced on Mount Washington’s summit.
Extreme wind events in Mount Washington Observatory’s (MWO) history
that were caused by the passage of an ETC and the development of a secondary
area of low pressure, which is characteristic of a nor’easter, include the fastest wind speed ever directly observed by
people on April 12, 1934 at 231 mph.
During a recent wind event on February 25, 2019, observers recorded a
gust of 171 mph, caused by a set-up similar to the 1934 World Record Wind.
While the secondary low is a characteristic phenomenon of a nor’easter, the
secondary development usually merges and overtakes the primary low or forms as
a single entity in the Mid-Atlantic and moves northeast along the coast.
were glued to the Hays Chart on February 25, 2019
It is important to note that the most extreme wind events experienced
on Mount Washington were not caused exclusively by nor’easters. They exhibited
many nor’easter features, but in these cases, a deep upper-level wave, commonly
cut off from the polar vortex, was intense enough to cause an anomalously deep
wave in tropopause pressure.
In early March 2021, such a system brought high winds gusting at 147
mph to the summit along with some of the coldest temperatures of winter 2021.
The temperature dropped to –28°F. With sustained 130 mph winds, wind chills
plunged to 80°F below zero. Meanwhile in the valley, power and heat outages as
well as significant damage occurred, including a downed tree in the Observatory’s
North Conway office parking lot.
ETCs, also referred to as mid-latitude or wave cyclones, develop as air
masses mix in an attempt to dynamically bring about equilibrium of temperature,
moisture, and pressure. When the two air masses meet, fluids are deflected to
the right (or left in the southern hemisphere) due to the Coriolis effect. This
creates counter-clockwise rotation around an area of low pressure.
ETCs are a type of cyclone, which are synoptic-scale low-pressure
systems that occur in the mid-latitudes, generally between 30°N and 60°N. They
are responsible for a majority of the inclement weather across the globe,
especially along the boundary between an eastern continental landmass and a
western coast of an ocean.
All nor’easters are ETCs, but not all ETCs are Nor’easters. In the
U.S., ETCs tend to affect the northern half of the country as Alberta Clippers
and commonly make their way to New England. Nor’easters affect the eastern
coastline with particular focus on the Mid-Atlantic and New England. Sometimes,
there is a combination of the two, and even cases where systems merge. The
previously mentioned record wind gusts measured by MWO all resulted from
merging systems. Strong and deep tropospheric waves affected the stratosphere,
destabilizing the jet stream and allowing the polar vortex to destabilize and
These systems are not to be confused with tropical cyclones
(hurricanes). Extratropical and tropical systems can and sometimes do look
strikingly similar on satellite, but differ in some very distinct ways.
Arguably the most visible and notable contrast between the two is the
comma-shape extension of an ETC that commonly extends to the south along the
cold front of the system. ETCs also lack a closed eyewall typically observed in
Dynamics wise, the development and evolution of ETCs involve strong
temperature and moisture gradients between air masses, known as baroclinic
zones, which is why these systems are also called baroclinic cyclones. As a
mid-to-upper-level wave approaches such a zone, the two air masses begin to mix
and the process of cyclogenesis ensues. This is in contrast to tropical
cyclones, which are more vertically surface-based, non-frontal, and develop
from convection over warm ocean waters in low horizontal wind shear (gradient)
Every ETC is an individual. Despite each storm’s unique features, they
do have many similarities, generally forming along boundaries of differential
air masses where temperature and moisture gradients occur with significant
vertical wind differences (shear). Cyclogenesis occurs along baroclinic zones
near an area in the jet stream where winds are the highest. Known as jet
streaks, these areas happen in the atmosphere’s lower and upper levels.
Lower-level jets tend to pass at elevations around the summit of Mount
Washington, assigning the Observatory an important responsibility of measuring
jet streak velocities.
As the cyclone progresses, the cold front rotates counterclockwise and
moves around the back of the cyclone with denser, cooler, and drier air.
Meanwhile, the associated warm front progresses more slowly. The warm front’s
air mass has to fight gravity as it lifts and mixes into a cooler air mass
ahead of the system. As the cold front sweeps around, the denser air undercuts
the less dense, warmer, and more humid air, forcing air aloft as well. Later,
when the cold front meets and mixes with the warm front, the cyclone begins to
Occlusion is when the cold air mass overtakes the warm front and
becomes cut off from the center of the low by being blocked off by the cold
air. Colder air begins to fill the air column, replacing the warm, humid air
which causes the system to weaken. Cold air in the column prevents lift and
decreases the temperature gradient enough for the cyclone to become
barotropically cold. The system becomes stacked and collapses on itself until
it dissipates along with the frontal systems associated with the ETC.
Atmospheric pressure can fall very rapidly when there are strong
upper-level forces on the system or there is extreme latent heat release as a
system moves from a dry continental air mass to a moist oceanic air mass.
When the pressure falls faster than 1 MB (0.030 inHG) per hour, the process
is called explosive cyclogenesis or bombogenesis, and these tend to be the
nor’easters well known in the Northeast.
Having discussed the life cycle of an ETC, what makes nor’easters
different from other ETCs? The difference lies in the track plus the heightened
potential of a nor’easters to undergo explosive cyclogenesis due to the
geographic set-up of North America. Commonly, areas of low pressure form on the
lee side of the Rocky Mountains as an upper-level wave feature crosses over the
range, then meets the warm, humid air lifting north from the Gulf of Mexico.
The upper-level troughs in the jet stream tend to dip farther south
than the wave features that form Alberta Clippers. The Gulf of Mexico is a very
warm body of water that helps feed the Gulf Stream, which moves northeast along
the eastern seaboard. Some of these areas of low pressure that come off of the
Rockies deepen as convection kicks off, and heat energy is absorbed by what
could develop into a nor’easter. As the beginning of the system moves over the
Appalachians around the Mid-Atlantic states, it becomes compressed and spreads
out to deepen again on the Appalachians’ lee side.
Having said that, an interesting feature also tends to occur on the
windward side, and this can be unique to North America, similar to the
injection of warm, humid air from the gulf. The cold air descending from the
north gets wedged between the Appalachians and the East Coast in a process
called cold air damming. Ultimately, the damming can enhance baroclinicity in
the lower levels and often form a secondary area of surface low pressure
separate from the initial trough and wave feature.
This newly developed center of surface low pressure begins to rotate as
warm air is pushed east and absorbs moisture from the warm Atlantic waters.
Because of the heightened baroclinicity and the amount of potential energy that
warm surface waters of the coastal Atlantic store and can release, explosive or
rapid intensification occurs.
With the arrival of another winter in the White Mountains, Mount Washington
will undoubtedly be treated with many ETCs and hopefully some more high winds,
cold temperatures, and snow from nor’easters.