The Feb. 12-13, 2014 snow-making machine: Under the hood

February 14, 2014

This massive and intense cyclone, known as “Snochi”, “Pax” and “The Love Storm”, unleashed it all: Severe thunderstorms, a southern ice storm, 15-20+” of heavy snow, and high winds. Herein we take a look at the atmospheric elements that made Snochi such a powerful and memorable Nor’easter.

A textbook, big East Coast snowstorm

When meteorologists take courses in atmospheric dynamics and synoptic meteorology, this is the very type of storm that is presented as an “archetype” for intensity, evolution and structure.

This cyclone’s technical designation is a “Miller Type A” developing as an energetic coastal low along the Gulf of Mexico, in conjunction with cold air damming along the East Coast.

Gulf and Atlantic moisture feeding the cyclone ensure plenty of raw material to manufacture heavy snow. A deep, cold air layer trapped against the Appalachians ensures that most of the precipitation falls frozen. And jet stream dynamics provide the sustained, vigorous uplift within thick cloud layers.

But there are regions within the snow-maker where snowfall becomes particularly focused. These sub-synoptic or mesoscale processes include stationary snow bands (as discussed by Wes Junker on 2/12) and pockets of elevated instability, which produce thundersnow. Snow rates of 2”-3”/hr within these localized, dynamically charged regions lead to large accumulation gradients, and the “boom” forecast scenario for some.

The rapidly changing thermal field is another mesoscale aspect. This was apparent in regions along and east of I-95, where a strong push of above-freezing air entered the system on Thursday morning. This hastened a change from snow to sleet to rain for many.

While it’s not possible to present all the details of this fascinating weather system, I’ll highlight a few of the more remarkable attributes in the sections below.

Jump starting the engine

An early indicator of this storm’s unusual vigor was an outbreak of intense thunderstorms over the eastern Gulf of Mexico and Florida, while the initial vortex was taking shape.

This is shown in the figure below. Notice the incredibly dense concentration of lightning strikes attending this squall line. A small cluster of severe weather including wind, hail and a tornado developed when the squall line swept across central Florida.

Figure 1. Intense band of Gulf thunderstorms in the developing warm sector of the Nor’easter. Lightning strike colors show the chronology of strikes (white = most recent, red = oldest).

For those of you who recall the March 13-15, 1993 “Storm of the Century”, a similar metamorphosis played out in the eastern Gulf of Mexico, including a violent squall line that crossed Florida.

The concentrated burst of convection injected a blast of energy into the developing storm. Think of the tremendous quantity of water vapor that must have condensed into rain; for every kilogram of vapor condensed, nearly a million calories of heat energy warms the cloud’s interior. I surmise that there must have been billions of kilograms of vapor processed by this squall line!

Big whirl: The storm’s satellite structure

A lot of condensational heating, and the generation of kinetic energy (created as warm air ascends and cold air sinks) spun up the engine, creating an intense vortex in the lower and middle atmosphere. The enormity of this system is most apparent from the vantage point of satellite.

Figure 2. False-color water vapor image of the Nor’easter, during its mature stage, 2 PM February 13. Key structural features are identified.

Shown in the figure is a very powerful Nor’easter with surface low pressure (990 mb) off the Delmarva Peninsula. The cyclone’s frontal wave structure consisted of a warm front and cold front bounding an expansive warm sector over the ocean. A line of thunderstorms developed in advance of the cold front, processing unstable, warm sector air.

Further north, an expansive precipitation shield of heavy snow spread northward and eastward along interior New England. This is the heavy band that earlier set up shop in the Mid Atlantic, dumping 15-20” along the Blue Ridge.

An extension of the flow deformation zone (discussed below) is evident along the comma head cloud structure. This zone terminates or “wraps up” around the upper-level vortex, which lies to the southwest of the surface low. During the afternoon, bands and cells of heavy snow, sleet and rain formed a wintry spiral around the upper low as it tracked across Virginia.

This patch of “end game” precipitation owed its vigor to unstable air (very cold air aloft overlying cooler air near the surface), producing lightning in the vicinity of Richmond. Additionally, the storm’s upper level vortex contained the most energetic region of spin (termed vorticity) – which, as it advances – induces air to rise just ahead of it. This snow was heavy and wet, with surface temperatures above freezing across much of the region.

Finally, the dry slot is quite prominent. At least one of the predictive models from the previous day indicated that Snochi would develop a robust dry slot, dividing the precipitation into two phases. This dry air current actually originates near the tropopause, descending miles to the ground in a wrap-around flow. Evaporation within the sinking, warming air clears away cloud layers. Orange colors in the image indicate extremely dry air at mid-upper levels.

Stoking the machine: Jet stream dynamics

There are dynamical processes at jet stream level that generate vigorous upward motion in a strong Nor’easter. Any process that removes air from the top of the vortex faster than it can accumulate at the bottom causes the storm’s central pressure to drop.

A classic “ventilation mechanism” became established on early Thursday morning. This is shown in Figure 3 (below). It involves mesoscale pockets of fast-moving air, embedded within the general jet stream flow. These pockets are called jet streaks. Two of these were located near the top of the intensifying vortex (labeled in the figure).

Figure 3. Classical dynamical meteorology at play in the February 12-13 snowmaker: Coupled jet streaks induced buku rising motion over the center of the storm (black circular region).

Jet streaks induce narrow columns of atmosphere to rise rapidly, like vertical chimneys. From dynamical meteorology, these chimneys are found in each jet streak’s “right entrance region” and “left exit region”. I have labeled these regions in the figure as “RER” and “LER”.

At times, the RER of a leading jet streak becomes juxtaposed with the LER of the following streak. Now we’ve got double the rising motion, in a spot parked right over the developing storm! Such coupled jet streaks are a recipe for rapid deepening of the surface vortex…and guarantee that heavy snow will develop in the storm’s cold sector.

One heck of a snow band

The warm sector of this storm remained mostly over open ocean, along the Gulf Stream. Intense thunderstorm cells fired repeatedly over the warm, humid and unstable air layer gliding off the Gulf Stream. Heat of condensation was continuously energizing the storm’s core.

But there is another “business end” in the classic, winter Nor’easter, contained on its cold side. Along the storm’s back edge, about 200 miles northwest of the center, lies the so-called deformation region. Wes Junker talked about this special place in his post on February 12.

The deformation zone is the snow-making “sweet spot”. Deformation means stretching and contraction of the airflow, brought about by air swirling around the primary vortex, and neighboring weather systems. Isotherms get squished together, tightening the thermal gradient on the back edge. This leads to vigorous ascent of warm air and sinking of cold air (recall that this process generates kinetic energy) along an intensifying frontal zone (the technical word is frontogenesis).

The result: The cyclone processes a tremendous amount of water vapor in this region, lifting it, where it quickly condenses and freezes into a swath of exceptionally heavy snow. One or more snowbands often become stationary, or nearly so, for several hours. The figure below illustrates the intense band that set up shop to the west and north of D.C. early Thursday morning. Its radar appearance (notice the red colors!) was truly ominous and correlated nicely with the primary snow “dump zone” northwest of D.C.

Thankfully, snow:liquid ratios were fairly high in the zone of heaviest snow accumulation. The snow was on the light and fluffy side, thanks to a very deep layer of subfreezing air (mid-20s) , and the fact that the most moisture-laden air remained along the coastline.

Figure 4. A radar snapshot from a summer thunderstorm? No; rather, an intense mesoscale snowband in the Nor’easter’s deformation zone. Inset shows the region of intense frontogenesis accompanying the deformation region (purple contours).

The models do a so-so job of predicting these mesocale bands. On Wednesday afternoon, several models predicted formation of a large band, but the position of the band varied tremendously from model to model. As stated by Wes Junker, the formation and positioning of the snow band is truly a “wild card” in the snow forecast.

So here you have the nuts and bolts of an exceptional snow storm, and a whole slew of technical terms to spout at your next power lunch or wine bar experience!

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