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Posted at 02:36 PM ET, 02/08/2013

Northeast blizzard: The science behind the storm


A Northeast storm that could rival the famed Blizzard of 1978 in New England is organizing.

This possibly historic blizzard is the result of interaction between packets of energy embedded within two streams of flow merging off the Mid-Atlantic coast — one from the north and the other from the south.

Related: Northeast hunkers down for major blizzard; Boston area possible snow bullseye

These “phasing streams” will lead to an explosively deepening area of low-pressure — a meteorological bomb — that will release vast quantities of snow from northern New Jersey across New England.

The jackpot for snow will be in New England, where a wide area of 18 to 24 inches of snow is likely, with localized amounts up to three feet. Add on top of that, winds gusting to near 60 miles per hour.

The two areas of flow bring key ingredients to the table for the prospects of a memorable storm.

The northern stream of flow, sweeping across the Ohio Valley, carries with it a wound-up packet of energy at high altitudes.

Meanwhile, the southern stream supplies deep moisture from an atmospheric river that extends to the tropics. Both the access to that moisture and an almost ideal configuration of high-altitude winds will help it become the dominant system, effectively drawing in the northern stream.

As the two streams of flow merge, the surface low-pressure area will rapidly strengthen — working synergistically from elements of the two streams to produce a monster storm.

The merge: in detail

Paul Kocin and Louis Uccellini in their two-volume book “Northeast Snowstorms” have documented that a number of major snowstorms featured interacting “jet streaks,” or packets of energy within the different streams.

In most historic Northeast snowstorms, these jet streaks interact to enhance the upper-level divergence. Said divergence reduces the mass at the top of a given column of air, promoting the upward motion needed to produce heavy precipitation. Think of sucking on a straw and how your beverage of choice rises up the straw into your mouth. The same general process happens with upper-level divergence.

By eliminating mass (air) at the top of the column, the pressure at ground level becomes less. As such, upper level divergence promotes the development of the low pressure.

The maps below do an excellent job of illustrating how two jet streaks in both streams of flow interact to maximize this divergence.


NAM model forecast of the upper level winds during the storm evolution. The strong winds (or jet streaks) are shaded in cyan and blue.

The two jet streaks, labeled 1 and 2, are embedded within the southern stream, while jet streaks 3 and 4 are associated with the northern stream.

The southern stream has two jet streaks aligned to maximize the upper-level divergence, or lifting, in the area circled in red.

Now look at the northern stream, which also has two jet streaks (labeled 3 and 4). Track jet streak 3 as it dives southward and notice how the dip in that northern stream sharpens with time while also changing its orientation.

By 7 p.m. Friday, the southern part of that northern stream dip (the dip or trough axis defined by the dashed line) is well east of the northern end. The jet stream is taking on “negative tilt.” Such an orientation maximizes the upper level divergence on its east side — square over New York and southern New England. At this time, the two different streams are starting to work in concert, leading to the rapid intensification of the storm along the coast.

By 7 a.m. Saturday morning, the two systems have merged.

The strengthening process in detail

As the streams of flow merge, the area of low pressure associated with the northern stream over the Ohio Valley (at the surface) weakens (or “fills”) and the southern low-pressure area near the East Coast rapidly deepens or “bombs out” (see below).


Evolution of the storm (low pressure centers) along with simulated precipitation at 6 hour intervals

The northern low weakens largely due to a strong high-pressure system over southern Canada (you can see it prominently in the 7 p.m. panel in the image above) packed with cold air. Because the cold air is denser, it’s hard to lift, making it more difficult for the northern low to survive

Meanwhile, the southern low on the coast rapidly strengthens as the northern stream trough takes on its negative tilt capturing and injecting energy into the southern stream system.

The southern stream low explosively deepens at the expense of the northern low for two other key reasons: the moisture associated with it and a growing temperature contrast along it.

The deep moisture feed

The map below shows the surge of moisture moving across the subtropics in association with the low (see animation). The total moisture with the system is over two standard deviations above normal. This is a moist system.


(NOAA adapted by CWG)

Remember how hurricanes derive their fuel for development from the warm waters of the tropical oceans and the deep convection (thunderstorms) and the resulting latent heat release . East Coast snowstorms also derive some of their energy from the release of latent heat. However, winter storms benefit from latent heat in a somewhat different manner than hurricanes.

Latent heat with a winter storm tends to build a ridge (or bump in the jet stream) out ahead of any upper level spin center (or packet of energy). The ridge development (see below — pay attention to the red dashed line) strengthens the upper level divergence which acts to deepen the low. This combination then increases the lifting which, in turn, results in more precipitation and latent heat release building the ridge even more.


The building ridge (at an altitude of about 18,000 feet or 500 mb) associated with the storm’s release of latent heat at different times.

The feedback between the spin center and ridge are important players in the development of the storm. The process is sometimes referred to as “self-development.”

A steepening temperature contrast

The interaction between the developing low pressure and the high pressure to its north acts to sharpen the temperature contrast in the storm environment further deepening the storm.

Note how the temperature lines near the New England coast squeeze together as the low deepens and closes in during the day today in the figure below. That squeezing together signifies that the storm’s front is strengthening and the slope of the front is steepening.


Pressures (black lines) and temperatures (colored lines) at an altitude of 5,000 feet at 7 a.m. today and 7 p.m. tonight

The steeper the slope of the front the faster air will be forced rise as it is lifted along the frontal surface. Such strong fronts usually are associated with banding precipitation with enhanced snowfall rates.

The moisture and temperature contrast together

For this storm, both the moisture transport and easterly component of the winds above the surface (symptomatic of the strengthening front) are highly anomalous. Diagnostics suggest they are more than six standard deviations from normal suggesting this will be a really rare event. Tremendous amounts of moisture are going to be transported and lifted along that frontal surface. That’s a great prescription for getting a major to historic snowstorm.

Conclusion

A number of factors are coming together to produce this mega-snowstorm/blizzard: lots of moisture accompanying a southern stream shortwave, upper level impulses in two streams of flow, a very favorable upper level wind pattern for cyclogenesis (strong deepening of the coastal low), a strong high north of New England to help strengthen the front along the coast, anomalously strong winds around the low to pump moisture into the region.

Wow, I only wish I was there.

By  |  02:36 PM ET, 02/08/2013

Categories:  Latest, Science, Winter Storms

 
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