Humanity has always used its mobility to settle where conditions are best for its temporary needs, from setting up residence near a plush grove of mongongo trees to retiring in a tropical paradise. The reasons are often economic: Shippers want a good port; drillers want to be over oil; power companies want a fast-flowing river to dam.
Sometimes the desires of man are at odds with the operation of the environment:
- Los Angeles has a lovely climate, but it sits on the San Andreas fault; large earthquakes are certain to, as Jerry Lee Lewis would say, shake one’s soul and rattle one’s brains.
- Saint Martin in the Caribbean Sea has 80 degree water all around, all year. But a category five hurricane occasionally passes through.
- Boston sits at the edge of a natural, deep-water harbor. But fierce nor’easters with hurricane-force winds and heavy snow visit almost every year.
In fact, Boston, New York, Philadelphia — all of what is known as the northeast I-95 corridor — will experience a storm this weekend. The weather Channel has named this storm Iola. Whether Iola will justify having a name and bring a foot of snow to the big cities or just be mainly a heavy rain event is still a tough question for forecasters.
A Nor’easter Is One Of Nature’s Ways Of Balancing The Energy Requirements Of The Atmosphere
The geometry of the earth-sun system requires that the poles be continually cooled relative to the equatorial regions. An equilibrium has to be reached. Fortunately, the earth has an atmosphere, which can efficiently move heat resources around, so that the temperature of the planet varies within a range of about 150 degrees Fahrenheit — and this includes ultra-frozen Antarctica and blazing-hot deserts. Most of the earth is livable most of the time. Compare this with an atmosphere-less world such as Mercury, where the temperature range is over a thousand degrees.
The atmospheric mechanism for reducing the contrast of polar and equatorial temperatures involves a process in which side-by-side air masses of differing density are repositioned with the dense air under the lighter air.
Dense air is cold and light air is warm (see Ideal Gas Law); dense air next to light has high potential energy. A nor’easter whirls the air around and ends up with dense air below light, which is a lower potential energy state.
The Contrast Of Air Masses Is Greatest In The Middle Latitudes
With the poles continuously running a solar input deficit while the equator runs a surplus, it is no surprise that the battleground between cold, dense air and warm, light air is somewhere in the middle latitudes. Particularly in the winter, cold air from Canada invades the eastern US, while warm air from the Gulf of Mexico and the Atlantic Ocean nestles up against the polar air masses. The cuddling turns violent when a dip in the jet stream aligns with the contrasting air masses.
The Many Wavelengths Of The Jet Stream
The jet stream circles the globe in mid-latitudes and averages a west to east flow. But waves ripple through the jet stream and these are associated with low pressure areas at the surface. Sometimes different wavelengths in the atmosphere combine to form a large wave — just as ocean waves can combine to create one mega-wave — and produce a major storm.
In the case of Iola, the effect is actually reversed. A large amplitutde wave is coming across the south and up the Atlantic coast, but another wave is moving across Canada and sapping some of the strength of the main storm.
In addition, the Canadian wave is forcing high pressure to move east, and the flow of cold air is reduced. The result will be rain farther inland than is otherwise possible.
Rain/Snow Line Probably Stays West of I-95
The Atlantic Ocean is warm (relatively) with temperatures mostly in the 40s and 50s. This not only adds fuel to the atmospheric potential energy fire, but also pushes rain inland as the storm approaches.
The cold, dense air at the surface is hard to dislodge, but this time the warm air should make enough progress into the big cities to keep the precipitation there mainly rain.
Vertical Structure Of A Nor’easter
The initial configuration of air masses that produces a mid-latitude storm is cold air adjacent to warm. As a low pressure center develops, warm air pushes against cold on the forward side (a warm front), while cold air undercuts warm on the trailing side (a cold front). As the storm develops, the cold front wraps around until it overtakes the warm front and squeezes the warm air upward. The boundary where the two fronts meet is called an occluded front.
When a storm has occluded, the potential energy has been converted to kinetic energy (wind), which then dissipates by friction. The storm acquires a vertical strcture of symmetrical low pressure from the ground to the jet stream. A system that has attained a closed vertical structure moves vey slowly as the low pressure fills in. When a storm reaches that stage near the Atlantic coast, the snow can continue for days.
This storm will move rapidly and not reach its final stage until it is well into Canada; as a result, the period of precipitation along the I-95 corridor will be short.
Forecasters Hedge Their Bets
The freezing point of water is unyielding: 32 degrees. When the 32 degree isotherm (line of equal temperature) lines up with I-95, forecasters lose sleep trying to figure out exactly where it will snow. Cold air is stubborn, and sometimes the snow holds in longer than expected.
Decoded Science is confident that this storm will produce mostly rain in Washington and Philadelphia, and just a little snow in New York. Boston is more problematical. The water in the Gulf of Maine is in the upper 30s. It is possible that Boston will get a major snowstorm, and some freezing rain is also likely in a narrow band near the 32 degree isotherm.
Amplification Of Jet Stream Waves
The many wavelengths in the atmospheric jet stream can combine to amplify a storm or dampen it. If waves overlap, they combine their energy. In this case a weak wave in Canada will sap some of the strength of the coastal storm by being out of synch with it. The result will be not a complete collapse of Iola, but a slight deflaton on the order of that of a New England Patriots’ football.
The Aftermath Of This Coastal Storm
The major effect of Iola may be to set the stage for a more substantial snow event in the big cities on Sunday or Monday by leaving cold air in its wake. Two waves with associated low pressure centers at the surface (Alberta Clippers) are spinning southeastward out of Canada. One of these might amplify as it approaches the coast — and it will be cold enough to snow.