The most thought provoking question I ask when giving aviation weather talks is…if the winds are from the east which way is the surface low going? Most are perplexed as they know weather systems usually move from west to east in the U.S. It’s a loaded question because surface winds do not dictate where systems move, one must look to the higher levels of the atmosphere.
It’s winter and mother nature is at her fury when it comes to storms. The weather map is much busier: winds are stronger both at the surface and aloft, lows are deeper, highs are stronger, fronts are more defined and isobars abundant. But what makes a good mid-latitude storm?
Air masses do not sit still, but nudge each other in a tug of war fashion. This frontal undulation eventually gives way as cold air begins to move faster than the warm air forming a kink (wave) causing converging air at the surface. Forced to move upward, the ascending air creates falling surface pressure spawning a new born low. A barometer at home or the latest METARs will denote decreasing pressure values. But a low in its infancy needs support from above to create a strong mid-latitude cyclone. At the surface the converging air rises, but air aloft must diverge outward to enhance and complete the cycle. This rising air is docile in speed, about 1 to 10 cm/sec (2 to 20 feet/min), compared to the explosive thunderstorm updrafts clocked at over 10 m/sec (>2000 feet/min). As this air slowly rises, it expands, cools to condensation forming cloud and eventually precipitation and is why low pressure and troughs gives us weather.
The weather chart meteorologists seek for surface low pressure development is the 500 millibar chart as this is where half the atmosphere exists -18,000 feet. This level steers the surface features and foretells a lot more. During my forecasting days, which relied entirely on paper charts, we would scramble to score this upper air chart first. This sought after “tool of the trade” not only depicts wind direction and speed, but where upper lows, troughs, ridges, and highs lurk. As well, temperature advection and rotation of the air in the troposphere called vorticity is conveyed. PVA (Positive Vorticity Advection) meant divergence aloft thrilling any meteorologist seeking surface low pressure development. There is also the whereabouts of jet streams that enters the forecast equation.
I mentioned we needed divergence aloft from an outflowing high pressure system perched above. Truth be told, we meteorologists look for the whereabouts of the upper lows and troughs as they are the surface low manipulators. In essence, a surface low needs low pressure upwind (westward) at altitude. If you drew an axis through the low pressure centres from the surface upwards, it would be tilted to the west. If this axis stacks up vertically, the air will stop rising and the low will begin to fill – fade away.
If you observe all the stages of development of a storm: birth, maturity and then succumbing to a meteorological death, it would take about a week. The multiple stages of lows are difficult to grasp in a private pilot ground school. Because it’s just not temperature differences, but also vertical dynamics that equally contributes to cyclogenesis – the birth of a low.
Where in North America does cyclogenesis occur to produce such winter storms? Due to its downwind location of the Rockies, and hence upper air support from upper troughs, Alberta produces the Alberta Clipper that whisks across the Prairies picking up momentum as it moves east. If you watch the TV series, Deadliest Catch, you would see crab fishermen eking out a living, contending with intense Aleutian Lows. Downwind of the Rockies in the U.S., cold air couples with warm air from the Gulf of Mexico, spawning the birth of the Colorado Low. Cold Canadian Arctic air swoops southward and butts head with the warmth of the Gulf Stream off Hatteras, North Carolina. A Hatteras Low, a.k.a. “East Coast Deepener,” “Nor’easter” or “Atlantic bomb” rears its ugly head. These storms have been known to dump a meter of snow in the Maritimes. In 1982, a Valentine’s storm knocked over the oil rig Ocean Ranger, killing 84 men as it succumbed to 80 to 90 knot northeast winds. Even the Great Lakes can trigger a good a storm due to diabatic heating from the open waters.
Two winters ago, my flight didn’t budge on the tarmac as a low morphed from the heat of the Great Lakes inundating Toronto Pearson with relentless heavy snow. Wimpy lows in their infancy can quickly transpire into meteorological bullies. As they track eastward they intensify explaining why the East Coast of Canada sees so much weather. As an ex-forecaster, I miss the excitement of watching a low intensify and forecasting its destructive path. In northern American latitudes, we have all experienced a snow day or two. Most of us have seen how our busy lives come to a standstill as a winter storm turns everyday business upside down. In a bizarre way, it brings us together. Whether it be out shovelling and meeting your neighbours for the first time or stuck at an airport when the first flight out may be a day away. But when a winter storm stops the hustle and bustle of life, people adapt. It’s why a noticeable baby boom occurs nine months later.
Air converges at a low pressure system and diverges (spreads out) at a high pressure system.
To complete the vertical cycle of a surface low, diverging air must be present aloft.
A mid-latitude cyclone lasts from 7 to 10 days.
Sometimes storms move slightly westward called retrogression. I have seen the same storm hit Newfoundland twice.
Regarding fronts, Norwegian meteorologist Vilhelm Bjerknes in 1919 identified the region of meteorological conflict as a “battleground between two air masses.” In reference to World War 1, Bjerknes termed the area as a front.