Bomb cyclones have made headlines this month

Last week, a massive Pacific storm, which was referred to as a “bomb cyclone,” battered the western regions of Washington, Oregon and Northern California. In fact, there were two bomb cyclones last week that left as many as 600,000 residents without power. The system last Friday was not as intense as the one from Nov. 19. From the Tuesday storm, winds were as high as 85 miles per hour in western Oregon and near 100 miles per hour off the coast of Vancouver Island in western Canada. The wind gusts were as high as hurricane-force winds. A Category 1 storm would have sustained winds of at least 74 miles per hour. Around a foot of rain was also reported in parts of Northern California near Sacramento resulting in area flooding. My relatives who are in Roseville, Calif., which is northeast of Sacramento, had over 3.6 inches of rain in one day.

Despite the severe conditions along the coastlines of British Columbia, Washington, Oregon and Northern California, the impact from those storms across the Inland Northwest were not as strong. However, the storm system late Friday and Saturday dropped near-record rainfall across North Idaho that took our November precipitation total in Coeur d’Alene to above-normal levels.

We have heard the term “bomb cyclone,” which is also referred to as “bombogenesis.” Other terms that have been used relating to a bomb cyclone include “weather bomb,” “meteorological bomb” and “explosive development.” According to articles from the Monthly Weather Review, other terms for this phenomenon which are “explosive cyclogenesis” and “meteorological bombs” were used by an MIT professor in the 1980s.

These massive storms will usually produce very heavy rainfall that results in flooding, damaging winds and blizzard conditions. A study from the Journal of Applied Meteorology and Climatology states that bomb cyclones are more frequent in the winter months, especially from December through early March. However, they can occur at any time during the year. The average number of these massive cyclones per year worldwide is 70.

Bombogenesis is a term used by meteorologists when a low-pressure system will strengthen rapidly within 24 hours. This process most often occurs in the mid-latitudes, between the tropical and polar regions with most of them having their greatest strength over the ocean. Low-pressure systems are also termed “cyclones.”

When a low-pressure system is forming or intensifying, air will rise and spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the Earth’s Coriolis force. As the air rises from a cyclone, the air pressure drops. If the air pressure drops at least 24 millibars, or .71 inches of sea-level barometric pressure within 24 hours, then the storm has undergone bombogenesis, which will be later termed a “bomb cyclone.”

According to a recent article by Accuweather, the average reading of a low barometric pressure reading during the winter season is 29.53 inches. When the barometer drops below 29.00 inches, then the storm will likely be intense. Last Tuesday, the massive storm over the Northwest had a sea-level pressure of 27.82 inches (942 millibars) off the coast of Washington. This is likely the lowest reading ever recorded in this area.

By the way, the lowest sea-level pressure ever recorded occurred Oct. 12, 1979, near Guam. Typhoon Tip had a pressure of 25.69 inches (870 millibars). Although this is the official record, scientists believe that pressures within a tornado, especially an EF4 or EF5 are even lower.

Meteorologists and climatologists point to the Superstorm of 1993 as one of the best examples of a cyclone (low-pressure system) that underwent bombogenesis. Also known as the “Storm of the Century,” the March 12-13 system strengthened from a barometric pressure reading of 29.41 inches down to 28.45 inches in about 24 hours. This storm was a “Nor’easter” that formed in the Gulf of Mexico and then moved through the eastern United States. Winds gusted to 100 miles per hour along with heavy snowfalls across parts of the East Coast. Over $5.5 billion in damage was reported in 1993, well over $10 billion in today’s dollars.

In terms of our local weather, we may see some scattered snow showers over the Thanksgiving holiday weekend. The recent storm that dropped 0.98 inches in Coeur d’Alene late last week put us over the November normal of 3.07 inches. At the Spokane International Airport, over 3.75 inches of moisture has fallen. Their average November precipitation is about 2 inches.

There is a chance of some rain or snow shower activity toward the end of next week. The following week may bring some snow to the lower elevations as colder air moves in. Stay tuned.

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Contact Randy Mann at randy@longrangeweather.com.