Marc Spilde, Forecaster & Shad Keene, Lead Forecaster
On November 25th and 26th a "bomb cyclone" moved into the area, bringing wide ranging impacts because of it's high winds and heavy snow. This low was unique because of it's track and it's depth. Our typical strong systems move from the southwest to the northeast, whereas this system tracked from the northwest to southeast.
It was also one of the deepest lows to ever impact the southern Oregon coastline. The lowest sea-level pressure measured was 971.2 mb (28.68 inches) at Buoy 46027 just off Point Saint George, CA. The storm set an all-time record for the lowest sea-level pressure observed in California at Crescent City with 973.6 mb (28.75 inches). Also, a new record for lowest sea level pressure for the month of November was set for the Medford Airport, 981.4 mb.
Impacts were severe. Wind gusts of 60 to 80 mph were common along the coast and at elevation just inland in SW Oregon and NW California, where widespread reports of downed trees and power lines caused power outages. A top instantaneous wind gust of 106 mph was measured at Cape Blanco Coast Guard station. It also brought 1-2 feet of snow to the higher passes along Interstate 5, which closed the road in both directions Monday night.
Marc Spilde, Forecaster — It sounds scary and sometimes it can be, but "bomb cyclone" simply refers to a storm that strengthens rapidly, with the barometric pressure falling at least 24 millibars (mb) or (0.71 inches of mercury) in less than 24 hours. These storm systems are often associated with strong winds and heavy precipitation and can cause significant weather hazards at sea and on land.
On Monday evening, November 25th, 2019, at its infancy (annotated as "1" in the image above), the low is only 1020 mb (30.12 inches). This GOES-West Air Mass RGB image shows a jet streak (denoted by the reddish colors) advancing quickly eastward through the north Pacific Ocean. The blue/purple colors to the north and west of the jet streak (near the Aleutians) indicate a cold air mass. The olive/green colors to the south indicate a warm air mass.
By early Tuesday morning, November 26th, 2019 ("2" in the image above), increasing temperature gradients in the tropo-sphere (reds very close to greens) and high-speed air associated with the jet stream aid in the formation of a baroclinic leaf (clouds aimed at Oregon), and, in this case, induce rapid cyclogenesis. At this point, the pressure is around 1000 mb (29.53 inches).
At maturity, by Tuesday evening ("3" in the image above), the low deepens into its classic "comma-head" shape and develops an eye-like feature as it undergoes warm seclusion just prior to moving onshore.
The lowest sea-level pressure measured for this storm was 971.2 mb (28.68 inches) at Buoy 46027 just off Point Saint George, CA. The pressure drop of more than 49 millibars in under 24 hours was more than double the necessary rate at which "bomb cyclones" are classified. The storm set an all-time record for the lowest sea-level pressure observed in California at Crescent City with 973.6 mb (28.75 inches).
A record lowest November sea level pressure was recorded at Medford with 981.4 mb (28.98 inches) and it was the second lowest pressure ever recorded at Medford (lowest was 978.0 mb or 28.88 inches set on January 20, 2010).