What’s up with that “bomb cyclone”? Plus a very mild first half of January

Published: Jan. 5, 2023 at 10:16 PM PST
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The last two days you’ve probably been hearing about the “bomb cyclone” off the California and Oregon coastline. So what’s the deal with it? Well, for one, it’s already dead. These storms come and go very quickly. But what is a bomb cyclone? This graphic explains it quite well.


The Pacific weather systems that regularly move across the region during the cool season typically contain an area of low surface pressure. These low pressure centers are also sometimes called “cyclones”. In this case, the low pressure center is over the mid-latitudes so we call those “extra-tropical lows” since they are “outside” of the tropics. As you know, these systems typically give us a round of rain and at least some wind.

A bomb cyclone is a surface low pressure center that deepens much more rapidly than a typical low. Specifically, the definition is generally agreed to be a 24 millibar pressure drop within 24 hours. For example a low pressure center that’s 995 millibars right now would need to drop to 971 mb. or lower within the next 24 hours to be classified a bomb cyclone. Since the pressure drops so quickly and (usually) quite low, the wind circling in towards these storms can get VERY strong. If the storm is moving rapidly eastward, northeast, or north, the damage can be extreme if they move near a coastline. Most of our major windstorms are bomb cyclones. This satellite image is from November 2019 showing one such storm. It followed an unusual path ESE toward the Oregon/California border, producing strong wind in that area.


One question I’ve heard...

Are bomb cyclones a new thing? Definitely not

No, they’ve always been around. Meteorologists have been regularly using this naming for at least 40 years. Other similar/related names: bombogenesis, a “bombing out” storm, meteorological bomb, etc... I remember in the 1990s saying a low was “bombing out” when talking with other meteorologists.

I can tell you I NEVER used the term “bomb” in any way in public forecasts until maybe 10 years ago!

It always seemed reasonable to me that I shouldn’t refer to a “bomb” in any sense on-air or in public. I figured it would be like yelling FIRE during a weathercast! But, somewhere between 2010-2015, someone in the media along the East Coast decided it would be okay to slip some meteorological lingo into their weather story. Some journalist somehow ran across the term, maybe in a NWS technical discussion? Why not throw in some “geopotential height” or a “tropopause fold”? That’s why I’m rolling my eyes as I write this. Somehow it just became acceptable, and possibly part of weather hype. I remember the good old days (10 years ago), when we just called it a “powerful storm”.

THAT is how the term came into common use. Some strange stuff...

Yesterday’s storm

The storm offshore bottomed out at around 955 millibars yesterday, but it died as it moved up the coastline today. We only saw a brief surge of southerly wind this afternoon in the valley. The big story was the strong easterly wind yesterday and last night. This was a “downslope” wind, not just a gap wind through the Gorge. Peak gusts made it to 50 mph in a few Cascade foothill communities like Yacolt, Hockinson, Estacada, & Sandy. Models did pretty well showing wind a bit outside of the “usual” east wind areas.


The 51 mph peak gust was the strongest there since the Labor Day windstorm in 2020. Of course that’s the event that blew up those fires on the west slopes of the Cascades, destroying hundreds of homes in one day. The wind gusted to 52 mph at PDX that day.



January is here and the we are now in a mild and wet weather pattern until further notice. The cold systems coming down from the Gulf of Alaska 2-3 weeks ago are history. Now we have a series of mild/wet systems approaching from the west and southwest for at least another week. Take a look at the next 10 days (rain and surface pressure) from the ECMWF model. Most of the time the main energy and jet stream will be aimed at southern Oregon and California. That leaves NW Oregon and SW Washington with regular periods of rain, but generally not heavy. Low pressure systems will generally be weakening as they move up the coastline too, so at this point I don’t see anything that screams STORMY. But in this pattern each system needs to be watched closely.


In a mild pattern like this, persistent cloud cover and breezy conditions typically mean our temperature doesn’t move much from day to day. High temperatures for at least another week will generally be around 50 degrees and lows around 40...plus or minus 5 degrees. Note the ECMWF ensemble forecast of high/low temps over the next two weeks is VERY stable.


And since we’ll be a bit warmer than normal, the mountain passes will only occasionally see snow. Most of the time the snow level will be up around 5,000′ or so over the next week


For those of you wanting snow this January? Very unlikely through at least the 18th or so. We’ve had quite a January “snow drought” the past few years. The last significant January snow was 6 years ago! So far it’s looking like another mild January.

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Wx Blog(kptv)