So What Exactly Is a Bomb Cyclone, Anyway? (Part 1)

The Thanksgiving Week bomb cyclone as seen over the Eastern Pacific. Source: COD Weather

​One of the biggest weather headlines out west has been the bomb cyclone that managed to drop surface pressures at stations along the Oregon coast to as low as 975mb (source: Wunderground)! Add to it the strong winds associated with the system and the heavy snow totals expected all over the Southwest through the Thanksgiving holiday and it’s easy to see that this November storm will surely be one for the record books. So how did we get here? And what exactly is an atmospheric river? Let’s dive into the science behind this bomb cyclone!

The Thanksgiving Week bomb cyclone as it inched its way onto the Oregon/California coast during the afternoon of 11/26/2019. Source: COD Weather

While at first it might sound like this system is something out of made-for-TV movie about a radioactive hurricane that’s about to dump sharks all over Oregon and California, the reality is that the naming convention comes from the idea that a low pressure system experiences explosive cyclogenesis, or rather a sharp increase in its intensity as it grows and matures. Generally speaking, atmospheric scientists use barometric pressure as the base for describing the intensity of a low pressure system. Sea-level pressure (SLP) lies around 1013hPa, and is commonly measured in fair weather conditions, i.e. sunny skies. The core of low pressure systems, as the name suggests, will be measured at much lower values than our SLP and are usually below 1000hPa. This was absolutely the case with this bomb cyclone, where the pressure was measured by Gold Beach, OR, to be at 975hPa around 7pm local time on 11/26/2019! As mentioned previously, the use of the meteorological term “bomb cyclone” is reserved for low pressure systems/cyclones that rapidly intensify over a short period of time: usually a decrease of 24hPa in a 24-hour period. Other frequently-used terms include meteorological bombs or just weather bombs, but the overall idea remains the same.
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500mb height anomaly map for the CONUS. Note the pinks over the West Coast, indicative of very abnormal pressure signatures that were forecast by the ECMWF model on the morning of 11/25/2019. Source: Pivotal Weather

As such, this system is currently packing a punch on the California and Oregon coast, bringing with it heavy winds and a trail of atmospheric moisture that will produce heavy rains and snow all across the Southwestern United States. This atmospheric moisture transport has been very evident in previous model runs going back several days given the anomalous strength of this system. These include the North American, Global Forecast, and European Models, all of which play an integral role in short, mid, and long-range forecasting (and will be talked about in greater depth in future articles!).  What’s interesting about these moisture transports across the Pacific is that in these sorts of instances they almost appear as currents that stretch out for hundreds of miles over much dryer environments. This is evident in model runs (that just came out earlier today!) by the Global Forecast Model (GFS) as seen below:

GFS IVT Map: Initialized at 10am PST on 11/26/2019 and valid for 4am PST on 11/27/2019. Source: Center for Western Weather and Water Extremes

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​In one of my upcoming pieces I’ll dive into this topic a bit more but for now, integrated water transport vectors (IVTs) are essentially  a measure of the amount of moisture being carried from one place to another with the wind. The feature we’re focused on covers a good chunk of the 30 to 35 degree region off the coast of California and snakes its way into the State and as far east as Utah. Generally, values of IVT over 200 suggest that there is a decent amount of moisture that is being carried by this flow and when the flow is as narrow as what this model run is suggesting, meteorologists will often refer to such a flow as an atmospheric river. And that is indeed what we are expecting to see unfold with this bomb cyclone as it continues its trek over the Western US. As it does so, heavy rain and snow will flow into the region, bringing with it conditions that will make holiday travels challenging for millions of people. In part two of this two-part article, we will be going over the aftermath of these sorts of systems with the help of one of my powder(snow) chaser colleagues as we visit some of the heaviest-impacted snow areas from this system!
 
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© 2019 Meteorologist Gerardo Diaz Jr.
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Sources:
https://www.wunderground.com/weather/us/or/gold-beach