Bomb Cyclones and Atmospheric Rivers

[ilovecornfields]

So, here’s the weather forecast for the next few days for where I live. I don’t remember reading about “bomb cyclones” and “atmospheric rivers.” Is this really a thing or just terms the media used to get us excited about weather? What implications does it have for flying? Should we avoid it altogether during these cataclysmic weather events ? @Scott Dennstaedt, PhD, maybe you can shed some light on this.

 

  ATMOSPHERIC CONDITIONS

 

    A cold front will move through the Central Coast today with

    partly to mostly cloudy skies with areas of drizzle/light

    rain. Rainfall totals should remain below one-tenth of an

    inch in most areas.

    This cold front will be followed by fresh to strong (19 to 31

mph) northwesterly winds and clearing skies tonight through

    Saturday.

    On Saturday, a storm with hurricane-force winds is forecast

    to rapidly intensify as it travels eastward across the southern

    Gulf of Alaska from 988 millibars to 948 millibars in less than

    24 hours. This storm will be classified as a "bomb cyclone."

    To be classified as a meteorological bomb, a storm needs lower

    24 millibars in 24 hours.

 

    The associated cold front is forecast to tap into subtropical

    moisture and take on the characteristics of an Atmospheric River

    (AR) as it moves into Northern California with heavy rain.

    Expected precipitation totals in Northern 

California will range

    from five-plus inches in the northern coastal mountains and the

    northern/central Sierra Nevada; three to five inches in lower

    elevations of the Bay Area and Sacramento Valley.  

 

    The ECMWF and GFS models continue to indicate that an intense

    cold front, strengthened by vigorous upper-level winds, will

    move through the Central Coast Sunday night into Monday morning

    with moderate gale-force to fresh gale-force (32 to 46 mph with

    gusts to 60 mph) southerly winds and between one and three inches

    of rain. The Santa Lucia mountains above Cambria could see as

    much as four inches. This system will produce multiple feet of

    snow in the Sierra Nevada above 8,000 feet.

[Scott Dennstaedt, PhD]

When I was working with the NWS back in the 80s as a researcher (starting as an intern), I did a fair amount of research on the concept on the cyclogenesis of extratropical cyclones, specifically looking at how numerical weather prediction models did at capturing such an event. I know the media and some weather organizations have coined the term "bomb-o-genesis" to mean rapid intensification of an area of low pressure.  These usually occur over the ocean and often create what folks back east call a nor'easter.    

These lows quickly become occluded and often deepen (pressure gets lower) very rapidly and the central pressures can approach 940 mb or lower.  Hurricane Ida (category 4) was 929 mb for reference. 

 

As one might expect, these major systems also become very barotropic with a closed upper level low and surface low stacked over each other. They are cold-core lows, but can certainly produce deep convection and when coupled with a good moisture source can bring significant precipitation potential inland.  The pressure gradient will produce strong winds at the surface and will likely carry with it some element of severe turbulence.