Pre-Thanksgiving travel got a whole lot harder this week in the western region of the US as what is known as a “bomb cyclone” had begun to impact the region on Tuesday, 26 November 2019. A bomb cyclone is caused by bombogenesis, which occurs when an extratropical cyclone rapidly intensifies as a result of the pressure in the center of the storm dropping at least 24 mb in 24 hours. In the case of this most recent cyclone, the storm dropped from 1002 mb to 975 mb in only 12 hours. A mid-latitude storm with a pressure of 975 mb indicates a very strong storm, in this case capable of producing hurricane force winds and winter storm warnings across the west coast. The rapid organization and development of the storm can be seen in Fig. 1.
Figure 1: Airmass RGB satellite imagery from 1730 UTC 26 November 2019 to 0130 UTC 27 November 2019. Source: NOAA
The rapid intensification of the storm is due to its location in relation to a strong jet. As can be seen in Fig. 2a, the area of low pressure shifted from the center of the jet to the northern portion of the jet exit region. Strong upper level divergence of air in this portion of the jet leads to rising air at the surface, which in turn lowers the pressure. Between 1200 UTC 26 November 2019 and 0000 UTC 27 November 2019, the pressure at the center of the storm dropped 27 mb as a result of this effect. Over this 12 hour period, the pressure, as seen in Fig. 2b, went from 1 standard deviation below the mean sea level pressure to 5 standard deviations below the mean sea level pressure. Coupled with the other mid-latitude cyclone currently over the central US, travel across the US for the Thanksgiving holiday is not going to be easy this year.
Figure 2a (left): Mean Sea Level Pressure (MSLP) in black, jet stream shaded in blue/pink. Source: http://www.atmos.albany.edu/student/abentley
Figure 2b (right): Mean Sea Level Pressure (MSLP) in black, standard MSLP anomaly shaded. Source: http://www.atmos.albany.edu/student/abentley