March 10th – 11th were exciting days for weather enthusiasts. To start out, California had significant precipitation. As shown in figure 1, the low-pressure system that brought precipitation to southern California was somewhat in the left jet exit region of the jet streak. This often indicates divergence aloft, causing rising motion, and ultimately an increased chance to strengthen a low-pressure system. Meanwhile the Midwest, a storm system was developing.
Fig. 1: A wind mat at 250 mb with sea level pressure contours in black, 1000-500 mb thickness in red and blue dashed lines, and the wind speed shaded based on the color bar (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=mslp_jet and Satellite and Radar Blog for March 11th.)
This was not the only dynamic mechanism causing the in system in California to exist though. Relative vorticity was in the area of the cyclone (the yellows and reds seen in the top of figure 2), as well as rising motion (the blue lines in the top of figure 2). The relative vorticity helped generate and strengthen the storm in the Midwest as seen in figure 2 as well by helping increase vertical motion. Vertical motion allows clouds to form more easily by allowing warm, moist air to rise, cool and condense. There was positive moisture advection, or moisture moving into a region, in the Midwest where the Midwestern system developed. This is shown in the bottom half of figure 2.
Fig. 2: (top panel) This is a map of 500 mb relative vorticity (shaded using the scale listed),wind barbs in knots, 500 mb geopotential height contours in black, and ascent in blue taken at 12 UTC on 2021 March 11 (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=rel_vort), and (lower panel) is a map of 850 mb height dew points where the green shaded regions are where dew point is equal to temperature. This means an air parcel is saturated and holding the maximum amount of moisture for the conditions (source: https://weather.cod.edu/analysis/).
The observations in figure 3 give an idea of what happened. If southern California is observed, the low-pressure system sweeps across the region. In the mid-west, it is possible to follow the development of the system along a stationary front. It is eventually strengthened to look more like a Norwegian Cyclone between the 06 UTC and 12 UTC maps on 2021 March 11. This can be identified by the occluded front north of Wisconsin and Michigan. The positive moisture advection and position of the jet are given to further evidence for the development of the low. It did end up strengthening.
Fig. 3: This is a progression of surface observations from the Storm Prediction Center. From top left to bottom right moving across the rows, the data shown is from 18 UTC 2021 March 10, 00 UTC 2021 March 11, 06 UTC 2021 March 11, and 12 UTC 2021 March 11 (source: https://www.wpc.ncep.noaa.gov/html/sfc-zoom.php).