The first week of April saw an interesting event bring about the potential for precipitation over Northern California and Oregon. This feature looked to be an upper-level cutoff feature, which indicates a geopotential height minimum. While upper-level cutoff features do not always lead to precipitation, there were several reasons to indicate this one might. The 300 mb wind map in Figure 1, for example, showed a small jet streak to the south of the upper-level cutoff on Monday 2021 April 05 at 12 UTC. The left jet exit region causes divergence aloft, which induces upward vertical motion and systems to strengthen. The height contours show the cutoff feature over Oregon and Washington. This is an example of how upper-level features do not always align with lower level features can still be related.
Fig. 1: A wind map at 300 mb with height contours (in dam) in black, and the fill pattern is the wind (in kts) forecasted for 12 UTC on 2021 April 05. The main feature to focus on is the cutoff feature moving eastward over Washington and Oregon (source: https://www.pivotalweather.com/model.php?m=gfs&p=300wh&rh=2021040518&fh=0&r=conus&dpdt=&mc=)
The 500 mb vorticity map in Figure 2 shows relative vorticity and rising motion. Looking between the 850 mb wind vs the 500 mb wind in Figure 3, the vorticity on the Figure 2 map seems to be shear vorticity. Shear vorticity is caused when air is moving at different speeds at different levels of the atmosphere, and if the shear vorticity is cyclonic shear vorticity, upward vertical motion is likely to occur downstream. Figure 2 confirms this. The movement of the vorticity into an area, called positive vorticity advection, can lead to strengthening of systems as a result of this upward motion.
Figure 2: ) 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 forecasted for 12 UTC on 2021 April 05. (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=rel_vort)
Figure 3: (left panel) This is a map of 500 mb wind (shaded using the same fill pattern as in Fig. 1), and height contours in black in dam forecasted for 12 UTC on 2021 April 05 (source: https://www.pivotalweather.com/model.php?rh=2021040518&fh=0&dpdt=&mc=&r=na&p=500wh&m=gfs), and (right panel) is a map of 850 mb wind with the same fill pattern as Fig. 1 and height contour in black in dam forecasted for 2021 April 05. A side note: the gray area on the 850 mb map is there because the terrain rises higher than 850 mb. This is normal for some parts of the western U.S. at higher elevations. (source: https://www.pivotalweather.com/model.php?m=gfs&p=850wh_nb&rh=2021040518&fh=0&r=na&dpdt=&mc=).
Looking at the height of the tropopause (the layer of the atmosphere where most weather takes place), temperature advection, and predicted surface precipitation, this specific system looked to be a cold core low when coupled with all the other elements. Figure 4 is a gif of these maps respectively. Cold core cyclones are stronger aloft due to thermal wind, which is a change in geostrophic wind between pressure levels. Lower thickness contours are to the left of the thermal wind vector in the Northern hemisphere, and these lower thickness contours tend to correspond with lower temperatures. With the cooler air at the center of the cyclone and temperature decreasing with height, the thermal wind gets stronger the higher up in the troposphere. This can lead to precipitation. The 700 mb temperature advection map and low tropopause height on the 2 PVU map in Figure 4 both show cold air moving into the region where the low is at the surface approximately (lower (higher) tropopause heights usually indicate cooler (warmer) air).
Figure 4: This gif has three images: the PVU 2 (potential vorticity unit) map is used to estimate the height of the tropopause, and it is coded using the fill pattern on that slide and has wind barbs as well to indicate wind speed. (source: https://www.tropicaltidbits.com/analysis/models/?model=gfs®ion=us&pkg=DTpres&runtime=2021040518&fh=6). The 700 mb temperature advection map had the fill pattern showing the change in temperature in K/hr, the blue contours are an estimate of cold air advection, and the red contours are an estimate of warm air advection. (source: https://www.tropicaltidbits.com/analysis/models/?model=gfs®ion=us&pkg=temp_adv_fgen_700&runtime=2021040518&fh=6). The surface map has sea level pressure contours (hPa) in black, thickness contours in red and blue (dam), and precipitation following the fill pattern. High pressure systems are the blue letter “H” and low pressure systems are the red letter “L”. (source: https://www.pivotalweather.com/model.php?m=gfs&p=prateptype_cat&rh=2021040518&fh=0&r=na&dpdt=&mc=)