Although it seems like the synoptic patterns have remained busy through April so far, this has not necessarily translated to the number of severe weather and tornado reports that we except for this time of year. In fact, through 15 April 2021, data from the Storm Prediction Center (SPC) there have only been 34 tornado reports across the continental United States (Figure 1), which is much lower than the average number of April tornadoes: 155. This weekend has the potential to bring the April 2021 numbers closer to average, as a fast-moving synoptic feature will bring severe weather to the Southern Plains and the Southeast on Friday and Saturday. In fact, the categorial convective outlook from the SPC has a day 1 enhanced risk (level 3 of 5) over the Southern Plains at 20Z 23 April 2021, while another enhanced risk is centered over southern Alabama and Georgia. According to the SPC, the severe threats will be multifaceted, with the possibility of large hail, damaging wind and tornadoes.
Figure 1: Preliminary tornado reports from 1-15 April 2021 (red dots). Source: Storm Prediction Center (SPC) Monthly Severe Weather Report Database
Figure 2: NOAA/NWS Storm Prediction Center (SPC) categorial outlooks issued on 23 April 2021 for a) Day 1, 20Z 23 April 2021 to 12Z 24 April 2021, and b) Day 2, 12Z 24 April 2021 to 12Z 25 April 2021. Source: NOAA/NWS SPC Convective Outlooks.
As of 21Z 23 April 2021, the broader area of low pressure in charge of this round of severe weather is situated over the Texas Panhandle with a minimum mean sea level pressure of 998 mb. A shortwave trough, or one that is smaller and faster-moving than a typical longwave trough, exists just to the west of the surface lows along the border of Texas and New Mexico. Like many of the systems that have moved across the United States this month, troughs have played a huge role in the strengthening and/or maintenance of the surface low as long as the center of circulation is to the east of the trough axis. Fortunately for this surface low, it is! Regions to the east of the trough axis will have positive vorticity (cyclonic motion) advected into its vicinity as a result of a vorticity maximum being located in the base of the trough, which will cause the air above the low to rise. At 500 mb (Figure 3), this can be observed through lower values of vorticity over the surface low (apart from thunderstorms along the dry line), and higher values of vorticity within the base of the trough. Near the surface, vorticity advection is often weaker than the 500 mb level since there are weaker winds at the center of the surface low, which indicates that there is also positive vorticity advection increases with height, thus allowing for cyclonic motions to occur. This process occurs through Saturday and Sunday morning (24-25 April 2021) as well, as the low pressure center remains to the east of the shortwave trough axis throughout its journey from the Texas Panhandle to east of the Delmarva Peninsula.
Figure 3: Loop of the NAM forecasted 500 mb relative vorticity (fill, bottom scale) and 500 mb height (black contours) over the Continental United States from 18Z 23 April 2021 to 18Z 25 April 2021 using three-hour intervals. Source: Pivotal Weather
There’s a little bit of a problem, however, even though the low pressure center is in an environment that would be conducive for the continual strengthening of the cyclone over this period. At 700 mb, the distribution of temperature advection in the vicinity of the surface low often counteracts the rising motions described by differential vorticity advection. Figure 4 shows geostrophic temperature advection along the 700 mb pressure level, where warm air advection is shown in red. If we apply the fact that warmer air is less dense than colder air, then the movement of warmer air into regions of colder air promote the lifting of air parcels. Since rising motions are expected in regions of warm air advection, low pressure centers at the surface will tend to move towards these regions, which partially explains the eastward progression of the low. As the forecast period begins at 18Z 23 April 2021, the low pressure center in the Texas Panhandle is situated between weaker areas of cold air advection to its west and warm air advection to its east. Shortly thereafter, the regions of cold and warm air advection become less evenly distributed around the low pressure center because of precluding thunderstorms, which can cause localized or regional cooling effects prior to the arrival of the surface low. Because of this, the low pressure center occasionally becomes embedded in regions of cold air advection between 0Z 24 April 2021 and 12Z 24 April 2021, which promotes sinking motions above the low. When combined with the rising motion expected from differential vorticity advection, these processes nearly cancel each other out, and allows for the surface cyclone to maintain its strength (998-1003 mb) through 12Z 25 April 2021.
Please note that this post is a forecast for 23-25 April 2021, and conditions can change. In the event of inclement weather, ensure that you have multiple ways to receive weather warnings and have a plan of action before the storm arrives.
Figure 4: Loop of the NAM forecasted 700 mb horizontal temperature advection (fill, bottom scale), 700 mb temperature (black contours, in degrees Celsius) and 700 mb horizontal frontogenesis (green contours) over the Continental United States from 18Z 23 April 2021 to 18Z 25 April 2021 using three-hour intervals. Source: Pivotal Weather