Just days after another deadly tornado outbreak wreaked havoc on the southeastern United States, another severe thunderstorm threat is in the forecast moving into Palm Sunday. This time, the risks for Dixie Alley are slightly lower, but the bullseye for significant severe weather is now focused over portions of the mid-Atlantic on Sunday afternoon (28 March 2021). In fact, the categorial convective outlook from the Storm Prediction Center (SPC) hints at a day 1 enhanced risk (level 3 of 5) over the mid-South at 20Z 27 March 2021, while another enhanced risk is centered over southeastern Virginia with an expansive slight risk (level 2 of 5) extending from Maryland to North Georgia for Palm Sunday (Figure 1). According to the SPC, the forecasted severe weather event will primarily come in the form of damaging thunderstorm winds or wind gusts of 50 knots (58 mph) or higher, with locally higher wind gusts to near hurricane force possible. There is also a lower, but still decent, chance of tornadoes and hail as the synoptic feature moves northeastward into southeastern Canada.
Figure 1: NOAA/NWS Storm Prediction Center (SPC) categorial outlooks issued on 27 March 2021 for a) Day 1, 20Z 27 March 2021 to 12Z 28 March 2021, and b) Day 2, 12Z 28 March 2021 to 12Z 29 March 2021. Source: NOAA/NWS SPC Convective Outlooks.
As of 21Z 27 March 2021, the center of the low pressure system that will be propelling the severe threat is situated over eastern Missouri and has a mean sea level pressure of 1007 mb (Figure 2). Weaker, broader areas of low pressure are situated along the cold frontal boundary to the north and to the southwest of the main area of circulation. Given the orientation of the jet stream beginning at 18Z 27 March 2021, the cyclone is located within the right jet entrance region of the jet streak situated over the Great Lakes and New England. To the south of the primary low pressure center is a weaker jet streak that encompasses the cyclone in its left jet exit region. The animation in Figure 3 shows the irrotational wind vectors in the upper troposphere, which simply describes the component of the wind without any rotation or directional changes associated with it. In the upper troposphere, regions of upper level divergence occur in locations where the irrotational wind vectors are pointed away from, and, to no surprise, includes Missouri. When a column of air is underneath an area where air diverges in the upper troposphere, the air density aloft will decrease, and causes upward vertical motions from lower levels of the atmosphere, which acts to either strengthen a surface low or weaken a surface high. Through the GFS (Global Forecast System) run at this time, the broader areas of low pressure begin to intensify as a result of these strong upward vertical motions as it moves into southeastern Canada on Sunday afternoon.
Figure 2: Surface analysis at 21Z 27 March 2021 with selected station plots. Source: National Weather Service Weather Prediction Center (WPC) Surface Analysis Archive
Figure 3: Loop of GFS forecast data over the Contiguous United States from 18Z 27 March 2021 to 06Z 29 March 2021 using six-hour intervals. Precipitable water (PW) is shaded using bottom left scale; 250-hPa jet velocity (in m/s) is shaded using bottom right scale. 300-200 hPa potential vorticity (PV) denoted in black contours; 600-400 hPa vertical velocity greater than zero denoted in red contours. Arrows denote irrotational wind component. Source: Alicia Bentley Real Time GFS Analyses and Forecast Maps.
If you look closely at how the jet stream is oriented in the previous figure, you may have noticed that there is also an upper level trough that dives into the Great Lakes as the low pressure center intensifies. Just like diverging upper level winds, troughs have their own ways of strengthening low pressure systems, as long as the center of circulation is to the east of the trough axis in the Northern Hemisphere. In a trough, the spinning motion, or vorticity, around its base is cyclonic (counterclockwise) due to the curvature of the trough, while the motion around a ridge is anticyclonic (clockwise). Granted that our cyclone is in the Northern Hemisphere, any parcel of air that is bounded by a trough to the west and a ridge to the east will have positive vorticity (cyclonic motion) advected into its vicinity, and will cause the air above to rise. Figure 4 confirms this process, as vertical ascent is greatest to the east of the trough, right where the low pressure system is forecasted to move. In terms of the severe threat for the mid-Atlantic on 28 March 2021, the contours for vertical ascent also account for the growth of strong thunderstorms that have smaller-scale updrafts.
Please note that this post is a forecast for 27-28 March 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 GFS forecast data over the Contiguous United States from 18Z 27 March 2021 to 06Z 29 March 2021 using six-hour intervals. Cyclonic relative vorticity is shaded using bottom scale. 500-hPa geopotential height denoted in black contours; 500-hPa temperature denoted in red dashed contours; vertical velocity greater than zero denoted in blue contours. Source: Alicia Bentley Real Time GFS Analyses and Forecast Maps.