Wait...Is That Maryland or Oklahoma? Discussing the Surprise June 5th, 2024 Regional Tornado Outbreak

        Is that Kansas? Is that Oklahoma? Nope, it's Maryland! 

Figure 1: Radar imagery of Germantown-Gaithersburg supercell 

that promoted a Tornado Emergency. Radar showing a clear debris 

ball and Tornado Debris Signature (TDS), indicating a tornado ongoing.  

On the afternoon of June 5, 2024, residents in Maryland and Northern Virginia were treated to an early preview of the new Twisters movie...only this time without the CGI. The region experienced a somewhat unexpected regional tornado outbreak, in which a total of 7 tornadoes touched down across Maryland, Virginia, and West Virginia. One such tornado tracked through the populous towns of Germantown and Gaithersburg, Maryland,  prompting the region's first ever Tornado Emergency (Figure 1).

        Though the DelMarVa region was outlined in a marginal risk for the potential for a few spin-ups, the outbreak came as a bit of a surprise to everyone. So what exactly happened that caught meteorologists off-guard? 

        When taking a look deeper at the environment in place, there were a few reasons why the day overperformed, which includes: 

• Locally higher instability and "fuel" for more vigorous thunderstorms 
• Locally enhanced "twist" energy provided by a couple of key elements 
• Rich low-level moisture that created lower cloud bases and made tornado formation "easier"

Priming the Atmosphere...

        Locally higher instability that initially forecast, especially at the lowest levels, contributed significantly to the more robust tornado threat. Figure 2 below shows the Surface-Based Convective Available Potential Energy (SBCAPE) values for the region. This measure essentially estimates the potential for air to rise (i.e. the power of the updraft) with higher values indicating higher potential for robust updrafts. As seen from the imagine, the supercells that spawned the tornadoes (including the Germantown-Gaithersburg tornado) were underneath a pocket of higher SBCAPE, allowing for more vigorous thunderstorms to develop.

        Instability was especially enhanced at the lowest levels. Weather soundings out of Dulles airport revealed an abundance of 3CAPE, which is a measure of CAPE from the surface to just 3 km up. Higher 3CAPE values indicate greater low-level buoyancy and stronger updrafts, which can make better use of the "twist" energy present (more on this in a second) by more efficiently tilting this energy into the storms updraft. Subsequently, the enhanced low-level instability present created an environment more conductive for rotating updrafts.

Figure 2: SBCAPE values across the DelMarVa region. Arrow shows a pocket of higher instability near the Germantown area. 

Getting Twisty...

        Beyond enhanced instability, the region also had enough "twist" energy available to create rotating supercells. Known as wind shear, this refers to the winds changing both speed and direction with height. This shear is needed to create rotating storms. 

        A closer inspection of the environment revealed some features that locally enhanced this "twist" and allowed rotating supercells to form. Figure 3 below shows a map depicting Storm Relative Helicity (SRH), which is a measure of this twist energy in M2S2. As clearly seen, the region was under a "bullseye" of enhanced helicity, with enough there to even produce a stronger tornado or two. 

Figure 3: 0-1 km SRH values over the DelMarVa region.  A bullseye of 250 M2S2 over DelMarVa indicates  enough shear for tornadoes, even some strong.

        Diving even deeper, this enhancement of wind shear stemmed from two key aspects: a remnant, weak low pressure system known as a Mesoscale Convective Vortex (MCV) and the resulting backed-surface wind out of the Southeast (Figure 4). Without getting too technical, this local backing of the wind enhances twist energy by creating more changing wind direction with height, which is a key factor in getting storms to rotate. 

Figure 4: Surface observations from the DelMarVa region. The circled areas show the wind barbs at observation points indicating winds coming from the
Southeast. 

All About That Base...

        Another small but critical factor that could help explain the more conducive environment was the low cloud bases. Figure 5 shows the Lifting Condensation Level (LCL) of the region, which is very simply the level at which the air becomes saturated and condenses to form clouds. This measure is often used to get an estimate of how low cloud bases will be, which is another factor in evaluating tornadic environments (lower cloud bases = better for tornado production). As depicted, the DelMarVa region had relatively low LCL heights of 750 M and under. Such low cloud bases promote easier development of tornadoes.  

Figure 5: LCL Heights in Meters in the DelMarVa region. LCL heights were 750 M and under, indicating low cloud bases

Forecasting Is Hard Man...

        Taken together, these features made the environment over the DelMarVa region quite favorable for tornadic supercells. These factors were not well forecasted with most short-range model guidance suggesting an environment far less potent. This event, in some respects, reminds me of the September 17, 2018 regional tornado outbreak in Richmond, VA. Though these two events had completely different set-ups, both share two commonalities: stronger instability than forecast and locally enhanced wind shear. In both of these outbreaks, models failed to pick up on these factors, leading to regional tornado outbreaks that caught both residents and local meteorologists off-guard. This event is a reminder that forecasting still has a long way to go and residents should always be weather aware. Check out the video below for a 4 second clip of the evolution of the supercell that produced the tornado in the Germantown-Gaithersburg area (pay close attention to the end of the "hook" appendage at the end of the storm for where the tornado is).