Several rounds of showers and thunderstorms are possible in the D.C. metro region tonight. The first wave of activity is possible between 7 and midnight tonight. A few of storms may be on the strong side, with downpours, frequent lightning, and gusty winds, but there is just a small chance of widespread severe storms.
We describe the storm setup in depth below, starting with a look back at Tuesday’s severe storms over Iowa, which reached rare intensity levels. But our meteorological situation tonight will be a very different one, much more benign.
Tuesday’s Midwest storms
Tuesday, the National Weather Service (NWS) Storm Prediction Center (SPC) issued a high risk outlook for exceptionally severe thunderstorms across a small portion of the Midwest, including parts of Iowa, Nebraska and Missouri. Such a risk, calling for 60 percent chance of widespread damaging storms, is rare – issued perhaps once or twice a year – and evokes national attention.
Figure 1 shows a dramatic line of storm cells, from weather radar, pummeling southern Iowa during the early evening. The storms developed along a stationary frontal boundary separating a high-energy (hot and humid) air mass from cooler northern air. The boundary interacted with intense wind shear and other triggers including a powerful low-level jet stream.
Quite the assortment of bow echoes and supercell storms developed, moving repeatedly across the same region from the northwest. Although I don’t show a color key in this figure, some of you may be struck by the magenta color in the core region of each cell. Magenta scales to an exceptionally large radar reflectivity factor i.e. how much energy is scattered back to the radar. Around D.C., a value of 50-55 dBZ means strong to severe thunderstorms, and this is colored red. In the image above, magenta codes for 65-70 dBZ. Energy levels this high typically mean a mixture of torrential rain and very large hail – baseball size, to be exact. Looking higher up in the radar scans (not shown here) I noticed frequent values of 75-80 dBZ. Values this extreme ping the very upper end of the theoretical scale, something that occurs with far less than 1% frequency, nationwide.
The hail unleashed was enormous, widespread and highly destructive (read: Hail in a Gale).
Figure 2 (below) shows the damage reports received as of this morning at NWS SPC. You see plenty of giant hail (green) and damaging wind reports (some of which topped 80 mph, blue) and a clustering of tornadoes over Iowa.
Figure 3 shows the storm-total rain accumulation, derived from NWS radar. Values exceed 6 inches in spots, from repeated “training” of severe storm cells over the same locations. Often in mixed severe-weather situations such as this, the flash flood component is inadvertently downplayed by the media, but flash flooding remains the number one severe weather killer.
Was yesterday’s storm complex a derecho? A derecho, by definition, is a convective-generated wind storm with a path length exceeding 250 miles, containing widespread severe wind gusts. It takes a few days to sort through damage reports and to conduct field surveys. A large bow echo complex developed across Missouri and Illinois overnight, a continuation of the Iowa convection. All told, over 400 severe wind reports have occurred. The complex likely will be labeled a derecho, but I caution that we should await an official verdict by NOAA SPC and/or regional Midwest NWS offices.
More severe weather possible in the Ohio Valley today
The headline across The Weather Channel’s website this morning states “15 Million At Risk: Derecho Possible”. NWS’s SPC thus far is taking a more conservative tact, outlining a slight risk region for the Ohio Valley, including a 30% damaging wind threat as far east as West Virginia. Typically, a widespread derecho event commands at least a moderate risk outlook. Such an upgrade is possible later today, depending on what develops.
And that’s the rub: Summertime derecho forecasts are troublesome. We can identify fairly well in advance the ingredients defining a potential derecho corridor – which include a frontal boundary, very large levels of air mass instability, and strong mid-level winds that parallel the boundary. But just because the “keg is full of powder” does not necessarily mean a derecho will develop. The organizing trigger is often subtle, or fails to materialize. And many derechos develop by a process called “upscale growth” meaning adjacent cells begin to congeal into a bow-shaped line of storms. But this process is essentially random growth…basically, a roll of the dice.
The Washington-Baltimore threat tonight
Figure 4 shows the weather system expected to impact the D.C. region tonight. A slow-moving cold front will sag through the region, as a weak wave of low pressure develops over the Mid Atlantic. Aloft, a weak wave of low pressure will amplify, triggering development of the surface low.
To get a significant, widespread severe weather event overnight in the Mid-Atlantic requires an extraordinary set of circumstances. I do not expect that these extreme conditions will materialize. During the summer season, severe storms are energized by large amounts of buoyant energy i.e. air mass instability, which is driven by the diurnal cycle of solar heating. With the primary lifting mechanism (front, surface low pressure and upper level wave) coming through overnight and early morning, this is simply the wrong time of day for a major severe outbreak.
Instability WILL be present, but not to the level supporting a derecho. In fact, the morning model guidance suggests that significant values of instability will remain to the south of the Washington region (Figure 5).
This makes sense, given the trajectory of the low pressure system directly over the Washington region: The system’s warm sector must track to our south, across central and southern Virginia, while the cool side envelops northern Maryland and southern Pennsylvania. This is what the models have been consistently advertising for several runs.
By mid-morning, the front and low pressure will be pushing south and east of the Washington region, moving us out of the system’s warm sector before significant solar heating can re-energize the air mass.
The other severe storms ingredient is wind shear, the change in wind speed and/or direction with increasing height. Different values and configurations of shear are a strong determinant of severe storm type, whether cells will organize into a squall line, or a bow echo, or spin up into supercells. A strong speed increase through mid-levels can organize weak supercells and bow echoes. Add a veering component in low levels (winds turning from southerly to westerly) and strong supercells capable of large hail and tornadoes develop.
Tonight’s wind shear is forecast to be modest, in a timeframe from about sundown until sunrise. Because of this shear, even small amounts of buoyant energy (instability), combined with uplift associated with the low pressure region, can lead to longer-lived, organized cells complexes.
Here’s a look at what the high resolution (mesoscale) models are predicting in terms of convective coverage and intensity tonight.
It’s possible that a large, intense bow echo may develop across the Ohio Valley later this afternoon and evening. A recent study found that whether such a large storm complex survives its transit across the Appalachians depends critically on the amount of instability available on the lee side (east of the mountains). With the predicted low values later tonight, any approaching convective system should weaken substantially.
Several models scenarios, including the bottom above, suggest that system remnants would essentially “split” the D.C. region with stronger, possibly severe storms moving to our south (in the more unstable air)…with a weaker complex (showers, embedded general thunderstorms) passing through north-central Maryland.
The most likely scenario for today includes a 20 percent chance of pop-up thunderstorms late this afternoon and early this evening, simply because of an unstable afternoon air mass. Some of these could be locally strong, but this is more likely to the south of the immediate Washington area, and along the higher terrain to our west.
After sunset, as the main piece of energy approaches, this includes frequent showers, some of which may be heavy, along with thunderstorms. Lightning could be fairly intense, and I cannot rule out isolated strong (40-60 mph) wind gusts. SPC states that locations in the Washington region have a 5% chance of severe (60 mph+) wind gusts. Some locations could pick up in excess of 1 inch rainfall. Isolated power outages would stem more from lightning than severe winds.
Given the most likely scenario outlined above, we will still keep our eye on any approaching, organized convective system, and its intensity trend. There is a low probability that such a storm could end up on our doorstep more intense than what guidance is currently predicting. Given the reduced skill in predicting individual derecho systems 12-18+ hours in advance, I cannot unequivocally state that we are completely out of the woods.
CWG will be monitoring the situation through the day and evening and will issue updates as needed.