A rapidly moving and potentially strong thunderstorm could pass near or directly through the Washington area early Tuesday morning. If the group makes full use of its potential and hits the area head-on, it could unleash torrential rain and catastrophic gusts – creating dangerous conditions for movement on Tuesday morning. However, it remains likely that the storms will not be particularly intense, or that they will only approach the Washington area, passing north or south. The most probable time for any storms will be between about 6 and 10 a.m., reaching first along Route 81 Interstate and lastly in southern Maryland. Early Monday afternoon, the storm band was just starting to take off in Wisconsin. By late afternoon, a thunderstorm was raging near the Indiana-Ohio border. Computer models generally project the cluster to intensify over the Ohio Valley Monday night through Tuesday morning before chasing southeast along the Appalachians to Pennsylvania, Maryland, North and Central Virginia and Virginia. The complex is expected to grow along the northern periphery of a heat dome, setting record high temperatures in the central United States. This zone – where hot, volatile air meets colder air and storms tend to break out – is sometimes called the “ring of fire”. The record-breaking heat wave extends east. over 100 million on alert There is an external possibility that the storm complex, known as the mid-range transport system, meets the criteria for a derecho – which is an extensive, fast-moving, long-lived, violent arc-shaped storm. However, if a derecho is formed, it is very unlikely to be as strong as the infamous event of about a decade ago, on June 29, 2012. The National Meteorological Service’s Storm Prediction Center has put the area at a 2 out of 5 risk level for severe thunderstorms on Tuesday. Areas in the northwest are at even higher risk – up to Level 4 out of 5 – “in anticipation of a highly organized MCS / possible derecho development”. The storm complex has the potential to create a zone of extensive wind damage that extends to many states as it approaches the Mid-Atlantic region early Tuesday. The complex will follow along a stationary front along the top of the heat dome as it moves from northwest to southeast. This front is reflected in the surface weather forecast map, valid at 8 a.m. on Tuesday. The solid crimson color over West Virginia indicates the possible location of the severe weather threat: Whether this complex will continue in an intense and catastrophic situation, east of the Appalachians, is extremely uncertain. Model views indicate that this is a possibility, as shown in the high-resolution NAM model simulation below. We define derecho as a fast-moving storm complex that creates a wind damage zone of at least 250 miles, with multiple, continuous gusts exceeding 58 mph and sometimes higher (ie in the range of 70-80 mph). But not all derechos are the same, as they seem to exist in a range of intensity and size. If this complex is considered derecho, its intensity can be mitigated from its morning time, when there is not enough sun to strongly destabilize the atmosphere. Derecho Science: The debate about what derecho is and what it is not The heavy derecho of June 29, 2012 arrived here late at night, one to two hours after sunset, after an afternoon when the temperature reached 104 degrees – the highest temperature in June in Washington recorded. This heat wave propelled an extremely unstable mass of air – one of the most unstable ever measured in the area – that could remain for a few hours after sunset. On the contrary, the derecho of June 13, 2013, one of the two that hit the Mid-Atlantic that day, managed to cross the Appalachians in the early morning hours, but ended up in a rather rough situation with few catastrophic winds from when it arrived in Washington. In other words, the time of passage of any storm group through the Appalachians and arrival on the eastern slopes seems to matter in terms of the derecho outcome. Chances are it may favor a less organized or weaker system when Tuesday’s thunderstorm arrives. However, as some models continue to simulate an angry storm arc affecting the DC area on Tuesday morning, we need to take this threat seriously. Even if the mountains take something from the storm complex and the atmosphere is a bit unstable, an extensive and deep “cool pool” containing high-velocity air could allow the system to be maintained at an intense level for a while east of the mountains. The CWG will monitor upstream growth trends, updated models and meteorological balloon observations early in the evening and post status updates at the top of this story.
