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* Strong thunderstorm up to 21 hours | Flood clock up to 23 hours *
15:10 – Strong to severe storms invading the northern suburbs of DC
A storm bow is moving south of Pennsylvania in northern Maryland, triggering a severe thunderstorm warning from northern Montgomery County north to the Mason Dixon Line. Includes Thurmont and Frederick. Storms – which stretch approximately from Hagerstown to Westminster – can cause gusts of up to 60 mph in addition to torrential rain and lightning.
This line of storms, booking south at 50 mph, could reach Beltway and the western suburbs of DC before 4 p.m. We will post our next update as they approach.
14:10 – Deadline for a strong thunderstorm is issued until 21:00
As intense storms have already erupted in the region, the National Meteorological Service has issued a warning of severe thunderstorms by 9pm. Additional storms – some of which may be severe – are expected to pass through the area by evening. Many storms will trigger torrential rain and dangerous lightning, while some can cause harmful gusts of wind and hail.
A strong thunderstorm watch has been released for parts of the District of Columbia, Maryland, Pennsylvania, Virginia and West Virginia until 9:00 PM EDT pic.twitter.com/N7SIHDuZsp
– Strong storm from NWS (@NWSSevereTstorm) June 22, 2022
The clock stretches from central Virginia to central Pennsylvania, where storms develop and pass south. It does not include counties along Chesapeake Bay, where storms are expected to be slightly less numerous and intense.
Remember that watching a strong thunderstorm means that the conditions are favorable for intense storms, but are not a guarantee. Be aware of the times. If a severe thunderstorm warning has been issued for your location, it means that a strong storm is imminent and that you should seek shelter.
The initial round of rains and storms that have already passed will pave the way for a new wave unfolding in southern Pennsylvania that arrives in the late afternoon. It can be more intense.
Original lunch article
The blissfully dry weather of the last few days is gone; a very warm, moist model takes its place. But cooler air lurks in the northeast and northwest. We are stuck in a transition zone where these contrasting air masses meet, a mature environment for intense thunderstorms.
Storms are most likely between about 3pm and 11pm, and some can be severe – containing destructive winds and hail in addition to heavy rainfall and dangerous lightning. Some areas may be affected repeatedly by severe storms – increasing the risk of flooding.
The National Weather Service has issued flood surveillance for much of the region, except in southern Maryland and counties near Chesapeake Bay, where rain and storms are likely to be less.
#Flood Watch for potential flash floods this afternoon and evening has been extended east in Corridor I-95. Heavy rain from thunderstorms can cause water to rise rapidly in streams, creeks and areas with poor drainage. pic.twitter.com/MtfzQDadcQ
– NWS Baltimore-Washington (@NWS_BaltWash) June 22, 2022
The heaviest rain and the greatest threat of floods are likely to be concentrated between Interstate 95 and 81. “1 to 3-inch rainfall is possible within hours, with larger amounts possible locally,” he warned. the meteorological service.
The total number of rains will be highly variable throughout the area, depending on where the heaviest storm cells are traced – which cannot be predicted before they start to form. Some areas can see less than a tenth of an inch, while some models show maximum amounts above 5 inches, which is a significant amount of rain. This amount of rain would require the formation and reform of severe storms by repeatedly tracking the same area – a phenomenon known as training. The biggest threat from training cells is west of Route 15, which runs from Frederick to Warrenton.
The Meteorological Service has placed the western half of our region at risk level 2 of 4 for excessive rainfall; our eastern regions are at risk of level 1.
The threat of flooding may be somewhat mitigated by the fact that June has been dry so far – but if 2 or more inches fall in a short period of time, it could quickly lead to flooding and poor drainage areas.
“Excessive runoff can lead to flooding of rivers, streams, streams and other low-lying and flood-prone areas,” the meteorological service said.
Remember never to try to drive through a flooded road, as the water level is difficult to judge. Turn around, don’t drown.
In addition to the threat of heavy rainfall, the Meteorological Service’s Storm Forecast Center has put the area at risk of a 2 out of 5 risk of severe storms that could cause “destructive winds and isolated hail.” An isolated tornado cannot be ruled out.
The most likely time for strong storms would be late afternoon and early evening, before the threat subsides into the dark. However, the risk of heavy rainfall can last up to 22 or 23 hours in parts of the area.
An unusual model is assembled in the next 12 to 18 hours.
As shown in the forecast surface diagram (valid at 20:00) below, we have an approaching cold front from the west. Above the bay and Corridor I-95 from the east, another slow-moving frontier is approaching: in fact, a strange direction, a process called retrogradation. A weak low pressure zone is expected to develop along this border.
Thus, the region will be located in an area where the humid, unstable air mass between the fronts is pressed from both directions. This is called air convergence and the result will be a large mass of air forced to rise.
Adding to the potency is the very high humidity content in the air. The morning meteorological balloon in Dulles revealed that the aggressive humidification of the deep atmosphere is underway, to the point where the “settling water” (the total liquid equivalent of water vapor depth) will be between 2 and 2.5 inches – a value that is quite excessive for our region in late June – close to record levels. These abnormally high values at 8 tonight are shown by the red bar on the map below.
So we have a very high moisture content, which is pressed up in the area between two fronts, in an atmosphere unstable enough to generate thunderstorms. These factors will intensify late this afternoon and are likely to persist until around midnight.
The deep air flow is also abnormal at this time of year, from the north – so the storm cells will develop in Pennsylvania and move south to the Baltimore-Washington region.
We believe that the retrograde front, draped on I-95, will act as a pipeline through which storm cells will fire repeatedly and will be traced from north to south. It is difficult to say a priori the exact counties / localities that are affected, but this effect of “training” can lead to impressive amounts of rainfall for some, up to 2 to 3 inches. One of the simulations of tonight’s high-resolution radar forecast model is shown below – you can choose the improved drill cage corridor along and west of I-95.
Another area of focused, heavy rain could be near or just west of Corridor I-81, where the increased rise of moist air from the mountains and the approaching cold front could rip out additional atmospheric moisture. Keep in mind, however, that the above radar simulation is just a rough guide on how storms can develop; the actual time and location of the storms may be quite different.
Locally strong storms can also generate damaging gusts of wind, intense lightning and, perhaps, even a weak tornado. We do not expect the severe weather cover to be as wide as the threat of floods. But cutting the wind (or increasing wind speed and changing altitude) is enough, along with the local “rotation” generated along the retrograde front, for the threat of an isolated tornado.
Damaging rectilinear winds in the form of downward gusts are more likely, in which the heavy mass of descending water in the cell streams drags the air down to the surface during a high-speed impact.
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