Get ready for a wild Monday around the Washington Area as the atmospheric conditions are forecast to be conducive to producing several tornadic thunderstorms. A strengthening low-level jet stream and increasing wind shear (winds that change direction with height) will be conducive for the development severe thunderstorms, some of which could produce several tornadoes.
The latest North American Model (NAM) indicates that the first period of thunderstorms may impact the Washington Region between 5 and 11 in the morning (image below, courtesy Tropical Tidbits).
After the initial batch of storms moves through, there may be a break with several peaks of sunshine possible before the final (and potentially more dangerous) line of thunderstorms accompanied by a strong cold front crosses our region between 12pm and 4pm. The image below (latest high-resolution NAM) shows the cold front moving from the Shenandoah Valley at 16z (12pm) crossing the Washington Area around 18z (2pm) and across the Chesapeake Bay by 20z (4pm).
The latest North American Model (NAM) showing EHI values between 9z (5am) and 00z (8pm) on Monday (image courtesy Pivotal Weather).
The Energy Helicity Index (EHI) is a combination of two indexes. By itself, it is the best index available for tornado prediction since it combines both CAPE and Helicity. The CAPE is the amount of pure instability present from a parcel of air that rises from the lower boundary layer. Helicity is the product of low level shearing (known as streamwise vorticity) and storm inflow directly into the streamwise vorticity. The Helicity is storm relative which means the Helicity is calculated from the storm’s frame of reference.
The highest EHI values are currently forecast between 15z (11am) and 21z (5pm) coinciding with the cold frontal passage during the afternoon hours on Monday.
Potential Rainfall Totals (European Forecast Model, courtesy Pivotal Weather):
Once the front passes, gusty northwest winds and much cooler temperatures will move into the DC Region Monday night and Tuesday. Another system may skirt to our south on Tuesday night and Wednesday producing cold rain showers in DC and Baltimore with a mix of rain and snow across the Potomac Highlands (including Skyline Drive).