Tornadoes Possible Wednesday Afternoon across DC Region

Threat for Tornadoes Increasing Wednesday Afternoon & Evening

Wind shear is defined as winds in the atmosphere that change direction and speed with height. Shear values are forecast to increase across the DC Region Wednesday afternoon & evening as the remnants of Fred move through West Virginia.  The image below shows shear values increasing above 40 knots across the Metro Area on Wednesday afternoon increasing the odds of some super cells developing that will be capable of producing large destructive hail, damaging winds and tornadoes.

Tropical Systems and Wind Shear

Land falling tropical systems begin to weaken as their circulation in the lower-levels of the atmosphere begin to interact with ground friction.  However, the mid and upper level winds remain stronger allowing for increasing wind shear along/east of the system’s main center of circulation, increasing the threat for rotating updrafts and tornadoes.

Energy Helicity Index (EHI) Values Impressive on Wednesday

0-1 km AGL EHI values greater than 1-2 have been associated with significant tornadoes in super cells.  The blue and especially yellow colors in image below suggest that the threat of some storms becoming tornadic will remain elevated Wednesday afternoon continuing through the late evening.

Timing Wednesday’s Thunderstorms

A broken line of thunderstorms (some potentially becoming rotating super cells) will move through during the peak heating of the afternoon, likely approaching the western suburbs after 3pm and clearing the eastern shore after midnight. (time stamp in upper left corner of image below).

Have a plan on Wednesday, know what to do during a tornado and have a weather radio or local media outlet available in case tornado warnings are issued for your area.

Timing Arrival of This Afternoon’s Potential Severe Thunderstorms

The latest high-resolution forecast models indicate that a disturbance over West Virginia will slide eastward into the DC Area this afternoon igniting a line of strong to severe thunderstorms.

Timing the Storms

The models indicate storms could begin anytime after 4pm, sliding through the immediate Metro Area between 5 and 8pm.

Storms will be capable of producing damaging winds in excess of 60 mph, hail and cloud to ground lightning.

Damaging Winds and Hail Possible this Afternoon

The Storm Prediction Center (SPC) has placed DC and portions Northern Virginia and Maryland in a “Slight Risk” for Severe Thunderstorms this afternoon, with the main threat being damaging winds in excess of 60mph and hail. 

Ingredients for Severe Thunderstorms

Increasing wind shear, steepening low-level lapse rates, dew points in the 70s (oppressive humidity) and temperatures in the 90s will provide the necessary ingredients for numerous thunderstorms to develop this afternoon and evening.

Timing the Storms

The latest high-resolution NAM model shows a nasty line of storms forming over the Shenandoah Valley around 2pm and then racing eastward across the Washington and Baltimore Metropolitan Areas between 4 and 7pm.

Flash Flooding

Storms may also produce torrential rainfall rates of 1 to 2 inches per hour causing flooding of low lying areas, underpasses and cause creeks and streams to rapidly rise out of their banks. Because of this the National Weather Service has issued a Flood Watch for portions of the DMV.