Radar & lightning:Latest D.C. area radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

4:05 p.m. update: As no severe weather has materialized in the D.C. area, I am going to cease the live updates.  We will continue to monitor the action and will post updates via Twitter.  For the forecast for tonight and Friday, stay tuned for our PM Update, which will be posted between 4:30-4:45 p.m.

(4:15 p.m. update: The tornado watch has been cancelled for the D.C. area but remains in effect for Howard and Baltimore counties, and counties adjacent to the Chesapeake Bay – but those will probably be dropped in another hour or two as storms are on their out)

3:50 p.m. update: Showers and thundershowers have reached the eastern half of the District and extend through much of western Prince George’s County and are headed northeast.  They are not severe and have had the beneficial effect of cooling temperatures into the 70s in their wake. There is no severe weather in the D.C. area (any intense activity is now north of Baltimore) and we would not be surprised to see the tornado watch dropped.

3:20 p.m. update: It’s quite possible the immediate D.C. area sees rather limited (if any) severe weather today.  The heaviest storms are in Pennsylvania and northeast Maryland.  Some rather ordinary thundershowers east of I-95 just south of the District continue moving to the northeast and will pass through southeast Fairfax County, northern Charles County, the southern and eastern half of the District (probably missing or just skirting NW D.C.), and Prince George’s County in the next one to two hours.  These could strengthen a bit and we’ll monitor.

2:55 p.m. update: There are some thundershowers in southern Prince George’s County and southeast Prince William County pivoting north towards the District. There are not severe. Some more intense storms have developed in northern Anne Arundel and eastern Howard County and are headed towards western Baltimore County. The main action is north of the Mason Dixon line where warnings are active west of Gettysburg and Harrisburg, Pa.

2:35 p.m. update: There are currently no severe thunderstorms in the D.C. area although some thundershowers are firing in Stafford County and southern Maryland. The closest intense activity is in the eastern panhandle of West Virginia and west central Maryland headed towards Martinsburg and Hagerstown. This activity will probably not affect the D.C. area. There is no imminent severe weather for the immediate D.C. area, but we will continue tracking storms in the region and will keep you updated.

Overview, posted at 2:05 p.m.: For the fourth straight day, the National Weather Service has determined the area is under the risk of hazardous thunderstorms.  After issuing severe thunderstorm watches daily Monday-Wednesday, it has hoisted a tornado watch for parts of the region through 10 p.m. tonight.

Area under tornado watch (National Weather Service) Area under tornado watch (National Weather Service, view big)

The watch includes Fairfax and Loudoun counties in Virginia, the District, and much of central and southern Maryland.  It does not include Virginia counties south of Fairfax and Loudoun.

Storm coverage and timing: The greatest coverage of storms and highest risk of severe weather may well be in northern Maryland and Pennsylvania based on radar trends.  In the immediate D.C. area and especially to the south, storm coverage may be more hit or miss and storms may not be as intense as further north.  Storms are mostly likely between 2:30 and 6 p.m., first in areas to the west and south.  Some lingering storm activity – possibly with heavy rain – can’t be ruled out between 6 and 9 p.m., but severe winds and/or hail are less likely as the evening wears on.

Storm impacts: In a sense, this is a low-end tornado watch.  The National Weather Service says there is 30 percent chance of 2 or more tornadoes in the watch area and 20 percent chance of a strong tornado, rated EF2 or higher on the 0-5 scale.

Somewhat more likely are damaging straight line wind events. The NWS says there’s a 50 percent chance of at least 10 incidents of damaging winds in the watch area.

It says the chance of hail is low, with a 20 percent chance of 10 or more instances of hail at least 1 inch in diameter.

Heavy rain and dangerous lightning are higher likelihood hazards in thunderstorms that affect the watch area.