1:05 p.m. update: As the worst of the storms are now reaching the Chesapeake Bay, and rain should diminish over the region between now and 3 p.m., I am concluding this live blog. Thanks for following along and for your reports!

12:45 p.m. update: The tornado watch has been discontinued from the District and to the west. It remains in effect for the eastern suburbs, but will probably be discontinued there by around 2 p.m.

12:42 p.m. update: Fortunately, this strong fall front has produced little in the way of severe weather in the region and, primarily, just beneficial rain. Amounts thus far are generally in the 0.5-1″ range (0.6-0.65 inches in the District), with some isolated higher amounts (1.31 inches in Sterling, according to WeatherBug). We might pick up another 0.1-0.2″ of rain but the back edge is now moving through the I-81 corridor and we should begin to dry out in the metro region between 2 and 3 p.m. The leading edge of the rain has now reached Annapolis.

12:25 p.m. update: Storms have now advanced into northern Anne Arundel and much of Prince George’s and Charles counties. Calvert and St. Mary’s counties: you’re next – the squall line should reach the area in the next 30 minutes. Locations along and west of I-95 are now in the clear in terms of a tornado threat.

12:13 p.m. update: The squall line now stretches from around Waldorf through Clinton and Bowie to around Baltimore. Wind gusts associated with the line remain mostly in the 35-45 mph range, with gusts of 41 mph and 43 mph measured at Reagan National and Manassas as the storms came through.

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.

Overview, 9:10 a.m.: The National Weather Service Storm Prediction Center has issued a tornado watch through 5 p.m. for a huge area from the Washington and Baltimore metro region all the way into central New York state including Philadelphia and New York City.

The combination of a strong cold front and low level spin in the atmosphere has primed the region for some gusty thunderstorms and perhaps a few tornadoes. The culprit is the same storm system – but in a weakened state – that produced up to four feet of snow in South Dakota, and a major tornado outbreak in Nebraska and Iowa.

Downpours and gusty winds are most likely in showers and storms that roll through the area today, but isolated tornado activity cannot be ruled out.  The best chance of tornadoes is east and northeast of the metro region this afternoon.

Timing: The worst of the storms have already passed through the I-81 corridor and should move through D.C.’s western suburbs between 9:30 and 11:30 a.m., the immediate metro region between 10:30 a.m. and 12:30, and the eastern suburbs between 11:30 and around 1:30 p.m.

ww0543_overview_wou Area covered by tornado watch. (National Weather Service)

Precautions: Remember, a tornado watch means there is a chance tornadoes will form in the region, so remain alert, and keep an eye on weather information.  A tornado warning – on the other hand – is more urgent.  A warning means a tornado has been detected by radar or spotted on the ground and to take action immediately. Go to the lowest level of a strong building and put as many walls between you and the outdoors as possible.

The NWS says there is a 30 percent chance of two or more tornadoes in this sprawling region, and a 20 percent chance of a strong tornado, rated EF2 or higher.

Earlier updates:

Noon:: The tornado warning that had been in effect for Stafford and Spotsylvania counties, and the City of Fredericksburg, has expired. I have not seen any reports of a confirmed tornado.

11:53 a.m. update: This squall line is trucking: it has already reached Mt. Vernon, Alexandria, most of the District and up north through Columbia and Ellicott City. It should reach La Plata, Clinton, Bowie, and Baltimore in the next 30 minutes or so. Main impacts from this line are heavy rain, reduced visibility and some wind gusts in the 30-50 mph range (generally below “severe” criteria).

11:45 a.m. update: Severe thunderstorm warning for eastern Stafford, western and southern Charles and southeast Prince George’s counties through 12:30 p.m. Storm with possible damaging winds to track through this region.

11:40 a.m. update: Squall line extends from Springfield to Arlington to Silver Spring. Should reach downtown just before noon and reach eastern suburbs by 12:15 or 12:30 p.m.. Gusty winds of 30-50 mph and downpours likely with this activity, with the possibility of some small hail.

11:30 a.m. update: TORNADO WARNING for Stafford and Spotsylvania counties, including Fredericksburg, through noon. Dopper radar indicated a possible tornado 17 miles southwest of Fredericksburg moving NE at 45 mph. Take cover in this path.

11:20 a.m. update: The squall line increased in speed and intensity and is moving inside the beltway now. Downpours and strong gusty winds – perhaps to 50 mph or so – are likely as this storm blitzes the metro area over the next 30-60 minutes.

11:05 a.m. update: The line of showers/storms continues marching eastward and is reaching the corridor from Manassas to Rockville. Doppler radar indicates the possibility of some gusty winds between North Potomac and Gaithersburg. The storms should reach the western side of the Beltway around 11:45 p.m. and move through the District just after noon.

10:35 a.m. update: The heaviest showers and storms now stretch from Warrenton through Germantown. As these come through, expect wind gusts of 30-45 mph and downpours. Manassas, Centreville and Rockville should see this activity move in during the next 30-45 minutes. Right now, there is no severe weather in the immediate area, but the storms to the southwest in central Virginia heading northeast continue to bear watching for the midday period.

10:20 a.m. update: The tornado watch has been extended to the south into central Virginia (including Fredericksburg and Charlottesville) and southern Maryland and discontinued in western Maryland. Strong to severe storms are currently impacting the Charlottesville area and extend northeast into Orange County. The D.C. area needs to watch these storms for midday impact.

10 a.m. update: The leading edge of the primary line of showers and storms has reached western Montgomery and Fairfax counties. More widely scattered showers have run out ahead of the line, with some on and off rain along and even a bit east of I-95. For now, the heaviest activity remains well west of the immediate metro region from Gainesville to Leesburg to Poolesville – but it is not severe. Storm motion is from southwest to northeast, and the immediate metro region should see the heaviest activity close to noon.

9:30 a.m. update: Showers and storms are currently advancing through central Frederick (including the city of Frederick) and eastern Loudoun counties. They are accompanied by gusty winds and downpours, but are not severe at the moment. As the line passed through Winchester earlier this morning, it took down several trees according to a report from the National Weather Service.