8:45 a.m. update: York County now has a flood advisory as storms moving through the area cause rapid rises in small stream and also urban flooding, the National Weather Service said.
For current road conditions, click here.
The area remains under a severe thunderstorm watch until 11 a.m. and a flood watch until 8 p.m.
7:40 a.m.update: York County is under a severe thunderstorm warning until 8:30 a.m., the National Weather Service announced.
A line of severe storms is moving east into the area at 45 mph, the NWS said. The storms are capable of producing hail that's 1 inch in diameter and damaging winds up to 60 mph.
These storms are also producing near-continuous cloud to ground lightning, the service said. Everyone is urged to take shelter inside as these storms pass through.
5:25 a.m. update: York County is under a severe thunderstorm watch until 11 a.m. and a flood watch until 8 p.m., the National Weather Service said.
All of central Pennsylvania is warned to watch for severe weather this morning as a line of thunderstorms moves east across the state, the service said. The storms have the potential for dangerous lightning, torrential downpours, damaging winds and hail, and residents should be prepared to take shelter as storms move through.
The flood watch means there's a potential for flooding along small streams and creeks and in areas with poor drainage, the service said.
For current conditions, check the abc27 radar here.
11:35 update: York County remains under a flood watch, according to the National Weather Service.
The flood remains in effect from 2 a.m. to 8 p.m. Thursday. The area is expected to get severe thunderstorms throughout the day Thursday, according to NWS.
The area could get one to two inches of rain, according to WHTM abc 27.
High temperatures are expected to be in the low 80s, both weather forecasters report.
4:41 p.m. update: York County could see a couple of isolated storms Wednesday night and severe thunderstorms throughout the day Thursday, according to meteorologists.
"What happens this evening depends on what happens in the Midwest, like in Illinois, Indiana, and how fast the storms move through those states," said Ryan Coyle, meteorologist with WHTM abc27. "There is a flooding threat Thursday because everything is already saturated, so you could get a flooding when you tack on one to three inches of rain."
Flooding issues are only expected for small streams and creeks, he said.
"But the main rivers, like the Susquehanna River, should be fine," Coyle added. "They can hold this water."
The county's weather issues could start off with a couple of isolated storms after 10 p.m. Wednesday, Coyle said.
Both Coyle and Erik Pindrock, meteorologist with AccuWeather.com, said they doubt the area will experience a "derecho," which Coyle described as a very large wind storm imbedded within a thunderstorm.
However, the county could get severe thunderstorms with powerful winds Thursday, they said.
"Over the next 24 to 36 hours, it's going to be very active and very dangerous for the York County area," Pindrock said. "There's a potential for damaging wind gusts, hail, flooding, downpours and even a potential for an isolated tornado in the area."
The potential for getting severe storms would wind down Thursday evening, Pindrock said.
"Throughout the day Thursday is the concern," he said.
The high temperature Thursday is expected to be near 80 degrees, according to Pindrock.
1:03 p.m. update: York County and most of central Pennsylvania is under a flood watch from Wednesday evening until late Thursday, the National Weather Service said.
A strong storm system will bring several rounds of heavy showers and thunderstorms through the region Wednesday night and through the day Thursday, the NWS said.
There is a potential for flooding along streams and creeks and in low-lying areas, especially those normally prone to flooding, the service said. Anyone living in those areas is advised to be prepared to take action if there is flooding.
Previous story: A potent storm is likely on its way to York County.
Although it hasn't shown up on the map yet, the bulk of the storm will probably hit the area during the day Thursday, in the late morning or afternoon, said John LaCorte, a meteorologist with the National Weather Service in State College.
"I wouldn't rule out the 'tornado' word," he said.
The weather that will hit this area on Thursday is the result of a gigantic line of powerful thunderstorms that could affect one in five Americans on Wednesday as it rumbles from Iowa to Maryland packing hail, lightning and tree-toppling winds.
Meteorologists are warning that the continuous line of storms may even spawn an unusual weather event called a derecho (duh-RAY'-choh), which is a massive storm of strong straight-line winds spanning at least 240 miles.
Wednesday's storms are also likely to generate tornadoes and cause power outages that will be followed by oppressive heat, said Bill Bunting, operations chief at the National Weather Service's Storm Prediction Center in Norman, Okla.
The threat: The risk of severe weather in Chicago, Indianapolis, Cincinnati and Columbus, Ohio, is roughly 45 times higher than on a normal June day, Bunting said. Detroit, Baltimore, Washington, Milwaukee, Pittsburgh and Louisville, Ky., have a risk level 15 times more than normal. All told, the area the weather service considers to be under heightened risk of dangerous weather includes 64 million people in 10 states.
"It's a pretty high threat," Bunting said, who also warned that the storms will produce large hail and dangerous lightning. "We don't want to scare people, but we want them to be aware."
Wednesday "might be the worst severe weather outbreak for this part of the country for the year," said Jeff Masters, meteorology director at Weather Underground.
You can have tornadoes and a derecho at the same time, but at any given place Wednesday the straight-line winds are probably more likely.
Last year, a derecho caused at least $1 billion in damage from Chicago to Washington, killing 13 people and leaving more than 4 million people without power, according to the weather service. Winds reached nearly 100 mph in some places and in addition to the 13 people who died from downed trees, another 34 people died from the heat wave that followed in areas without power.
Derechoes, with winds of at least 58 mph, occur about once a year in the Midwest. Rarer than tornadoes but with weaker winds, derechoes produce damage over a much wider area.
Wednesday's storm probably won't be as powerful as 2012's historic one, but it is expected to cause widespread problems, Bunting said.
The storms are the type that will move so fast that "by the time you see the dark sky and distant thunder you may have only minutes to get to safe shelter," Bunting said.
Around here: If the storm remains intact after crossing the Appalachian Mountains, which would be rare for a derecho, it should hit the Washington area by late afternoon or early evening, he said.
For Washington, Philadelphia and parts of the Mid-Atlantic the big storm risk continues and even increases a bit Thursday, according to the weather service.
LaCorte expects the bad weather to hit this area and says residents should stay tuned to storm updates, as they change rapidly.
This area won't see oppresive heat in the wake of the story, he said.
Today's high is 84 degrees, and temperatures will hover in the 70s this week, according to the weather service.
York Dispatch staff writer Mollie Durkin contributed to this report.