610PM EDT Update:
A line of severe storms with winds up to and possibly in excess of 60 MPH continues to move through southeastern Pennsylvania. Philadelphia is now under the warning, but shouldn't see the storms for another hour or so.
537PM EDT Radar Update:
Allentown and Reading PA are the first in line for a new line of severe storms. Philly and Trenton are the two biggest cities next in line if the storms hold up.
515PM EDT UPDATE: The line isn't getting the organization it needs to be a powerful system, so the likelihood of a derecho is waning. I'm not feeling too confident that the line is as bad as it could be. I still defend the use of calling this a possible derecho, because the conditions were there for it to turn into a dangerous situation, but it didn't follow through. Better to be safe than sorry.
Regardless, the storms still have a history of producing damage to trees and winds around 60 MPH. Warnings are now in effect for the areas around Pottsville, Allentown, and Reading PA.
A strong line of storms is starting to develop in northern Pennsylvania right now, and renowned severe weather expert Dr. Greg Forbes sounded the alarm a few minutes ago:
A derecho (widespread damaging windstorm associated with a line or bow of thunderstorms) could drive from north-central to southeast PA and possibly into MD, NJ, and DE this afternoon and evening.
The NWS is tracking a line of storms northeast of State College, PA that has a history of producing winds of 60-75 MPH. The line shows no signs of weakening right now, and continues to move southeast at 30 MPH. Here's the wording the NWS is using in their warnings:
* SEVERE THUNDERSTORM WARNING FOR...
COLUMBIA COUNTY IN CENTRAL PENNSYLVANIA...
MONTOUR COUNTY IN CENTRAL PENNSYLVANIA...
NORTHERN SNYDER COUNTY IN CENTRAL PENNSYLVANIA...
SOUTHERN SULLIVAN COUNTY IN NORTH CENTRAL PENNSYLVANIA...
UNION COUNTY IN CENTRAL PENNSYLVANIA...
NORTHUMBERLAND COUNTY IN CENTRAL PENNSYLVANIA...
NORTHEASTERN CENTRE COUNTY IN CENTRAL PENNSYLVANIA...
SOUTHERN LYCOMING COUNTY IN NORTH CENTRAL PENNSYLVANIA...
SOUTHERN CLINTON COUNTY IN NORTH CENTRAL PENNSYLVANIA...
* UNTIL 530 PM EDT
* AT 423 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
LINE OF SEVERE THUNDERSTORMS. THESE SEVERE STORMS EXTENDED FROM
KASSON BROOK TO MUNCY VALLEY TO MUNCY TO MCELHATTAN TO KEATING...
MOVING SOUTHEAST AT 30 MPH. THESE STORMS ARE CAPABLE OF PRODUCING
QUARTER SIZE HAIL...AND DESTRUCTIVE WINDS IN EXCESS OF 70 MPH.
The development of the line over the next few hours will determine how severe it is and how long it lasts. Environmental conditions are favorable for the line to maintain intensity and/or strengthen. If the line is able to continue to organize, it could turn into a small "powerhouse" that produces a long swath of intense winds and wind damage. The term "derecho" only means that this could be a long-lived, widespread wind damage event.
Here's the line of storms as of 4:30PM EDT:
If you live in southeastern Pennsylvania, northeastern Maryland, or anywhere in New Jersey or Delaware, you need to keep a very, very close eye on this system. These storms are fast paced and can develop or dissipate rapidly and without much reaction time. Prepare for the potential for wind damage and/or power outages. Also prepare for the potential that the line could shift more more southerly and threaten the Baltimore/Washington DC area.
Even if this alert turns out to be a false alarm, it's better to be prepared than to be blindsided.
NOTE: A "derecho" is just a line of storms that produces extensive wind damage over a span of 240 miles. Even if the storms only last 100 miles, it's still dangerous and can produce a lot of damage. Don't let the term "derecho" fool you. Any severe storm is dangerous no matter what it's called.
See the Storm Prediction Center's "About Derechos" page for more information, and see my writeup from the system that went from Chicago to Atlantic City.