Question 1

What does the PJM footprint cover?

Accepted Answer

PJM Interconnection operates the bulk power system across all or part of 13 states and the District of Columbia, serving more than 65 million people from northern Illinois eastward across Ohio, Pennsylvania, and the Mid-Atlantic, and southward into Virginia, the western Carolinas, eastern Kentucky, and eastern Tennessee. The footprint straddles the Appalachian Mountains, which split it into a coastal Mid-Atlantic regime and a continental Ohio Valley regime — the same cold-air outbreak can settle west of the mountains while the coastal plain stays milder, and coastal storms can hit the Mid-Atlantic without producing significant precipitation west of the ridges.

Question 2

What does the PJM generation mix look like?

Accepted Answer

PJM has the most natural gas combined-cycle capacity of any U.S. ISO, much of it commissioned in the past decade adjacent to Marcellus and Utica shale gas production. Nuclear contributions come from a large fleet — Limerick, Peach Bottom, Salem, Hope Creek, the restarting Three Mile Island Unit 1 (now Crane Clean Energy Center), Davis-Besse, Beaver Valley, and others. Coal continues to retire across the Ohio Valley and Appalachia. Wind is concentrated in northern Indiana, the Pennsylvania ridges, and West Virginia. Solar is growing rapidly in the southern Mid-Atlantic. Hydro is modest, with Conowingo on the Susquehanna and several pumped-storage projects.

Question 3

What weather events most stress the PJM grid?

Accepted Answer

Mid-Atlantic summer heat across Washington, Baltimore, Philadelphia, and Newark drives the highest summer peaks, often accompanied by derechos and severe storms (the June 2012 derecho is the regional benchmark). Winter Arctic outbreaks across the Ohio Valley produce the most severe cold events — Winter Storm Elliott in December 2022 forced significant generator outages on the coldest morning. Recurving hurricanes (Sandy 2012, Ida 2021) deliver damage well inland, and ice storms across Kentucky, West Virginia, and the Virginia mountains produce widespread distribution outages. Data center load growth in Northern Virginia is now the single largest source of forecast load growth in the U.S.

Question 4

How does the Appalachian divide shape PJM weather?

Accepted Answer

The Appalachian Mountains split the PJM footprint into a coastal Mid-Atlantic regime and a continental Ohio Valley regime. Cold air outbreaks tend to settle west of the mountains while the coastal plain stays milder; conversely, coastal storms frequently impact the Mid-Atlantic without producing significant precipitation west of the ridges. Wind-flow direction relative to the ridges controls whether ice or snow falls on a given event, which has direct consequences for distribution outage patterns and which NWS alerts get issued where.

Question 5

What weather alerts dominate the Mid-Atlantic in summer?

Accepted Answer

The Washington, Baltimore, Philadelphia, and Newark metros routinely see Heat Advisories (forecast heat indices of 100°F to 104°F for two or more hours) and Excessive Heat Warnings (105°F or higher) during July and August. Severe Thunderstorm Warnings, Flash Flood Warnings, and occasional Tornado Warnings accompany the strong cold fronts that break the heat. The June 2012 derecho — warned for in real time by Severe Thunderstorm Warnings tagged for destructive winds — remains the regional benchmark for damaging-wind events.

Question 6

How does the National Hurricane Center's outlook affect PJM?

Accepted Answer

The NHC's Tropical Weather Outlooks track Atlantic systems through the June–November season. Hurricane and Tropical Storm Watches and Warnings can be issued for the New Jersey shore, the Delmarva Peninsula, and Tidewater Virginia 36 to 48 hours ahead of expected onset, with Storm Surge Warnings posted along the coastal counties. Even inland PJM territory sees major impacts from recurving systems — Sandy in 2012, Ida in 2021, and several smaller events drove significant inland flooding and distribution damage well into Pennsylvania and West Virginia.

Question 7

What weather drives PJM's most severe winter alerts?

Accepted Answer

Arctic outbreaks moving south out of central Canada drive the most extreme winter cold across PJM, with Wind Chill Warnings issued when chills are forecast to fall below approximately -20°F across the Ohio Valley and northern Mid-Atlantic. Synoptic Winter Storm Warnings are most common along the I-95 corridor for nor'easters and Alberta clipper events. The December 2022 Winter Storm Elliott was the recent benchmark cold event, with sustained sub-zero wind chills across the central footprint.

Question 8

Why do ice storms cause so much disruption across PJM?

Accepted Answer

The mid-elevation terrain across western Pennsylvania, West Virginia, Kentucky, and the Virginia mountains is highly prone to freezing rain when warm air aloft overruns a sub-freezing surface layer. The NWS issues Ice Storm Warnings when ice accumulations are expected to reach a quarter inch or more — enough to bring down tree limbs and weaken overhead distribution. The Appalachian ridges and valleys see some of the most sustained ice storm restoration challenges in the country, with damage spread across thousands of distribution feeders.

Question 9

What sources are referenced for the historical events on this page?

Accepted Answer

Specific event references on this page (the June 2012 derecho, Hurricane Sandy 2012, Hurricane Ida 2021, Winter Storm Elliott in December 2022) are drawn from the FERC and NERC Inquiry into Bulk-Power System Operations During Winter Storm Elliott (October 2023), the NWS Weather Prediction Center storm-event summaries, the National Hurricane Center's Tropical Cyclone Reports at nhc.noaa.gov, and PJM's published event analyses and capacity performance reports at pjm.com.

Metric	Actual	Normal	Departure
CDD	1436.1	1170.6	+265.5
HDD	2952.6	2836.3	+116.3

PJM

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PJM (PJM Interconnection) — Frequently Asked Questions

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