Question 1

What does the ISO-NE footprint cover?

Accepted Answer

ISO New England operates the bulk power system for Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont, serving roughly 14.5 million people across 6.5 million customers. The territory is geographically constrained, transmission-import limited, and historically dependent on natural gas. Significant offshore wind procurement under state-led RFPs is now in active build-out along the Atlantic coast, with Vineyard Wind, Revolution Wind, and South Fork Wind in operation or commissioning.

Question 2

What does the New England generation mix look like?

Accepted Answer

Natural gas dominates ISO-NE generation, frequently exceeding 50% of energy production on a typical day. Nuclear contributions come from Seabrook in New Hampshire and Millstone in Connecticut. Hydro is a mix of in-region (mostly Maine and Vermont) and large imports from Hydro-Québec via the Phase II HVDC tie. Coal and oil generation now exists almost exclusively as winter reliability backstop, dispatched only during cold snaps when gas supply is constrained. Behind-the-meter solar has grown rapidly, suppressing midday demand on the system.

Question 3

What weather events most stress the ISO-NE grid?

Accepted Answer

Winter is the defining stress season for ISO-NE — the system's installed capacity requirement is set by winter, not summer, peak, a relative rarity among U.S. ISOs. The signature challenge is natural gas pipeline constraint during cold snaps: the regional gas distribution system serves heating customers under firm contracts first, leaving gas-fired generators with interruptible supply short of fuel. The system relies on dual-fuel oil-burning capability, LNG send-out from Distrigas in Everett, and pre-positioned heating-oil inventory. Nor'easters, ice storms, and recurving hurricanes (Sandy 2012, the 1938 hurricane) round out the major weather threats.

Question 4

What is a nor'easter and how is it warned for in New England?

Accepted Answer

A nor'easter is a coastal storm that develops or intensifies along the Gulf Stream and tracks up the Eastern Seaboard, with winds blowing from the northeast onto the New England coast. The National Weather Service issues Winter Storm Watches 24 to 48 hours ahead of significant snow, then Winter Storm Warnings or Blizzard Warnings for higher-end events, alongside Coastal Flood Warnings, High Wind Warnings, and Beach Erosion Statements for coastal communities. Nor'easters are the dominant winter storm threat for Boston, Providence, Hartford, and the southern New England coastline.

Question 5

What weather drives New England's coldest winter days?

Accepted Answer

Brutal cold in New England comes from modified Arctic air masses spilling out of central and eastern Canada. The NWS issues Wind Chill Watches and Wind Chill Warnings when wind chills are forecast to fall below approximately -25°F (regional thresholds vary). Northern New Hampshire, Maine, and Vermont can see actual air temperatures into the negative double digits; the southern New England metros are typically less extreme but more impactful for grid load due to population density. The February 2023 wind chill event that produced -108°F readings on Mount Washington is the recent reference case.

Question 6

How does the National Hurricane Center's outlook affect New England?

Accepted Answer

Hurricane and tropical storm impacts on New England, while less frequent than further south, can be severe. The NHC's Tropical Weather Outlooks and individual advisories track systems that may recurve up the Eastern Seaboard, and Hurricane Watches/Warnings are issued 48 and 36 hours ahead of expected onset. Hurricane Sandy in 2012 and the September 1938 hurricane (the 'Long Island Express') remain the two most-studied events. Even as a post-tropical system, a recurving hurricane can deliver damaging winds and storm surge to coastal Massachusetts, Rhode Island, and Connecticut.

Question 7

What summer weather events drive New England's heat alerts?

Accepted Answer

The NWS issues Heat Advisories for southern New England when forecast heat indices reach 100°F or higher for two or more hours, and Excessive Heat Warnings at higher thresholds. Boston, Hartford, Providence, and the Connecticut River Valley see the most frequent summer heat alerts. The signature event is the multi-day heat wave (three or more consecutive days at or above 90°F), which compounds health risk and drives the highest summer cooling load on the system.

Question 8

Why do ice storms cause so much disruption in New England?

Accepted Answer

Ice storms occur when supercooled rain falls into a sub-freezing surface layer and accretes on conductors, tree limbs, and structures. 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 lines. The hilly terrain of central New Hampshire, Vermont, and western Massachusetts is particularly vulnerable, and the December 2008 ice storm remains the regional benchmark for slow, multi-week distribution restoration.

Question 9

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

Accepted Answer

Specific event references on this page (Hurricane Sandy 2012, the September 1938 hurricane, the December 2008 ice storm, the February 2023 Mount Washington wind chill event) are drawn from the National Hurricane Center's Tropical Cyclone Reports, NWS Gray and NWS Boston event summaries, the Mount Washington Observatory's published observations at mountwashington.org, and ISO New England's published winter readiness assessments and event reports at iso-ne.com.

City	Wed May 27	Thu May 28	Fri May 29	Sat May 30	Sun May 31	Mon Jun 1	Tue Jun 2	Norm
Boston, MA	8860	7154	6953	6547	6952	6751	7255	70/52
Worcester, MA	8657	7251	7150	6544	7048	6947	7452	68/47
Providence, RI	8559	7452	7152	6746	7050	6949	7353	70/50
Hartford, CT	8759	7554	7554	6947	7251	7250	7553	72/50
New Haven, CT	8461	7755	7756	7148	7253	7151	7354	71/50
Stamford, CT	8661	7756	8057	7250	7554	7453	7656	72/52
Bridgeport, CT	8460	7755	7856	7149	7253	7252	7355	72/52
Springfield, MA	8858	7553	7552	6846	7349	7249	7751	72/47
Lowell, MA	8757	7151	7049	6445	6948	6848	7352	69/47
Cambridge, MA	8860	7154	7053	6548	6951	6751	7355	70/52
Manchester, NH	8654	7249	7046	6343	6846	6946	7350	70/45
Portland, ME	7754	6549	5947	5745	6248	6248	6551	66/46

City	Wed May 27	Thu May 28	Fri May 29	Sat May 30	Sun May 31	Mon Jun 1	Tue Jun 2
Boston, MA	14	490.04"	77	770.24"	18	23	10
Worcester, MA	11	33	71	710.05"	13	30	14
Providence, RI	24	440.32"MOD	69	690.06"	14	26	13
Hartford, CT	240.05"	12	42	42	13	25	12
New Haven, CT	180.05"	9	31	31	10	20	10
Stamford, CT	190.03"	9	27	27	9	17	11
Bridgeport, CT	17	9	29	29	9	19	10
Springfield, MA	24	13	51	51	13	29	12
Lowell, MA	13	450.12"	780.03"	780.19"	19	26	13
Cambridge, MA	14	490.06"	76	760.24"	19	24	10
Manchester, NH	12	23	780.08"	780.25"MOD	23	26	14
Portland, ME	1	610.02"	820.14"	820.56"MOD	35	20	10

City	Wed, 5/27	Thu, 5/28	Fri, 5/29	Sat, 5/30	Sun, 5/31	Mon, 6/1	Tue, 6/2
Boston	10 mphModerateW	7 mphLowN	5 mphLowNE	11 mphModerateNW	9 mphModerateNNW	7 mphLowN	5 mphLowSE
Worcester	10 mphModerateW	8 mphModerateNNW	6 mphLowWNW	9 mphModerateWNW	8 mphModerateNNW	6 mphLowNNW	5 mphLowNNW
Providence	9 mphModerateWSW	7 mphLowNNW	5 mphLowWNW	11 mphModerateWNW	7 mphLowNW	5 mphLowN	4 mphLowN
Hartford	7 mphLowWSW	9 mphModerateNNW	7 mphLowNW	11 mphModerateNW	8 mphModerateNNW	6 mphLowNW	3 mphLowENE
New Haven	6 mphLowSW	7 mphLowNNW	6 mphLowW	9 mphModerateWNW	7 mphLowNW	5 mphLowNW	4 mphLowE
Stamford	7 mphLowWSW	10 mphModerateNNW	7 mphLowWNW	10 mphModerateWNW	9 mphModerateNW	7 mphLowNNW	6 mphLowENE
Bridgeport	7 mphLowSW	8 mphModerateNNW	7 mphLowW	10 mphModerateWNW	8 mphModerateNW	6 mphLowNW	5 mphLowE
Springfield	7 mphLowWSW	9 mphModerateNNW	7 mphLowNW	12 mphModerateWNW	9 mphModerateNNW	7 mphLowNNW	4 mphLowNNW
Lowell	9 mphModerateW	7 mphLowNNW	4 mphLowWNW	9 mphModerateWNW	6 mphLowNNW	4 mphLowN	4 mphLowWSW
Cambridge	10 mphModerateW	8 mphModerateN	5 mphLowNE	11 mphModerateNW	8 mphModerateNNW	6 mphLowN	5 mphLowSE
Manchester	5 mphLowSW	8 mphModerateNNW	5 mphLowWNW	9 mphModerateW	7 mphLowNNW	4 mphLowN	3 mphLowNW
Portland	9 mphModerateW	6 mphLowN	6 mphLowENE	14 mphModerateENE	10 mphModerateNNW	6 mphLowNE	6 mphLowSW

Date	HDD Fcst	CDD Fcst	HDD Norm	HDD Dep.	CDD Dep.
Wed May 27	0	6.9	5.3	-5.3	+6.9
Thu May 28	2.4	0.3	5.3	-3	+0.3
Fri May 29	3.5	0.6	5.3	-1.8	+0.6
Sat May 30	8.5	0	5.3	+3.2	0
Sun May 31	4.8	0	5.3	-0.5	0
Mon Jun 1	5.5	0	5.3	+0.2	0
Tue Jun 2	2	0.1	5.3	-3.3	+0.1
7d Total	26.7	7.9	37.1	-10.5	+7.9

Metric	Actual	Normal	Departure
CDD	62.0	0.0	+62.0
HDD	3832.0	3598.8	+233.2

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