Question 1

What does the non-CAISO Western footprint cover?

Accepted Answer

The Western Interconnection is the synchronously connected bulk power system spanning 14 western U.S. states, two Canadian provinces (British Columbia and Alberta), and a portion of northern Baja California, Mexico. Outside the CAISO footprint, it is fragmented into roughly 38 individual balancing authorities, including Bonneville Power Administration (BPA), PacifiCorp East and West, Arizona Public Service, NV Energy, Salt River Project, Idaho Power, PNM, and many others. The Western Electricity Coordinating Council (WECC) is the regional reliability entity, but it is not a market operator. The April 2026 SPP western expansion carved several balancing authorities along the Front Range out of the historic non-CAISO Western footprint.

Question 2

What does the Western generation mix look like?

Accepted Answer

Generation across the non-CAISO Western footprint is dominated by Columbia River basin hydroelectric (BPA, Mid-Columbia, and Snake River projects), which can supply more than half of Pacific Northwest demand in a high-water year. Wind is concentrated in the Columbia Gorge, Wyoming, and the Mountain West. Solar is rapidly growing across Arizona, Nevada, Utah, and New Mexico, with the Desert Southwest hosting some of the highest-capacity-factor solar resources in the country. Coal continues to retire (Intermountain, Springerville, San Juan, Naughton, Jim Bridger). Natural gas combined-cycle and peakers fill the dispatchable middle. Geothermal in northern Nevada and Palo Verde Nuclear (Arizona, the largest nuclear plant in the United States) round out the mix.

Question 3

What weather events most stress the Western grid?

Accepted Answer

The non-CAISO West encompasses two of the most weather-extreme power territories in the country. Pacific Northwest winter storms determine snowpack accumulation, which in turn determines next-summer hydroelectric availability — a low snowpack winter can cut Columbia hydro capacity sharply months later. The Desert Southwest experiences the nation's most extreme summer heat, with Phoenix exceeding 110°F regularly and overnight lows often failing to drop below 90°F during multi-day heat domes. Wildfire smoke and direct flames threaten transmission corridors across virtually the entire footprint (the 2020 Labor Day fires in Oregon, the 2021 Caldor and Dixie fires that derated key California-Oregon transmission paths). Drought years compress hydro across the Columbia, Colorado, and California river systems simultaneously.

Question 4

What weather alerts dominate the Desert Southwest in summer?

Accepted Answer

The National Weather Service issues Excessive Heat Warnings across Phoenix, Las Vegas, Tucson, and the broader Desert Southwest when forecast highs are expected to reach roughly 110°F or higher (criteria vary by zone). Multi-day heat domes that lock in 115°F+ daytime highs and overnight lows that fail to drop below 90°F are the most consequential events, since residential and commercial cooling load stays elevated 24 hours a day with no diurnal recovery. Phoenix's annual Excessive Heat Warning days have grown sharply over the past decade.

Question 5

How are atmospheric river storms warned for in the Pacific Northwest?

Accepted Answer

Atmospheric rivers — narrow corridors of concentrated water vapor streaming off the Pacific — produce most of the Pacific Northwest's annual precipitation in just a handful of winter events. The NWS issues Flood Watches, Flood Warnings, High Wind Warnings, and Winter Storm Warnings across the Cascades and Coast Range when strong AR events are inbound. Beyond immediate flood and wind impacts, the snowpack accumulated during these storms determines next-summer hydroelectric availability across the Columbia and Snake River systems.

Question 6

What weather drives Red Flag Warnings across the Western Interconnection?

Accepted Answer

The NWS issues Red Flag Warnings when the combination of low humidity, strong winds, and dry fuels creates conditions for rapid wildfire spread. Across the West, Red Flags are most common in summer and early fall, particularly during downsloping wind events along mountain ranges and during dry-lightning episodes. Wildfire smoke from regional fires can degrade solar output across multiple states for days at a time, and direct fire impacts on transmission corridors can force lines out of service for safety.

Question 7

How does the National Weather Service warn for monsoon weather across the Southwest?

Accepted Answer

The North American Monsoon brings an annual surge of moisture into Arizona, New Mexico, and surrounding states from roughly mid-June through September. Monsoon-related alerts include Severe Thunderstorm Warnings, Flash Flood Warnings, and Dust Storm Warnings (Haboob events). The signature events are intense thunderstorm complexes that drop heavy rain on burn scars and wash out roads, and dust walls that reduce visibility to near zero across the Phoenix and Tucson metros.

Question 8

What weather drives Winter Storm Warnings across the Mountain West?

Accepted Answer

The NWS issues Winter Storm Warnings across the Mountain West when significant snow, blowing snow, or ice accumulation is expected. The Cascades, Sierra Nevada, Wasatch, and Rockies all see frequent multi-foot snow events through the November–April stretch. Wind Chill Warnings accompany Arctic outbreaks across Wyoming, Montana, and Idaho when wind chills fall below approximately -25°F. Snowpack accumulated through these storms drives runoff that determines summer hydro availability across the entire Western Interconnection.

Question 9

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

Accepted Answer

Specific event references on this page (Pacific Northwest atmospheric river events, the 2020 Labor Day fires in Oregon, the 2021 Caldor and Dixie fires, Desert Southwest heat dome events, the SPP April 2026 western expansion) are drawn from NWS Portland, Medford, Phoenix, and Las Vegas event summaries, CAL FIRE and U.S. Forest Service incident records, the U.S. Drought Monitor at droughtmonitor.unl.edu, and Western Electricity Coordinating Council reliability assessments at wecc.org.

City	Wed May 27	Thu May 28	Fri May 29	Sat May 30	Sun May 31	Mon Jun 1	Tue Jun 2	Norm
Seattle, WA	7254	7853	6049	6448	6649	7154	7954	66/49
Portland, OR	8156	8454	6746	6846	7249	8457	8557	68/50
Spokane, WA	8255	9257	7949	6746	7045	7648	8248	68/45
Boise, ID	8556	8655	8052	7349	7650	8253	8958	73/48
Las Vegas, NV	8261	8363	8264	9170	9671	9974	10274	90/67
Reno, NV	5439	6142	6644	7246	7649	8152	8552	76/47
Phoenix, AZ	9367	9366	8967	9471	9973	10476	10676	96/68
Tucson, AZ	9462	9461	9261	9364	9867	10170	10370	92/62
Salt Lake City, UT	8657	8757	7950	7251	7752	8255	8757	73/50
Albuquerque, NM	8653	8954	8952	8655	9058	9259	9158	80/53
Billings, MT	9360	9761	9860	8253	7649	7750	7951	71/45

City	Wed May 27	Thu May 28	Fri May 29	Sat May 30	Sun May 31	Mon Jun 1	Tue Jun 2
Seattle, WA	3	58	580.03"	9	1	1	6
Portland, OR	4	25	250.07"	0	1	1	3
Spokane, WA	740.09"	69	690.28"MOD	260.06"	12	6	4
Boise, ID	480.05"	16	40	400.08"	5	5	2
Las Vegas, NV	3	5	5	0	0	0	0
Reno, NV	920.11"	650.04"	29	12	7	4	3
Phoenix, AZ	0	0	0	0	0	0	0
Tucson, AZ	0	0	0	0	2	3	7
Salt Lake City, UT	13	1	42	47	13	8	3
Albuquerque, NM	5	16	16	4	0	5	25
Billings, MT	15	4	15	740.13"	680.35"MOD	290.04"	21

City	Wed, 5/27	Thu, 5/28	Fri, 5/29	Sat, 5/30	Sun, 5/31	Mon, 6/1	Tue, 6/2
Seattle	5 mphLowNNE	4 mphLowN	7 mphLowSSW	5 mphLowNE	5 mphLowESE	6 mphLowE	5 mphLowNNE
Portland	7 mphLowNNW	5 mphLowWNW	7 mphLowW	6 mphLowNW	6 mphLowNW	6 mphLowNNW	5 mphLowNNW
Spokane	1 mphLowSSE	1 mphLowN	7 mphLowS	10 mphModerateSSW	8 mphModerateSSW	3 mphLowS	1 mphLowESE
Boise	8 mphModerateW	9 mphModerateE	5 mphLowN	10 mphModerateWNW	5 mphLowNW	3 mphLowNNW	3 mphLowNE
Las Vegas	17 mphHighSSW	11 mphModerateS	9 mphModerateSSW	4 mphLowS	4 mphLowSSW	5 mphLowSSE	4 mphLowSW
Reno	5 mphLowW	2 mphLowNNW	4 mphLowW	3 mphLowW	2 mphLowWSW	2 mphLowNNW	1 mphLowWSW
Phoenix	7 mphLowS	6 mphLowS	6 mphLowSW	4 mphLowSW	3 mphLowSW	2 mphLowSW	3 mphLowW
Tucson	9 mphModerateS	9 mphModerateS	7 mphLowSW	5 mphLowW	3 mphLowW	3 mphLowW	5 mphLowW
Salt Lake City	12 mphModerateSE	8 mphModerateSSE	5 mphLowS	5 mphLowS	3 mphLowNNE	4 mphLowWSW	4 mphLowNE
Albuquerque	6 mphLowENE	13 mphModerateESE	13 mphModerateESE	12 mphModerateW	4 mphLowWSW	5 mphLowESE	8 mphModerateESE
Billings	7 mphLowSSE	6 mphLowSSE	5 mphLowS	9 mphModerateWNW	11 mphModerateWSW	7 mphLowWNW	6 mphLowNNW

Date	HDD Fcst	CDD Fcst	HDD Norm	CDD Norm	HDD Dep.	CDD Dep.
Wed May 27	1.9	6.3	3.7	4	-1.8	+2.3
Thu May 28	1.2	7.5	3.7	4	-2.5	+3.5
Fri May 29	2.8	4.8	3.7	4	-0.9	+0.8
Sat May 30	3.5	5	3.7	4	-0.1	+1
Sun May 31	2.5	6	3.7	4	-1.2	+2
Mon Jun 1	0.6	8.2	3.7	4	-3	+4.2
Tue Jun 2	0	9.7	3.7	4	-3.7	+5.7
7d Total	12.5	47.5	25.9	28	-13.2	+19.5

Metric	Actual	Normal	Departure
CDD	303.2	146.6	+156.6
HDD	1982.5	2392.3	-409.8

WECC

Temperature Forecast

Precipitation Chance

Recent History (7 days)

WECC (Western Interconnection) — Frequently Asked Questions

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