Degree Days & Energy Demand
Base Temperature
The reference point used in degree-day calculations — conventionally 65°F (18°C) in the United States. Below this temperature, heating is generally needed; above it, cooling is generally needed. The base temperature reflects the historical observation that most buildings neither heat nor cool when outdoor air is near 65°F.
CDD (Cooling Degree Days)
A measure of how much cooling a building is likely to need on a given day. Calculated as the number of degrees the average daily temperature exceeds the 65°F base temperature. A day with an average temperature of 80°F yields 15 CDDs. CDDs accumulate through the warm season and are the primary indicator of electricity demand for air conditioning.
Departure from Normal
The difference between an observed or forecast value and the 30-year climatological average (normal) for that location and date. A departure of +8°F means conditions are 8 degrees warmer than the long-term average. Departures are used throughout the site to contextualize whether conditions are anomalous relative to historical expectations.
HDD (Heating Degree Days)
A measure of how much heating a building is likely to need on a given day. Calculated as the number of degrees the average daily temperature falls below the 65°F base temperature. A day with an average temperature of 40°F yields 25 HDDs. HDDs accumulate through the cold season and are the primary indicator of energy demand for space heating.
Normal (30-Year Average)
The standard climatological baseline published by NOAA, calculated from the most recent 30-year period (currently 1991–2020). Normals represent the expected average temperature, precipitation, or degree-day total for a given location and calendar date. Comparisons to normal are used throughout the site to identify whether conditions are above, near, or below historical averages.
Season-to-Date
The cumulative total of heating or cooling degree days from the start of the current heating or cooling season through the present date. The heating season conventionally runs July 1 through June 30; the cooling season runs January 1 through December 31. Season-to-date totals help energy analysts track whether cumulative demand is running ahead of or behind a typical year.
Forecast Models
ECMWF (European Centre for Medium-Range Weather Forecasts)
A European intergovernmental organization that produces what is widely regarded as the world's most accurate global numerical weather prediction model, commonly called "the Euro." ECMWF forecasts are run twice daily and extend to about 15 days. The model has historically outperformed U.S. models on medium-range forecasts, particularly for storm track and temperature anomaly prediction.
Ensemble Forecast
A set of multiple model runs, each starting from slightly different initial conditions or using different model physics, used to represent forecast uncertainty. Where ensemble members agree, forecasters have higher confidence; where they diverge, uncertainty is greater. Both GFS and ECMWF operate ensemble systems (GEFS and ENS, respectively).
GFS (Global Forecast System)
The primary global numerical weather prediction model operated by the National Weather Service / NOAA. GFS runs four times daily and produces forecasts out to 16 days. It is the most widely used free weather model in the world and forms the backbone of most public and commercial forecast products in the United States.
Model Agreement
A qualitative assessment of whether the GFS and ECMWF models are producing similar or divergent forecasts for a given period. High model agreement generally increases forecaster confidence; low agreement — particularly in the 6–10 day range — signals greater forecast uncertainty and is noted throughout the site when evaluating extended outlooks.
NAM (North American Mesoscale Model)
A regional numerical weather prediction model run by NOAA that covers North America at higher spatial resolution than the GFS. The NAM is better suited for short-range (0–60 hour) forecasts of convection, precipitation, and local terrain effects. It is run four times daily.
Climate Outlooks
Above / Near / Below Normal
The three probability categories used in CPC outlooks. "Above normal" and "below normal" indicate enhanced probability of temperatures or precipitation finishing above or below the historical median for the period. "Near normal" (also called "equal chances") means no strong statistical signal in either direction. Note that these are probabilities, not forecasts of exact values.
CPC (Climate Prediction Center)
A center within NOAA that produces official U.S. outlooks for temperature and precipitation probabilities at lead times from one week to one year. CPC products are widely used by energy traders, utilities, and commodity markets to anticipate seasonal demand. The center is part of the National Centers for Environmental Prediction (NCEP).
CPC 6–10 Day Outlook
A CPC product showing the probability of above-normal, near-normal, or below-normal temperature and precipitation for days 6 through 10 from today, covering a 5-day window. It is updated daily and is one of the most-referenced products in energy market weather analysis.
CPC 8–14 Day Outlook
Similar in structure to the 6–10 day outlook but covering days 8 through 14, providing a slightly longer-range view of temperature and precipitation tendencies. Forecast confidence decreases at these lead times, so the outlook is best interpreted as a probabilistic signal rather than a specific forecast.
CPC Monthly & Seasonal Outlooks
CPC also produces one-month and three-month (seasonal) temperature and precipitation outlooks, updated monthly. These longer-range products are used for capacity and fuel planning and reflect climate signals such as ENSO (El Niño / La Niña) and teleconnection patterns rather than individual weather systems.
ISO / RTO Regions
CAISO (California Independent System Operator)
The independent system operator that manages the electric transmission grid and wholesale energy market for approximately 80% of California's electric load. Founded in 1998, CAISO serves most of the state with the exception of major municipal utilities such as the Los Angeles Department of Water and Power (LADWP) and Sacramento Municipal Utility District (SMUD). It coordinates the dispatch of power plants, operates real-time and day-ahead markets, and ensures grid reliability across its service territory.
ERCOT (Electric Reliability Council of Texas)
The independent system operator responsible for managing the flow of electric power to approximately 90% of Texas's electric load, covering 254 counties. ERCOT operates as a standalone interconnection with minimal ties to the Eastern or Western Interconnections, making it unique among major U.S. grid operators. It runs competitive wholesale markets for energy, ancillary services, and manages transmission across its footprint, which includes some of the largest electricity demand centers in the country.
ISO (Independent System Operator)
A nonprofit organization that coordinates, controls, and monitors the electric transmission grid within a defined geographic footprint. ISOs manage real-time energy dispatch, operate wholesale electricity markets, and ensure reliability across their service territories. In the U.S., ISO and RTO are often used interchangeably, though RTOs are federally certified under FERC Order 2000 and typically have broader transmission planning authority.
ISO-NE (ISO New England)
The independent system operator for the six-state New England region: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. ISO-NE manages the bulk power system, administers wholesale electricity markets, and coordinates long-term planning for the region's grid. It was established in 1997 and is overseen by the Federal Energy Regulatory Commission (FERC).
MISO (Midcontinent Independent System Operator)
A nonprofit RTO that manages the bulk electric transmission system and wholesale energy markets across a 15-state footprint stretching from the Canadian border south through Louisiana, as well as the Canadian province of Manitoba. MISO is one of the largest energy markets in the world by geographic footprint and total installed generation capacity. It operates day-ahead and real-time energy markets, capacity markets, and ancillary services markets.
NYISO (New York Independent System Operator)
The independent system operator that manages New York State's bulk electric transmission system and wholesale electricity markets. NYISO divides New York into 11 load zones (A through K), running from western upstate New York through Long Island and New York City. It was established in 1999 and administers the state's competitive energy, capacity, and ancillary services markets.
PJM
A regional transmission organization (RTO) that coordinates the movement of wholesale electricity across all or parts of 13 states — Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia — plus the District of Columbia. PJM is the largest RTO in the United States by peak load and operates one of the world's largest competitive wholesale electricity markets, including energy, capacity, and ancillary services markets.
RTO (Regional Transmission Organization)
Functionally equivalent to an ISO, an RTO is a FERC-certified entity that manages transmission and wholesale markets across a multi-state region. RTOs typically have broader authority over transmission planning than ISOs. PJM, MISO, and SPP are designated RTOs; CAISO and NYISO are designated ISOs. The distinction is largely regulatory — operationally, both manage grid reliability and wholesale energy markets.
SEEM (Southeast Energy Exchange Market)
A bilateral automated energy exchange launched in October 2022 that facilitates 15-minute real-time energy trading among utilities in the southeastern United States. Unlike a traditional ISO or RTO, SEEM does not centrally dispatch generation or set prices through a single auction; instead, it enables participating utilities — including Duke Energy, Southern Company, Tennessee Valley Authority (TVA), and others — to transact directly with one another in an automated marketplace. SEEM serves Alabama, Florida, Georgia, Mississippi, North Carolina, South Carolina, Tennessee, and portions of Virginia.
SPP (Southwest Power Pool)
A nonprofit RTO that manages the electric transmission grid and wholesale energy markets across a 14-state footprint covering much of the central United States, including Kansas, Nebraska, Oklahoma, and portions of surrounding states. SPP was founded in 1941 — making it one of the oldest grid operators in the country — and became a FERC-certified RTO in 2004. It operates day-ahead and real-time energy markets and coordinates transmission planning across its region.
WECC (Western Electricity Coordinating Council)
The regional reliability organization responsible for the Western Interconnection — the interconnected electric grid covering the western contiguous United States, Canada's British Columbia and Alberta, and a small portion of northern Mexico. In the context of this site, the WECC footprint refers to the non-CAISO western states: Washington, Oregon, Idaho, Montana, Wyoming, Utah, Nevada, Arizona, New Mexico, and Colorado. Unlike the eastern ISOs and RTOs, the western grid does not have a single independent system operator; power is managed by a combination of utilities, federal power agencies (BPA, WAPA), and market operators.
NWS Alert Levels
Advisory
The lowest tier of NWS weather alert, issued for conditions that may cause inconvenience or pose a limited hazard. Advisories do not typically threaten life or property but indicate that conditions warrant awareness and caution (e.g., Wind Advisory, Winter Weather Advisory, Dense Fog Advisory).
Extreme
The NWS severity classification for the most dangerous weather events — those posing an exceptional threat to life and property. Extreme-severity events are rare and include categories such as Tornado Emergency or Particularly Dangerous Situation (PDS) designations within a larger warning product.
Warning
A high-urgency alert issued by the NWS when a hazardous weather event is occurring, is imminent, or has a high probability of occurring. Warnings indicate that life or property is at risk and that immediate protective action is recommended (e.g., Tornado Warning, Flash Flood Warning, Winter Storm Warning).
Watch
A preparatory alert issued when conditions are favorable for the development of a hazardous weather event, typically within the next 24–48 hours. Watches cover larger geographic areas than warnings and indicate that people in the affected region should monitor the forecast closely and be prepared to take action (e.g., Severe Thunderstorm Watch, Winter Storm Watch).
Data Sources
CPC (Climate Prediction Center)
See Climate Outlooks section above. CPC is the NOAA center responsible for the extended-range temperature and precipitation outlook maps displayed in the CPC Outlooks panel.
EIA (U.S. Energy Information Administration)
The statistical and analytical agency within the U.S. Department of Energy. EIA collects, analyzes, and disseminates energy information including electricity generation, consumption, prices, and grid operations data. Peak load and generation statistics shown on the site are sourced from EIA publications and data APIs.
NCEI (National Centers for Environmental Information)
A NOAA data center that manages one of the world's largest archives of atmospheric, coastal, geophysical, and oceanic data. NCEI publishes the official 30-year climate normals (1991–2020) that serve as the baseline for temperature and degree-day departure calculations throughout the site.
NOAA (National Oceanic and Atmospheric Administration)
The U.S. federal agency responsible for weather forecasting, climate monitoring, and ocean science. NOAA encompasses the National Weather Service (NWS), the Climate Prediction Center (CPC), the National Centers for Environmental Prediction (NCEP), and NCEI, among many others. The majority of weather data shown on this site originates from NOAA systems.
NWS (National Weather Service)
The primary public weather forecasting agency in the United States, operating under NOAA. NWS maintains a network of local Weather Forecast Offices (WFOs) that issue point forecasts, zone forecasts, and weather alerts for the entire country. The forecast data and active alerts shown on this site are retrieved directly from NWS APIs (api.weather.gov).