How GasCast works

Every number in GasCast comes from public weather and energy data. This page explains where each one comes from, when it updates, and how we calculate it, so you can judge it for yourself.

Data sources

When things update

GasCast checks for new data about every 15 minutes, all day. The top of the app shows when each source last updated and when the next one is due.

Degree days

Natural gas demand follows temperature. Degree days turn temperature into a demand number:

Heating degree days (HDD) track heating demand, which is mostly gas burned in homes and businesses. Cooling degree days (CDD) track air conditioning, much of which is powered by gas-fired power plants.

GasCast weights each city's degree days by how many people live there, so a cold day in Chicago counts for more than a cold day in a small town. It then adds up the next 16 days and compares the total with normal, the long-term average for those same dates.

Depending on the season, the app leads with whichever one matters more right now: HDD in the heating season, CDD in summer.

Run-to-run changes

The big number on the Today page is how much the 16-day total moved compared with the previous run of the same model. That's what traders watch: whether the latest forecast got colder or warmer than the last one.

• For HDD, + means colder (more heating demand) and − means warmer.
• For CDD, + means hotter (more cooling demand) and − means cooler.
• "Since this morning" compares with the first update of the day, so you can see the whole day's drift.

Confidence

Weather models are run many times with slightly different starting conditions. These runs are called an ensemble. When they land close together, confidence is High. When they spread out, it's Medium or Low.

The "likely range" is where 80% of the runs land (the 10th to 90th percentile). Confidence usually drops the further out you look: the first week is much more reliable than week two.

Corrected forecast PRO

Every model has habits. For example, one might run a little warm a week out. GasCast keeps track of each model's recent forecasts and checks them against NOAA's measured degree days once those days have happened.

The corrected forecast nudges each day by that model's recent average miss at that lead time. It's deliberately cautious:

• A correction is only applied once enough days have been checked.
• It starts small and grows as more checks come in, and recent misses count more than old ones.
• Days without enough history are shown raw and marked that way.

You can always switch between Corrected and Raw to see both.

Storage estimate PRO

EIA publishes the official weekly storage change on Thursdays. Before it comes out, GasCast estimates it from the weather that actually happened that week, since weather is the biggest driver of how much gas goes into or out of storage.

• The estimate comes with a likely range and how far off it has typically been.
• After EIA releases its number, GasCast grades the estimate and shows last week's result next to the new one, so you can see the track record.
• It is our estimate, not EIA's number, and it's always labeled that way.

Storage figures are working gas in the Lower 48 states, in billion cubic feet (Bcf), the same measure EIA reports and traders quote.

Supply, grid & maps PRO

Supply & demand: the other side of the market. GasCast shows daily power burn (gas used to make electricity), estimated from EIA's hourly gas-fired generation and compared with the same days last year. Monthly figures show dry production, LNG and pipeline exports, and residential, commercial, industrial and power demand, with EIA's forecast for the coming months. The balance is supply minus demand: positive means gas is going into storage.

LNG feedgas: interstate pipelines must publicly post how much gas is scheduled at each delivery point. GasCast reads the scheduled deliveries into the LNG export terminals it can track, converts them from dekatherms to Bcf/d (1.037 Dth per Mcf), and shows the daily total and which terminals moved. These are nominations, not metered flow, and terminals or pipelines GasCast doesn't track are listed so the total is never mistaken for the national figure.

Power grid: hourly electricity demand and gas-fired generation for the major US grid regions (ERCOT, PJM, MISO, CAISO and more), from EIA.

City heat map: each city's forecast temperature compared with normal for the next two weeks, as a weekly average or day by day. Press Play to step through each day, and see the average for each region.

Outlook maps: NOAA's official 6–10 day, 8–14 day and weeks 3–4 temperature outlooks.

Forecast evolution: a grid of every forecast day across the last several runs. Each cell is colored by whether that day got colder or warmer than the run before, so a steady trend stands out even when the 16-day total barely moves.

Patterns & past seasons PRO

Weather patterns: the Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Pacific/North American pattern (PNA) are large-scale patterns that help steer cold air into the US. GasCast shows NOAA's 2-week ensemble forecast for each one, with the likely range. In winter, a negative AO or NAO and a positive PNA usually favor colder weather in the East.

Similar past seasons: GasCast compares the last 45 days of measured degree days (smoothed over 7 days) with the same dates in every season since 1981 and shows the closest matches, plus what happened in the 30 days after. It's a way to put this season in context, not a forecast.

Tropics

Active storms come straight from the National Hurricane Center. A storm is flagged when it's in or near the Gulf of Mexico, where it can shut in offshore production, disrupt LNG export terminals and cut power demand along the coast. Tap a storm for NHC's latest advisory and forecast cone.

What GasCast doesn't do

GasCast shows weather and forecast information. It does not tell you when to buy or sell, and nothing in it is trading advice. Forecasts can and do change, especially more than a week out.