When Forecasts are Issued and Available

HydroForecast issues forecasts once or multiple times a day depending on the type of forecast to incorporate the latest weather and observation data. This article explains how to interpret forecast initialization and issue times, how frequently forecasts update, and what inputs go into them.

• Forecast Timing: Initialization vs. Issue
• See Weather Input Timestamps
• HydroForecast Short-term Release Schedule
• HydroForecast Seasonal forecasts are issued once per day
• Examples on Frequent Updates to HydroForecast Short-Term
• HydroForecast Seasonal forecasts are issued once per day

Forecast Timing: Initialization vs. Issue

When viewing forecasts, you’ll see two timestamps:

• The initialization time refers to the start time of the forecast
• The issue time is when the forecast was published and made available in the dashboard or API

HydroForecast's issue times are later than the initialization time because we rely on global weather forecasts, which take a few hours to run. Here’s what you’ll see in the HydroForecast Dashboard:

Because HydroForecast relies on global weather models that take time to run, issue times are typically ~8 hours after the initialization time. Weather models like ECMWF, GFS, GEFS, and HRRR have varying processing times. HydroForecast waits for the freshest available inputs before issuing forecasts. For example:

• ECMWF: ~7.5 hour delay
• GFS: ~5.25 hour delay
• GEFS: ~6.5 hour delay
• HRRR: ~1.5 hour delay

We add small buffers (~30 minutes) to ensure completeness and reliability. The exact inputs used depend on your forecast region. For more information on our weather forecast providers see NOAA’s documentation on their forecast release timing and ECMWF's forecast dissemination schedule.

 The figure below highlights how we incorporate the most up-to-date weather forecasts available in the 00 UTC HydroForecast initialization. 

See Weather Input Timestamps

You can now view the initialization times of the weather inputs used in your short-term forecasts (ST-3) directly in the dashboard.

• Select a short-term forecast from the dropdown
• Hover over the i icon.
• A tooltip will show the initialization time of each input (e.g., HRRR: 18 UTC).

This transparency helps you understand how forecasts stay up to date—even when different weather models update at different cadences.

HydroForecast Short-term Release Schedule

HydroForecast Short-term releases two primary forecasts per day, one for the 00 UTC init time and one for the 12 UTC init time. 

Here are the approximate times of day the 00 and 12 UTC primary updates are first available:

Every two hours between the primary updates, HydroForecast Short-term checks for adjustments to the latest forecast to ensure that your forecasts reflect the latest conditions as quickly as possible. During these checks, we incorporate:

• New near-real time flow observations if available 
• Updated weather inputs (e.g., HRRR, GFS) that have been released

While forecasts may update several times throughout the day, we store only the final forecast from each initialization time (00 UTC and 12 UTC). This means that when you look back in time to view forecasts from days in the past, you will only be able to access the last updated version of the 00 UTC init and 12 UTC init. 

Examples on Frequent Updates to HydroForecast Short-Term

HydroForecast in Europe without hourly data:

 ✅ ECMWF and GEFS as weather inputs 

✖️ No data assimilation

HydroForecast was issued twice per day shortly after ECMWF and GEFS were released. 

HydroForecast in Europe with hourly data:

 ✅ ECMWF and GEFS as weather inputs 

 ✅ Yes data assimilation

With data assimilation, HydroForecast updates more frequently so the final update was just after 06 UTC and 18 UTC for this customer.

HydroForecast in the United States with no flow data:

 ✅ ECMWF, GEFS, HRRR & GFS as weather inputs 

✖️ No data assimilation

GFS and HRRR, both of which update more frequently than ECMWF and GEFS.  Since HRRR updates more frequently, we use the 05 UTC HRRR in the first HydroForecast release.

Every two hours we check for new weather, which means that there will likely be a new forecast every 2 hours with updated HRRR. 

We don’t ingest HRRR after 11 UTC, so without data assimilation or unexpected delays to weather inputs the final forecast update is at 14 UTC (using the 11 UTC HRRR as an input). After which the 12 UTC forecast cycle takes over and is issued at 20 UTC. With hourly data, it is possible to have more frequent updates due to data assimilation between 14 UTC and 20 UTC.

Forecast Delays

If a forecast is significantly delayed, we will notify affected customers. For the 00 UTC initialization, we expect forecasts to be released prior to 09 UTC and for the 12 UTC initialization we expect forecasts before 21 UTC. Our system has internal checks in place to notify the HydroForecast team of any unexpected delays and we have an on-call engineer at all times to ensure operational readiness.

HydroForecast checks for new updates every two hours, but no forecast delay notification will occur if an expected update doesn’t occur at the 2-hour mark. This prevents unnecessary alerts during routine weather model delays.

HydroForecast Seasonal forecasts are issued once per day:

For more information on the weather inputs into seasonal please see our support doc on How HydroForecast Seasonal works.