When forecasts are issued
When we reference forecasts, there are a couple of times to note:
- The initialization time refers to the start time of the forecast
- The issue time is when the forecast was published
HydroForecast's issue times are later than the initialization time because we rely on weather forecasts which take a few hours to be produced.
HydroForecast Short-term has two primary updates each day using the 00 and 12 UTC (GMT) weather forecasts. Every two hours between the two primary updates, HydroForecast Short-term makes adjustments to the latest forecast based on near-real time flow observations and weather forecast data from NOAA's High Resolution Rapid Refresh (HRRR). There are also adjustments made to the forecast every six hours based on NOAA's Global Forecast System (GFS). Here are the times of day the 00 and 12 UTC primary updates are first available.
00 UTC Forecast (the morning forecast in the US):
Initialization time: 00 UTC aka 16:00 PST/19:00 EST of the day before
Issue time: 08 UTC aka midnight PST/03:00 EST
12 UTC Forecast (the afternoon forecast in the US):
Initialization time: 12 UTC aka 04:00 PST/07:00 EST
Issue time: 20 UTC aka noon PST/15:00 EST
Below is a table outlining the estimated timing of forecast update frequency (in UTC). The left column shows the 0z initialization (init) update timetable, and the right column shows the same for the 12 UTC initialized forecast. All of the latest forecasts may be accessed via our API and on the HydroForecast dashboard. We store 2 forecast per day, which are highlighted in bold. The stored forecast correspond to the last update of the 00 UTC and 12 UTC initialized forecasts. 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 00 UTC init; 18 UTC issue and 12 UTC init; 06 UTC issue. Please note, the table times are not exact and you can expect forecasts will be issued typically within plus or minus 30 minutes of the issue time.HydroForecast Seasonal forecasts are issued once per day:
Initialization time: 00 UTC aka 16:00 PST/19:00 EST of the day before
Issue time: 01 UTC aka 17:00 PST/20:00 EST