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

All of the latest forecasts may be accessed via our API and on the HydroForecast dashboard. We store 2 forecasts per day. 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 and 12 UTC init. 

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