Controls & Map

Pick a retrospective date, then select a river basin or an individual reservoir by clicking it on the map.

Status & Gauges

Four status cards show the current classification for each drought category. Six gauges compare current conditions to the historical record for this time of year -- for every gauge, a lower reading always means more severe drought conditions.

Current & Outlook

The Current tabs show the selected series (soil moisture, streamflow, precipitation, temperature, or reservoir storage) leading up to the selected date. The Outlook tabs show a historical-analog projection of that series, or of drought-severity probabilities, for the weeks following the selected date. An Interpretive Summary alongside these views gives a concise written summary of the selected condition.

Status Thresholds

Flash Drought uses a soil-moisture percentile: how today's root-zone soil moisture ranks against the same calendar day in every prior year on record.

• Warning: triggers if any of the following are true:
  • an active flash-drought event is already underway;
  • soil moisture has been drying for 10 or more days in a row, is at or below the 10th percentile, and ranks among the most severe 20% of flash-drought conditions on record for this time of year; or
  • a sharp short-term drying signal is combined with soil moisture at or below the 20th percentile.
• Advisory: soil moisture has been drying for 10 or more days in a row and is at or below the 30th percentile.
• Watch: triggers if either of the following are true:
  • soil moisture is at or below the 30th percentile; or
  • soil moisture has been drying for 5 or more days in a row and is at or below the 40th percentile (a lower bar, since the drying trend itself is the concerning signal).
• Normal: none of the above.

Meteorological, Streamflow, and Storage Drought each use a percentile for their own variable (rainfall/temperature combined, streamflow, and reservoir storage, respectively) compared to the historical record for this time of year. All three use the same cutoffs:

• Warning: at or below the 7th percentile.
• Advisory: between the 7th and 16th percentile.
• Watch: between the 16th and 31st percentile.
• Normal: above the 31st percentile.

These thresholds are expert-judgment starting points, not independently validated cutoffs (see Status & Coming Soon).

Similar Analogs

The Outlook views use a historical-analog approach rather than a deterministic weather forecast: they identify past dates with the most similar conditions and show the range of what actually happened next, historically, following those similar conditions. Each historical date is compared against the selected date using:

• Recent precipitation vs. normal for this time of year (3-month window)
• Recent temperature vs. normal for this time of year (3-month window)
• Recent streamflow vs. normal for this time of year (1-month window)
• Current reservoir storage vs. normal for this time of year
• Current soil moisture vs. normal for this time of year
• Recent trend in soil moisture -- wetting or drying
• Recent trend in reservoir storage -- rising or falling
• Recent trend in streamflow -- rising or falling

For an individual reservoir, its parent river basin's overall storage condition is included as a ninth variable. All variables are standardized against history and weighted equally; matches must be at least 20 days apart so a single drought episode isn't counted more than once, and must fall within about two weeks of the same time of year (so a summer date is only compared against other summers, not winters).