US Extreme Weather and Climate Change Dashboard
Variables on this page
Every variable below is evaluated over a full reliable-trend window against this site's combined "detected change" standard (IPCC's likelihood criterion plus a magnitude-vs-variability check -- see the homepage or Methodology for the full definitions). Shown as alternative ways of characterizing this same phenomenon -- some from independent sources, others a different construction on the same underlying data (each variable's own note says which).
Flooding
WeeklyUSGS HCDN-2009 reference gauges, weekly national index. Unit: % of reference gauges above threshold.
Reliable trend since 1957 — gauge coverage grows from a single reporting gauge in 1900 to near-full network by 1990 (the same organic-growth pattern the heat-wave HCN network shows); 1957 is the first year weekly coverage never drops below 50% of the 742-gauge network. Stricter percentile thresholds mean fewer gauges qualify in a typical week, by construction — expect the 99.9th-percentile series to sit at or near zero most weeks. See the Methodology page.
Source: USGS NWIS daily discharge, HCDN-2009 reference gauges
Computing trend…
Flooding — Peak Flow (Annual Maximum Series)
AnnualTraditional USGS flood-frequency-analysis construction: % of reference gauges whose single highest daily discharge that year exceeded that gauge's historical 90th percentile of annual peaks -- looks at each year's one worst day, not moment-to-moment exceedance like the percentile index above. Unit: % of reference gauges above the AMS threshold.
A different construction from the day-of-year percentile series above: this looks only at each gauge-year's single worst day, not moment-to-moment exceedance. Reuses the same downloaded NWIS discharge data and the same 742-gauge HCDN-2009 reference network; calendar year (not USGS's conventional Oct-Sep water year) for consistency with every other index on this site. See the Methodology page.
Source: USGS NWIS daily discharge, HCDN-2009 reference gauges
Computing trend…
Heavy Precipitation Extremes
AnnualStandard ETCCDI Rx1day/Rx5day heavy-precipitation indices from HCN station data -- a distinct source and construction from the streamflow percentile index above, with meaningfully higher IPCC AR6 detection confidence (streamflow is confounded by dams/land-use/urbanization in ways station precipitation isn't). Unit: mm.
Heavy precipitation is not the same as flooding. IPCC AR6 WG1 Ch.11 (River Floods): "There is not always a one-to-one correspondence between an extreme precipitation event and a flood event, or between changes in extreme precipitation and changes in floods." That is why AR6 has meaningfully higher confidence in heavy-precipitation trend detection than in river-flooding trends specifically -- station precipitation isn't confounded by dams, diversions, land-use change, or urbanization the way streamflow is. Compare against the streamflow percentile/peak-flow panels above.
Source: NOAA GHCN-Daily, HCN reference stations
Computing trend…
A reading of, say, 25% does not mean "25% of the country is flooding" — it's the fraction of the reference gauges that are currently at or above the historical 90th percentile for that particular day of the year. It's a measure of how widespread unusually high flow is across the USGS HCDN-2009 reference network, not a share of national land area or population affected. Full methodology on the Methodology page.