Folsom Lake siteid=748 · CA

Reading the HRRR weather strip

The top chart is the lake's top-2 m mixed water temperature forecast. This bottom chart shows the atmospheric weather over the same time window — it's the input the GLM physics model uses to predict the water-T curve.

Three traces

• Air temperature (°F, left axis) — solid red line.
• Wind speed (mph, right axis) — dotted cyan line. Wind drives mixing in the top few meters; spikes here usually shave a degree or two off the surface water temp shortly after.
• Cloud cover (0–100 %, hidden axis) — faint grey area wash behind the lines. Heavy cloud blocks solar gain; the water-T line above will plateau or dip a few hours later.

Aligning the two panels

Both panels share the same x-axis range and a vertical orange "now" marker. Pick any moment on the time axis and slide your eye up to the water chart to see how the lake responded — typically a damped, lagged version of the air-temp swing because water has huge thermal inertia compared to air.

What to look for

• Water swing ≪ air swing — normal. A 20 °F overnight air drop usually nudges the top 2 m of water by only 3-6 °F.
• Wind spike + water dip — wind-driven mixing pulled colder water up from below the thermocline. Bigger effect on shallow lakes.
• Heavy cloud + flat water-T — solar gain shut off; temperature plateaus.
• Strong air swing + flat water-T — usually means the lake is fully mixed (no stratification) and is acting like one huge thermal mass. Could also mean the GLM is under-coupled — worth checking the calibration if it persists.

Adverse-condition markers

With the ⚠ adverse conditions toggle on (the default), hazard markers are drawn along the top of this strip — and cold-water markers directly on the water-T line above. The shape says what the condition is (▲ wind, ✕ thunderstorm, ◆ heavy rain, ● fog, ■ cold water) and the color says how bad (yellow = caution, red = danger). Hover any marker for the time window and peak values behind the flag; the full thresholds are in the key beneath the chart.

Source

Hourly HRRR (NCEP High-Resolution Rapid Refresh, 3 km CONUS, refreshed every hour) via the local Open-Meteo mirror. Falls back to ERA5 archive for dates older than HRRR's ~14-day window. Source tag shown next to the chart title.