What “year to date vs. normal” really means

Cumulative lines answer a specific question

The “This year” line adds each completed day’s estimated rainfall from January 1 through the most recently published date. The “Normal” line adds the average rainfall for the same calendar days during the 1991–2020 reference period. Comparing the same dates avoids placing a partial year next to a full annual average.

A rising gap means the selected point has accumulated more rain than the reference by that date; a falling gap means the reference has caught up faster. One storm can move the line sharply, while a long dry period can gradually reduce an earlier surplus.

Normal is a reference, not a promise

Climate normals describe a historical average. Individual years commonly finish above or below that value, and the normal does not say what must happen next. Being below normal in June does not guarantee a wet autumn, just as being above normal does not remove future drought or fire-weather concerns.

The reference is calculated for the selected weather-data grid. An official airport or cooperative station may have a different normal because it observes another point and may use a different record and quality-control process.

Why the comparison lags recent totals

Rain by Zip uses ERA5 reanalysis data through Open-Meteo for both cumulative lines. Reanalysis combines observations with a weather model to produce a consistent historical grid. It is published later than near-real-time radar, so the comparison intentionally stops about a week behind the recent rainfall cards.

The lag prevents unpublished days from being counted as zero. It also means Today and Past 24 Hours should not be added directly to the displayed year-to-date figure unless every intervening day and data source is handled consistently.

Responsible ways to use the chart

The chart is most helpful when paired with the recent totals and historical daily table. Together they show immediate conditions, the sequence of recent events, and the longer cumulative context without pretending that one number answers every rainfall question.

• Use it to describe the direction and size of a year-to-date departure at the selected grid point.
• Inspect the line over time instead of focusing only on the final number.
• Compare locations cautiously because terrain and grid resolution matter.
• Do not interpret the chart as a forecast, flood assessment, drought declaration, or official climate record.
• Use NOAA, local climate offices, and water-management agencies when an official determination is required.