Method and accuracy

How close we usually are

• Rain

  radar estimate, NOAA MRMS

  Usually within about 10 to 15 percent of a backyard gauge.

• New snow, cities

  snowfall analysis, NOAA NOHRSC

  Usually within about 25 percent or 1 inch of volunteer reports.

• New snow, mountain towns

  snowfall analysis

  Reads low at high elevation, often by about a quarter.

• Lake-effect snow

  snowfall analysis

  Varies street by street, so a gauge a mile away can differ a lot.

Will this match my gauge?

Volunteer gauges count the day from about 7 AM, and so do we, close enough for most places. Each day on this site is the 24 hours ending 12:00 UTC, labeled by the date it ends. That is 7:00 AM Eastern Standard Time, 8:00 AM Eastern Daylight Time, and 7:00 AM Central Daylight Time; each town page names its own local time. A gauge read at 8 AM, or one sitting under a tree, will differ a little.

Rain: radar estimate

Rain totals come from NOAA MRMS, a national radar analysis checked against rain gauges. We add up its hourly and daily grids for the spot where your town sits. Month and year totals count the days that end in that month or year, so the day labeled Oct 1, which ends at 7 AM Eastern that morning, counts toward September.

Month and year totals start with the day ending on the 2nd, because the day ending on the 1st is mostly the last day of the month before.

Snow: snowfall analysis

Snow totals come from NOAA NOHRSC, a national snowfall analysis. New snow is the depth of fresh snow that fell. Liquid is the same snow melted down to water, from the radar estimate. A foot of snow can melt to an inch of liquid or less, so the two numbers are not the same thing, and we always say which one you are reading. Snow is measured from October to May; outside those months a day reads not measured, not zero.

Your town's record

The rank line compares the biggest day of the latest storm with the nearest long-record NOAA GHCN-Daily weather station within 10 miles, back to the 1930s at many stations. The rank is 1 plus the number of record days that were strictly bigger; ties are reported separately. With no such station nearby, the rank line is hidden.

Compared with normal

Normal is our own average of that station's complete years from 1991 to 2020 (NOAA's current normals period) for the same dates. We need at least 20 complete years, otherwise the line is hidden rather than estimated. The station's observer day and our 7 AM day differ slightly, so a comparison is a guide, not a ruling.

Volunteer gauges

The three nearest CoCoRaHS gauges within 15 miles are read by hand once a day, usually around 7 AM. They are shown as measured and labeled volunteer gauge.

When a number is missing

Running late: when the newest radar hour is more than 3 hours old, we keep showing the last good totals, label every one with the time it ends at, and say so in a note at the top of the page, never as if it were current. A button on the page asks whether newer data has arrived. No radar coverage: towns in Alaska and Hawaii are outside the area NOAA's radar covers, so we show gauges and the record only, never 0.00. Dry week: we say no rain or snow in the last 7 days, once.

Words we use

Rain gauge

A rain gauge is a measuring tube that catches rain. Volunteers read theirs each morning and report the total.

Radar estimate

Weather radar sees rain falling everywhere at once. NOAA turns that into a rain total for every square kilometer and checks it against gauges, so every town gets a number, even without a gauge.

Trace

A trace means a few drops or flakes fell, but less than 0.01 inch of rain or 0.1 inch of snow, too little to measure.

Melted to water

Snow is measured two ways: how deep the new snow is, and how much water it holds once melted. A foot of fluffy snow can melt to less than an inch of water.

Normal

The average for the same dates over the 30 years from 1991 to 2020, at the nearest weather station with a long record. It is the official normal, so "wetter than normal" means wetter than that average.

Storm

A run of days in a row with at least 0.10 inch of rain or 0.5 inch of snow. The storm total adds up those days.

Snow season

The snow season starts on October 1 and runs to the end of May, so "this season" counts the new snow since October 1.

New snow and snow on the ground

New snow is what fell in the time period. Snow on the ground is what is lying there now, including older snow, so it can be deep even on a day with no new snow.