Rain chance describes a probability

A National Weather Service precipitation probability describes the chance of measurable precipitation at the forecast point during the stated period. A 40% value does not tell you that rain lasts for 40% of the afternoon, or that 40% of an inch will fall. It also does not give the amount if precipitation occurs. See the NWS explanation of precipitation probability.

On the Mammatus homepage, read the hourly or daily period beside the percentage. A daily value and an hourly value may refer to different windows. The daily display uses the highest available probability from that day’s forecast periods; it is not a sum of hourly chances.

Rate is different from accumulation

A precipitation-rate map describes how quickly liquid-equivalent precipitation is falling at the represented time. In Model Lab, precipitation type and rate can use different palettes for rain, snow, freezing rain, and sleet. A rate in inches per hour is not the total depth of snow that will accumulate.

For a calculation example, a constant rate of 0.20 inches per hour lasting three hours would produce 0.60 inches of liquid equivalent. A single forecast frame does not say that the rate remains constant for three hours. You need the time evolution to estimate that interval’s amount.

Check where a running total starts

Some model accumulation products start at the beginning of the selected run. Their values usually grow as forecast time advances. Other products cover a fixed interval, such as the six hours ending at the valid time. Read the parameter note before comparing them.

For a running total from the same run, an interval amount can be calculated by subtracting the earlier total from the later total at the same grid location. If the total is 0.25 inches at F006 and 0.75 inches at F012, the F006–F012 interval contains 0.50 inches. Adding those two running totals would double-count earlier precipitation. The calculator below demonstrates the subtraction.

Keep the accumulation window consistent

Do not subtract values from different initialization times. An older run’s total at 00:00 UTC can include hours that a newer run has not accumulated. Compare the same start and end times, not just matching map colors or matching final valid times.

A decreasing total is a reason to check the product and timestamps. It can indicate a reset or a mismatch in the inputs. The calculator flags that situation instead of silently converting the negative difference into zero rainfall.

A smooth map still has a source grid

Regional zooms can expose the source model’s grid spacing, particularly around isolated precipitation maxima. Mammatus interpolates continuous shading to make the field easier to read. That presentation does not make a coarse grid an exact neighborhood forecast.

Simulated reflectivity describes a model’s representation of radar echoes, not an observation from a radar at that future time. Use actual radar and official forecasts for current conditions. For flash-flood risk, a rainfall total alone is incomplete: the timing, ground conditions, terrain, and drainage also matter.

TRY THE CALCULATION

Find an interval from running totals

Use values from the same model run, parameter, and grid location. This is a worked calculation, not a current rainfall forecast.

0.75 − 0.25 = 0.50 inches during the interval (12.7 mm).

A fixed-window accumulation is already an interval total. Do not apply this subtraction to unrelated runs or reset accumulations. Your entries stay in this browser.

Sources & corrections

This guide explains the Mammatus website’s behavior and uses the original product definitions below. The publisher is Mammatus Weather; it is not an agency forecast or an endorsement by a data provider.

Found an error? Send a correction with the page link and the detail that needs checking. Read our sources and methods.

Explore precipitation in Model Lab