• What Precipitation Accumulation images are available?
• How does the 1-Hour Precipitation Accumulation work?
• How does the 3-Hour Precipitation Accumulation work?
• How does the Storm Total Rainfall work?

What Precipitation Accumulation images are available?

The past 1- and 3-Hour Accumulation products show estimates of the amounts of rain that have fallen during the past 1 and 3 hours, respectively. The Storm Total product shows estimated rainfall amounts over the entire storm beginning and ending at the times indicated at the bottom of the key in the scientific version of the NEXRAD graphic.

The computer programs that create these products use as their input the reflectivity data gathered during the volume scans (as is displayed in the composite reflectivity product and the four tilts of the base reflectivity product). The programs assume that all reflectivities are from liquid rainfall, and then use equations to convert the reflectivities to inches of rainfall. Hail, which has unusually high reflectivities, can cause these products to overestimate rainfall amounts, as can ice pellets (sleet). Snow has unusually low reflectivities, about the equivalent of drizzle, and is usually underestimated on these products. The precipitation accumulation products cannot be used to estimate the liquid equivalent of the snowfall or the amount of snow that has fallen. The level of detail with the NEXRAD Rainfall Accumulation data allows users to pinpoint counties and individual towns that have received the most rainfall, and gives emergency planners and others information they need to plan rescue operations and evacuation procedures during flood situations.

How does the 1-Hour Precipitation Accumulation work?

The past 1-Hour Accumulation product is updated every 5 or 6 minutes (depending on whether there is severe weather in the area) when the radar is in precipitation mode.

How does the 3-Hour Precipitation Accumulation work?

3-Hour Precipitation Accumulation is updated once an hour when the radar is in precipitation mode, and does not become available until precipitation has been detected for three consecutive hours. Each update represents a running total of the rainfall over the past 3 clock hours.

How does the Storm Total Rainfall work?

Like the Surface Rainfall Accumulations, Storm Total Rainfall products begin to keep track of rainfall amounts as soon as precipitation is detected any where within the range of the radar. These products will reset themselves to zero rainfall after a full hour has passed during which no precipitation has been detected within range of the radar. Accordingly, Storm Total Precipitation Accumulations may accumulate the rainfall from a continuous series of single storm events into one overall total rainfall accumulation. Storm Total Precipitation Accumulation products are updated every 5 or 6 minutes (depending on whether or not there is severe weather in the area) when the radar is in precipitation mode.