These products indicate probabilities of rainfall exceeding certain thresholds, a 'best estimate' peak rainfall amount, and probability of cloud-to-ground (CG) lightning. Rainfall forecasts are for the highest rainfall amounts within square regions 40 km (~20 miles) on a side. Lightning forecasts are for the probability of two or more strikes within the same regions.
Probabilities are for 3-hour rainfall exceeding 0.1, 0.5, 1, and 2 inches (approximately 2.5, 12.5, 25, and 50 mm). Rainfall amount forecasts are for rainfall in the categories 0.1-0.49, 0.5-0.99, 1-1.99, and 2+ inches.
Input to the products includes a national radar composite, infrared satellite imagery, cloud-to-ground lightning strike observations, and numerical weather prediction model forecasts. All data are input to an extrapolative/statistical algorithm that analyzes the intensity, position, and short-term movement of rainfall and convective systems, and general atmospheric conditions (humidity, stability, and vertical motion) conducive to rain.
The probabilistic output takes the form of color-filled contours on county-state map backgrounds. Categorical amount forecasts take the form of color-filled boxes. The box format accounts for the fact that the rainfall amount forecasts refer to the maximum rainfall values anywhere within the entire box, not the amount at any one place. For ease of viewing, the output for the conterminous United States is presented in six overlapping sector maps.
Forecasts are updated at approximately 0:55 each hour, and cover the succeeding 3-hour period. Production can be interrupted due to communications outages or system maintenance downtime. Problems will generally be corrected only during regular business hours.