The National Weather Service had to change the way it maps storms just to illustrate Harvey's unprecedented rainfall

As frequent weather checkers will know, the National Weather Service signifies total rainfall on a map using an array of colors: varying shades of green mark areas where total rainfall ranges from 0.1 inch to 1.5 inches, while the deepest shade of red marks areas where the total rainfall is between 10 and 15 inches.

Prior to Tropical Storm Harvey, the National Weather Service's scale topped off at 15-plus inches of rain. But Harvey's record rainfall — which battered the Texas coast over the weekend and prompted historic flooding in the Houston area — forced the National Weather Service to add a few new colors to its key. Now, a deep purple hue, representing rainfall of 15 to 20 inches, shades much of southeastern Texas; two lighter shades of purple were also added to depict 20 to 30 inches of rain and "greater than 30 inches of rain."

Both colors are already on the map:

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"Some perspective on the amount of rainfall that Tropical Storm Harvey has put down across Southeastern Texas and Southwestern Louisiana. We've had to update the color charts on our precipitation graphics in order to effectively map it," the weather agency explained in a statement Monday.

In some areas of Houston, the total rainfall in the last 72 hours was as high as 39.72 inches. Some areas are expected to get up to 50 inches of rain.