Major Herndon Storm: What You Need To Know

HERNDON, VA — A flash flood watch is now in effect for the entire D.C. and Baltimore region Friday afternoon into Sunday afternoon as a rare type of storm bears down on the area. Here's everything you need to know about this huge rainstorm:

What on earth is this storm?

The Capital Weather Gang calls this a "very unusual storm system to develop in late July," and notes that it resembles a winter Nor'easter. But, because it's in the middle of the summer when the air is more humid, it will generate a lot more precipitation than a winter storm would.

How much rain are we going to get in Herndon?

That's a moving target. Yesterday, the European model projected as much as 6.8 inches for Reston/Herndon, whereas now we're in the 5.5-inch range. The National Weather Service rainfall forecast is more conservative, and indicates that our area is going to get in the range of 3.5 inches.

For comparison's sake, 6 to 7 inches of rain is the equivalent of 60 to 70 inches of snow.

When is it going to hit, and when will most of the rain fall?

The storms will begin to develop during Friday afternoon, so the morning commute should be OK. The flash flood watch will last all the way into Sunday, so there will be periods where the rain is heavy and periods where it is light.

At least it will be nice and cool. Temperatures will be in the 70s during the duration of the storm.

Here's a general schedule of what you can expect, according to the Capital Weather Gang:

Friday afternoon and evening: Widespread showers and storms.

Friday night: Continuing waves of showers and storms.

Saturday: Scattered periods of rain, sometimes heavy, and sometimes with thunder. Temperature may drop into the 60s.

Saturday night: Intermittent showers, but mostly to the east and northeast of D.C. Temperature may dip into the 50s.

Sunday morning: Shower taper off.

Image via Flickr user Jo Naylor