On Friday morning, the D.C. region woke up to torrential downpours and flash flood warnings. It was an early start to an extreme rain event that’s going to continue through at least Saturday morning.
When it’s all said and done, as much as 6 inches of rain will have fallen in the D.C. area. Widespread totals of 2 to 4 inches are likely, and it’s all going to happen over the course of 24 to 36 hours. A flash flood watch is in effect through Saturday afternoon, which means we could see a torrent of water start to flow down streets and creeks at any moment.
This event is bonkers for a variety of reasons. Let us count the ways.
1. It’s more like Snowzilla than a summer storm.
In the words of the National Weather Service this morning:
A SYSTEM MORE REMINISCENT OF THE COOL SEASON …
In the Mid-Atlantic, really big snowstorms almost always have a strong easterly component to their winds that develops as the surface low strengthens as it tracks to our south. Those easterly winds are sometimes referred to as the cold conveyor belt. It helps pull in moisture off the Atlantic Ocean.
2. The low pressure system is unusually low, the moisture is unusually moist and the winds are unusually windy.
This storm has a greater than 5 standard deviation to its easterly winds. That means it’s rare! The figure below shows the return frequency for having that strong of easterly winds. The various colors on the map indicate how often such winds will occur. The brown shade would occur during this time of year once every 10 years.
By contrast, the red areas don’t occur enough to gauge the proper return ratio except to say that during a 40-year period they were not observed between 1979 and 2009. This storm is a rare bird for late July.
We can create the same kind of maps for moisture and pressure. The transport of moisture into the D.C. region is impressive in this story. On top of that, the pressure is very low for this time of year.
Ensemble anomaly fields jumping out for Mid Atlantic rain event Fri-Sat. Moisture levels & transport, unusually deep low. Here’s SREF. pic.twitter.com/BwJoRkm1AT
— Alex Lamers (@AlexJLamers) July 27, 2017
3. The data suggests this storm is a 1-in-50 year event. But it’s probably more.
But that doesn’t even take into account the fact that it’s the middle of summer. As we said in No. 2, this isn’t a typical summer storm.
So, on average, this kind of storm would happen once every 50 years, all else being equal. However, noting that it’s July and not January, we’re thinking it’s actually more like 1 in 100 years — maybe even longer.
Looking at return frequencies (e.g. Silver Spring, MD), a rainfall of ~5" in 6 hours qualifies as a 1 in 50 year event. pic.twitter.com/SXS9njAZJS
— Ben Noll (@BenNollWeather) July 28, 2017
4. This forecast is rarely issued here.
This chart sums up how ridiculously uncommon it is to have the National Weather Service issue a forecast like the one we have in effect today. Notably, the precipitation forecast percentile is 99 percent. Remember how in school they would tell you that you were in the 99th percentile? It meant you were really smart. In this case, it means D.C. is going to be really rainy.