Understanding summer rainfall patterns in Virginia

RICHMOND, Va. (WWBT) - Recently, we have had some “scattered” showers and thunderstorms in the forecast and sometimes there have been First Alert Weather Days in place on these days to alert our viewers to the potential for some of those storms to be “strong to severe.”

The terminology “strong to severe” means that SOME of the storms that do develop could have the potential to produce damaging wind gusts, hail, or other hazards.

The NBC12 First Alert Weather team is committed to giving you the First Alert to that possibility so that you are “weather aware.” We know you have busy lives and may not be paying attention to the weather as closely as we are every day.

Summer showers and storms are tricky to forecast in any one spot because when they do develop, they tend to be more “scattered” or what we like to call “hit or miss” in nature.

Can you remember a time when you heard rumbles of thunder at your house, perhaps multiple times, but it doesn’t even rain? You find out later that your friend just miles away got a huge downpour. Your flowers and garden are seemingly parched and not a drop to be found!

I realize it’s frustrating; it’s just the nature of summer precipitation patterns here in Virginia. Let’s take a look at what happened across the area on Tuesday, Aug. 15. Below is a map of rainfall totals across our region.

Notice that many areas stayed dry, many areas had at least some rain, and other areas got a significant amount of rain (1/2 to 1″ or more). In many cases, it was feast or famine. Southeastern Virginia down into North Carolina had the most!

So when we put together a forecast, we like to give you the following terms to hopefully explain the “coverage” of rainfall that we anticipate to help you better plan your day:

Isolated means many areas stay dry. 20% or less of our viewing area should expect rain.

Scattered would imply 30% to 60% of the area will have the opportunity for rain. However, many may still miss out.

Numerous/Widespread would mean a much larger portion of our viewing area can anticipate some rainfall. Even when widespread, there are still some communities that can do much better than others in terms of rainfall totals.

As I mentioned, the anticipation of summer rainfall can be very frustrating because on most days, the showers and thunderstorms we get are hit-or-miss in nature. It’s possible your neighborhood gets “hit” 2 or even 3 days in a row. Then again, other areas get hit 2 or 3 days in a row, and you get nothing on those days!

So how are we able to get that more widespread rainfall in the summer? It’s rare, but sometimes a stronger summer “cool” front will move into our area and help to trigger more numerous showers and thunderstorms across our region. Perhaps giving 70% or more of us the opportunity for some beneficial rainfall.

We often have to wait until later August and September for a decaying tropical system to move into our area, offering more widespread rainfall. These typically come in from the Gulf Coast or the southeast Atlantic coastline.

Depending upon the track of that storm, they can also bring the threat of significant wind and even severe weather in the form of weak tornadoes. While these “weak” tornadoes tend to be on the lower end of the scale, they can cause significant property damage. Our hope is always to get the benefit of the rain without a widespread wind or tornado threat.

We also need to be wary of too much of a good thing. Tropical systems are loaded with moisture and can often bring multiple inches of rain, which can result in localized or even more widespread flooding concerns.

More widespread precipitation events are far more common in the cooler months of the year: Fall, Winter, and Spring. These are caused by what we call “synoptic” scale or larger scale low-pressure systems that move into our region.

So how dry are we currently across central Virginia? The US Drought Monitor data as of Aug. 8 shows that overall, we are doing pretty good in central Virginia. There are some abnormally dry areas and even some pockets of moderate drought showing up in Northern Virginia, around DC and into southern Maryland.

That said, in the summer months, the evaporation rate is quite high and going even 2, 3, or more days without any rainfall can cause conditions to change rapidly. We’ll have new data available Thursday to show you to see if anything has changed in the short term.

Other than some isolated to widely scattered showers and thunderstorms in our forecast for Thursday, the weather pattern looks exceptionally dry for Friday into the weekend and even next week as well. Looks like we will all need to be watering a good bit more than usual in the coming week or longer.

Be sure to download the NBC12 First Alert Weather app for up-to-date information and to receive severe weather alerts. You can find the app in the Apple Store and on Google Play.

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