Flash flooding was a huge problem for many folks here in the Greater Boston area on Thursday morning. In fact, heavy rain came down so fast at times that several people found themselves either stuck or driving through flooded roads just to get to work, especially south of downtown Boston. 

That was a common scene across parts of Interstate 93 south toward Milton after 2-4 inches of rain fell in a short period of time. And some communities reported even more rain than that – up to 6 inches!

So how did all of this happen? Well, we had several ingredients working against us all at the same time.

One of those ingredients is a stationary front. It’s been parked across the Mid-Atlantic and the Northeast for the past couple of days. With the front in place, it’s kept our weather unsettled, with areas of low pressure riding along the front like a highway, giving us uplift in the atmosphere and on/off rain and storm chances here in New England. But Thursday was much different.

A second ingredient really added to the tropical downpours we saw. What’s that ingredient, you ask? It was moisture or water vapor. We call that Precipitable Water, or PWAT. Precipitable water values give us an idea about how much moisture is available in the air. It does not, however, tell us how much rain we’ll actually receive on the ground. Instead, PWAT is a tool to let us know how much moisture we have to work with. In other words, how much moisture there is available in the atmosphere over an area to fall as rain, giving us a heads up about potential flooding. 

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Some of our forecast models indicated that PWAT values for Thursday would be about two inches or more, indicating very high moisture content over our region. Precipitable water values of an inch or less generally means low moisture content.

Another ingredient that worked against us was some mid-level energy (we call it a shortwave) above the surface that moved through our region. That energy gave some of the rain an extra push, if you will, for heavier downpours to happen.

On top of that, these rain showers moved through the region slowly, allowing the water to pile up quickly on several roads, creating a nightmare for many drivers.

So, all these ingredients combined for flash flooding to happen. 

Remember, if a street is flooded, turn around, don’t drown! Find another route to get to your destination.

As we continue moving through the day, the threat for rain and flooding will taper significantly. We’ll still see mostly cloudy skies for much of the afternoon. Some peeks of sun are possible. High temperatures will be in the upper 60s and lower 70s. 

A few isolated showers or storms could develop toward central and western Massachusetts on Friday, but much of Boston will see partly sunny skies and highs in the mid to upper 70s. 

Then this weekend, we’ll see warmer temperatures. Highs will be in the low 80s on Saturday and Sunday. We’ll see clouds and sunshine. 

Hotter weather is expected next week.