First Alert: Heavy Rain, Flooding Into the Weekend

A First Alert has been issued for Philadelphia, Pennsylvania suburbs, Delaware and South Jersey from 3 p.m. Friday to 3 p.m. Saturday for heavy rain and possible flooding.

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A First Alert has been issued for Philadelphia, Pennsylvania suburbs, Delaware and South Jersey from 3 p.m. Friday to 3 p.m. Saturday for heavy rain and possible flooding.
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A WET JULY ADDS TO FLOOD THREAT
nIt has been an unusually wet July all across our area. Even for July, which is our wettest month of the year, on average. Take a look at some of the numbers:
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Some places have been much wetter than these official locations. The map below shows which counties have been the wettest this month.
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Some places have been much wetter than these official locations. The map above shows which counties have been the wettest this month.
This wet pattern has made much of our area vulnerable to flooding. Some parts of our area, such as Upper Montgomery and Northern New Castle County, only need 1-2 inches of rain in a 6-hour period for flooding.
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The areas in brown can withstand 4 inches or more. That’s quite a contrast. So, the vulnerability of some places is MUCH more than others. If you live in one of those bluish regions, and in a normally flood-prone area, you should especially be prepared for significant flooding Friday and/or Saturday.
nANY CHANGES FROM YESTERDAY?
nOur most reliable computer models have been painting a wet and stormy picture. I’ve focused on the European Model-the world’s best overall. It was the first to suggest this storm and the heavy rain, at the same time other models suggested a sunny weekend. First, the surface map that came out Wednesday valid for Saturday at 8 a.m.
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The LOW pressure center is clearly near Rehoboth Beach, Delaware. The more lines there are around the LOW, the more intense it is. That’s a pretty strong storm for a non-tropical system in July.
Now look at the same model 24 hours later. This is also valid at 8 a.m. Saturday:
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EuroWX.com
The forecast is almost identical, which gives us extra confidence that the EURO has the right idea on the location and intensity of the storm. If we look a little closer, we see the center of the LOW is a bit farther south than the Wednesday forecast. It’s also a bit slower.
Now let’s compare the rainfall forecasts from the Wednesday forecast, followed by the Thursday forecast:
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The basic pattern is very similar. It’s clear that a band of very heavy rain will extend west to east, from around West Virginia to South Jersey. It’s a little hard to see the numbers, but look for the reddish colors. The scale on the right shows that they represent AT LEAST 4” of rain. That area on both maps covers parts of Delaware and South Jersey. And the purplish colors inside the red area are AT LEAST 6” of rain! There’s no sense of trying to find the exact amount for your neighborhood this far ahead of time. The only real change is the storm track a bit farther south suggests that the heaviest rain would likely stay south of Philadelphia. But that’s almost trivia-they are very close!
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EuroWX.com
Sometimes, a computer model can give a wild solution. That’s why each model is run a number of times with small changes in initial conditions. These are called “ensembles”. The EURO has 51 ensemble members. So, to see if the above solution is representative (and can be trusted as legitimate possibility), the ensembles should suggest a similar solution-on average. Averaging 51 model runs would probably lead to a much weaker LOW. But in this case, it doesn’t. Once again, the Wednesday forecast is followed by the Thursday forecast:
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Once again, the solutions are practically identical. The latest forecast is just a little slower than the one from yesterday.
nThose maps are the average of the 51 ensemble members. That’s a pretty impressive LOW near our coastline at 8 a.m. Saturday (putting it mildly). Since the EURO is the best model overall, I call this “The best of the best.”
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tropicaltidbits.com
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NBC10
Here are a couple tips for flash flooding, including avoiding flood waters and evacuating as soon as water starts to rise.
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Cortesía Brandon Scneider/9news
THE OVERALL FORECAST
nAs I said earlier, unless this model is WAAAAY off, we’re going to get a lot of rain between Friday afternoon and Saturday. The exact timing of the heaviest rain and the spot where the heaviest will occur are yet to be determined. But I would suggest making sure any gutters near your house are cleared of debris. There may be quite a bit of water running into them.
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NBC10
The slower movement of the storm is why we’ve adjusted the FIRST ALERT for our entire area from 3 p.m. Friday to 3 p.m. Saturday. And we’ve emphasized that the greatest flood threat is in Delaware and South Jersey, and flooding is least likely in the Poconos, Lehigh Valley, and Berks Co. Unfortunately, the slower movement suggests that clouds and cool weather will linger into Sunday, along with some showers-especially near the shore. We just hope that the slow-down causes even more rain to fall. We’re already predicting plenty.
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