Coastal Storm to Bring Heavy Snow, Rain to Region Tuesday

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Severe Weather Central

Weather Forecast
March 11, 2017
By PhillyinFocus

A First Alert has been issued for Monday overnight through Tuesday. Why? A major late-season storm is expected to pass the Mid-Atlantic and Northeast. The result? A lot of snow for some, and heavy rain for others. That’s what it’s looking like, for now, at least.

Let’s explain.

A storm center producing snow and forcing more cold air across the region (after an already cold air mass will be hanging over us) is expected to shift in from the west. At nearly the same time, a coastal low will form off the southern Atlantic coast.

That coastal low is expected to chug up the east coast, and arrive at our doorstep by Tuesday afternoon. Tons of moisture, strong wind gusts and mild air will be forced into a cold air mass. It’s the perfect ingredients for a stormy Tuesday.

Who sees what?

That depends on where you are—it’s all about neighbourhood forecasting. We’re calling the First Alert “two pronged.”

The first component of the First Alert is for the PA Suburbs, Lehigh Valley, and Berks county. These areas now look to see all snowfall during the event. In addition, it appears these neighborhoods will see the heavier bands of snow which will lead to the highest accumulations. For now, we’re expected 6-10+” for the Suburbs, and 10+” for Upper Montgomery, Upper Chester, and Upper Bucks counties through to the Lehigh Valley and Berks County.

The “+” on the forecasted values is our way of letting you know this storm still has a long way to go. It needs to interact with the ocean waters up the coastline, and any land elements as it churns up the coast and edges near areas like the Carolinas. So, there is still a potential for much higher values than 10” in these areas. Remember, this is our initial forecast.

The second component of the First Alert includes the I-95 corridor, Wilmington-Philadelphia-Trenton, and our southern Neighborhoods, Delaware, South Jersey and the Shore. It’s likely moisture will enter Monday night, bringing snow, potentially heavy snow near the corridor. But it now looks more plausible that these zones will see a changeover to heavy rain, possibly thunderstorms during Tuesday morning and afternoon as the low nears the Jersey coastline and wedges warmer air and gusty conditions on land.  This may also lead to minor coastal flooding along the Jersey shore and DE beaches.

As a result, snow totals will likely be lower in these zones. Possibly only 1-3” in Southern DE and the Jersey Shore, 1-6” in South Jersey, and a tight gradient on higher amounts near the I-95 corridor. Take a look at the map below. Of course, as we watch the storm track, this may need adjustments.

Why would it change? As explained above, the storm still has to form and move up to us. Once it starts moving our way on Monday, different computer models, which aren’t in entire agreement at this point, will likely start to streamline. If the center of the coastal low gets too close to land, that pushes more rain into Philadelphia (which means less snow time). If it pulls farther out to sea, that could mean higher amounts of snow closer to Philadelphia and surrounding areas. It’s all about the track! Take a look at 3 separate computer models for the same time Tuesday morning. Notice the differences in intensity (the deep blues, yellows, and oranges), precipitation type (rain or snow), and the differences in the storm center (the red L) location.

So, it’s not a question of WILL we get rain and snow on Tuesday. It’s a question of who will see the most of what. And that depends ALL on the track.
We’ll have a CLOSE eye on the track over the next 2 days, and be constantly updating you.

Stay with the NBC10 First Alert Weather team for the latest weather updates including alerts, maps and forecasts.

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