First Alert Weather Blog: Snow Possible for Friday | NBC 10 Philadelphia
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NBC10 First Alert Severe Weather Central

First Alert Weather Blog: Snow Possible for Friday

Glenn has the details on snow expected to hit parts of our area Friday.

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    NEWSLETTERS

    We could see some snow in parts of our area Friday. NBC10 First Alert Weather chief meteorologist Glenn Schwartz has the details.

    (Published Wednesday, March 8, 2017)

    NOT AT ALL RARE IN MARCH
    Did you know that we average more snow in March than December in Philadelphia? Yup. The long-term average is 3.3 inches in March vs. 2.1 inches in December. We’ve even had snow in April, but not much in recent years (We haven’t even seen an inch of snow in April since 2002-3). But, believe it or not, we got more than NINETEEN inches of snow on Easter weekend in 1915 (no, that’s not a misprint: 19 inches of snow on April 3-4)! And we’ve had measurable snow as late as May 8, 1898.

    As for March, we had a foot of snow as recently as two years ago. And we’ve had measurable snow in March in 10 of the past 16 years. Yes, we were spoiled in February, with the warmest one ever recorded, along with only 2 inches of snow. So, snow is NOT rare in Philadelphia in March.

    WILL IT BE COLD ENOUGH TO SNOW FRIDAY?
    It’s March. Temperatures in this area will reach the 60s this week. Can we really get any snow that goes beyond flurries or snow showers by the weekend? Again-yup. The air cold enough for snow is not very far away. Look at the difference between temperatures around 5000 ft. for 1 a.m. Friday vs. 1 p.m. Friday. The temperature at that level in Philadelphia is about 37 degrees, which would clearly suggest any precipitation at that time would be rain. But only 12 hours later, that temperature drops to 18 degrees Fahrenheit. That’s a drop of about 20 degrees!

    That map for 1pm shows that it might be cold enough for snow down to much of Delaware and South Jersey. The warmth of daytime in March will be fighting the colder air just above the ground.

    All precipitation around here at this time of year starts as snow up in the clouds. As the snow falls and hits warmer air closer to the ground, it melts and becomes rain. If it’s cold enough at 5000 ft. and snowing harder than flurries, it doesn’t matter if the ground temperature is above 32 degrees-the snow will eventually reach the surface. I’ve seen it snow with ground temperatures as high as 46 degrees! Of course that doesn’t last long-the snow cools things off fairly quickly.

    Here is one computer model forecast map for 10 a.m. Friday. Blue represents snow, and the darker the blue, the heavier the snow. This is still too far out for most short-range high-resolution models, so we’ll see what might change tomorrow. 

    SO WHAT’S LIKELY TO HAPPEN FRIDAY?
    Of course, the ground will be plenty warm Thursday night. And the precipitation could start as rain from the Philadelphia area southward. Even if we change to snow during the A.M. rush, it would take quite a snow burst to stick on any roads, let alone our main roads. So, the main weather threat for A.M. rush Friday looks to be lower visibilities due to the snow. Farther north, the snow would start earlier, but it still would take hours (or a heavy snow burst) to stick on roads.

    But as the day goes on, snow can accumulate on grassy surfaces (and your car), mainly north of the PA turnpike, with temperatures staying in the 30s. As of now, most computer models show the snow moving out before sunset. So, the timing may work in our favor-at least for those who are hoping for nothing worse than wet roads. 

    WHAT ABOUT THE SUNDAY THREAT?
    Tuesday’s computer models were generally in agreement on keeping the Sunday storm weaker and farther south than they were Monday. It doesn’t happen often, but, as of now, it appears that Monday’s Canadian model was the best. It had the storm weaker and much farther south than the others. Sound familiar? The Tuesday Canadian model is so far south that Virginia doesn’t even get snow. But, as is common with that model, it develops a big storm a day or two later. We’ll see if that part of their forecast holds by tomorrow.

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