Forecasters say an unpleasant period of heavy snow, with wind gusts up to 30 mph, is expected during Tuesday morning’s drive, with several inches possible north and west of the city.
A computer model run Monday was forecasting a storm resembling a meteorological fireball to explode off the mid-Atlantic coast during the morning, dumping snow in defiance of the recent warm spell. Temperatures reached 50 degrees for the fifth straight day in Philadelphia on Monday.
A winter weather warning was in effect for Bucks, Chester and Montgomery counties for 3 to 5 inches of snow, and a winter storm warning for Upper Bucks counties, with up to 7 inches.
A Coastal Flood Watch is in effect until 10 a.m. Wednesday for areas along the Delaware River, and a Jersey Shore Advisory is in effect until 3 a.m. Wednesday for minor flooding.
Snow accumulations could vary from nothing south and east of the city to several inches in Philly’s neighboring counties, and the only certainty was that totals would vary radically depending on temperatures and elevations.
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Discussing the forecast late Monday, the National Weather Service said the latest track guidance on models indicates “a longer period of snowfall in areas closer to the I-95 corridor.” He added that the weight of the snow may cause some power lines to fall.
Just 15 miles apart can make a big difference in who gets what, said Tom Kines, chief meteorologist at AccuWeather Inc.
“We were hemming and hawing all day,” he said. “I guarantee we’ll be wrong somewhere about this.”
Rain is expected to fall late Monday night, mix with snow and turn to snow before or near dawn, with heavy rain during the morning.
The AccuWeather forecast was for 1 to 3 inches of snow in the immediate Philadelphia area, and 3 to 5 inches in the northern and outer western suburbs.
The weather service was forecasting 1 to 2 inches in the city, and several inches in suburban Pennsylvania.
Paul Dorrian, an Arkfield meteorologist based in Valley Forge, was forecasting 3 to 6 degrees across the entire region.
Temperatures during snowfall are expected to range between 33 to 35 degrees.
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Why all this uncertainty?
temperature: It was warm and expected to stay above freezing during the snowfall in the immediate Philadelphia area. It will be cold enough for snow to fall at the 5,000-foot level, and snow could stick when the temperature exceeds 32 degrees at the surface, Kennis said. By coating the ground, it creates a cooler surface. But Kines said the temperature of 33 or 34 when it snows can be crucial in determining the amount of accumulation in a particular area.
Terrain: Higher areas, such as Roxboro, Chestnut Hill, and mountainous areas in Pennsylvania counties adjacent to Philly should see higher accumulations than areas to the south and east. Temperatures decrease slightly with altitude. Additionally, when the weather is mountainous, the snow has a shorter distance to fall, and therefore less time to melt.
The storm itself: On Monday, the hurricane was in the Tennessee Valley and was expected to migrate into the ocean somewhere off the mid-Atlantic coast and intensify quickly. When it exploded, strong northerly winds to the west of the center would attract cold air, causing the shift. It’s still unclear exactly where the storm will mature and how intense it will be, and meteorologists say that’s all what makes these forecasts difficult.
“It’s a definite thing,” said Alex Starman, a meteorologist at the Weather Service office in Mount Holly.
The rain was scheduled to stop by early afternoon.