The National Weather Service has issued a Winter Storm Warning for Philadelphia, Baltimore, and Washington effective tonight through Monday. A warning indicates that heavy sleet, freezing rain, and or snow are imminent and expected within the next twenty-four hours. At this time, Philadelphia is expecting around 6 to 9 inches of snow, while Baltimore and Washington are expecting 9 to 12 inches of snow. Precipitation will begin as light rain before quickly transitioning over to sleet (and perhaps a brief period o freezing rain) during the overnight hours and eventually all snow by daybreak. In addition to the heavy snow, temperatures will fall from the lower 30s around sunrise and into the 10s during the afternoon. Winds gusting over 20 miles per hour may cause blowing and drifting snow, particularly across central Maryland where the heaviest snow is anticipated. As temperatures will be very cold, all roads are expected to be impacted with snow and ice.
An arctic front will drop from north to south across the bay region during the overnight hours. At the same time, an area of low pressure will move along the front and spread precipitation over the area. Initially, temperatures will be mild enough so that precipitation falls in the form of light rain in Washington, Baltimore, and Richmond. Between midnight and 6 a.m., temperatures will nosedive and fall through the 20s. This will allow for the precipitation to change to sleet and then to snow. The transition from rain to sleet to snow will occur first in the Baltimore area and then Washington DC. Temperatures may be cold enough in Philadelphia so that the precipitation begins as a mixture of rain and sleet or just plain sleet. However, all locations will be reporting moderate to heavy snow with falling temperatures during the morning and early afternoon.
Intense snowfall rates can be expected in the convergence zone, a place where warmer, subtropical air to the south merges with fresh arctic air to the north. This will result in tremendous uplift or convection, and will produce a wide swath of moderate to heavy snow across the region. Since the arctic front will be to our south, we will be on the cold side of the storm and will see mostly snow from this system. Farther to the south, in southern and southeast Virginia, a prolonged period of mixed precipitation is likely before the changeover to snow. When all is said and done, a zone of 9 to 12 inches of snowfall can be expected in Baltimore and Washington with a few isolated spots receiving up to 15 inches of snowfall before the precipitation tapers to flurries during the late afternoon. Farther to the north, snowfall amounts will be lighter but still significant across northern Maryland, southern Pennsylvania, and the greater Philadelphia area where 6 to 9 inches of snow can be expected. A slight shift in the band of heaviest snow could alter the snowfall forecast to a certain degree, but in either case the major cities in the Middle Atlantic will receive a plowable snowfall.
As we move into the nighttime hours and into early Tuesday, the sky will clear and arctic air will become entrenched in the Middle Atlantic region with near-record low temperatures on Tuesday morning. Lows may fall to near zero just to the north and west of Philadelphia, Baltimore, and Washington, with single digits around Annapolis. During the day on Tuesday, high pressure will build over the region with plenty of sunshine; however, temperatures will be unseasonably cold with highs only in the mid 20s. The average high for this time of year is near 50 degrees!
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