The National Weather Service has posted a Winter Storm Watch for Philadelphia, Baltimore, and Washington effective from late Sunday night through Monday afternoon for the potential for a significant accumulation of freezing rain, sleet, and snow (5 or more inches.) A watch indicates that conditions are favorable for the above conditions, but not imminent. A warning may be issued if the weather service determines that the conditions are imminent, or they may change the watch to a Winter Weather Advisory of data indicate a change in the expected precipitation types and amounts.
On Sunday night, our attention will turn to an area of low pressure moving along a frontal boundary. The system will not feature a strong center of low pressure, like a nor'easter, but rather an area of intense convergence caused by two air masses pressing into one another. Warm, subtropical air to the south will battle frigid air to the north. The area of convergence between the warm air to our south and the arctic air to the north will be the track upon which an area of low pressure will develop Sunday into Monday. Areas to the north of the convergence zone will experience a moderate to heavy snow event, with areas to the south receiving a soaking rain. In the middle, a wide range of precipitation types is expected including rain, sleet, snow, and even freezing rain. A significant period of icy conditions can be expected in this zone.
At the moment, Philadelphia, Baltimore, and Washington appear to be north of the convergence zone with mixed precipitation on Sunday night changing to heavy snow on Monday. Philadelphia will likely have a shorter period of mixed precipitation resulting in higher snowfall amounts. A significant accumulation of more than 5" of snow certainly appears likely in these areas. A shift of 20 or 30 miles will have a significant impact on where the heaviest snow falls with this system. In addition to the snow, temperatures will plummet into the 20s during the day with winds gusting over 25 miles per hour at times. Clearing on Monday night will be accompanied by bitterly cold temperatures with low from 0 to 10 degrees by Tuesday morning.
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