National Weather Service United States Department of Commerce

Heavy Rainfall for Portions of the West and Central U.S.; Monitoring Fire Weather Conditions for New England

Tropical moisture continues to flow across portions of the Great Basin, desert Southwest and California. Isolated instances of flash flooding remains possible. Meanwhile, a storm and associated frontal boundary will focus showers and thunderstorms for the center of the nation. For New England, dry and breezy conditions could enhance favorable fire weather conditions. Read More >

Overview

A late season snow storm blanketed Central PA with a general 6 to 10 inches of snow on Friday the 16th, the day before St. Patrick's Day, 2007. This storm developed quickly, as it developed along an old frontal boundary that produced some rain (that combined with snow melt to cause minor flooding), and even ended as some slushy snow in the Northern PA counties on Thursday (the 15th). However, the front had pushed well off to the south and east of PA on Friday morning, and the primary low pressure center developed in the Carolinas during the day on Friday (the 16th), moving to the northeast. Despite the placement of the surface low on Friday morning (just deepening over South Carolina), the deep easterly flow of moisture into the state Thursday Night and Friday Morning began producing light snowfall in the southern half of the state before sunrise on Friday. The jet stream then dipped sharply to the south over the Appalachian Mountains, and intensified the storm rapidly during the day on Friday. The snow started as a fine, needle-like snow for most of the area, with rain falling in York and Lancaster. The rain changed to sleet, then to snow, during the daylight hours over those lower Susquehanna Valley Areas, with New Holland (Eastern Lancaster County) not changing completely to snow until 8 pm Friday. The rest of the region saw the heaviest of the snow fall during the Afternoon and Evening on Friday, as the also snow became a heavier, wetter, more-dendritic snow. See the Skew-T, Log-P diagrams below for representative atmospheric soundings for KLWX (Sterling, VA) and KPIT (Pittsburgh, PA) at 12z Friday (8 am) and 00z on Saturday (8 pm EDT Friday). Note the warm layer aloft on Friday morning at both LWX and PIT, which gets erased by the evening, showing the change to a snow-supportive sounding/atmosphere.
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