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Overview

Sally made landfall near the Alabama-Florida border as a hurricane early Wednesday morning (9/16). Sally then slowly tracked to the northeast through southern Alabama on Wednesday. Meanwhile, a large trough aloft was centered across the eastern half of Canada with the base of the trough extending southward from the Midwest to the northern Mid-Atlantic. Sally started to accelerate and turn more to the ENE Wednesday night into Thursday morning (as it approached the Carolinas) when it started to get picked up by the larger trough. Additionally, a rather strong cold front (for the last days of summer) was starting to drop toward the Mid-Atlantic from the north (but would remain north of the local area through the day on Thursday). Sally already had a history of producing extreme amounts of rainfall across Florida, Alabama, and Georgia before any rain started to move into the local area.

By the middle of Thursday morning, a large area of moderate rain had overspread most of the AKQ forecast area (and even areas of western VA). 250 mb analysis noted a 90-100 knot jet streak developing across PA that was moving to the east-northeast (while strengthening) during the day on Thursday. Much of our area was situated in the rear right quadrant of the jet streak, which is one of the regions favorable for large-scale ascent. This in addition to the strong deep-layer moisture advection was responsible (in part) for the fact that moderate rain continued throughout much of the area through Thursday afternoon. By early Thursday evening, a few flooding issues were reported in and around Richmond due to 2 to locally 3 inches of rain. By the middle of the evening, post-tropical cyclone Sally was tracking ENE through the central Carolinas while the cold front was approaching northern portions of the forecast area. There was actually a slight decrease in the average rainfall intensity from about 9 PM through midnight. From midnight through about 6 AM, the cold front crossed most of the region and was south of all areas except for coastal NE NC. During this time, Sally continued to accelerate to the ENE and moved to a position to near the NC Outer Banks. As it interacted with the approaching front, the thermal gradient from the surface to just above 850 mb tightened in a WSW-ENE oriented corridor near to just NW of the center of the storm (roughly from Emporia VA-Hampton Roads). This resulted in strengthening frontogenesis from the surface to just above the 850 mb layer across portions of south and SE VA. Combined with an environment with PWs of ~2.3", this resulted in a narrow, but intense band of rain moving across south/SE VA from midnight-5 AM Friday morning (9/18). While the heaviest rain only lasted 3-4 hours at any one location, rainfall rates of  1-2" per hour were observed at times. Combined with the rain that fell earlier in the day, numerous instances of flash flooding were observed from southern Brunswick County to Wakefield to Hampton Roads. The heavy rain quickly came to an end by 6-7 AM as Sally pulled offshore. Generally 3 to 6 inches of rain fell across central/southern VA, with lesser amounts across far NW and SE portions of the forecast area. There were a few locally higher totals in the 6-7" range. The very heavy rainfall resulted in river flooding issues across our forecast area (that are currently ongoing), particularly in the Meherrin, Blackwater, and Nottoway River basins.

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