Excessive Rain Event
Thursday-Friday September 30 to October 1, 2010
A unique upper level jet stream pattern evolved over the eastern United States in the days preceding what actually were two significant rain events for the East Coast and the Northeast, with the event from the 30th of September through October 1 yielding enormous amounts of rainfall for eastern New York and western New England. Had antecedent conditions not been abnormally dry across the region (moderate drought conditions according to the U.S. drought monitor in western New England) the 3" to 5" rainfall with locally up to 8.5" all coming in a fast 24 hour window, would have likely yielded significant and potentially major regional flooding. As it was, with so much water coming so quickly, flooding was widespread, with most of it, however, being minor including many flooded roads and basements, along with many of the region's smaller rivers and streams, such as the Canajoharie Creek, the Schoharie Creek, the Rondout Creek, the Mettawee river, the Hoosick river, and the Batten Kill, going into minor flood stage. Minor flooding also occurred along the Mohawk and Hudson rivers with moderate flooding along the Esopus Creek in Ulster County.
An anomalously strong low pressure trough at the jet stream level carved itself out over the southeast U.S., due in large part to strong high pressure blocking which existed over the sub-tropical Atlantic. A series of cut-off low pressure systems formed within the trough from early Monday Sept. 27 through Thursday Sept 29 over the Tennessee valley. The combined flow around the cut-off lows and the strong sub-tropical ridge over the central Atlantic created a narrow channel of deep tropical southerly flow along the Atlantic seaboard. Stuck between the mean trough position over the Southeast, and the sub-tropical ridge over the central Atlantic was a stalled front where several surface storms developed and tracked north, feeding on the moisture rich tropical air, which not only had it's origins over the Caribbean, but also a connection to the sub-tropical Pacific. The end result from the blocked, semi-stationary pattern was the series of storms riding up the eastern seaboard producing excessive rains over especially eastern North Carolina and Virginia from the 27th and 28th, with moderate rains over New York and New England, to the blockbuster rounds of rain that followed from the 29th through October 1. Added to the mix, was the brief development of tropical storm Nicole over western Cuba late on the 28th, only lasting as a tropical system for less than a day. Many thought, and incorrectly reported, that it was the remnants of Nicole that lead to the excessive rainfall across the Carolinas and the Northeast. However, Nicole was essentially a non-entity, with the plume of tropical moisture already in place and well established, the torrential rains would have occurred had Nicole been there or not.
The first storm came up the Appalachians passing to the local region's west from late Monday into Tuesday Sept. 27-28. A moderate rainfall resulted with much of the local area busting into the storm's very warm and humid sector on the 28th where scattered downpours occurred. As the first low went by, it drove a weak cold front through the Northeast which stalled along the New England coast on the 29th. Mild, but dry air followed the front on Wednesday the 29th, which set the stage for significant overrunning to develop on the 30th as the next wave of low pressure approached.
The next storm came up the coast early early Thursday, September 30 bringing an initial round of torrential rain to especially Berkshire County during the morning. Rainfall totals in the Berkshires ranged from 1.75" to 2.25" with lesser amounts in New York. The axis of heaviest rain then shifted to central New York by the late morning where it stalled through the day producing torrential rains and flooding. Light showers or drizzle occurred over much of the Hudson valley and western New England through the afternoon as the old stalled front drifted into the region as a warm front in response to the storm tracking north through central PA. Warm and humid air surged into western New England through the early afternoon, arriving in the Capital Region during the early evening with a jump in both temperature (near 80° at 8pm in North Adams, MA, mid 70s into the Capital Region) and dewpoints climbing into the low 70s. A period of strong SSE wind, which had been blowing just off the deck at around 2000' in elevation due to a tight pressure gradient, mixed down to the ground in the warm and humid air mass during the evening. Scattered minor tree and power line damage occurred, especially in the higher elevations east of the Hudson river and in western New England. Wind gusts for a brief time during the evening in Capital Region approached 40 mph which had followed calm winds that had prevailed through the day.
As the storm moved through east-central New York during the evening, passing northeast, it swung the torrential zone of rain that had been parked over central New York into eastern New York between 7pm and 11pm. Cooler air filtered back into the Hudson valley between 10pm and 11pm setting up a new overrunning surface and a pattern for efficient heavy rain production through the night as a second storm drove north along the coast. This storm subsequently tracked east of the initial low, passing to the Capital Region's east effectively keeping the Hudson valley and western New England in a zone of torrential rain from 11pm through 6am with rainfall amounts during this period ranging from 2"-4". A third weak low then came up after daybreak Friday sending one final burst of torrential rain up the Hudson valley and across western New England through noon. Rainfall tapered to showers and ended from west to east, finally pulling out of Berkshire and Litchfield counties by 7pm on Friday, October 1, followed by some breaks of sunshine in the Capital Region during the early evening.
The map below illustrates the 7-day rainfall totals for the nation with the area from North Carolina to New York the obvious bulls eye with record breaking rainfall. 2.68" of rain fell at Albany on September 30th setting a 24 hour rainfall record for that date. This was followed by 3.04" of rain on Friday October 1, also setting a 24 hour record rainfall for that date.
The Albany National Weather Service Doppler storm total rainfall estimate. Based on regional rainfall observations, the radar overestimated rainfall by up to 2 ". However, the distribution clearly and accurately illustrates the scope of the torrential rain in that it affected the entire region
WeatherNet 6 Event Total Rainfall reports from September 30-October 1, 2010
Flooding Reports: Note: These are a small number of reports used by the National Weather Service for the purposes of verifying the flood warnings that were issued and therefore do not represent the extent of the flooding which occurred.
Wind Damage Reports: Although damaging wind was not widespread with the storm, as much of the region remained in a fairly cool and stable air mass through the day with light and variable wind. A brief period of high wind occurred from the late afternoon into the night in the warm humid air mass that briefly moved into the Hudson valley and western New England. The strongest wind gusts with subsequent minor tree and power line damage was realized in the higher elevations of western New England. Also, a line of particularly torrential downpours moved into the Mohawk valley and Adirondacks affecting Fulton and Hamilton counties during the evening. Doppler radar indicated 60-80 mph winds above the ground with the line with pockets of that strong wind briefly mixing down to the ground. Fortunately, a temperature inversion (warm air over cool air) provided a stable enough layer so most of that wind remained aloft.