Saturday, September 8, 2012
Severe Weather Outbreak
An outbreak of severe thunderstorms occurred between 2pm and 6:30pm across eastern New and western New England on Saturday September 8. A moderate risk for severe thunderstorms had been outlooked by the Storm Prediction Center for the region on the 7th with that forecast continued through the 8th for the anticipation of a widespread and significant damaging wind event associated with a strongly forced squall line and for the potential of discrete supercells in advance of the line creating an isolated tornado threat. A tornado watch was issued for the region at 1pm on the 8th and continued through 6:15 pm in New York and through 7pm in western New England. There were 42 official reports of damaging wind in the twenty one county CBS 6 coverage area, no tornadoes, and no hail.
So, all the big parameters were coming into place for a large severe event. likely marked by some discrete thunderstorms running in advance of a strongly dynamically forced squall line immediately along the leading edge of the cold front. However, there was one exception, one missing ingredient which in the end was the reason why the event was not more widespread and damaging than it was. The missing ingredient was a lack of overall instability with convective available potential energy values (CAPE) running from 500 to a maximum in the Hudson valley of 1000 j/kg. Despite very warm and humid surface conditions, temperatures aloft were not particularly cold. The colder the air aloft the more instability and thus taller stronger updrafts. Also, lapse rates were meager, meaning the temperature drop with height was nominal, another indicator of the lack of significant cold air aloft. So, the big forecast challenge for this event was how much would the strong dynamics in the atmosphere compensate for the fairly weak and meager instability.
In the end, the weak instability prevented much of any severe thunderstorm development from occurring in advance of the main squall line. This was a positive for the region as any discrete thunderstorms would have spun like tops and likely produced tornadoes. This scenario in fact did play out over and near New York City during the morning as discrete cells developed in a more unstable and strongly sheared environment and went on to produce several tornadoes. Instead, locally from the late morning through the early afternoon, low topped showers developed in Dutchess and Columbia counties quickly spreading north through eastern Rensselaer, Berkshire, and Bennington counties before moving out of the region early in the afternoon. Had greater instability been in place, these showers would likely have grown into damaging thunderstorms.
The squall line itself did materialize as expected and went on to produce pockets of damaging wind as it advanced. Storm tops, however, were shallow, generally 20,000 feet on average or lower at times. In fact through much of the squall line's existence very little lightning occurred due to the shallow tops which meant there was limited mixing of rain drops and ice crystals which is necessary to cause charge separation and cloud electrification. Storm tops, however, grew a bit to near 30,000' on average late in the afternoon, especially in the mid Hudson valley where instability was a little higher. These taller storms caused a marked increase in lightning in this region. Storms to 30,000' also developed in the squall line across southeast Hamilton, northern Saratoga, Warren, northern Washington, and Rutland counties where frequent lightning also occurred. These taller storm segments in the line also produced pockets of damaging wind. No hail occurred, likely due to the shallow nature of the t'storms in the squall line.
The squall line was narrow and generally moved through any one location quite quickly with anywhere from a half and hour to an hour of trailing strata form rain behind it. Temperature drops of approximately twenty degrees occurred with the line's passage with decent shelf clouds noted along its leading edge. The following three Instant Doppler 6 images show the progress of the classic squall line as it moved into and through the region.
Instant Doppler 6 Radar Images
3:43 pm September 8, 2012
4:43 pm September 8, 2012
This is a local storm report map generated at the Albany National Weather Service office, showing the storm reports (wind damage) as well as the severe thunderstorm (yellow) and Tornado (red) warning polygons that were issued by the Albany office for this event.
This is the Storm Prediction Center (SPC) national severe reports map for September 8, 2012 illustrating the significant number of damaging wind reports over the Northeast.
WeatherNet 6 Rainfall Reports for September 8, 2012
|Town||County||Rainfall Report||Town||County||Rainfall Report|
|Savoy, MA||Berkshire||0.80"||Pittsfield, MA||Berkshire||0.70" to 1.06"|
|Milton||Saratoga||0.45"||Clifton Park (Oaks)||Saratoga||0.50"|
|Ulster Park||Ulster||0.38"||West Shokan||Ulster||0.67"|
|Landgrove, VT||Bennington||0.85"||Danby, VT||Rutland||0.56"|