18 September 2012 – Severe low-topped convective lines with flash flooding (+ Null TOR Watch box)

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This was a multi-hazard event for the Albany CWA, with reports of damaging winds, flash flooding, and even some minor river flooding in the Catskills. A tornado watch box was issued for the entire CWA except for southern VT, however no tornadoes occurred in the Northeast.

Synoptically, a full-latitude trough extending from Hudson's Bay all the way southward to the U.S. Gulf Coast states was responsible for the inclement weather. This type of upper level pattern looked more indicative of October or November than September. Anomalously high PWATS of +2 to +3 STDEV and southerly wind anomalies of at least +4 to +5 STDEV pointed towards a heavy rain event. However, the magnitude of rainfall was greater than expected and is discussed below. As is typical in this part of the country, multiple frontal boundaries accompanied the passage of the upper trough. The boundary that was most active in terms of the severe weather and flash flooding was a pre-frontal trough well out ahead of the main cold front. The pre-frontal trough passed through east central New York and western New England during the late afternoon through evening hours, while the cold front moved through overnight.

The severe weather/wind aspect of this storm system was rather complicated. With a very strong environmental wind field aloft (60-70 kt at 850mb and 50-60 kt at 925mb), most of the Albany CWA was already under a Wind Advisory with the Taconics, southern Green Mountains, Berkshires and Litchfield Hills under a High Wind Warning. It was reasoned that it would not take much to mix down the strong winds located not too far above the surface. Low-topped convective lines developed during the mid afternoon over central NY and northeast PA and moved eastward into our area. There was very little to no CG lightning associated with this convection, but there was enough momentum transfer to mix down strong winds as these convective lines passed through. Several Severe Thunderstorm Warnings were issued to account for this. Throughout the entire event, there was very limited instability. Surface based or mixed layer CAPE likely did not exceed 250 J/Kg across the area. This was evidenced by the SPC RUC hourly mesoanalysis. Also, mid level lapse rates were poor throughout the event, well less than 6.0°C/km. Thick cloud cover lingered through the day. The magnitudes of low to mid level wind shear and helicity were substantially high during the event, but there was not nearly enough CAPE to develop or sustain any tall updrafts. SRM radar data did not depict any significant rotation within any of the convective lines.

The heaviest rainfall occurred along a wide stripe from the Catskills in BGM's CWA extending northeastward through the eastern Catskills in the ALY CWA, through Albany County. Rainfall amounts of 4 to 7 inches were common in this swath. The torrential rain fell in mainly a 3-5 hour period during the afternoon and evening hours and was mainly a result of training low-topped convection. Initially terrain played a role during the more synoptic warm-advection precip during the morning and early afternoon as some Catskill gauge sites in favored upslope areas had 2 to 3 inches before the training convection began. However, rainfall from the afternoon/evening hours of 3 to 5 inches from the Catskills north and east was fairly uniform and resulted from the low-topped training convection. Freezing levels were around 11-12 kft, and most of the convection had the highest reflectivities of 45-50 dBZ below the freezing level resulting in very efficient warm cloud rain processes in the presence of deep moisture.

Despite dry antecedent ground conditions, flash flooding occurred first in portions of Greene and Ulster County due to extreme rainfall rates and 5 to 7 inches of rain falling in less than six hours. Flash flooding also occurred into portions of the Capital Region, where 3 to 5 inches of rain fell in a relatively short amount of time. Most of the flash flooding reported was due to road closures in urban areas. Due to a high volume of runoff from the eastern Catskills, the Esopus Creek at Mount Tremper rose above minor flood stage and even approached moderate stage. The Schoharie Creek at Prattsville shot up very quickly over 7 feet in only a few hours, but came up just shy of flood stage. Had preceding ground conditions not been so dry, river flooding would have been worse and more widespread, especially in the Catskill basins. Reservoir levels were very low, so flooding did not occur downstream of Gilboa on the Schoharie Creek.

Above: The Hazardous Weather Outlook and Area Forecast Discussions highlighted the potential for damaging wind gusts associated with convective lines.

Above: Day 1 1300 UTC Convective Outlook from SPC on September 18 highlighted the potential for severe weather. Categorical probabilities, as well as tornado and damaging wind probabilities are included.

Above: Soundings from 12 UTC September 18 from Albany NY (KALB), Buffalo NY (KBUF), Chatham MA (KCHH), Upton NY (KOKX), Sterling (IAD) and Pittsburgh PA (KPBZ) showing limited instability and poor mid-level lapse rates. The only sounding which depicted any significant forecast CAPE was down at Sterling. Across the Northeast though, very limited instability was present and this regime did not change much throughout the day.

Above: Visible satellite loop shows thick cloud cover persisting throughout the day across east central NY and western New England. This cloud cover significantly limited instability build-up. Water Vapor imagery and 500 hPa heights show a full latitude trough extending from Hudson’s Bay all the way to the Gulf Coast. This was quite a dynamic system that is more common during the cold season.

Above: Loop of MSLP (blue contours), 3-hr pressure tendency (color shading), and METAR plots. Low pressure passed to the north and west of the area, allowing for a pre-frontal trough and cold front to affect our region.

Above: A Mesoscale Discussion graphic from SPC highlighted the pre-frontal trough and cold front. The severe weather in the Northeast CONUS was associated with the pre-frontal trough, as the low level jet moved east of the region by the time the cold front moved into the area.

Above: These images show the progression of Tornado Watches issued throughout the day across the Northeast. After no tornadoes were reported across the region, the last watch that was issued for the day was a Severe Thunderstorm Watch.

Above: Mesoscale Discussions from SPC show the progression of their thought process issuing the watch boxes. The main concern was tornadoes spinning up along QLCS structures due to a large magnitude of low level shear and helicity.

Above: A loop of the VAD Wind Profile from the Albany County Radar (KENX) shows strong winds aloft throughout the afternoon and into the evening. 50 kt wind speeds were common 2000-3000 ft above the ground.

Above: The Regional Hazards map for September 18 at 5:10 am EDT shows Wind Advisories were in effect across much of the Northeast, with High Wind Warnings for higher elevation areas such as the Taconics, southern Green Mountains of VT, Berkshires, and Litchfield Hills.

Above: Maps from the Global Ensemble Forecast System (GEFS) for the 18 UTC September 18 forecast initialized at 12 UTC. First image shows Precipitable water anomalies of +2 to +3 STDEV across the region. Second image shows widespread swath of +5 STDEV of v-component (southerly) wind anomaly.

Above: SREF forecasts initialized 15 UTC on September 17. The first image shows probability of greater than 2 inches in 36 hours. Note the likely values (60-70 percent) of chances of exceeding 2 inches in the Catskills and Adirondacks. The second image shows probability of greater than 3 inches in 36 hours. Note only low chance values (20-40) percent of exceeding 3 inches in the Catskills.

Above: GEFS forecasts initialized 12 UTC on September 17. The first image shows probability of greater than 2 inches in 36 hours. Note only low chance values (20-30 percent) of chances of exceeding 2 inches in east central NY. The second image shows probability of greater than 3 inches in 36 hours. Note the absence of any probability greater than 5 percent of chances of exceeding 3 inches.

Above: SPC analyzed plots of 250, 500, 700, 850, and 925 hPa heights at 12 UTC September 18. At 250 hPa, also notice the shaded colors indicating the position of the upper level jet and implied divergence associated with the right entrance region of the jet moving into the Northeast and Mid-Atlantic states.

Above: SPC analyzed plots of 250, 500, 700, 850, and 925 hPa heights at 00 UTC September 19. At 250 hPa, also notice the shaded colors indicating the position of the upper level jet and implied divergence associated with the right entrance region of the jet now over the Northeast and Mid-Atlantic states.

Above: SPC RUC Mesoanalysis loop of Surface-based CAPE and CIN from 1600 UTC to 2300 UTC September 18. Notice the 250 J/Kg CAPE contour barely makes it into the Albany CWA, with the more substantial instability displaced farther south over the Mid-Atlantic states.

Above: SPC RUC Mesoanalysis loop of 0-1 km and 0-6 km shear from 1600 UTC to 2300 UTC September 18. Magnitude of low level and deep layer shear was substantial across the region.

Above: SPC RUC Mesoanalysis loop of Supercell Composite Parameter and Significant Tornado Parameter from 1600 UTC to 2300 UTC September 18. These plots indicate the potential for supercells and tornadoes was rather low across the Northeast CONUS, with better potential across the Mid-Atlantic states.

Above: Radar reflectivity loops for the duration of the event. The first loop shows mainly the stratiform rain that affected the area, while the second loop shows when low-topped convective lines with torrential rainfall moved across the area. Warning polygons are also plotted, with yellow polygons representing Severe Thunderstorm Warnings and the brighter green polygons representing Flash Flood Warnings.

Above: First image is a Multi-sensor Precip Estimation (MPE) map, which takes into account radar estimates combined with actual gauge readings. The MPE display is more crude than the radar estimates in terms of resolution, but are likely more accurate. The second image is a zoomed-in map of Dua-Pol radar precip estimates (STA).

Above: First image is Dual-Pol radar precip estimate (STA) and second image is legacy radar precip estimate (STP) for the duration of the event. When compared to the MPE and many rain gauge reports, the STA had a better representation of rainfall than the STP. The STP significantly underestimated the rainfall in many locations.

Above: SPC severe weather reports for September 18, including how the reports fit into the 1630 UTC Day 1 Outlook. Blue dots indicated wind damage reports. Only one tornado (red dot) was reported along the east coast and occurred in north-central North Carolina.