11-12 September 2013 Severe/Flash Flooding/Heat

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Multiple warm season hazards occurred over a two-day stretch from Wed Sep 11 to Thu Sep 12, as an anomalously warm and moist air mass was entrenched over the region while several frontal boundaries crossed the area. Sep 11 had more in the way of severe storms, while on Sep 12 there was more flash flooding than severe storms.

Warm and very moist/humid air started to surge northward into the region during the early morning of Sep 11, which set the stage for building extreme instability by early afternoon. A strong and expansive upper level ridge that had resided across the central/southern CONUS for several days shifted northward into our region accompanied by significant warming aloft along with the surge of moisture. From the 12Z Sep 11 ALB sounding, the 850mb temperature was an incredible +20C and the PWAT had increased to around 1.60". Of most significant note was the existence of a stout EML that had moved over the region from the Midwest/Great Lakes with 850-700mb lapse rates of 7.0C/Km. With ample sunshine through the late morning and early afternoon, instability built to extreme levels. The special 18Z ALB sounding revealed over 4000 J/Kg of SBCAPE, which was the product of very high surface dewpoints and the EML with 850-700mb lapse rates increasing to 7.4C/Km! The SPC RAP mesoanalysis page showed between 3000-6000 J/Kg of SBCAPE across our forecast area by mid-afternoon, with the highest values from the Capital Region southward. The page also revealed tremendous amounts of moisture for mid-September, with dewpoints in the mid 70s and a 370K theta-e contour even indicated!

Before severe thunderstorms commenced, the big story was the heat and extreme humidity which resulted in very high heat indices. Surface dewpoints actually rose steadily through much of the day. It is hypothesized that the lower mid-September sun angle limited deeper mixing and allowed for dewpoints to climb into the mid 70s in the Hudson Valley. Surface temperatures managed to reach the lower 90s, which resulted in Heat Index values topping out at 105 at ALB, GFL, and POU! Due to the rising trends in the dewpoints and temperatures, a short-fused Heat Advisory was issued for much of the Hudson Valley and Litchfield County at 1235 pm.

Convection during the afternoon/evening of Sep 11 was forecast fairly well by some of the high resolution models, especially the HRRR which showed initiation over the higher terrain of west/central PA then rapidly building northeastward through south/central NY, and eventually eastern NY and western New England. The forcing was somewhat weak/subtle, as there was a small scale disturbance over PA which got the convection going, but with extreme instability not much forcing was needed for scattered to numerous thunderstorms to develop. Limiting widespread convective development as well were meager 0-6 km bulk shear values of only 20-30 kt. A Severe Thunderstorm Watch was issued at 233 pm for most of the forecast area except for Dutchess/Litchfield counties. The best deep layer 0-6 km shear was displaced to the north of our area over northern NY and New England, however there was around 30-35 kt of shear across the southern Adirondacks eastward to the Lake George/Washington county area, which likely played a crucial role in organizing and maintaining a bowing line with significant wind/lightning from Saratoga county eastward through Washington/Bennington counties. The storms across the central and southern portions of the area were generally less organized due to lack of shear and were more pulse-type and/or brief multicell clusters. One of the stronger organized storms produced 1 to 1.25 inch diameter hail and developed in Fulton County and eventually merged with other cells in Saratoga County to produce the severe line mentioned previously. Six SVR warnings were issued across the area. There was a lot of lightning associated with thunderstorms, with some structures/trees likely being struck.

The convection that occurred on Thu Sep 12 was more widespread due to greater large scale ascent ahead of a deep upper level trough progressing eastward across the Great Lakes and southeast Canada, although was generally less severe due to lower amounts of instability. There were at least three distinct surface boundaries, with a pre-frontal trough over eastern NY during the early afternoon, a cold front farther back across western NY, and a secondary cold front back over the upper Great Lakes. The day started out with more cloud cover than the previous day, especially across the western/southern Adirondacks. There was a narrow time-window of clearing for areas from mainly the Hudson Valley south and east, where SBCAPE of 1000-2500 J/Kg developed according to the SPC RAP mesoanalysis page. The special 16Z ALB sounding revealed around 1200 J/Kg of SBCAPE and a weak mid-level capping inversion, but with noticeably weaker 850-700 mb lapse rate of 6.4C/Km, which indicated the EML from Wednesday had moved east of the region. The deep layer shear was stronger than on Sep 11 across most of the area, with 30-40 kt of 0-6 km bulk shear with a unidirectional wind profile, which indicated the main threat would be wind with a very low probability for severe hail. A Severe Thunderstorm Watch was issued at 1241 pm for the entire forecast area, although it seemed conditions were too cloudy/stable across northern Herkimer/Hamilton counties for severe convection.

There appeared to be a better threat of isolated Flash Flooding on Sep 12, due to continued anomalously high PWATs and possible several bands/clusters of convection with downpours moving across the area. This threat was mentioned in the HWO, although it was decided to not issue any Flash Flood Watches due to expected isolated coverage.

Four SVR warnings were issued, mainly across northeast portions of the Capital Region, Rensselaer/Columbia counties, Bennington/northern Berkshire, and the Mid-Hudson Valley. These areas are where the highest instability had developed. There were sporadic reports of wind damage, however the bigger threat ended up being Flash Flooding. From radar estimates and reports, there were three main areas that were impacted by Flash Flooding. Portions of the Capital Region from Albany and especially northeastward to Latham, Cohoes, Troy and Brunswick received between 2 and 3 inches of rain in less than two hours, with rapid runoff occurring over urban areas. Another area of potentially greater rainfall of 3 to 5 inches (according to radar estimates) occurred across southeast Windham County in and near Brattleboro. Multiple reports of Flash Flooding came in from Brattleboro with road washouts. The third area was across southern Rensselaer and northern Columbia counties, where 3 to 4 inches of rain fell according to both reports and radar estimates. Several bands of convection moved across this area with multiple reports of Flash Flooding and even a mudslide. A senior citizen housing complex had to be evacuated in Nassau. There were Flash Flood Warnings for two of the three areas.

The intensity of the convection as well as rainfall rates weakened during the evening by sunset, and especially after dark with loss of instability. Heavy rainfall was generally confined to convective bands which were not widespread, so there were mainly minor rises on main stem rivers with no flooding. Williamstown on the Hoosic River did briefly get to caution stage, but no other point did.

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Above: GIS map of maximum temperatures for Sep 11 and Public Information Statement of individual reports. Most valley locations reached the lower 90s.

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Above: The 1300Z SPC Convective Outlook for Sep 11 included a Slight Risk for most of the ALY forecast area. The best potential for any tornadoes was expected to be well north across northern New England, where low level shear was maximized. The biggest threat for our area was wind with a 30% contour from around Albany northward. Large hail was also a threat due to high magnitude of instability and very steep mid level lapse rates.

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Above: 1200Z Soundings Sep 11 for Albany, NY (ALB), Buffalo, NY (BUF), Chatham, MA (CHH), and Upton, NY (OKX). Note the existance of an EML over the region evidenced by steep 700-500 mb lapse rates. Also there is a unidirectional wind profile for all four sites.

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Above: Special 1800Z soundings were conducted by ALB and Gray, ME (GYX). Note the extremely high magnitude of instability with 3000-4500 J/Kg of surface-based CAPE! Also, the EML was still over the region depicted by 850-500 mb lapse rates well over 7.0C/Km! Deep layer 0-6 km shear was unidirectional with magnitude around 25-30 kt. Combined with high instability, this was sufficient for storms to organize into multicell clusters and lines.

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Above: Infrared satellite loop and MLSP analysis (left) shows mainly clear skies during the morning of Sep 11 across eastern New York and western New England, with convection quickly developing across Pennsylvania and expanding northeastward into south-central and eastern New York during the afternoon. Water Vapor loop and 500 mb height contours (right) depict an expansive late-summer ridge extending northward into northeast CONUS, providing a very warm and humid environment. Note an upper level jet in Quebec, with our region in vicinity of the right-entrance region of the jet.

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Above: Loop of MSLP (blue dashed contours), 3-hr pressure tendency (color shading), and METAR plots. Note the extremely high dewpoints in the lower to mid-70s across the region.

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Above: Mesoscale Discussion (left) from SPC issued at 1730Z talked about the likelihood of the issuance of a watch during the afternoon into the evening. The Severe Thunderstorm watch (right) was then issued at 1835Z.

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Above: SPC RAP Mesoscale analysis loop from 1900Z Sep 11 to 0200Z Sep 12 of SBCAPE (left) and 0-6 km bulk shear (right). Note the extreme values of SBCAPE of 3000-6000 J/Kg over east central New York and western New England at 1900Z. The best shear was located to the north across northern New England and southeast Canada.

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Above: SPC RAP Mesoscale analysis loop from 1900Z Sep 11 to 0200Z Sep 12 of 700-500 mb lapse rate (left) and surface theta-e (right). Note the steep lapse rates of greater than 7.0C/km and theta-e values of greater than 360K southeast of Albany.

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Above: 0.5 reflectivity loop from 1903Z Sep 11 to 0257Z Sep 12. Warning polygons are also plotted, with yellow polygons representing Severe Thunderstorm Warnings.

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Above: 4-panel FSI image from 2146Z Sep 11 (left) and Correlation Coefficient from 2155Z Sep 11 (right). The storm shown in the FSI cross-section is from Fulton County, which produced 1.25 diameter hail. Note the elevated core of >65 dBZ and the 50 dBZ contour reaching almost 40,000 ft. The Correlation Coefficient shows hail spikes from two different storms, one from the Fulton County storm and the other from a storm in Oswego County. Note the abrubt decrease in CC values seen by the blue colors.

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Above: Dual-Pol rainfall estimate Storm Total Accumulation (STA) through 2355Z Sep 11 (left) and legacy radar rainfall estimate Storm Total Precip (STP) (right). Note the heaviest rainfall occurred to west of our area and impacted BGMs forecast area and there was likely some hail contamination in some of the higher estimates.

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Above: SPC severe weather reports for Sep 11, including how the reports fit into the 1300Z Day 1 Outlook. Blue dots indicate wind damage reports and green dots large hail of 1 or greater in diameter.

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Above: The 2000Z SPC Convective Outlook for Sep 12 included a Slight Risk for most of the ALY forecast area. The main threat was damaging winds based on less magnitude of CAPE, but greater 0-6 km shear than Sep 11. There was even a 30% probability contour for wind damage from Albany southward.

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Above: 1200Z Soundings Sep 12 for Albany, NY (ALB), Buffalo, NY (BUF), Chatham, MA (CHH), and Upton, NY (OKX). Note significantly less forecast CAPE than the previous day, as well as mid-level lapse rates not nearly as steep. These observations indicated the EML that was present the previous day was well east of the region.

Above: Special 1600Z sounding from ALB continued to confirm modest instability and mid-level lapse rates, with a unidirecitonal wind profile.

Above: Infrared satellite 24-hour loop and MLSP analysis (left) shows convection from the evening of Sep 11 weakening, with clearing taking place during the late morning into early afternoon of Sep 12 and convection re-developing thereafter. Water Vapor loop and 500 mb height contours (right) shows a strong upper trough progressing southeastward across the Great Lakes and southeast Canada.

Above: Loop of MSLP (blue dashed contours), 3-hr pressure tendency (color shading), and METAR plots. Note a pre-frontal trough moved into eastern New York during the mid to late afternoon hours, while the real cold front lagged back across western New York.

Above: Mesoscale Discussions from SPC issued at 1602Z (left) and 1624Z (middle) talked about the likelihood of the issuance of a watch during the afternoon into the evening. A Severe Thunderstorm watch (right) was then issued at 1640Z. The main concern was for downbursts.

Above: SPC RAP Mesoscale analysis loop from 1700Z Sep 12 to 0100Z Sep 13 of SBCAPE (left) and 0-6 km bulk shear (right). Note SBCAPE values of around 1000-2500 J/Kg across much of the area except for the western Adirondacks where conditions were more stable. Shear magnitude increase to between 30-40 kt across the area that was most unstable, but not until early evening.

Above: SPC RAP Mesoscale analysis loop from 1900Z Sep 11 to 0200Z Sep 12 of 700-500 mb lapse rate (left) and surface theta-e (right). Note 700-500 mb lapse rates were much weaker than the previous day, with less than 6.0C/Km across much of the area. The highest values of theta-e resided just south of Albany.

Above: 0.5 reflectivity loop from 1614Z to 2356Z Sep 12. Warning polygons are also plotted, with yellow polygons representing Severe Thunderstorm Warnings and green polygons representing flash flood warnings. Training convection with heavy rainfall was more common compared to Sep 11.

Above: Dual-Pol rainfall estimate STA through 2356Z Sep 12 (left) and legacy radar rainfall estimate STP (right). There were three distinct areas of heavy rainfall which lead to flash flooding based on the purple/blue shaded areas estimating 2.5 to 4 inches of rain. The STA did a better job with the heavy rainfall estimates based on rain gage comparisons.

Above: SPC severe weather reports for Sep 12, including how the reports fit into the 1300Z Day 1 Outlook. Blue dots indicate wind damage reports and green dots large hail of 1 or greater in diameter, and red dots represent tornado reports.