July 13, 2014 - Severe Weather (Rotating Storms No TORs)

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This event impacted only a small portion of the forecast area, 4 southeastern counties (Ulster, Dutchess, southern Columbia, Litchfield), with damaging winds. One storm in particular caused a significant amount of tree damage across southern Ulster County and in the city of Poughkeepsie. This event was not well-forecast though, as there was a canopy of cloud cover in place with some breaks across the southern counties during the mid to late afternoon. It was anticipated that the magnitude of the instability would not be great enough to develop tall updrafts capable of producing severe thunderstorms; however the magnitude of deep and low level wind shear was significant.

This event was not well-anticipated by WFO ALY or the Storm Prediction Center (SPC). We were anticipating possible minor/urban flooding and gusty winds, but not severe weather. The late afternoon SPC Severe Weather Outlook included only a small section of the western Mohawk Valley in a Slight Risk area, with the rest of the ALY forecast area in a 5% threat area for wind (See Text Outlook). The cluster of wind damage reports in the ALY CWA were outside (east) of the Slight Risk area.

The available instability was the biggest question on this day due to cloud cover, but just enough instability developed across the southern tier counties to tap into the strong deep layer shear. Most of the storms with the relatively taller cores displayed broad rotational characteristics. The cores from storms in our area were not particularly tall though, as none had 50 dBZ cores even reaching the -20C level of around 25,000 feet. So hail was not a significant threat, but downbursts from mesocyclones were.

There was a very delicate balance with the CAPE and shear for this evening. The CAPE across most of the ALY forecast area did not cross the threshold for organized, rotating storms to develop, but there was just enough across the southern counties of around 1000-1500 J/Kg. The lower magnitude of CAPE farther north resulted in just general very shallow convection with no rotation. Some breaks in the clouds were noted in the visible satellite imagery from mid to late afternoon across the areas that had a larger magnitude of CAPE and eventual severe storms.

In terms of the shear environment, 0-6 km bulk shear values of 35-40 KT were common across the area, with low level shear values supportive pf potential severe weather (0-1 km shear was 20-25 KT and the 0-3 km SRH was 200-300). This led to organized rotating updrafts where sufficient instability was present. Mid-level lapse rates were marginal, but the 850-500 mb lapse rate did increase to 6.2C/Km from the 00Z ALB sounding.

Synoptically, the convection was forced by a pre-frontal trough seen in MSLP analysis well ahead of a cold front over the Great Lakes Region. Aloft, the convection developed ahead of an approaching short wave trough over Lake Ontario evident by drying noted in water vapor imagery.

This event proved to be difficult in deciding between issuing warnings for severe thunderstorms or tornadoes for the rotating storms that had well-developed mesocyclones. The first rotating storms near the Dutchess/Columbia border did produce damage (trees down). This first report of the day was missed since it was a rather shallow storm that showed rotational characteristics. That storm allowed us to get a baseline for issuing Severe Thunderstorm Warnings on any future storms that looked similar. This worked well in terms of diagnosing storms that produced wind damage for the rest of the event.

The real challenge came from deciding whether or not to issue warnings for tornadoes, especially on one storm in particular that moved into southern Ulster County from Sullivan County. This cell was long-lived and had a history of good rotation all the way from the southern Tier of NY near Binghamton. The mesocyclone that moved into Ulster County had consistent, but broad circulation.

To investigate the rotation and potential for tornadoes, the latest CSTAR research methodology was used and we heavily relied on the V-R shear tool and SRM in FSI to calculate the 0.5 gate-to-gate shear values across 0.5 nmi and the Vm of the mesocyclone respectively. The values were consistently in the Category I (or weakest) group where there is "no clear signal between tornadic and non-tornadic mesocyclones". Each radar scan showed the Category I values, so we did not feel there was a high probability of a tornado. Also, there was no well-defined hook echo on the reflectivity that would have given us better confidence to issue a TOR. This methodology seemed to work very well for this event, as there were multiple cells that displayed rotation, but did not reach the criteria and did not have strong gate-to-gate shear in the lowest level. There were several reports of trees down associated with this cell, but there were no indications the damage was from anything other than from straight-line winds. The mesocyclone passed over portions of the city of Poughkeepsie and resulted in several downed trees. Again, the rotation was broad from SRM data and indications were the damage was from straight-line winds associated with the mesocyclone.

Above: SPC Convective Outlook from 1300Z on July 13 shows the main Slight Risk area and associated 15% probability contour for damaging winds to be west of the Albany forecast area. There was a 5% contour for damaging winds across much of the Albany forecast area.

 

Above: The Albany Hazardous Weather Outlook and Area Forecast Discussion mentioned the potential for thunderstorms with heavy rainfall, but did not perceive a threat of severe weather.

Above: 1200Z Soundings July 13 for Albany, NY (ALB), Buffalo, NY (BUF), Upton, NY (OKX), and Pittsburgh, PA (PIT). Note the lack of forecast instability and the weak mid-level lapse rates. However, deep layer and low level wind shear was sufficient for storm organization and possible rotation.

Above: The Water Vapor imagery (left) shows a distinct area of mid-level drying, indicative of a trough advancing eastward across the eastern Great Lakes proving the large scale lift supporting the convection. The Infrared Satellite imagery (right) shows cooling cloud tops across the southern tier and southeastern NY associated with the developing convection. More widespread convection occurred across PA, MD, WV and OH.

Above: The visible satellite imagery shows breaks in the cloud cover during the afternoon hours across the central-southern tier and southeastern NY, which allowed for sufficient instability to develop. Note farther north the cloud cover was more extensive, thus limiting the instability.

Above: Updated 2000Z SPC Outlook did not expand on the threat area for severe thunderstorms producing damaging winds for the late afternoon/early evening hours into SE NY and SW New England.

Above: Mesoscale Discussion from SPC during the late morning talked about the potential for isolated Severe threat across SE NY and SW New England. There was a Severe Thunderstorm Watch from Sullivan and Delaware County in NY and points south and west, however SPC indicated the threat of severe storms to the east of this region was lower.

Above: As severe thunderstorms continued to maintain strength moving eastward into the southern portions of the Albany forecast area, SPC issued a short-fuse Severe Thunderstorm Watch reacting to the ongoing severe weather threat.

Above: Loops from SPC Mesoanalysis archive show the SBCAPE (left) and 700-500 mb lapse rates (right) from 1800Z July 13 to 0100Z July 14. Note the area of 1000-1500 J/kg of CAPE that builds south of Albany during the afternoon hours. Mid-level lapse rates remained poor through the day, which resulted in less of a hail threat.

Above: Loops from SPC Mesoanalysis archive show the 0-6 km bulk shear (left) and 0-1 km bulk shear (right) from 1800Z July 13 to 0100Z July 14. The 0-6 km shear values in the 30-40 knots range were common across the area where severe thunderstorms developed. The 0-1 km shear was fairly strong, increasing to 20-25 knots by early evening. The increasing low level and deep layer shear aided in storm rotation.

Above: KENX 0.5 Reflectivity (left) loop and 0.5 Storm-Relative Motion (right) from 2036Z to 2358Z. Note in SRM imagery several cells that exhibited strong broad rotation with well-developed mesocyclones.

 

Above: SPC severe weather reports for July 13, including how the reports verified with regards to the 1300Z Day 1 Outlook. Blue dots indicate wind damage reports. Note the cluster of several wind damage reports across SE NY into NW CT.