21 July 2010 Severe Weather Outbreak EF1 tornado in northwestern Connecticut

 

This event was fairly well forecasted. Upstream from Albany Tuesday July 20th, an strengthening upper level jet was over the Great Lakes Region. Two potent short waves riding along a stationary front just to north along with a surface cold front, was the culprit for the event. Our region, mainly north of Albany, was also under the left exit region of a 250MB 100KT Jet. A southerly flow brought increased humidity. As the short waves move in aloft, the air cooled resulting in SBCAPES between 1500 to 3000 J/K. The capes were actually highest from Albany north late in the day. Perhaps the greatest harbinger of all, was that the 0-6KM bulk shear was forecast to increase to over 40kts by Wednesday afternoon. The shear was mainly unidirectional and the LFC was initially over 3000 feet AGL, not very conducive for a tornadic outbreak.

 

SPC initially had our region in slight risk from Tuesday. However, after seeing the Bulk Shear increase markedly, they upgraded all of our area into moderate risk by 11 AM. Then, by around noon, they issued a PDS Severe Thunderstorm Watch, which was reissued before 8PM to continue to 1AM. Coordination with SPC was excellent.

 

Convection began to develop quickly by midday in County warning area. One of the first cells formed right at the apex the Mohawk Hudson Valley, possibly the result of a southerly Mohawk Hudson Convergence, well ahead of the synoptic forcing. This cell while quickly developing into a thunderstorm, took awhile longer to become severe, once it reached the Taconic Hills. The first warning was issued at 124 PM and it verified with a nice lead time of 30 minutes.

Cells initially erupted upstream in central New York by late morning. Outflow boundaries form them likely formed the second significant cell, which formed in Dutchess County, then tracked into Litchfield. The storm turned to the right. It is possible that it eventually interacted with a combined outflow boundary from storms well south as well as a sea breeze to produce an isolated supercell. There was considerable damage with this storm. The original clusters of storm from central New York worked into the southern Catskills and Mid Hudson Valley and developed into more of a line producing more wind damage and even some large hail.

 

A little later, more storms, probably firing off the Lake Ontario breeze front, moved into the southeast Andirondacks and work down the Mohawk Valley into the Greater Capital District. There were scattered damage mainly wind with these cluster of storms, especially in Saratoga County.

 

For cells in Dutchess/Litchfield moving away from the KENX radar (especially right-moving supercells), it is important to utilize the KOKX radar since it may be closer and sampling the storm better. In this case, with the supercell that developed in NW Litchfield County, the SRM from the KOKX radar was first to show strong (>50 kt) gate-to-gate shear in the 0.5 degree scan, and consistently showed it for several scans. We are now receiving 8-bit data from surrounding RDA's, so the data quality is good.

 

After sunset it was becoming less clear which storms (even the stronger ones) were mixing significant winds to the ground. Initially, a line of storms formed across the Southern Adirondacks and moved into the Mohawk Valley, producing wind damage. Three distinct cores then formed after some time, with polygons issued for the northern and southern cell, but not for the middle cell. There was a period of time where we did not receive reports AND radar signatures with both reflectivity strenghts and core depth decreasing as well as diminishing base velocity signatures for the middle cell that moved into Saratoga county. At the same time the other two cells were still quite robust with strong reflectivity cores and sharp reflectivity gradient. The only storm that produced damage in term of reports received was the supposed weaker middle storm. So, the lesson learned is be wary of storms that may seem to be weakening, but then move into a more populated area because there is a greater potential for reports of downed trees. The other two cells, while impressive looking on radar, could not be verified.

 

 

Above: Upper air data from the Storm Prediction Center at 12Z 21 July for 850 hPa (left), 500 hPa (center) and 250 hPa (right). At this time, an upper level impulse can be seen approaching from the western Great Lakes, with higher magnitude westerly boundary layer winds pushing into the region at 850 hPa. Upstream winds at 500 hPa were 50 Kt or greater, a threshold the SPC uses for increased severe weather potential. An upper level jet segment was also upstream, with our region potentially in the left exit region later in the day.

 

 

Above: Upper air data from the Storm Prediction Center at 00Z 22 July for 850 hPa (left), 500 hPa (center) and 250 hPa (right). There is a wind shift noted at 850 hPa but the boundary layer winds were relatively weak, at 30 Kt or less. Winds had increased to 50 Kt at 500 hPa, but the position of the upper level jet segment at 250 hPa was questionable in terms of whether our region was in divergent or convergent upper flow.

 

 

Above: Surface analysis from the Hydrometeorological Prediction Center valid 12Z 21 July. Note the stationary front from southern NY through about Long Island.

 

 

Above: Skew-T analyses from the Storm Prediction Center at 12Z 21 July from Albany, NY (ALB; left), Buffalo, NY (BUF; left center), Chatham, MA (CHH; center), Upton, NY (OKX; center right), Maniwaki, Ontario Canada (WMW; right). Generally between 1000-2000 J/Kg CAPE was predicted later in the day, and some increased shear was noted around OKX in the 0-3 Km SRH. There was abundant moisture available for convection seen in K indices.

 

 

Above: Skew-T analyses from the Storm Prediction Center at 18Z 21 July from Albany, NY (ALB; left), Buffalo, NY (BUF; left center), Chatham, MA (CHH; right center), Upton, NY (OKX; right). CAPEs were nearing 2000 J/Kg, but note the increased shear, especially at KALB, seen in the 0-3 Km SRH.

 

 

Above: Skew-T analyses from the Storm Prediction Center at 00Z 22 July from Albany, NY (ALB; left), Buffalo, NY (BUF; left center), Chatham, MA (CHH; center), Upton, NY (OKX; center right), Maniwaki, Ontario Canada (WMW; right). Note some of the CAPES exceeded 2000 J/Kg and the shear at OKX increased as the 0-3 Km SRH indicated.

 

 

 

Above: Severe Weather Outlook for day one from the Storm Prediction Center, issues at 1230 UTC 21 July, and probabilities for hail, wind and tornadoes.

 

 

Above: Severe Weather Outlook for day one from the Storm Prediction Center, issues at 1300 UTC 21 July, and probabilities for hail, wind and tornadoes.

 

 

Above: Severe Weather Outlook for day one from the Storm Prediction Center, issues at 1630 UTC 21 July, and probabilities for hail, wind and tornadoes.

 

 

Above: Mesoscale discussion 1395 from the Storm Prediction Center.

 

 

Above: Mesoscale discussion 1398 from the Storm Prediction Center.

 

 

Above: Mesoscale discussion 1401 from the Storm Prediction Center.

 

 

Above: Mesoscale discussion 1405 from the Storm Prediction Center.

 

 

Above: Mesoscale discussion 1408 from the Storm Prediction Center.

 

 

Above: Loop of LAPS CAPE (left) and MSAS surface dew points and winds through the day of 21 July. The LAPS CAPE display may have been contaminated by questionable observations, noted by the bullseyes of maxima, and the relative minima over southern NY through NW CT. The MSAS loop does show a persistent dew point gradient over southern NY and southern New England, evidence of the old stationary front. However, there really was not enough surface winds to provide much temperature advection or moisture convergence.

 

 

Above: Storm Prediction Center mesoscale analyses from 21 July of CAPE (left), Lifted Condensation Level (center left), 0-6 Km shear (center), 0-3 Km helicity (center right), and dew point and winds (right). The CAPE display offered similar information as the soundings and LAPS displays, while the LCL display showed levels above 1000M, not necessarily supportive of tornadoes. The Shear displays showed some shear supportive of rotating thunderstorms, while the surface dew points reflected what the MSAS showed.

 

 

Above: Storm Prediction Center mesoscale analyses from 21 July of low level lapse rate (left) and a series of midlevel (700-500 hPa) lapse rates (left center, right center and right). The low level and mid level lapse rates were not unusual, and did not signal enhanced updrafts, but still, the magnitude of the lapse rates suggested enough instability for potential severe weather.

 

 

Above: Loops of radar reflectivity from KENX and KOKX and a mosaic of radars across the region during the afternoon and evening of 21 July, some with Severe Thunderstorm Warning and Tornado Warning Polygons overlayed. Note the hook or appendage on the storm that tracked through NW CT that produced the tornado, seen from multiple radars. Also, note the widespread nature to the severe weather, illustrated by all the severe warning polygons.

 

 

Above: Loops of Storm Relative Velocity from KENX and KOKX during the afternoon and evening of 21 July. Note the enhanced rotation in the storm that tracked through NW CT that produced the tornado. The rotation was seen on multiple radars.

 

 

Above: Severe Weather reports over the NWS Albany, NY county warning area.

 

 

Above: Severe Weather reports over the NWS Albany, NY county warning area.

 

 

Above: Severe weather reports across the U.S. on 21 July.

 

Any comments, questions or suggestions, please mail to:Neil.Stuart@noaa.gov