July 2, 2014 - Severe Weather & Flash Flooding

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The severe weather and flash flooding that occurred on July 2 was supported by a prefrontal trough/height falls that was located well ahead of a slow-moving cold front across the Great Lakes region. This cold front was associated with a rather deep long wave upper-level trough that had several pieces of shortwave energy rotating through it. One of these pieces of energy ejected out of the long wave trough and helped initiate convection unusually early in the day across the forecast area - during the late morning hours.

The forecast area was primed for potential severe weather and possible flash flooding, with the region being under the right entrance region of a slightly anti-cyclonically-curved 100 knot upper-level jet (with the additional synoptic features mentioned above). The forecast area was also underneath increasing mid and upper-level deformation (caused by the approach of Hurricane Arthur from the SE), which further increased dynamic lift. The wind field was relatively weak, with the 850-300mb mean wind only around 20 knots. Despite this relatively weak wind field, bulk shear values in the 0-6km layer were around 35 knots with even better shear of 40 knots and greater further to the west. The shear was considered top heavy in this situation, so the threat of tornadoes was considered low.

An unusually unstable thermodynamic environment across the region was aided by a warm (temps in the mid to upper 80s) and increasingly humid air mass that had been in place since the prior weekend, as a Bermuda high remained parked across the Mid-Atlantic region. The Bermuda high, combined with the approaching upper-level trough, allowed for an excellent return flow of moisture associated with S-SW low level winds that helped advect plenty of moisture into the region. By the morning hours, surface dew points had risen into the upper 60s and even lower 70s, with anomalously high PWAT values approaching 2.0 inches.

However, this return flow brought with it a capping inversion in the mid-levels that made it less clear whether enough forcing would be available to promote convection. However, given similar setups during several past convective events, the pre-frontal trough typically provides enough low level lift to overcome any mid-level caps in place, especially given sufficient moisture and instability. Temperatures at 700 mb approached 10C while 500 mb temps were in the -5C to -10C range, both extremely warm and often indicators of capping that may be too strong to promote convective development. Any thunderstorms that would form, however, had the potential to be extremely efficient with warm rain processes, with FZL heights above 14 kft combined with the high PWAT values, so heavy rain was possible. Hail was not expected to be a significant threat due to the warm temperatures throughout the profile, but could not be ruled out given the expected moderate to high instability. Wind, in addition to the heavy rain potential, was considered the greatest threat of the day due to a nearly unidirectional wind profile with tall storm having the potential to produce wet microbursts.

The Storm Prediction Center had highlighted the entire forecast area in a Slight Risk with the 1300Z Outlook, with a 15% contour for both hail and wind. By mid-morning, concern was growing that it was going to be a very active day as temperatures were rising faster than previously forecasted and low-level moisture continued to increase, with widespread readings of 70F dew points and even dew points reaching as high as 75-76F in some spots! The Weather Prediction Center placed the entire forecast area in a Slight Risk for excessive rainfall as well. It was decided that a 16Z special ALB balloon launch would be conducted to support the potential severe and heavy rainfall threat. During the preparation of the balloon around 11am, thunderstorms began to develop in central NY/PA and the Storm Prediction Center quickly issued a Severe Thunderstorm Watch for the entire area.

The 16Z sounding revealed an atmosphere primed for convection. Surface-based CAPE was slightly over 2000 J/kg. The mid-level 850-500 and 700-500 mb lapse rates had steepened slightly to 6.5 C/km. Surface temperatures had warmed into the upper 80s and low 90s due to mostly clear skies during the morning hours. Downdraft CAPE (DCAPE) was over 1000 J/kg, further supportive for the potential of damaging winds with the stronger storms. Visually, the sounding revealed a lot of CAPE within the hail growth zone. PWAT values were ~1.8 inches. 0-6 km shear magnitude had increased to 45 knots, so with the unidirectional wind profile, multicell clusters/broken line segments were expected with perhaps a few discrete supercells possible as well. 0-1 km shear remained weak at only 10 knots, so the threat of tornadoes remained low. With their updated 1630Z Outlook, SPC increased the probability for wind damage to 30% across the southeast 2/3 of the Albany forecast area.

Initially, a few discrete cells formed between 15Z to 17Z, but with so much instability in place, several additional thunderstorms developed and the mode turned quickly multicellular. Severe Thunderstorm Warnings were issued for several of these cells as they moved from SW to NE across the southern 2/3 of the forecast area. There were several reports of wind damage associated with the severe thunderstorms. The most impressive of these was a cluster of storms that moved into Schoharie County and exhibited a wall-of-wind signature on base velocity with widespread inbound values between 60-70 knots indicated. Another radar feature that played a large role in the issuance of warnings was the presence of top-heavy KDP columns (KDP values 3+ deg/km near the top of the column). ZDR and CC values were slightly depressed in the updraft core as well signifying that there was a lot of melting of hailstones going on in the mixed phase region in addition to large water droplets being easily lofted into the updraft. These KDP columns were able to be tracked as they descended towards the surface and often coincided with peaks in the base velocity data.

In addition to the severe threat, a flash flood threat also set up as groups of multicells began to train/redevelop over the same areas. The two main areas where this occurred were in the immediate Capital District across eastern Schenectady/western Albany/southern Saratoga counties, and across the Mid-Hudson valley (Ulster, Dutchess, Columbia) into Litchfield county. Additional poor drainage flooding occurred especially in portions of Windham and Berkshire counties. Several reports of Flash Flooding occurred across portions of the Capital District and Mid-Hudson valley. These thunderstorms produced high rainfall rates, with several spotters reporting up to 2 inch/hour rainfall rates.

The thunderstorms moved east of the forecast area by the early evening hours. A second round of thunderstorms developed across the western Mohawk valley during the evening hours, with a couple more reports of wind damage. This cluster of storms weakened considerably as it moved east into northern portions of the Capital Region with the loss of daytime heating.

 

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Above: SPC Convective Outlook from 1300Z on July 2 discussed the potential for severe weather in the Northeast with a Slight Risk for the entire ALY forecast area. Equal probabilities for wind damage and large hail were displayed, but the discussion mentioned CAPE and near unidirectional shear combined with weak mid-level lapse rates would yield mainly a wind threat from multi-cells and bows/LEWPS.

Above: The Albany Hazardous Weather Outlook and Area Forecast Discussion highlighted the potential for Severe Thunderstorms and locally heavy rainfall preceding the event.

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Above: 1200Z Soundings July 2 for Albany, NY (ALB), Buffalo, NY (BUF), Upton, NY (OKX), and Pittsburgh, PA (PIT). Note the potential for forecast SBCAPE to exceed 2000 J/Kg at ALB and OKX. Also there is a unidirectional wind profile with increasing deep layer shear to the west over BUF and PIT at 1200Z.

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Above: Loop of MSLP (blue dashed contours), 3-hr pressure tendency (color shading), and METAR plots. A well-defined pre-frontal trough can be seen moving eastward across the region during the late morning and early afternoon, which provided the low level forcing for convection. Also note the extremely high dewpoints in the lower to mid-70s across the region.

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Above: Mesoscale Discussion from SPC during the late morning talked about the increasing potential for severe thunderstorms across the region. A Severe Thunderstorm Watch was then issued at 1115 am EDT.

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Above: A special 1600Z sounding was conducted at ALB, which further bolstered the confidence in severe thunderstorm development. The observed SCCAPE was around 2000 J/Kg, with 1030 J/Kg of DCAPE which signaled the potential for strong evaporative downdrafts. Also of note was the 0-6 km wind shear increased to 45 knots, while the 0-1 km wind shear was only 10 knots, thus signifying a relatively low tornado threat with a unidirectional wind profile above 1 km. The PWAT continued to be very high at 1.77.

Above: The 1630Z SPC Convective Outlook still had a categorical Slight Risk for the area, but increased the probabilities for large hail and damaging winds from 15% to 30% (within 25 miles of a point). The threat for wind seemed greater based on the 1600Z ALB sounding though.

Above: Mesoscale Precipitation Discussion from WPC issued at 110 pm highlighted a threat for Flash Flooding in addition to the severe weather potential.

Above: Loops from SPC Mesoanalysis archive show the SBCAPE (left) and DCAPE (right) from 1400Z to 2200Z. Note the SBCAPE increases to generally 2000-3000 J/Kg across much of the Albany forecast area, thus indicating high levels of instability. Also note the DCAPE in the 800-1000 J/Kg range, especially from around the Capital District south and east. This signified a threat for strong evaporative downdrafts associated with convection.

Above: Loops from SPC Mesoanalysis archive show the 0-6 km bulk shear (left) and Effective shear (right) from 1400Z to 2200Z. Note the increase in shear by early afternoon across the region to between 35 to 45 knots, which allowed for storms to organize into multi-cell clusters and small bows.

Above: Loops from SPC Mesoanalysis archive show the 0-3 km lapse rate (left) and Sig Severe parameter (right) from 1400Z to 2200Z. Note the steep 7.0 to 7.5C/Km low level lapse rates developing from late morning to early afternoon, which was another favorable factor for convection producing strong wind gusts. Also note the Sig Severe parameter was maximized over the much of the region.

Above: Precipitable Water loop from SPC Mesoanalysis archive from 1400Z to 2200Z shows values approaching 2 inches across the region, which supported the threat for heavy rainfall associated with convection.

Above: KENX 0.5 Reflectivity (left) loop from 1605Z to 2205Z and 0.5 Base Velocity (right) from 1716Z to 2037Z. Note on the Reflectivity loop merging and training cells over the Capital District during the early to mid-afternoon, with a solid line of convection developing south and east of Albany later in the afternoon. Also note significant wall-of-wind signature (deep blue color) on Base Velocity loop crossing Delaware and Schoharie Counties between 1715Z and 1815Z.

Above: Dual-Pol rainfall estimate Storm Total Accumulation (STA) (left) and legacy radar rainfall estimate Storm Total Precipitation (STP) (right). Note there were three main areas where the heaviest rainfall occurred. One area in a stripe over northern and northwest portions of the Capital District, a second area over southern Windham County in Vermont, and a third across the Mid-Hudson Valley, southern Taconics and southern Berkshires. There was generally good agreement between estimates from the STA and the legacy STP with some minor differences.

Above: SPC severe weather reports for July 2, including how the reports verified with regards to the 1300Z Day 1 Outlook. Blue dots indicate wind damage reports and green dots large hail of 1 or greater in diameter.