7 July 2013 Severe/Flash Flooding

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A handful of severe thunderstorm and flash flood events occurred on a day with weak/subtle forcing and key features. The synoptic scale pattern featured a predominant Bermuda High centered off the Mid Atlantic coast. The 500 mb ridge axis extended northward into our region, although an increasingly diffluent flow aloft developed across the Northeast CONUS due to a warm-core cut-off low moving into Ohio. While the Water Vapor imagery was generally clouded by significant mid and upper tropospheric moisture, the initial 12Z NAM and GFS showed a weak upper level disturbance/vort max moving out of Ohio and northwest PA into Upstate NY during the afternoon.

Looking at the important mesoscale features across our area, there was a batch of showers and associated cloud cover that moved through the northern half of the CWA during the morning hours, while areas to the south had full sunshine which allowed for rapid heating. This set up a rather significant instability gradient across the area by Noon, which showed up well in the MSAS Equivalent Potential Temperature analysis, with values in the 350-360K across southern areas and 340-345 north. The theta-e gradient reached 15-20K across the eastern third of the forecast area, sufficient for thunderstorm development. The instability gradient resulting from differential heating appeared to be crucial for convective initiation during the early afternoon. The high resolution mesoscale models, such as the HRRR and the local HIRESWRF showed convective cells initiating across the area during this time as well. The models concentrated most of the initial convection across east-central NY and western New England, which ended up being a good forecast.

The 12Z ALB sounding showed the potential for severe weather and possible flash flooding. The forecast SBCAPE was around 1500 J/Kg, although Albany was right along the instability gradient, so there was expected to be much higher CAPE of 2000-3000 J/Kg south of the Capital Region across the Mid Hudson Valley, Taconics, Berkshires and Litchfield Hills. The PWAT was quite high at 1.84 inches and expected to increase to over 2 inches by afternoon. These values were around +2 STDEV above normal for early July. The computed 0-6 km shear was 41 kt at 12Z, although upstream at BUF the magnitude was less at 22 kt. So the expected shear for our area during the afternoon was around 25 to 35 kt, which would be sufficient for storm organization into multi-cell clusters or lines given the near unidirectional profile. The freezing level was quite high over 14,000 ft, but given enough instability especially over southern areas, both hail/wind potential existed. Mid level lapse rates were rather marginal around 6.0 to 6.5 deg C/km, but not as poor as we've seen in some of the non-severe convective days this season. K-index values were in the mid 30s, which combined with recent very wet ground conditions signified a flash flood threat, especially for areas that could receive repeated or training convection. The combination of high freezing levels, high K index values (greater than 30), and a tall/skinny CAPE profile promoted very efficient warm rain processes given the high PWAT values in place.

The HWO mentioned the potential for a few severe thunderstorms with the main threats damaging winds and also isolated flash flooding.

Convection occurred mainly in two phases. The first developed near the Capital Region and in northern portions of the Mid Hudson Valley and Berkshires during the mid to late afternoon. The strongest cells developed in the better instability near the southern edge of the sharpest Theta-E gradient across extreme eastern Green County and Columbia County. A few of the updrafts had very impressive deep and tall cores, with 50 dBZ between 40,000 and 50,000 ft! Both severe hail up to golf ball size and damaging winds were reported in Columbia County. Although we received hail reports to verify some of the SVRs issued, we also did receive several reports of sub-severe hail, as the warm thermodynamic environment promoted melting of hail as it descended towards the ground.

Unfortunately, significant updrafts kept re-developing across southern portions of Columbia County for multiple consecutive hours. A Flash Flood Warning was issued at 224 pm, once radar estimates showed 1 to 2 inches of rain across the area with more heavy rain continuing to train over the same locations. The first reports of flash flooding came in around 345 pm with a portion of the Taconic State Parkway closed. Additional reports with more road closures and even a bridge washout occurred between 4 and 5 pm. By this time, the STA depicted an area of 4 to 5 inches of estimated rainfall, with a larger swath of 3 to 4 inches surrounding the max amounts. The worst flooding and heaviest rainfall occurred in Taghkanic.

Both the STP and STA did extremely well handling this rainfall event, with little differences between the two. FFMP when using both the legacy and HPE also did extremely well in this case highlighting the affected basins well. The 3hr FFG was around 3 inches, and some locations in Columbia County received this amount in just over an hour.

The second phase of convection occurred during the evening hours, as additional showers and storms developed across south-central NY likely associated with the upper level disturbance moving through the area, with even more significant diffluent flow aloft by that time. This convection was shallower and did not produce any severe weather; however, heavy rainfall occurred with efficient warm cloud processes. As had occurred several times in the past few weeks, this convection moved across the already flood-ravaged area of southern Herkimer County between 8 and 10 pm. A Flash Flood Warning was issued at 844 pm, with flash flooding reported in Little Falls with multiple roads closed between 838 and 900 pm. The statement was upgraded to a Flash Flood Emergency at 919 pm with several reports of high water in Little Falls and coordination with NYS OEM resulted in them deploying vehicles for potential swift-water rescues. The flooding in southern Herkimer County occurred with about half as much rainfall as what fell in Columbia County earlier in the day, however, this was the 4th or 5th flash flood event or emergency that occurred in southern Herkimer County since June 28.

 

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Above: The Hazardous Weather Outlook and Area Forecast Discussions from early in the morning mentioned the potential for convection, although the threat for severe storms and flash flooding became a bit more apparent later in the morning with the aid of the 12Z ALB sounding and observations.

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Above: The Storm Prediction Center (SPC) Day 1 Outlook from 1630Z did not have any of the Albany forecast area in a slight risk for severe weather.

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Above: 1200Z Soundings on July 7 from Albany NY (ALB), Chatham MA (CHH), and Upton NY (OKX) depict an environment suitable for potentially severe storms based on forecast SBCAPE values of 1500-3000 J/Kg from ALB southward to the OKX corridor, with also 30-40 kt of 0-6 km shear sufficient for storm organization.

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Above: A 216 pm EDT Mesoscale Discussion from SPC talked about the possibility of a few multi-cell clusters capable of producing severe hail and wind, but the probability of issuing a watch was only 20 percent at that time.

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Above: Visible satellite loop (left) shows extensive morning cloud cover over northern areas with manly clear skies south of Albany, which contributed to differential heating and developing an instability gradient. The 500 mb pattern and Water Vapor loop (right) depict a Bermuda High over the western Atlantic with the ridge axis extending northward into southern New York and New England, while a closed low tracked northeastward through Indiana and Ohio, resulting in increasingly diffluent flow over east central NY and western New England. Note a 100 kt jet max along the eastern periphery of the low.

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Above: Loop of MSLP (blue contours), 3-hr pressure tendency (color shading), and METAR plots. The loop shows a surface trough and weak/small cyclone moving into the region during the afternoon.

Theta_e

Above: Loop of surface Equivalent Potential Temperature (K) from MSAS analysis. Loop depicts a building instability gradient from the late morning into the afternoon. Note the 10-20 degree gradient from north to south by early afternoon.

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Above: Loop of surface Moisture Flux Divergence (g/Kg/12 hr) from MSAS analysis shows an enhanced area of Moisture Flux Convergence over the Albany forecast area during the afternoon.

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Above: The SPC 2000Z Day 1 Outlook expanded the slight risk area to include portions of the Albany forecast area from the Capital Region southward once convective trends indicated the threat for wind/hail had increased.

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Above: A severe thunderstorm watch was issued for Ulster and Dutchess counties late in the afternoon for convection upstream over PA, although the brunt of the severe weather occurred in Columbia and Berkshire counties.

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Above: SPC RAP Mesoanalysis loops of Effective Bulk Shear (left) and Surface-Based CAPE (right) from 1600Z to 2100Z. Note the best shear of 30 to 40 kt over the northern half of the Albany forecast area, however the magnitude CAPE was very high from around the Capital Region southward with values of 2000-4000 J/Kg.

0.5_refl

Above: 0.5 deg KENX radar reflectivity loop. Warning polygons are also plotted, with yellow polygons representing Severe Thunderstorm Warnings and the brighter green polygons representing Flash Flood Warnings. Note explosive convective development across Columbia and Berkshire counties, which coincided with the large magnitude of CAPE. Training cells over Columbia county resulted in flash flooding. Additional convection then developed farther north along the old Theta-e gradient and eventually farther west over southern Herkimer county where flash flooding occurred once again due to heavy rain on saturated grounds.

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Above: Dual-Pol radar precip estimate STA (left) and and legacy radar precip estimate STP (right) for the duration of the event. Estimated amounts from both products are similar. Note the large bullseye of 4 to 5 inches across southern Columbia county. These amounts were corroborated with a 4.84 gage report in Taghkanic.

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Above: SPC severe weather reports for July 7, including how the reports fit into the 1630Z Day 1 Outlook. Blue dots indicated wind damage reports and green dots are 1 to 2 inch diameter hail.