June 25 2014
Flash Flood and Severe
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A slow moving cold front worked southward across the region through Wednesday night. The air mass ahead of this front was warm and very humid with dew points climbing at or just above 70F. Showers and thunderstorms occurred along and ahead of the boundary with the main threat from gusty winds and heavy rainfall.
Models were hinting the best convergence and resulting higher QPF was to be across the Adirondacks-Mohawk Valley. This was where the Flash Flood Watch was initially issued on the midnight shift Wednesday. Furthermore, per water.weather.gov, this region has seen more wet conditions in the past couple of weeks than the remainder of the forecast area. Watch was expanded Wednesday afternoon into the Helderbergs and Schoharie Valley given upstream radar trends. The watch was then canceled for those aforementioned regions and hoisted for Capital Region, Catskills, Mid HudsonValley, Berkshires, Southern Vermont and Northwest Connecticut during the early evening hours Wednesday evening per radar trends and reports we were receiving.
Training echoes, high freezing levels (at or above 14,000 feet AGL) and very efficient warm precipitation processes resulted in flash flooding. KBUF sounding at 12Z was a classic tall and skinny profile in the highly anomalous PWAT regime. LAPS data during the event suggested SBCAPES were near 2000 J/KG with 0-3km Helicity between 100-150 m2/s2. Several deeper convective elements did exhibit rotation on the radar with several reports of wall clouds (and one image on social media that looked like a funnel), but no damage was observed or relayed from where the rotation was observed on the radar. With the lack of reports, as the rotation continued into Southern Vermont, no reports of damage continued in Bennington.
The rotation exhibited was consistently shallow and weak from scan to scan on the radar (it was never completely consistent). The LCL's in a tropical environment were favorable for an isolated spin-up. The greater concern was an isolated wet microburst though DCAPE values were generally in the 200-300 J/kg range based on the 12Z soundings and SPC RAP mesoanalysis data.
The axis of maximum precipitation was well south of where the guidance suggested which is why the initial Flash Flood Watch was issued for our northern and western areas. Once the evolution of the radar echoes and the temperature trends became apparent through the day, the support for the heaviest precipitation shifted south into the high CAPE and highest PWAT environment. The Weather Prediction Center sent MPDs highlighting this fact. Surrounding NWS offices shifted their Flash Flood Watches south before we did based on upstream conditions and we did the same later notifying our surrounding offices. The Northeast River Forecast Center agreed it was a good idea to shift the Flash Flood Watch. In the future, it may be a good idea to adjust the Flash Flood Watch area sooner, when temperature and radar trends are apparent. There was a well-defined temperature gradient and strong convection typically develops along the leading edge of the rain in the high CAPE and high PWAT environment. Again, the model guidance QPF was consistent prior to the event but the actual heaviest rainfall was well south and east of the guidance.
Weather Prediction Center guidance prior to the event actually had the highest predicted rainfall in the area southeast of the Capital region pretty much where it happened. It did not capture the second maxima though just north of Albany. Consensus from all the model predicted rainfall guidance indicated the axis would be to the northwest.
Dual Pol radar rainfall estimates were generally good except for some limitations near the periphery of the radar coverage such as in the southern Schoharie County area. Neighboring radars estimated considerably more rainfall in southern Schoharie County but their lowest elevation angle is over 6000 feet above ground, while KENX lowest elevation angle is at 1000-2000 feet. So the radars were sampling different layers in the rain/thunderstorms.
The neighboring radar did better estimate rainfall with the Schoharie County small area of heavy rain (it may have actually slightly overestimated), but it should be emphasized that it was very localized there. Our radar still showed 2-3" (and ground truth verified these amounts!), but when it falls in the wrong spot, such as when training and in complex terrain (it was very isolated here compared to other locations), then it becomes very challenging. It seemed areas that got 3-6" of rain in less than 3 hours...and in most cases in less than 2 hours had the worst flash flooding. This was a case where a cut-off low was not involved, and it was with a slow moving cold front too. These are tough, especially when the antecedent conditions were fairly dry too. It might have been a good idea to coordinate with the Weather Prediction Center about why they placed the heaviest rainfall to the SE of the Capital region (which went against most model guidance QPF but turned out to be correct).
An Emergency Management personnel member claimed in Catskill (Greene Co.) they were having microburst winds with a thunderstorm. A Severe Thunderstorm Warning was issued for a marginally strong thunderstorm. No downed trees were reported in SE Greene or Columbia Counties. This was frustrating, since there was an elevated core and the spotter report. We need to take down the name and number of people with this ground truth to follow-up on, when it is reported in the future. This information usually leads to "some" verification.
Above: Storm Prediction Center day 1 severe weather outlook and probabilities for damaging wind. Severe weather chances were very limited.
Above: Weather Prediction Center day 1 excessive rainfall outlook and Quantitative Precipitation Forecast. There was a slight risk for rainfall amounts to exceed flash flood guidance and the predicted rainfall was around 2 inches in parts of the NWS Albany forecast area.
Above: Soundings from 1200 UTC 25 June for Albany, NY (KALB) (left), Buffalo, NY )KBUF) (center) and Upton, NY (KOKX). Notice the deep moisture across the region indicated in the soundings.
Above: Upper air analyses at 1200 UTC 25 June from the Storm Prediction Center for 850 hPa (left), 500 hPa (center) and 250 hPa (right). Upper dynamics were not very strong but the region was in the right entrance region of an upper jet segment.
Above: Mesoscale Precipitation Discussion from the Weather Prediction Center.
Above: Mesoscale Precipitation Discussion from the Weather Prediction Center.
Above: Mesoscale Precipitation Discussion from the Weather Prediction Center
Above: Loops of Storm Prediction Center mesoscale analyses for Precipitable Water (left), Surface based CAPE (center left), Moisture Convergence (center right) and midlevel lapse rates (right). Note the axis of highest PWAT values and axis of enhanced low level moisture convergence, while there was some instability but relatively limited instability.
Above: Loops of station plots and MSLP analyses beginning at 1500 UTC 25 June and 0000 UTC 26 June. Broad low pressure was over our region contributing to the low level convergence over the region.
Above: Loop of visible satellite imagery with observations overlayed and water vapor satellite imagery with 500 hPa heights overlayed. The spin of the upper level energy is evident and the clearing in southern areas of NY contributing to an increase in the low-level thermal gradient and enhanced low-level convergence supporting deep convection along the low level thermal gradient.
Above: KENX radar loops of base reflectivity and base velocity. Note the lines of thunderstorms organizing into one large band of thunderstorms along the thermal gradient south and east of the Capital District.
Above: KENX storm total precipitation ensing 0938 UTC 26 June and GIS analysis of observed precipitation.
Above: Local storm report listing the severe weather and flood reports.
Above: Public information statement with a list of observed rainfall amounts.