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Heavy Precipitation Supercells of 29 May 2012

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Part IV: Hydrology

Situational awareness of antecedent rainfall conditions was a major factor in warning decision making for flash flood warnings on Tuesday afternoon. Showers and thunderstorms which moved through during the overnight hours into the morning on the 29th produced rainfall amounts ranging from one to two inches. Also, the month of May to date had been wet, with rainfall from May 1st - 28th ranging from 2.5 to over 5 inches in isolated locations. Forecasters recognized that it would not take a lot of additional rainfall to produce flood problems.
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Figure 22 shows a Northeast Mosaic of storm total rainfall between 12 UTC on 29 May to 12 UTC on May 30th; this does not include the rainfall during the overnight hours on May 29th. Highlighted are two areas that had over 5 inches of rain throughout the event.

These areas included near Lowell, Vermont and another across eastern Essex County, New York into southern Addison County, Vermont. These two areas had some of the most serious flooding. The area near Lowell in northern Vermont had more devastating damage due to the mountainous terrain. These slow moving, training heavy precipitation supercell thunderstorms, created these treacherous conditions.
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Figure 23 shows one hourly rainfall rates during the height of the storms on May 29th. Rainfall rates surpassed three inches an hour in Addison County, VT. Two inch rainfall rates were observed across the northern tier of Vermont. This heavy rain flooded many roads in Addison County, causing them to become impassable.

The more serious damage in northern Vermont, near Lowell and Belvidere, washed out entire roads. Many rivers jumped their banks flooding fields and farmland, and washouts along rural dirt roads caused them to need to be rebuilt.
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Figure 24 is a hydrograph from the Missisquoi River at North Troy. Runoff from the storms across northern Vermont flowed into the Missisquoi River and then northward to the Canadian border where the North Troy River gage is located. During the morning hours the gage height rose from just below 2 feet to over 4 feet following heavy rain from thunderstorms which moved through the region overnight.

With next round of heavy rain during the afternoon, the gage height rose about another 4 feet, just cresting below flood stage Tuesday night. In twenty-four hours the gage height rose over 6.5 feet, peaking less than a foot below flood stage.
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Figure 25 is a Google map background showing two of our flash flood warning polygons issued and a plot of the associated flash flood reports across the region.

The blue raindrops represent locations where flooding was reported. These are the towns that were hit the hardest by the heavy rain which caused flash floods. Many other areas had minor flooding issues.

Figure 21: KCXX 1.3° Velocity (left image) and Reflectivity (right image) at 2000 UTC on 29 May 2012, along with Tornado Vortex Signature (green triangle) and hail signature (pink icon).
Figure 22: Northeast Mosaic of Storm Total Precipitation from 12 UTC on 29 May to 12 UTC on 30 May 2011
Figure 23: One Hour Precipitation Rate from 20Z on 29 May 2012
Figure 24: Hydrograph the Missisquoi River at North Troy, VT
Figure 25: Google map background with flash flood warning polygons (green shapes) and flash flood reports


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Burlington
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S. Burlington VT 05403
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Page last modified: June 9, 2012
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