Crozet Weather Almanac: May 2018

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Chart courtesy Heidi Sonen and Roscoe Shaw.

Rain, Rain, Go Away

May started off really dry. You probably don’t even remember since it has rained so much since then that you wonder why they ever dismantled the Ark in Old Trail.  Yes, newbies, there was once a huge, almost biblically-sized ark right across from Mi Rancho Restaurant in the center of Crozet’s biggest housing complex.  We almost needed it last month.

Crozet’s ark under construction, circa 2006. Photo: Michael Marshall.

After only 0.13” of rain through May 13th, rain fell on 15 of the final 18 days of May.  While that’s not exactly 40 days and 40 nights, it was a lot.  Heidi is the official rain-gauge-emptier and she poured 7.82” from our bucket by the end of the month.  But rainfall amounts were wildly variable from place to place and some places got that much in just a couple of hours on May 30th. That leads us to our next topic… 

Radar Estimated Rainfall

My first job after grad school was as a storm chaser for NOAA.  That sounds crazy but actually it was mostly boring with an important purpose.  A brand new type of radar called NEXTRAD was being installed and our job was to drive into as much terrible weather as we could and gather data. Then our observations could be compared with the radar. The result was the first computerized radar algorithms that could automatically warn of dangerous weather in real time.

Radar estimated rainfall image from the weather app RadarScope on May 30, 2018. The darkest red indicates over 8” of rain. RadarScope is an excellent radar app and worth paying a few dollars to own. Courtesy Heidi Sonen and Roscoe Shaw.

One of the spin-offs of this approach is radar estimated rainfall.  In the old days, a forecaster could only guess how much rain was falling in a flood scenario.  Rain reporting during the pre-internet days was slow and sparse.  Now, the radar automatically calculates total rainfall as it falls with resolution as fine as the neighborhood scale.

 During the torrential downpours on the evening of May 30, radar estimated rainfall allowed the flooding to be narrowed down precisely. We “only” had 2.6” at our house n Crozet but nearly 10” of rain fell in just a few hours from North Garden to Ivy to Free Union.  Flood warnings were issued appropriately and the rainfall estimates were fed automatically into hydrology models which then calculated forecasts for stream and river flows. Small streams flood and recede quickly near the most intense rainfall while the bigger rivers rise more slowly downstream with a time lag.  

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