STP 05-31-2015: A Handful Of Storms Beat The Odds, But Little Rainfall

Issue Date: Sunday, May 31th, 2015
Issue Time: 9:05 AM MDT

Summary:

With a ridge overhead, thunderstorm activity was effectively squashed statewide on Saturday. By late afternoon, a few rogue storms were able to form over the Northwest Slope, Front Range and Southeast Mountains. However, the storms looked more impressive that they actually were: highest observed rainfall was only 0.05 inches. No flooding was reported yesterday. See our map below for the radar estimated rainfall in your area.

We are taking this lull in action to begin to appreciate just how active May was; not so much from the flooding aspect, but certainly from the total precipitation aspect. One tool we commonly use for our Flood Threat Bulletin forecasts is the precipitable water (PW). This measures the amount of water vapor (not liquid, only gaseous water!) in the overhead column of atmosphere. In Colorado, when PW exceeds about 0.7 inches, this may be the first indication of a heavier rainfall threat. When values exceed 1 inch, there is a very high likelihood of action somewhere in the state. The charts below, courtesy of NOAA’s Earth System Research Lab, show the PW from this past May. The 0.7 inch line is marked by a thick black line. Impressively, during almost every day except for a handful, PW exceeded 0.7 inches in at least one of the three main stations we use: Pueblo (purple), Grand Junction (green) and Boulder (blue).

IPW_historicalSo how does this compare to normal? Well, we also included a chart of 2014, a more typical year. Note that only 7-10 days exceeded the 0.7 inch threshold. Meanwhile, also shown is 2012, the year of an intense spring/summer drought. Note that during that year only a couple of days exceeded PWs of 0.7 inches, but Grand Junction did not exceed 0.7 inches for the entire month! This is a quick way to get a perspective on just how unusual this May has been.
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Storm Total Precip Legend