Monsoon Musings

Convection associated with the North American Monsoon had a major impact on southern Utah (and even parts of northern Utah) over the past few days when several notable flash floods occurred.  

One was in Moab where a state of emergency was declared after downtown and environs were flooded severely on Saturday night.  

Video of Main Street in Moab turned into a river. A car in this video was pinned to the curb by the Main Street water current pic.twitter.com/I4bt3rAtvo

— Swat Lake City (@Neildos) August 21, 2022

Radar imagery during the period showed a cluster of thunderstorms moving slowly eastward.  In the image below, an especially strong cell is due east of Moab (southeast of CNY, the 3-letter identifier for Canyonlands airport) over the headwaters of Mill Creek, which became a raging torrent.  

Source: NCAR/RAL

On the previous day, Friday, Aug 19, multiple hikers experienced a flash flood in the Virgin Narrows in Zion National Park, apparently near the Temple of Sinawava near the end of the Zion Canyon Scenic Drive.  One hiker is still missing, and search and rescue operations are ongoing.  

If you are planning outdoor activities in southern Utah, the National Weather Service issues a flash flood potential rating for several national parks and recreation areas at https://www.weather.gov/slc/flashflood.  Consult it as you would an avalanche report before backcountry skiing, adjusting plans accordingly.  In many instances, these flash floods are unsurvivable.  

Finally, I've seen some call the event in Moab a 100-year event.  The one hundred years here represents an average recurrence interval of precipitation in a given period at a point.  It is possible to obtain these at the NOAA Precipitation Frequency Data Server.  

One needs to be very cautious when using and interpreting these recurrence intervals.  First, they are based on prior observations from a relatively limited number of sites, indicated below.  

Source: https://www.weather.gov/media/owp/oh/hdsc/docs/Atlas14_Volume1.pdf

And the period of record varies depending on the station.  Monsoon thunderstorms are very hit-and-miss and extreme events are often undersampled by a limited number of stations with short periods of record.  This results in important uncertainties in the estimates of the return intervals of extreme events.  

Second, these are recurrence intervals at a point.  Monsoon thunderstorms can be very localized.  The probability, for example, of say an inch of rain in an hour at a point is much lower than it happening in any given region. Don't be surprised if you hear of two 100 year events happening in a given region like southern Utah in the same year.  It happens.   

Finally, these return intervals are based on prior observations.  The climate is changing.  Their representativeness is declining as the climate is warming.