Extreme Precipitation Events

There is broad scientific consensus that as the climate warms, the frequency and intensity of extreme precipitation events have increased and will continue to increase.  The plot below (left) from the NOAA U.S. Climate Extremes Index shows the one-day precipitation trend for the past century (note the steep increase since the early 1980s).  Meanwhile, operational forecast model skill continues to be higher for mass fields (height, pressure, wind) than for Quantitative Precipitation Forecasts (QPF), especially with respect to heavy and extreme precipitation events.  This is largely due to the many physical parameterizations necessary to produce model QPF.  The plot below (right) shows that NWS Weather Prediction Center (WPC) human forecast skill for heavy and extreme events exceeds that of NWS operational models by 20-40% per year


  • Analyzing historical and expected trends in blocking patterns and their relationship to extreme precipitation and flooding events   

  • Understanding the synoptic-dynamic and thermodynamic mechanisms that cause extreme precipitation events, particularly the importance of anticyclones and air mass conditioning  

  • Investigating the synoptic-dynamic differences between the most extreme precipitation events (top 1%) and other heavy events (e.g., top 10%)

  • Improving predictability of extreme precipitation events   

Read our 2015 article on the 2013 Alberta Flood (the costliest natural disaster in Canadian history) in Monthly Weather Review

Read our 2014 Weather and Forecasting article on synoptic typing and precursors of heavy precipitation events in Montreal

Below are ncep cfsr animations of composite potential temperature (k, shaded) on the dynamic tropopause (2 PVU surface), weighted potential temperature anomalies (K, solid contours), and statistical significance (95% and 99% confidence intervals, dashed red contours) for:

(left) top 1% of warm-season precipitation events at Montreal,                                   (middle) median 50 top 5% warm-season precipitation events, and                                     (right) median 50 top 10% warm-season precipitation events.

The animations are every 6 hours from 2 days prior (t = -48 h) to the onset of the heaviest precipitation until onset time (t = 0 h).  

Note that in the top 1% composite, eastern North America is marked by a much warmer DT and substantially larger potential temperature anomalies (21 K vs. 10 K) over Montreal by t = 0 h.  This suggests that the air mass in top 1% is pre-conditioned with warm, humid, low stability air thanks in part to a strong ridge over the eastern seaboard.  Theory holds that for a given amount of ascent (trigger), more precipitation will result within a warmer, more humid air mass.