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Challenges of Forecasting Summer Rainfall

Posted Friday, June 6th —

Last Thursday, June 5th was an occasion when rainfall vastly exceeded forecasted amounts.  Thursday was particularly unique in that unexpectedly heavy rainfall affected the area not once but twice in the same day!  The morning rains developed in an atypical manner (more on that later), while rain had not been expected until much closer to mid-day.  In general, there are several possible causes of summer rains being more or less abundant than forecast.  Any or all of these can produce inaccurate forecasts of rainfall amounts.

  • Warm season rain events have more moisture to draw upon, and may be concentrated in small cells or pockets of intense rainfall.  This can produce an “all or nothing” type effect in which there can be a fine line between heavy rain and mere sprinkles, both in time and distance.
  • Formation of storms may occur at the boundary of warm, humid air and cooler “outflow” remaining from earlier storms.  These boundaries are often either undetected or handled poorly by the major forecast models. This effect definitely enhanced the strength of Thursday’s late afternoon thunderstorms.
  • Forecaster experience may not help overcome the lack of help from the forecast models, because a weather pattern which usually produces little to no rain may produce heavy rain in a small fraction of events.

Specifically, I have identified a handful of reasons that Thursday’s rains were unusual.

  • TIMING – Overnight storms typically die out around sunrise.  These storms developed just after sunrise.
  • SHORT-TERM MODELS had forecast the morning rains to stay to the north of the area.
  • OTHER STORMS were occurring at the same time over Arkansas and Missouri.  These often would have limited the moisture inflow needed to support heavy rainfall in southwest MN.
  • UPPER-LEVEL SUPPORT for the storms was weakening during the day.

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