Spring Outlook Outlines Risks of River Flooding

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GRAND RAPIDS, Mich. (Feb. 21, 2014) — Above-average precipitation, a deep late-season snowpack, and ice-choked rivers are all ingredients that will make some form of flooding likely on West Michigan rivers this spring.

Thursday, the National Weather Service office in Grand Rapids issued its first official spring flood outlook, describing the statistical chances of local rivers reaching various levels of flooding through March, April, and May.

Here is the link to the full report.   It can be a bit complicated to decipher because of the use of river stages and percentage probabilities.  The outlook compares current conditions with average conditions in past seasons.  It uses an assumption based on long-range forecasts that precipitation will remain near normal through the spring months.

In short, the risk of flooding this spring is well above normal; it will be difficult to avoid at least some minor flooding on West Michigan rivers and streams.  For example, the chance of minor flooding on the Grand River is reported as at least 75% from Ionia to Grand Rapids; that compares with chances of less than 20% in an average year.  “Minor flooding” causes minimal property damage, but some inconvenience to the public.

The next step up is “moderate flooding,” describing water that can affect a number of buildings, along with covering roads near streams.  Moderate flooding can often force localized evacuations of homes or businesses.  An interesting note is that even though the Grand River hit record stages in April of 2013, the flooding was only officially in the moderate category.

The average likelihood of the Grand River experiencing moderate flooding through West Michigan is less than five percent; that is, moderate flooding should occur less than once every 20 years in the long run.  However, this flood outlook gives the Grand River a much higher chance of such conditions.  The probability of moderate flooding at Ionia is 33%; 27% at Lowell; 43% at Ada; and 38% for downtown Grand Rapids.

Last April’s record flood stage in Grand Rapids was 21.85 feet.  The current outlook lists the chance of reaching 21 feet at 38%, and the chance of reaching 22.5 feet at 25%.  Interpolating between the two, you see that there is essentially a one in three chance that flooding this year will be as significant as in 2013.  While that is still less than a coin-flip chance, it is much higher than average — and much higher than anyone who lives along the river should feel comfortable with.

The Thornapple River also shows some high probabilities of flooding, with both the Hastings and Caledonia forecast points indicating a 44% chance of moderate flooding.  Flooding along the Thornapple remained just below the ‘moderate’ threshold in April of 2013.

It’s easy to put too much weight into these statistical forecasts — remember, they are only a best guess based on a snapshot of current conditions.  The spring flood outlook will be updated on Thursday, March 6th, with adjustments depending on how the weather behaves and any changes to the expected precipitation in the forecast.

Photo courtesy: National Weather Service, April 20, 2013

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