Highlights:

• Rainfall amounts of 50 to 100 mm predicted from Sunday through Tuesday
• Ponding and localized flooding expected in areas of poor drainage
• Local watercourses will rise and many could reach bank full conditions or spill their banks
• LTVCA preparing to operate the McGregor Creek Diversion Channel.
• Thames River should be expected to rise up enough to flood the sidewalk in downtown Chatham.
• Next week water from the Upper Thames will move downstream flooding the low lying river flats from Delaware down to Chatham.
• High lake levels could aggravate drainage issues in shoreline areas.

Message:

Weather forecasts are calling for a very significant rainfall event beginning tomorrow (Sunday) morning and lasting through into Tuesday. There is still quite a bit of uncertainty in the various forecasts but total rainfall accumulations for the three day period range from around 50 mm upwards to 100 mm. Generally, the areas with higher rainfall are predicted to be further east in the watershed. The majority of the rain is expected to fall on Sunday. With the recent rains over the last few days, the ground is still wet and has less ability to absorb this new rain. In addition, there are strong easterly winds in the forecast that could lead to shoreline flooding that will be addressed in a separate Flood Watch.

The amount of rain in the forecast is significant for this time of the year. Even the lower rainfall amounts in the forecasts should be expected to cause ponding and localized flooding in areas with poor drainage. Local ditches, creeks and streams should be expected to begin rising Sunday afternoon and some could reach bank full conditions or spill their banks that evening or overnight.

The LTVCA is preparing to operate McGregor Creek Diversion Channel to protect the south end of the city of Chatham from flooding. Even with the lower rainfall amounts predicted, water levels on McGregor are expected to reach the water level thresholds for operations sometime overnight Sunday or early Monday.

The high water levels on Lake St. Clair are having an impact on water levels in the Thames River all the way up into the city of Chatham. The Thames River in these downstream areas is higher than would typically be expected given current flows in the river. Late Sunday or early Monday, as the river rises in response to the rain water coming from our local tributaries, the river will rise and is expected to flood the sidewalk along the river in downtown Chatham. Other low lying floodplain areas adjacent to the river in these downstream areas should also expect flooding.

As we move into early next week, the water from the Upper Thames will begin making its way downstream. Given the rainfall amounts in the forecast, flooding of the low lying, mostly agricultural, river flats from Delaware down to Chatham should be expected. Effects in the Delaware area could be seen by late Monday or early Tuesday. Over the following two or three days that water will move downstream affecting other areas. The higher rainfall amounts in the forecasts could produce more severe flooding, but it is too early to predict what else might happen.

The impact of the heavy rains could be worse in low lying shoreline areas and communities. Due to the high lake levels, the groundwater table is high and storm water sewer systems and local watercourses are full with lake water. As a result, rainwater is not draining properly from these areas. This alone could lead to additional ponding and localized flooding. In addition, any water from upstream making its way downstream on these watercourses into these shoreline communities and areas could cause additional flooding. The community of Shrewsbury is one example of such a community that could be affected by this.

In order to reduce nuisance flooding, residents should ensure that catch basins and other drainage areas are clear of debris.

People should take extra caution and avoid the river, ditches, and streams. The combination of slippery banks and fast moving cold water can be dangerous. Standing water can also present unseen hazards. Children and animals should be kept away from the water.

Officials will continue to monitor the situation and update this advisory if necessary.

This message will be in effect until May 24, 2020.

Message Contact: Jason Wintermute ([email protected])