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CINDY DAY: New moon, high tides and storm surges

A school bus crosses over the Canso Causeway through storm surge on Oct. 5, 2011.  - Jack Ronalds/File
A school bus crosses over the Canso Causeway through storm surge on Oct. 5, 2011. - Jack Ronalds/File

There’s been a lot of talk of wind gusts and rainfall totals as we watch Chris make its way toward Newfoundland’s Avalon Peninsula. This storm is going to produce a fair amount of both, but Chris is also a marine storm and is having a powerful impact offshore.

Cindy Day
Cindy Day

Gale warnings are in place just off Nova Scotia’s Eastern Shore up to Forchu and then over to the south and southwest shorelines of Newfoundland. Storm warnings have been issued for  July 12 for the east coast ,  Cape St. Francis and south. There, the south wind around the storm will be as high as 55 knots, or just over 100 km/h.

Then there’s the issue of storm surge. That term comes up quite frequently during hurricane season, but it’s not well understood.

What exactly is a storm surge? The next few days will bring particularly high tides along the coast of Nova Scotia. A trough of low pressure will pass over the province this afternoon giving strong southwesterly winds later July 11 and 12. These strong southwesterlies combined with the higher than normal tides will result in higher than normal water levels and some rough and pounding surf along parts of the Fundy and Atlantic coastlines. Storm surge is a coastal flood of rising water commonly associated with an area of low pressure, but low pressure is not the only consideration when issuing a storm surge warning: wind direction and the phase of the moon are also considered.

Right now, the phase of the moon is of particular concern. This month’s new moon is July 12. When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide and it’s happening now.

When the storm surge of a hurricane comes in at high tide, the affect is worse as the height of the high tide is added to the storm surge to produce a storm tide.

That storm tide, combined with brisk south winds, will bring particularly high tides and higher than normal water levels along Newfoundland’s east coast - Cape St. Francis and south late July 12. This has the potential to create localized flooding.

High wind and stormy sea conditions will subside July 13 - Friday afternoon. 

Cindy Day is the chief meteorologist for SaltWire Network.

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