Friday, September 01, 2017

Patchy frost tonight - rain from Harvey Sunday

The remains of tropical storm Harvey will interact with a frontal system, producing rain over southern Quebec Sunday. Meanwhile hurricane Irma has developed in the eastern Atlantic. (NOAA)
Frost Advisory
September has started on a rather cool note, with cold high pressure over Ontario producing unseasonably chilly temperatures, along with gusty northwest winds. Montreal was 8C (48F) Friday morning, recovering only to 16C (61F) in the afternoon. Locations northwest of the city were even colder Friday. Ottawa established a record low for September 1st at 4.7C (40F), the previous was 6.1C (43F) set in 1967. The forecast low tonight is 3C (37F) in Ottawa, with scattered frost. A frost advisory is in effect tonight for portions of northern and central Ontario, the Ottawa Valley, West Quebec as well at Mont Tremblant, Sainte-Agathe and the Saquenay, Lac St Jean region. Frost advisories are also in effect for the Adirondacks of New York and northeast Vermont. Scattered frost is likely just before sunrise Saturday.

Saturday will be milder, with abundant sunshine and a high temperature near 21C (70F). The nice weather however will be short-lived. The remains of tropical storm Harvey, currently over Kentucky, will be moving north into the Ohio Valley. The system has weakened significantly since producing catastrophic flooding in Texas and Louisiana. Harvey still managed to drop 100 to 200mm of rain in the Mississippi Valley as well as Tennessee. Isolated tornadoes have been reported as well. Harvey will interact with a frontal boundary sliding across the Great Lakes, producing rain across southern Quebec on Sunday. I originally thought the rain would remain to our south, but the front will act like a straw, drawing deep tropical moisture into the St. Lawrence Valley. We can expect 15-25mm (1 inch) of rain Sunday, along with gusty winds and cool temperatures. Conditions should improve slightly for Labour Day Monday, with partly cloudy skies and slightly warmer temperatures.

Hurricane Irma
Of note, a new hurricane has developed in the open waters of the Atlantic about 1600 miles (2500km) east of the Leeward Islands. Irma is a major hurricane with 110mph (175km/h) winds, moving west northwest at 13 mph (20km/h). The storm is forecast to strengthen. It is very early in the life cycle of this system, but forecasters are watching it carefully to see what the impact may be to the Caribbean and possibly North America by late next week.

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