Rex Block provides perfect stretch of late summer weather to Montreal

There are a few signs around of the changing seasons, such as fields of pumpkins and a few trees showing their colours. The weather however has a different agenda, feeling more like late July and August, with highs in Montreal in the upper 20s, and humidex readings in the 30s.

The weather does not feel like September at all these days, thanks to a Rex Block feature established over eastern North America. So what is a Rex Block? The phenomenon was first discovered by Meteorologist Daniel F. Rex in the early 1950s. The Rex Block occurs when strong high pressure is located poleward, just to the north of low pressure, blocking its movement. The high pressure in our current example is nearly stationary at all levels in the atmosphere, basically cut off from the typical westerly flow that moves weather systems along in our latitudes.

What this means for practical weather is that whatever weather you have currently, you will continue to enjoy until the ridge breaks down or is finally nudged east. That can sometimes take days to weeks to occur. For Montreal, we have not had any precipitation since September 7. Over the last few days, the high pressure has been directly overhead, allowing for clear skies, light winds and well-above normal temperatures. Daytime highs have been more typical of late July and August. The mercury soared into the upper 20s over the weekend, along with elevated humidity making it feel almost uncomfortable at times. Overnight lows have been more typical of what our daytime highs should be for mid-September around 16C to 18C (60 to 65F).

A Pivotal Weather image showing Monday mornings weather map displaying the Rex Block in action. A Rex Block occurs when high pressure in all levels of the atmosphere, is located directly north of low pressure preventing it and other weather systems from moving, creating a roadblock in the atmosphere. The pattern can last from days to weeks. 

Expect at least another few days of this weather, with widespread sunshine through Wednesday, warm and definitely humid at times. Highs will generally be close to 30C (85F) for most of us in southern Quebec and eastern Ontario.

While we have been enjoying the nice weather, the Middle Atlantic and southeastern United States are under the low pressure portion of the Rex Block, with persistent showers, thunderstorms and an easterly onshore winds that has resulted in some coastal flooding. Early Monday morning, the National Hurricane Center was monitoring low pressure located 140 km south of Cape Fear, North Carolina. This system will slowly meaner northwest at 6km/h over the next 24 hours, producing heavy rain and rough seas. The Rex Block will result in the storm making very little northward progress for the time-being.

By Thursday, high clouds may stream north from this system into southern Quebec, but I don't expect any precipitation at this time. The only threat for rainfall at this time may come from a backdoor cold front wrapping around the northern edge of the aforementioned high pressure.

Rex Block or not, the good news is that if you like warm temperatures, it appears daytime highs will remain well-above normal through at least the end of September.

Long stretch of near-perfect late summer weather forecast for Montreal

An early taste of fall colours on Ile Perrot this past weekend. However, don't be fooled by the cold weather as summer is set to make a return, with abundant sunshine and above normal temperatures forecast into the upcoming weekend.

After the wettest meteorological summer on record for the city and an early taste of fall, we are in store for an extended period of sunny, warm weather.

Strong high pressure anchored over southern Ontario, will remain in place well into the weekend. This will provide the Montreal region with clear, cool nights and sunny, seasonable warm days. As the week moves along and into the upcoming weekend, a southwest flow will develop, allowing for temperatures to rise above normal. High temperatures will be close to normal values of 20C (68F) and 10C (50F) for Wednesday, but rise into the middle 20s beyond and perhaps upper 20s by the weekend. No precipitation is in sight.

Strong high pressure over southern Ontario will keep the remains of hurricane Francine well to our south, saving the region from heavy rainfall. (NHC)

Tropical Storm Francine

On Tuesday afternoon, tropical storm soon to be hurricane Francine was located 610km southwest of Morgan City, Louisiana, heading northeast at 14km/h. Francine has 100km/h winds and was expected to strengthen into a hurricane late Tuesday. The storm is forecast to make landfall along the central Louisiana coast on Wednesday afternoon. Hurricane warnings are in place, with conditions expected to deteriorate quickly along the food-prone Gulf Coast. A storm surge of up to 3 metres is possible, along with 100-300mm of rain.

Once inland, Francine is expected to weaken while moving north into the Mississippi Valley. The good news for Montreal and southwestern Quebec, is that the aforementioned high pressure will deflect Francine well south of our area, where the storm will weaken and fill over time.

Wet Summer

As mentioned, Montreal just experienced the wettest summer on record. From June 1st to August 31st, 492.9mm (19.4 inches) of rain fell on the city, smashing the long-standing record of 431.1mm set in 1893. Despite the persistent clouds, rain, thunderstorms and very humid conditions, we still managed 18 days with the temperature at 30C (86F) or higher at Trudeau Airport from June to August.

Montreal did manage some very fall-like weather over the weekend. On Saturday, a strong cold front produced up to 20mm of rainfall across the region, accompanied by gusty winds and dropping temperatures. The overnight low on Sunday morning was a chilly 7.1C (45F), only rising to 13.4C (55F) on Sunday. The was the coldest high temperature for the date, surpassing the previous low maximum of 13.9C (56F) set in 1938.  Despite the early taste of fall weather, the long range forecast is calling for above normal temperatures for tour region into October.