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.