Gulf flow: This flow develops with the presence of a trough over western North America. A strong southerly flow from the Gulf of Mexico develops ahead of this trough, bringing in hot and humid conditions all the way north into Canada. This type of flow usually brings Manitoba its most humid days of the summer, and will often develop only a handful of times a year. (Fact: It was thanks to a gulf flow that Carman, Manitoba, broke the all-time Canadian humidex record of 53 on July 25, 2007). This type of flow is also notorious to bringing southern Manitoba some of its most severe thunderstorms of the year as the trough moves east, clashing with this hot humid airmass.
This is the type of airmass that will be affecting Manitoba Thursday through to Saturday. As this southerly flow develops, moisture will gradually increase tonight and Thursday, before peaking Friday. It will be a very mild night tonight thanks to breezy southeast winds, with lows only in the low teens. Highs tomorrow will be in the high twenties in most areas, with a few 30°C readings possible, especially in southwestern Manitoba. It will generally be mainly sunny in the morning, and early afternoon, before clouds begin to increase in the afternoon.
The first chance of thunderstorms will be Thursday late in the afternoon and in the evening. This will be thanks to a low pressure moving into southwestern Manitoba, with building instability/moisture and lift into the area with the gulf flow. These storms will likely be isolated, although with the high instability and lift, any that develop could be strong.
Friday is looking to be a scorcher. Uncertainty still exists in how far north the warm front will go, although it appears as though this is when the heat and moisture will be at its maximum. Skies will be mainly sunny to start, although cloud will begin to increase in the afternoon, similar to Thursday. Hottest areas look to be in the southeast. Generally, highs between 30 and 32°C look likely along and south of the Trans-Canada. This along with dewpoints in the mid teens will give humidex values into the mid thirties. Things would be much cooler west of the cold front, with highs only in the teens. Stay tuned for updates on the heat, as uncertainty exists.
With the cold front, some strong thunderstorms will be possible in the afternoon and evening. I will provide updates on this risk in the comment section of this post, and in the thunder forecast tab at the top of the page. On top of those storms, more elevated storms and rains are possible Friday night and Saturday before things finally calm Sunday.