An interesting weekend is in store for much of Ontario as a large autumn Colorado Low is forecast to push into the province. For the most part, forecast models are struggling to resolve some of the details at this point in time.

Thursday morning, the initial warm front from the large developing Colorado Low will push in with an elongated frontal boundary extending back into the mid-western United States and Rockies. The warm front will bring the first wave of showers and thunderstorms as it pushes through southern Ontario. Instability parameters are not too high along and behind the front so nothing severe is expected in terms of winds, but some heavy rain is likely which could meet the severe criteria in terms of rainfall. This is depicted in the image below.

 The real fun will start on Friday when the large upper trough crests the Rockies which will cause the low to organize and deepen (strengthen) as depicted in the image below. From the get go we can see there is a great deal of disagreement between forecast models as to where exactly the low’s center will be located and where the associated frontal boundaries will lie.

Compared below are the NAM (first image) and GFS (second image) forecast models and it looks as though a hybrid of the two should give an appropriate solution for the forecast this weekend. Given the position and strength of the upper trough, the location of the low as per the NAM seems reasonable but with the ridge of high pressure building through Manitoba and northwestern Ontario, it seems as though the GFS has the better solution with where the warm front should be positioned. Ultimately, the bottom line here is those along and south of the warm front will see periods of rain and thundershowers but have well above seasonal temperatures and even humidity.

Saturday doesn’t get any easier to forecast, either. As depicted in the images below, the NAM (first image) pushes the warm front well into Quebec where the GFS (second image) keeps it through the Ottawa Valley. Here is another tough call as the ECMWF and GLB models agree with the GFS but the NAM model isn’t totally out to lunch either. The high in the Atlantic Ocean will help reinforce the southerly flow behind the warm front but at the same time the ridge to the north and northwest will act against it. Furthermore, the Ottawa Valley tends to hold on to the cooler air longer so once again it would seem that a hybrid solution of the models is needed. The main message to grab from this analysis is that an unsettled pattern continues with scattered showers and thunderstorms for southern Ontario on Saturday.

On Sunday, the model disagreement continues lending again to the uncertainty with when the associated cold front will push through. This will be depicted once again in the images below. Looking at the temperature forecast for Sunday evening, it can be seen that the GFS (first image) model is much faster than the GLB (second image) model. As these lows develop and the cold air surges south to the southeast, it becomes rather difficult to slow the progression of the cold air down. Taking this into account, it would be wise to lean more with the GFS solution here. It is somewhat worrisome when the GLB and ECMWF forecast models do agree a little better. None the less the cold front’s passing will bring more rain and likely usher in some gusty winds as well. The main issue here is whether this will occur Sunday evening through the night or on Monday.