Many people are probably wondering what happened to the forecast of a nice weekend for southern MB (myself included). Just yesterday, the forecast was calling for sunshine all weekend and highs of 14 today and a balmy 19 on Sunday. Today, the weekend forecast dropped the high to only 11 today, and wait for it, a chilly +6 for Sunday (a 13 degree drop) with clouds and a chance of showers. Why the big flip? The simple answer.. poor model handling of a Hudson Bay vortex. Over the past few days, the model was indicating that the Hudson Bay vortex would stay to our north and just affect northern MB. However, based on the newest information, the model is now showing this vortex diving deeper into southern MB this weekend, sending that chilly airmass further south than earlier thought. The result.. instead of a nice sunny mild weekend, we will be turning cloudy and considerably cooler than had this airmass stayed to our north.
Note that all forecasts beyond Day 2 (tomorrow) are produced automatically from the model with no human intervention. Normally, this provides adequate accuracy for the Day 3-5 forecast. However, there are occasions when the model solution can change dramatically from one run to another, resulting in sudden and drastic changes to the forecast. This is particularly evident when the automated forecast is produced from a single model solution (in this case, using only the Canadian GLB model) A more promising forecast system, known as the ensemble forecast system, uses a blend of several model solutions to come up with a better forecast. Ensemble forecasts are generally more accurate (or at least, less wrong) and have less variability and "flip-flopping" It is hoped that EC will be moving to this new forecast system in the near future to produce more accurate long range forecasts and to help minimize those drastic forecast flips such as what we saw this weekend.