Sometimes with weather it is a matter of a few hundred kilometres, and that small change early on can magnify into big changes later in the forecast period.
This is exactly what happened with the previous forecast. An area of low pressure, forecasted to impact southern regions early in the forecast period, was weaker and travelled farther north than expected. This in turn did not allow the area of arctic high pressure to build southward; instead, warm air was able to build northward, bringing with it our first taste of summer.
If you have been following the weather models, you’ll see they have been bouncing back and forth a little bit between a warm very latitudinal pattern and a cooler more zonal pattern. The warm latitudinal pattern seems to be winning out, so I will go with that, keeping in mind that there is a chance for the cold pattern to re-establish itself.
For this forecast period, it looks like a deep trough of low pressure will form off the West Coast. This will force the flow over our region to become southerly. This will allow for very warm air to move northward, pushing daytime highs into the mid- to upper 20s for most of this forecast period. Along with the warm air, humidity levels will increase, which will help keep overnight lows from falling much below 10 C.
With the increase in humidity will come increasing chances for thunderstorms. Unfortunately, the strong southerly flow looks like it might keep most storm systems to our west as they develop to our southwest, then move almost due northward. We may see some rainfall around Friday, May 21 and again early during the week of May 24. Let us keep our fingers crossed!
For those thinking about planting frost-sensitive plants on the long weekend, the models hint at a brief shot of cold air around May 25 or 26, so double-check the five-day forecast before planting.
Usual temperature range for this period: Highs, 14 to 26 C; lows, 1 to 10 C.