December was a bit warmer and wetter than usual.
“It’s a significant departure from normal,” says Jesse Ellis, a weather forecaster with the Southeast Fire Centre. “But it’s not record-breaking.”
The SEFC has released its summary for the weather of December 2019. And it says the end of the year — and decade — was marked by the influence of airflows coming in from the southwest.
While a modified Arctic air mass brought cold and dry air to the first part of the month, it didn’t last.
“This cold and dry air was soon over-run by a developing moist, westerly flow that initially brought snow December 1-2, turning to mostly rain by the 3-7,” Ellis says. The remainder of the month was dominated by a succession of Pacific frontal systems within a prevailing westerly flow aloft.
“When we have a succession of system after system after system, it doesn’t allow any Arctic air to sneak in from the north or the east,” he says.
That’s why the start of the month was actually colder than the end, despite the solstice’s lower sun angle and less solar heating near month’s end.
“The absence of any fresh push of Arctic air during the rest of the month was largely the reason for the mean monthly temperature being 2.3 degrees above average,” says Ellis. That’s the biggest variation from average temperatures since February of last year.
And warmer weather, of course, means more rain than snow when there’s precipitation. Ellis says 40 per cent of total precipitation fell as rain this year, compared to 35 per cent on average. The month’s total precipitation was 8 per cent above average.
The mid-to-end of the month saw wave after wave of Pacific storms heading inland. An atmospheric river event (a.k.a. Pineapple Express) produced the greatest daily snowfall (14.8 cm) on Dec. 19. However, that was before a warm southwesterly flow drove temperatures and freezing levels upwards over the following two days as precipitation continued to fall. Rainfall totals exceeded 15 mm from Dec. 20-21.
“The warmest temperature of the month (+7.5 degrees) was reported on Dec. 20 under cloudy skies with light rain, breaking the record for that day set in 2004,” says Ellis. A new daily maximum mean temperature record of +4.4 was set the following day.
No monthly temperature records were broken. The highest mean temperature for the month of December was +1.3, back in 1979.
Ellis says this warm, wet weather is expected to continue until the mid-month. By then he says computer models are hinting of the return of cold and dryer Arctic air.