April 2021 Review

 It's time for our look back at April 2021, but for this month there's a caveat:

The Blatchford weather station near downtown Edmonton had no readings from April 1st through April 8th. That is the longest outage at Blatchford in 28 years, going back to 1993. But rather than leaving all those days blank we have instead substituted the temperatures from the South Campus station which should hopefully be a close enough match.

High Temperatures

April 2021's Highs were generally pretty close to average, with the month starting a little warm and then getting a little cool near the end. The two days closest to the top of the range were April 1st's 18.9°C, and April 30th's 22.4°C was a warmest-in-30-years High. We didn't really get near the bottom of the range with April 27th's 4.4°C as the coldest High.

Overall there were 18 Highs which were warmer than the 30-year average and 12 which were colder.

On average in April we will have 2.5 days with Highs below 0°C, but in 2021 we didn't have any. In April we also average 2 days which hit 20°C, and in 2021 we squeaked in above that with 3: 20.3°C on the 16th, 20.8°C on the 21st, and 22.4°C on the 30th.

Low Temperatures

For the Low temperatures April 2021 started above average, but the final two weeks were mostly below average, and were also mostly below freezing. Overall 14 Lows were above the 30-year average compared to 16 below.

On average in April we have 17 Lows below freezing, and 2021 was a little cooler than that with 20. The coldest Low was -6.1°C on April 23rd. Looking at the blues and pinks in this chart, normally the freezing Lows start to become rare after about mid-April, but this year we had a solid week of frosts right near the end of the month.

It's possible that the frost which we had on April 28th will end up as the final frost for the season, but we won't really know for a few more weeks:   

The Final Frost?

Last spring in 2020 the final frost at both Blatchford and Stony Plain was very early on April 19th. Meanwhile at the International the final frost came nearly a month-and-a-half later on June 5th. 

More typically the 30-year average date of the final frost for Blatchford is around May 6th, Stony Plain is a few days later on May 10th, and the airport is a few weeks after that on May 23rd. Please keep in mind though that the date of the "average" final frost means that half of the time it happens before that date, and half of the time it happens after.

Warm and Cold Months

As we have already seen, for April 2021 the Highs were a little bit above average while the Lows were a little bit below. The average High of 12.1°C ranked as Edmonton's 49th warmest April since 1880, while the average Low of -1.6°C was in the 81st spot.

Looking at the recent history of warm and cold months April 2021 is a small orange bubble, at 0.6°C warmer than the 20th century average. Looking at the line for April there are still lots of blue bubbles, including 2020. And the last time that we had a notably warm April was in 2016.

Across Canada

Looking across Canada, in the west the temperatures for April 2021 were fairly close to the 20th century average, ranging from Calgary at 0.7°C above its average down to to Winnipeg at -0.7°C below its average.

In central and eastern Canada it was generally warmer though, with Ottawa and Montréal at 3°C warmer than their long-term averages.

Consistent April

In Edmonton April is one of the months which has seen the least temperature change since modern records began in 1880. This chart has the average temperatures for January, April and July:

• For our coldest month, January's average Highs have increased by 8°C and the Lows have increased by 10°C.
• For our warmest month, July's Highs have increased by 1°C while its Lows have increased by 4°C.
• But for April the average Highs have actually decreased by 1°C, while the average Lows have increased by 2°C. 
• October isn't shown here (although it can be added using dropdown on the lower left) but it is basically right overtop of April, and its Highs have decreased by 0.6°C while the Lows increased by 2°C.

So for the shoulder-season months of April and October Edmonton's temperatures are not very different from where they were a century ago. All of the other months have seen bigger shifts, with the biggest changes happening in January and February.

Are April's temperatures flat~ish everywhere? Here we have the January, April and July histories again, but this time they are for Vancouver, Winnipeg and Montréal:

• Vancouver is obviously a lot warmer than Edmonton, or the rest of Canada. Its Lows have gone up by around 2°C for January, April, and July. For the Highs January has warmed by 2°C while the averages for April and July are both 0.3~0.4°C cooler than a century ago.
  
  
• Winnipeg is a lot like Edmonton, unsurprisingly. January has warmed a lot, and July has warmed a bit. April is pretty flat, but both the High and Low are up by about 1°C.
  
  
• Finally for Montréal the Highs for January, April, and July are all up by about 2°C. For the Lows April and July are both up by about 1.5°C, while January is up by 3.5°C.

Those are just a few examples, and all of the other months and cities can be accessed through the dashboard. This is just to show that Edmonton's experience isn't completely unique, but also that things do vary depending where you are in the country.

April Snowfall

In April 2021 the International Airport recorded 5.2cm of snow. That's below April's average of 15cm, and was just a bit below the 5.5cm that we got in April 2020.

Winter 2020-2021 Snowfall

For the winter of 2020-2021 so far the International has had a total of 73.5cm, which is below the average of 116cm, and is a bottom-5 winter for the International, going back to 1960.

The "winter" isn't necessarily over though, because in May we have roughly of 50/50 chance of seeing more snow. Our average snow for May is 5cm, but that is split between a bunch of years with no snow, and bunch of years with 7, 10 or 20cm+ of snow. (May 2003 had 38cm of snow at the International).

Across the country Edmonton's 73.3cm of snow was below all of these other cities, except for Winnipeg at 66.2cm and Vancouver at 12.6. And this year Calgary doubled us with 148cm. So Edmonton and Winnipeg both had very low snowfalls this winter, but for the rest of these cities the totals were all pretty typical.

Total Precipitation

April is the month where we typically start to transition from snow into rain, and in April on average we get 28mm of total (snow + rain) precipitation.

For April 2021 the International had the most precipitation of all of the stations in the region with 12.8mm. Blatchford had 5mm and Stony Plain had 6.6mm.

In recent years April 2015, 2019 & 2020 were all similarly below around 10mm, so April 2021 is low, but not really unusual.

For 2021 so far though all of the months have had below average Precipitation. The International has had the most overall with 40.9mm, which is its 3rd lowest January-April going back to 1960. Stony Plain's 22.9mm is its 2nd lowest going back to 1966. And Blatchford's 19.2mm is a bottom-10, going back to 1880.

May Temperatures

Looking ahead to next month, May has not had a High below freezing since 1959, but about one third of the time there will be a few days that stay below 5°C. 

On the other side of things we should probably see a week or two of days above 20°C. And May 2015, 2016 & 2017 all had almost a week of 25°C days, while 2018 had almost 2 weeks. And from 2016-2019 we had 4 May's in-a-row with a day which hit 30°C.

For Low temperatures, about a quarter of the time our final frost will be in April, and so May will have no Lows below 0°C. More typically though, May will have a handful of frosty nights.

May Snow

And no one want to hear this, but the International records snow about half of the time in May. Recently May 2016, 2017 & 2018 were snowless, but 2019 had 4 snowfalls and 2020 had 1.