Seasonable Weather with A Chance of Showers

Feeling chilly after the weather we’ve had lately, more seasonable weather has returned to Southern Manitoba with a slight chance of showers tonight across most areas of the RRV.

The large captured upper low over Northern Manitoba

A GOES East multispectral satellite imagery from Tuesday evening showing the large captured low (denoted by the red L) anchored over Northern Manitoba.

A large upper trough is entrenched over Manitoba, bringing with it cooler air and clouds. Over the next several days, our temperatures will only climb to around the 20°C mark, right around the normal daytime high at this time of year of 21°C.

For today, temperatures will climb to about 23°C before the sky clouds over as a weak cold front passes and our winds increase out of the northwest to about 20km/h with gusts to 40km/h. Clouds will clear out in the evening and we’ll drop to a chilly low of 8°C.

Skies will cloud up again on Thursday as we struggle to hit 20°C. Well have a slight chance of showers through Winnipeg and the Northern RRV through lunch and the early afternoon, however it won’t amount to much if it does materialize.

Temperatures will slowly climb at the end of the week into the weekend. Friday will see mainly sunny skies with a high of 21°C as we have one more day under the influence of the upper trough. As we move into the weekend, warm air pushes in once again, and we should see above-normal highs near 25°C under sunny skies.

A Cooler Weather Regime

Cooler, more seasonable, weather is going to move into Southern Manitoba for the week ahead as a large upper trough swings into the region.

GEM-REG 500mb Winds valid Tuesday Morning

GEM-REG 500mb wind field valid Tuesday morning. A deep upper low is straddling the SK MB border with a trough extending to the SW denoted by the dashed black line.

A strong upper low sitting over the northern Prairies is in the process of stalling out over Northern Manitoba as it’s been captured by a significant long-wave trough. While today will be quite pleasant with sunshine and daytime highs across the Red River Valley near 28°C, the aforementioned trough will be moving into our region tomorrow.

With this system stalled out over the province, we’ll be stuck under cooler air for the rest of the week; daytime highs will sit around the low-20’s under a mix of sun and clouds or cloudy skies. Winds are expected to be fairly light through the week and overnight lows will sit around 10-13°C.

While Northern Manitoba will be fairly wet this week, it looks like we’ll be relatively dry in Southern Manitoba. The best chance for rain looks to be on Wednesday night into Thursday as a small reinforcing trough swings through the Red River Valley. Things then look to improve slightly over the weekend as an upper ridge starts to push across the Prairies, however another powerful low pressure system is forecast to move through Southern Manitoba and bring another batch of cooler air across the Prairies next week as it ushers in another cold trough.

August Continues Winnipeg’s Heat Streak

August closed out with another 30°C+ day, rounding out what ended up being another above-seasonal month, continuing Winnipeg’s above normal temperature streak to 14 consecutive months.

August 2012 Summary

2012 Departure from Normal Temperature, with Year-to-Date mean

2012 departure from normal (or monthly anomaly of) temperature. The year-to-date mean is also plotted.

August 2012 closed out with an average temperature of 19.2°C, 0.51°C above the normal of 18.7°C. At +0.51°C above normal, the month of August ended as the closest-to-normal month so far this year; the next closest was May which ended at 0.8°C above normal. The warmest day in August 2012 occurred on August 29th, where the mercury soared to 35.4°C, just shy of that day’s record of 36.1°C set in 1972. No new daily record high temperatures were set this August. The coldest night this August was on the night of August 18/19th where the temperature dipped all the way down to a chilly 4.5°C. This was well above August’s record low temperature of –1.1°C set in 1888. New new daily record low temperatures were set this August.

2012 Departure from Normal Precipitation, with Year-to-Date total

2012 departure from normal (or monthly anomaly of) precipitation. The year-to-date total is also plotted.

August ended with a significant deficit of precipitation compared to normal. Usually we see close to 80mm[1] of precipitation, however this month we saw just 44mm. This continues the Winnipeg’s precipitation deficit to 3 consecutive months.

In other miscellaneous August statistics:

  • We finished the month with 6 days at or above 30°C.
  • August 2012 ended up as only the 45th warmest August on record.
  • The greatest precipitation event this august was 23mm on August 4.
  • This month’s precipitation accumulation of 44mm pales in comparison to the record wettest August that occurred in 1985, where Winnipeg received a whopping 218mm of rain.
  • We quadrupled the record driest August’s (1915) total precipitation amount of 10mm.

2012 So Far

As mentioned before, August’s deviation from normal temperature of 0.51°C has continued the above normal trend for 2012. Every single month this year has been above normal temperature-wise and August marks the 14th consecutive month of such weather.

September 1st marks the start of meteorological fall, so we can take a quick look at Summer[2] 2012. Temperature-wise, we ended up as the 14th warmest summer, although it was an extremely close race…

Top 15 Warmest Summers on Record
Year Average Summer
1 1988 21.0°C
2 1983 20.6°C
3 1961 20.4°C
4 1955 20.2°C
5/6 1930 20.1°C
5/6 1963 20.1°C
7/8 1933 19.9°C
7/8 1995 19.9°C
9 1991 19.87°C
10/11/12 1919 19.83°C
10/11/12 1921 19.83°C
10/11/12 2006 19.83°C
13 1936 19.767°C
14 2012 19.761°C
15 1932 19.7°C

If you round to tenths, then there is a 5-way tie for 10th place, however looking closer, we can see the finer details and sort things out.

Year-to-Date Temperature Anomalies for Winnipeg, MB

Year-to-date temperature anomaly, by month, for 2012 (red) compared to the other 139 years on record for Winnipeg, MB, with the five warmest years (orange) and five coldest years (blue) noted.

Looking again at our Winnipeg temperature anomaly climatology, we’ve added in August of this year, and we can now see that we are the most above normal that we’ve ever been. While we were chasing 1987 last month, with August’s +0.51°C temperature anomaly we’re now 0.25°C above 1987’s August YTD anomaly. At this point, it’s pretty much guaranteed that 2012 will end upon the top 5 hottest years on record, it’s just a matter of where exactly we end up.

June, July and August all had below-normal amounts of precipitation which has left us with a 118mm deficit for the summer months. Typical summer rainfall is 247.5mm, which puts this summer at only 52% the normal amount of precipitation. This is a troubling trend; 2011 placed as the 5th driest summer on record, and 2012 managed to sneak into the top 10:

Top 10 Driest Summers on Record
Year Summer Rainfall
1 1929 76.7mm
2 1886 77.2mm
3 1961 91.0mm
4 2006 91.5mm
5 2011 93.0mm
5 1889 96.0mm
7 1894 97.1mm
8 1915 114.8mm
9 1936 121.4mm
10 2012 129.5mm

Due to our wet spring, however, we’re “only” 67mm below our normal precipitation amounts on the year so far.

The Rest of 2012

While seasonal forecast have very little skill, there are a few things that can give us an idea of how the rest of the year is going to shape up. With Arctic sea ice at a record low, it will take longer than normal to establish a pool of cold Arctic air. In particular, the delayed onset of ice over Hudson Bay can inhibit the development of a true Arctic vortex over the area. This feature sets up a strong NW flow over the NWT and eastern Prairies and is often responsible for the advection of Arctic air southwards into the Prairies and Northern Plains. The inhibition or absence of this feature means that, while cold outbreaks can never be ruled out, it’ll be much more difficult for the cold air to actually stick around.

Another feature to keep an eye on is El Niño, the feature responsible for bringing warmer waters to the eastern equatorial Pacific. When El Niño is weak, it’s effects can be limited to areas further west in the Prairies[3], however in moderate-to-strong El Niño events, Southern Manitoba will often see warmer-than-normal winters. In weak-to-moderate events, precipitation can be increased over the region as the storm track cuts through the area, but in stronger events, the storm track is often to our north and Winnipeg enjoys relatively warm, dry winters in a bit of a no-man’s-land in the middle of a strong split upper flow.

That being said, it’s quite likely that the onset of winter will be delayed due to the delay in development of a pool of cold Arctic air, and it’s all but guaranteed that 2012 will go down as one of Winnipeg’s warmest years on record.

  1. August’s 1981–2010 normal precipitation amount is 79.4mm.  ↩
  2. Meteorological summer runs through June, July and August.  ↩
  3. British Columbia and Alberta, mainly. Saskatchewan marginally.  ↩

Elsewhere in Weather News: September 1st, 2012

Tropical Depression Isaac: Update

What was once dubbed Hurricane Isaac when it made landfall as a category one hurricane, is now considered a ‘tropical depression’. Isaac continues to pose a threat a week after it made landfall but it has weakened significantly, and will continue to do so, as it penetrates further into the southern half of Continental United States. Although this hurricane was “only” rated a category one when it made landfall, comprising of maximum sustained winds of 135km/h at peak intensity, you would have not guessed the incredible amounts of rain it brought into Louisiana and surrounding Gulf States. Here are a couple of the most impressive rainfall totals recorded throughout the whole event:

-In Gretna, LA, the National Hurricane Center (NHC) unofficially reported 466mm of rainfall.
-In New Orleans, LA, 510mm of rainfall was reported, breaking New Orleans’ old daily rainfall record by 48mm (a record thsat was set by Hurricane Katrina).
-In Kiln, MS (west of Gulfport) 433mm of rainfall was reported.

Not only did Hurricane Isaac bring with it enormous amounts of rain, the storm surge that accompanied it was quite severe. Wind-driven storm surge is mostly caused by the severe winds accompanying a hurricane. These winds push ocean waters towards land similar to a wave but larger in size (in this case with south winds) and in turn, ocean levels are higher than the normal tide. This phenomena makes for serious flooding along the Gulf Coast since the terrain along the Gulf of Mexico is not mountainuous – it is more of a gradual slope. When ocean water then rises, it is easy for it to move inland quickly.

Isaac storm surge

Hurricane Isaac’s wind direction depicted with arrows. Blue boxes along the shoreline show areas where winds were favorable for storm surge. Green arrows (north winds) is where storm surge was less favorable because of the wind direction. Picture was taken just before landfall on the night of August 28th. (Source: NOAA)

In Louisiana, two people lost their lives in the Plaquemines Parish, one of the hardest hit parishes, as the tide rolled in and consumed their house. This now brings the Hurricane Isaac death toll in the United States to four, with damages listed in the hundreds of millions. In Louisiana and Mississippi, power was out to over a million people and it was estimated that 75% of New Orleans was out of power when the eye wall passed closest to the city. For a storm being of weak category one, it did an extensive amount of damage. This was in part caused by it’s broad area of precipitation, large area extending out from the eye of tropical storm force winds, extremely slow movement, and storm surge levels you would expect form a category two or three hurricane.


Residents evacuating in Lafitte, LA by boat because of flooding. (Source: Yahoo News)

This week in the tropics there are three other notable storms: Hurricane Kirk out in the middle of the Atlantic which poses no threat to land; Tropical Storm Leslie which does not pose a threat to the United States but as it strengthens, could pose a threat to Bremuda; and lastly, Hurricane Ileana out in the Eastern Pacific poses no threat to land but could cause high surf to the Baja Peninsula.