Another sultry night.. relief coming Wednesday

We got to 34.8C at Winnipeg airport today, just 0.2C off the record high for today with humidex values peaking at43C. And after today's steamy weather, there won't be much relief tonight. It will be another very warm night across southern MB, especially here in Winnipeg and the Red River valley where gusty southerly winds will persist tonight, keeping temperatures from dropping much. As a result, temperatures will remain above 30 well into the evening, gradually dropping to an overnight low of only 26C. That would be Winnipeg's second warmest low temperature on record, behind only July 12 1936 when a low of 28.3C was recorded. That will mean a very uncomfortable night for sleeping for those without A/C.

Relief however is on the way on Wednesday. A heat-wave ending cold front will be coming through by midday or early afternoon. So after another warm and muggy morning with temperatures peaking at 32C by midday, winds will shift into the northwest in the afternoon sending temperatures and humidex values down. By evening, temperatures will likely be down to the 25C mark, and 20c by midnight with lows of 16C by morning. In addition, the cold front will likely be accompanied by showers and thunderstorms that could bring locally heavy rainfall and possibly some severe winds or hail. Look for more comfortable conditions and seasonably warm temperatures for Thursday into the weekend.

Heat Wave - Day 3

It's already 27C at Winnipeg airport as of 8 am, after an overnight low of only 24C, a record warm night for July 24th (and one of the warmest minimum temperatures on record in Winnipeg). I think we have a good shot at hitting 35C today provided we stay sunny all day. With dewpoints of 23c, that will give peak humidex values of 45 this afternoon, the steamiest day of the heat wave so far.

Tonight will be another warm one.. with a low of only 25C, maybe even 26 or 27C as we get a southerly breeze all night. If so, that will be Winnipeg's second warmest minimum temperature on record (warmest minimum was an amazing low of 28.3C on July 12 1936 following Winnipeg's hottest day ever of 42.2 on July 11) However, that low of 25C will likely not be the minimum for Wednesday. The heat-wave ending cold front will be coming through by midday, so temperatures will be dropping noticeably in the evening, likely below 25C by midnight.

Heat Wave - Day 2

After a sizzling Sunday, it's a meltdown Monday as the hot and humid airmass persists over southern MB. As of 11 am, humidex values had already surpassed the 40 degree mark over many localities thanks to some very high dewpoints today.. including an unbelievable 28C dewpoint reading at Carman. (I suspect this value is erroneously high, as the dewpoint at Carman always seems to be a couple of degrees too moist compared to neighbouring sites.) Nonetheless, dewpoints across southern MB are in the 22-25c range today which will make today's 35C temperatures feel oppressive. Take it easy out there.

By the way, I replaced the barometric graph plot on my main page with a dewpoint/heat index plot to better gauge how sticky it is over the next few days.

The heat is on!..

It was a sizzling Sunday across southern Manitoba with temperatures of 32 to 33C and humidex values of 40 to 44C. As they say, it wasn't the heat.. it was the humidity, or more accurately, the dewpoints which were in the 21 to 24C range across southern MB. These dewpoints are in the uncomfortable range, and makes it more difficult for the body to cool off by evaporation. (see graph from my weather station showing dewpoint and heat index from today. Notice how the dewpoints dropped late in the day!)

And it looks like the hot humid weather will continue for the next 2 to 3 days before a cold front brings more comfortable conditions by Wednesday evening. Until then, expect more of the same as today with afternoon temperatures of 32 to 35C, and humidex values of 40 to 45C. Stay cool!

By the way, you may notice that my weather station displays "heat index" rather than the humidex. The "Heat Index" as shown on my station is an equivalent temperature based on air temperature and humidity (or more accurately, the dew point). It is similar to but NOT the same as the Canadian "humidex" value, which employs a slightly different equation and gives different results than the American "Heat Index". Generally the Heat index will register about 3-5C lower than the humidex. My station can only display heat index. A simple way of calculating the humidex is to add the temperature and dewpoint (in C) then subtract 13. For example, if the air temperature is 32C and the dewpoint is 22C then the humidex is 41C (32 + 22 - 13 = 41C) Environment Canada issues humidex advisories when humidex values of 40C or greater are expected. Whatever the case, both indeces are designed to tell you how hot it "really" feels based on the moisture content of the airmass. The basis for this is that when it's humid, it becomes harder for the body to cool since evaporation from the skin (i.e sweating) is less efficient. Thus, 30C with 60% humidity will feel "hotter" than 30C with 30% humidity because the body will not be able to cool as quickly at higher humidity levels. In reality, it's not any hotter, it's just more uncomfortable.

The dew point is the best indicator of how humid or "sticky" an airmass is. The following table gives a rough idea of dewpoint and comfort of an airmass..

DewPoint
< 10C ..............dry
10-14C ............comfortable
15-17C ............moderately humid, still comfortable for most people
18-21C ............humid, uncomfortable for most people
22-24C ............very humid, uncomfortable for everyone
>24C ..............oppressive, stifling

In our latitudes, dewpoints above 24C are rare although they do occur sometimes in southern MB in the growing season due to contribution from evapotranspiration from plants and crops. Dewpoints of 27C or more are usually only found in tropical areas near warm oceans.

Western heat wave spreading east this weekend

The heat wave that has gripped the western Prairies for the past week with temperatures in the mid to upper thirties will be spreading east over southern Manitoba beginning this weekend. A ridge of high pressure over southern Manitoba on Thursday will move east into the Great Lakes by Friday. A return southerly flow west of the ridge will draw increasingly warm air from the western U.S. and Canada into southern MB over the weekend, with temperatures expected to climb into the low to mid thirties by Sunday. The hot weather is expected to continue through much of next week with temperatures likely persisting in the thirties most of the week. Little precipitation is expected over the period, except for a possibility of some showers or thunderstorms Tuesday and Wednesday as a weak cold front tries to move through southern MB.

Officially, a heat wave is defined as a period of at least 3 consecutive days with maximum temperatures of 32C or greater. This is a throwback from the Fahrenheit days when we used 90F (32C) to define a "hot" day. Unofficially, we have started to define a heat wave in Canada as a period of 3 days or more with maximum temperatures of 30C or more. It's a nice round number, and it's a good threshold for most Canadians to define as a "hot" day. Either way you slice it, it looks like an extended period of hot weather is coming to southern MB!