Monday, September 14, 2009

Summer in September continues.. cooler weather next week

With a high of 27C this afternoon, today marked the tenth day of 25C or greater temperatures in Winnipeg so far this month.. matching the total number of 25C days recorded all of July. And it looks like we'll add a few more 25C days this week as the summer-like weather continues over southern MB with sunny skies and well above normal temperatures through Wednesday. There will be a brief cooldown Thursday into Friday as a backdoor cold front tries to push cooler air into southern MB. However, windy and warmer conditions will return this weekend with gusty southerly winds pushing us back into the mid to upper 20s. Long range models are indicating a change to cooler weather by next Monday as we approach the official first day of fall on Sept 22nd. In the meantime, enjoy the summer weather denied to us the past 3 months!


  1. I cannot believe how humid the airmass is for this time of year!!

    The dewpoint is very high!

  2. Heat reords for September 1948 (ROBs post) will be almost impossible to exceed. Link on my name

  3. Rob With this Heat and Humidity why no severe or even minor Thunderstorms.

  4. It takes more than heat and humidity to produce thunderstorms. You need lift and instability to get the parcels of air rising to form thunderclouds. Instability in the atmosphere occurs when you have cooler air aloft while it's warm and humid in the low levels. Right now, it's not cool enough aloft to produce that instability. Also, you need something to start lifting the air parcels to initiate the convection (a "trigger") like a front, or a surface boundary. Again, right now, there is no trigger for generating that lift. So we're stuck with sunshine and warm temperatures!

  5. If EC's forecast of 6 more days of 25C+ weather verifies, Winnipeg will match 1940 as the most number of 25C days in September (16) Thursday and Friday were looking questionable for hitting 25C, but latest runs show cooling may not be as pronounced those days as earlier thought.

    For the record...

    Greatest number of 25C days in September (Winnipeg, since 1872)

    1. 1940 ....... 16
    2. 1948 ....... 15
    3. 1897 ....... 14
    4. 1967 ....... 13
    5. 1938 ....... 12
    6. 1906 ....... 11

    Winnipeg will have 11 as of today.

  6. Rob, can you help me understand what fundamentally changed to allow us to shift from month after month of cool wet weather to the lovely September we're enjoying? Was there some kind of blocking issue that has since departed? It's hard to ponder what is one of the nicest Septembers in memory in the same year that brought one of the worst summers I can recall? Thanks

  7. Mark..

    The simple answer is that a persistent upper ridge over the West Coast finally broke down and built east over the Prairies this month. The more complex question is what caused this to happen, and why did it take so long? This gets into complex global circulation patterns and atmosphere-ocean interactions that are beyond my level of expertise. In reality, I don't think anyone has a really good explanation why things were so cool for so long, and why all of a sudden, things changed. There are various factors that likely played a role (like La Nina, volcanic eruptions, Arctic oscillations, solar cycles, etc) but exactly how did they interact to affect the overall weather pattern? This type of pattern has happened before, but it's rare. And it's tough to explain or predict rare events mainly because we have little data or experience to work from.

    Also, keep in mind that we've been above normal for "only" 3 weeks.. that's still far behind the prolonged below pattern we've been in. Who knows, perhaps we'll fall back into a below normal pattern next week that will last a while. Or we'll get into an extended period of above normal conditions that will last through the winter. The reality is we simply don't have the skill to say with confidence one way or the other when it comes to long range predictions, which for the most part, are still no better than a coin flip.

  8. One person I know has suggested that warm ocean temperatures in the northern Pacific generated the giant ridge over the west coast of North America. Due to the huge size of this ridge, a huge trough built in over central north america. Because the ridge allowed the ocean to continue to get warmer, the ridge stayed put. As a result of the ridge staying put, the trough also stayed put.

    Now don't ask my whether this is actually true, it is just an educated guess based on ocean temperature trends. Also, don't ask why the ocean in the northern Pacific warmed in the first place...because I don't know either. Finally, the big question is why that ridge broke down, presumably the ocean didn't cool off suddenly (that takes months).

    In order to have a warm winter, we want cool northern Pacific temperatures (based on this theory). That would generate a trough in the northern Pacific and a ridge over central North America.

    Interesting analysis by a climatologist, will be interesting to see how it applies to this winter.

  9. Yes, warm pools in the north Pacific have been known to strengthen those West Coast upper ridges.. but it's a chicken and egg thing. Does the warm pool help establish the ridge, or vice-versa, or a combination of the two? Also, what determines how long the warm pool will last, as this case was much more prolonged than previous cases. Again, it's a complex ocean-atmosphere interaction problem that we still have a long way to study before we can get a better handle on long range predictability.

  10. Jim..

    I've input some numbers for September, and it looks like we may be on pace to have the warmest September on record in Winnipeg, which currently stands at 16.8C in 1948.

    As of the 15th, the average temperature at YWG airport has been an amazing 19.7C.. and it looks like we'll maintain that average through the 20th. For the last 10 days of the month, I input a daily average of 11.5C, which would be 10 days of below normal temperatures consistent with what the GFS and ensembles are showing. That would still give us a final monthly mean of 17.0C, which would top 1948 as the warmest September on record here.

    If the last 10 days are cooler than 11C, than we'll finish just below the 1948 mark. So depending on how cool those last 10 days of the month are will determine whether we set a new record this month. But at this point, it's looking possible.

  11. Rob If it takes sending you to the mediterranean to bring back their weather, next time we'll do it sooner. Better than a coin flip, and easier than heating or cooling the pacific.

  12. One of the major differences this year is the much warmer overnight temps. In both 1940 and 1948 (16.8) there were 15 days of Single Digit Low Temps . Also in 1940 two nasty frosts (-2.8C) to offset 5 days in (40) and 4 in (48) of > 30C

    This Sept only one SDLT so far and one predicted in the next 6 days.

  13. Rob!

    What is your thought on the Farmers Almanac's forecasting techniques????

    They call for a very cold winter this year!!!

  14. Rob puts it very well how long range forecasts are unreliable. These outlooks cannot be relied upon with any expectation of accuracy.

    This past summer is a great example... the hudson bay low/ west coast ridge upper pattern did not give way until recently. This is exactly what I was concerned with back in June, when many were convinced we had 'turned the corner'.

    The farmer's almanac is not based on science.. enough said.

  15. Rob...

    When you say 25C+, would 24.6C count in that total, or does it have to be 25.0C and higher?

    P.S. The mean temp at my place so far this month is 19.9C.

  16. Scott.. By 25C days, I mean days of 25.0C or higher.

  17. I don't put a lot of stock in Farmer's Almanac forecasts.. especially since they don't say how they come up with their predictions. They say they have something like 80% accuracy, but when they're objectively verified, they're actually quite poor. People might remember the odd time they're right, whereas they have no problem remembering when official forecasts are wrong.

    With the emergence of an El-Nino pattern developing and continuing into next spring, odds are that western Canada will have a milder and drier winter than normal. The strength of that El Nino will determine how far into central Canada that mild pattern will extend. A weak El-Nino won't do much for us, but a strong El Nino likely will give us a milder than normal winter. Current ENSO outlooks are calling for a weak to moderate El Nino this year (index of 1.5-2) which would favour a more variable winter for us, but likely with more mild spells than the past couple of winters.

  18. Another question...

    Why does fog form on some days and not others. Today for instance, the humidity in Steinbach has been 100%, or nearly 100% for several hours and the wind has been calm. Is an inversion required for fog?

  19. Following this superb SEPT is anything remotely like 1963 in the cards for us?

    Awed historically by the events of Late November and musically by a 4 Season night in Late December October 1963 was golf weather at its leaves, no mosquitos and lots of sunshine. Sept 63 was also pleasant.

  20. Scott,

    Radiation fog forecasting is a very tricky proposition. Even when all the conditions that we are aware of are met for fog, it sometimes will not form. We know that some of the ingredients for radiation fog are:

    - Clear skies (maximize cooling of the low-levels)
    - Moist low layers with little or no dewpoint depression
    - Light winds

    One of the greatest challenges we have with fog forecasting is that we have an inadequate representation (in reality, and in the model) of moisture (quantities and fluxes) in the low-level atmosphere. The UPS Fog Forecasting Technique tries to alleviate this lack of data but instituting a conceptual model to understand the vertical distribution of moisture. They state that one should take the dewpoint temperature at the time of maximum mixing during the daytime (usually the hottest point of the day) and assign that value as your "crossover temperature." They say that in addition to the known requirements for radiation fog, one should also check to see if the overnight temperature is dropping below the crossover temperature. If not, fog is unlikely.

    Lastly, for many radiation fogs, some form of mixing is required. This often means that a location can have no dewpoint spread for hours overnight and be clear, but shortly after sunrise, when the sun has been able to heat the ground even just a small amount, fog forms as the saturated air is lifted and mixed.

    Fog is a tricky, tricky thing, and can often be frustrating to even an experienced forecaster (unless they're "experienced" enough to not care too much about fog ;) )

  21. Thanks to everyone for some very educational posts. They help people like me understand, at least a little bit, how weather operates. I've noticed how the temperature stays up well into the night. My AC has run until 3:00 AM some days. When it's 19 °C at 11:00PM, that's great. After the miserable time we've had, I'm enjoying this elevated temperature event.

  22. BRAD
    Surface Buoy Temp on Lake Winnipeg is 23C today 7C warmer than the last 6 years avg.

    Soil temps are also in the mid teens as well .

    As these temps equal and exceed dew points in many cases do they effect the Fog Potential.

  23. Rob!

    If we hit 31 C on Saturday will that be a record for Winnipeg????

  24. The record high for Saturday Sept 19th is 31.2C set in 2004 (another year without a summer) I'm not sure if we'll beat it.. but we'll be close. 850 temps of 19C are forecast Saturday with a southerly valley wind. That usually equates to a high of 28-30C for Winnipeg at this time of year.. but if low level temps are higher, or winds are lighter, we may break the record. We'll come close on Sunday as well when the record is 27.8C.

    Just phenomenal weather to be talking about, records or not. We easily went above 25C again today, the 13th day of the month at 25C or more. We are virtually assured of at least tieing the 1940 record of sixteen 25C days in a September. We may even get another one by the end of next week based on the latest GFS run which has now backed off on the extent of cooling next week. That increases the possibility of this September finishing as the warmest on record in Winnipeg if that pans out.

    Exceptional temperatures for Sept 17th over western SK today with an amazing high of 37.0C in Rosetown!

  25. Brad

    I reread your R fog post particularly crossover point. Answer is obvious...


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