Sunday, September 07, 2008

Lake effect season is here

This morning is providing a good example of a common phenomenon in southern MB in the fall - lake effect precipitation. Lake effect precipitation develops when you have cool air aloft moving over the relatively warm lake waters, creating narrow bands of instability precipitation to the lee of the lakes. You need at least a 13C difference between the lake water and the temperature at 850 mb, and winds need to be well aligned in the lowest 10000 feet (i.e. little directional shear) to form these narrow bands. This morning we have 850 temperatures of +2C over 18C lake waters, along with low level winds aligned in a 330-340 degree orientation giving lake precipitation to the southeast of Lakes Manitoba and Winnipeg. Typically these lake effect bands form in the cool air behind a cyclonic storm system and will dissipate as subsidence moves in with an approaching ridge of high pressure.

Early in the season, these precipitation bands are in the form of rainshowers. As we get further into the fall with colder temperatures aloft (i.e. 850 temps of -5C or colder), these lake effect bands will produce narrow swaths of snow to the lee of the lakes.. sometimes quite heavy. See an example from October 11, 2006 for an early season lake effect snow event over southern MB.. notice that the lake effect bands from that case are oriented virtually identical to those this morning (except it's rain today, not snow!) Lake effect bands off Lake Manitoba commonly set up between Portage and Elie, but can come into Winnipeg if the low level flow is in the right direction, from around 300 degrees, as seen in this example from last September.


  1. Thanks for the explanation Rob!
    It seems like environment Canada has a very tough time getting the forecast right!
    They call 30% chance of showers for today in there 5:00 AM forecast
    Then in their 11:00 forecast it is bumped up to 60%.
    Lucky for them they have that 11:00 update forecast or there predictions for the day would never be that accurate!

  2. Several rounds of heavy downpours affected central Winnipeg today. Hopefully all the rain from the past 2 days will add enough moisture to the boundary layer that frost will not be an issue. Having said that.. EC's forecast low of 5 C for Winnipeg airport is too optomistic I believe. With clearing and a light NW drainage flow... the stage is set for a cold night at YWG.

    Puzzlingly, EC did not put out a frost warning for S Saskatchewan even though the forescast low for Regina is 1 C (!!). The NWS in Bismark put out a frost advisory for western N Dakota and NWS Grand Forks mentions potential for scattered frost in the Devil's Lake basin. It is all dependent on what the cloud cover does tonite... but with 850 MB temps close to zero and a surface high moving from SE ALberta to western N Dakota by morning.. its gonna be a close call.

  3. Rob!
    How do you gain access to those radar images???
    Those are not the ones that are displayed on Environment Canada's website!!!
    Where do you get these ones from???

  4. Daniel..

    Those are password protected radar images from EC that are currently not available to the general public. There are hopes to get these type of images to the public in the future, which are quite a bit more detailed and useful than the radar products currently available on Weatheroffice.

    By the way, those lake effect rainbands off Lake Manitoba produced a narrow swath of 15-20 mm of rain in the High Bluff-Oakville areas yesterday according to the CWB mesonet.

  5. Patchy light frost reported this morning north and east of Winnipeg over St Andrews and Dugald. Officially a low of +2.9C at YWG airport this morning.

    Lowest min temps this morning over the traditionally colder SE corner of the province.. with lows of 0.7C in Vassar (CWB), +1.0C in Sprague (EC), -0.5C in Marchand (MB-Ag).

  6. Interesting set up tonite. Inverted trough from a low taking shape in the Dakotas will pass thru Winnipeg. A very strong southerly low level jet of 45 kts is progged to feed into the low and associated shortwave. Deep moisture will be drawn up and elevated convection is expected to initiate on the nose of that jet.

    The GFS depicts a very sharp cut off to the forcing from that LLJ... in fact putting Winnipeg in a light northerly flow by midnite. However, Winnipeg and SE Manitoba will be influenced by the right entrance region of an upper jet maxima moving up in the SW flow.

    Question is... will the large scale forcing from this feature be able to generate precip further NW for Winnipeg? Right now it looks like the best bet for widespread heavy rain will be way down over SE N Dakota on the nose of the LLJ.

  7. Hey Rob - it looks like we followed suit in Southern Ontario yesterday and last night with some lake effect showers coming in off of Lake Huron and Georgian Bay. Certainly is another sign that summer appears to be over.

  8. Kuhny..

    Yes, it's that time of year again.. along with hurricanes! I see you guys may get a good soaking from Ike by the end of the weekend!

  9. take a look at the SPC outlooks!
    they are calling for no organized storms forecast!

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