Precipitation Forecast

[Tato]

Is it just me or the weather folks have no clue about the amount of precipitation? Yesterday the said less than 1mm; I forgot an empty bucket on my backyard yesterday and there were about 10cm in there by the time it was all over... likewise with the snow forecast, they call it for flurries and I got a foot worth of snow in my driveway...

[57]

Precipitation is very localized and if you're any distance from where they measure, you can see a dramatic difference. Forecasting also cannot take localized storms into account and is based (mostly) on what happened in the past when there were similar conditions. Here's an example:

http://www.digitalhome.ca/forum/show...08#post1429208

It isn't until they actually "see" the (localized pop-up) storms on their radar that they can predict the amount of precipitation more accurately, along with watches and warnings.

It's actually quite amazing that they sometimes have long range (3-5 days) forecasts that predict a storm based on something like a Colorado Low dumping a certain amount of snow on a particular area. Of course, due to the random nature of weather, a change of wind direction, jet stream, etc can make the storm go further south or north and miss us completely.

These days with hot weather in Southern Ontario, daytime heating means it's possible to get "pop-up" storms almost every day in the late afternoon - that's why they usually have a 30% or so probability.

Another factor is the GTA "heat island" effect, which means it's warmer here than in the surrounding area, therefore the air can hold more moisture, so we get less precipitation (unless the storm is dense and fast-moving). It can also deflect storms/fronts around us. Heavy snow in the GTA usually comes when the wind comes from an unusual direction like the south across the lake.

Although storms are difficult to forecast, "regular weather" like temperatures and fronts and regular rain are pretty good, with typically 80% accuracy for 1 day and 50% accuracy for 2 days. After that - too much randomness.

If you go to the radar sites right now, you can see the pop-ups forming - one minute there's nothing on the radar, minutes later there's a thunderstorm (yellow/red colours)...

http://www.intellicast.com/National/...e&animate=true

Edit - Line of storms headed towards GTA right now. May knock out BTV uplink between 8:30- 9:00PM...

[Jake]

Very localized. I recorded 4.8 mm in 15 minutes yesterday afternoon at my rain sensor. Yet biking home the asphalt was dry in spots. I still got drenched though.

[CamDAB]

Localized terrain, heat island effect (mentioned by 57), proximity to any one of the great lakes, and general frontal or pressure stability can have a gigantic effect on precipitation levels.

Was watching the thunderstorms as they left the Hamilton area last night, and caught an interesting occurrence.

Lightning in the ice crystal top of one rapidly intensifying storm was discharging into upper clear air. Haven't see that before... The storm, south of Hamilton, intensified for about 5 minutes, then just as rapidly, calmed down. This shows just how volatile frontal storms and pop-ups can be.

Environment Canada's radar is updated every 10 minutes on or about 7 minutes after the hour, so, 07, 17, 27 etc. So, for the Toronto area, the nearest radar image is located here:

http://www.weatheroffice.gc.ca/radar..._e.html?id=WKR

There are other links on that page for other parts of Ontario.

Intellicast is an amazing site for radar, maps, NEXRAD radar, hurricane info, lightning strikes etc.

I've been using both EC and Intellicast for years. Another great hobby... :-)

Cameron

[YXU]

Thanks Cameron,
I hadn't heard of Intellicast before. Being using MyRadar App and Envirnonment Canada. Intellicast's iPad App is simply awesome! Great features for a free weather app.

Let's hope some rain appears in SW Ontario soon. Fields are looking pretty dusty and I hear corn production and other crops are going to be hurting.

[57]

I'm guessing someone didn't see the second link in post 2? You're welcome.

[waterug]

Could someone explain the difference between "risk" of a thunderstorm and "chance" of a thunderstorm? I assume it's a difference in probabilities, but, which is the more "likely"?

[phycomp]

You should try living in the UK, every day the weather is dramatically different, impossible to plan anything outdoors! Must move to Italy pronto!

[dirtroad]

Tato... Is it just me or the weather folks have no clue about the amount of precipitation?

Q. Don't you you know what happens to little boys and girls who lie?

A. They become Weather Forecasters!

We recently had a system roll through southern Manitoba, most places got a light rain, where as Steinbach got close to 80mm or over 3 inches of rain over a short period of time.

[frenchhomer]

Another site that I like for radar is called rain alarm, google it and check it out.