A few years ago, I figured out that medium range weather forecasts, week 1 and week 2, are very useful for planning snowboarding, climbing, and skateboarding trips around Canada and the US. If you plan on chasing storms, or are simply trying to figure out what conditions are like these are very useful tools. In order to use them, you need to have some knowledge of hemispheric and local weather patterns, know some meteorological lingo, and sit down for an hour or two to digest some somewhat technical information. Recognize that this is fairly advanced stuff, but after some time, you will develop a feel for these products and how they relate to where you live or are traveling to.
Medium range forecasts are found at the NWS Climate Prediction Center and Canadian Meteorological Centre. Instead of daily forecasts that you are used to, these forecasts cover periods of time, typically a week and give a 500 mb chart, a technical discussion (forecast), and probabilities that temperature or precipitation will be above or below average.
We all know that weather forecasts degrade in accuracy with time. The same is true using broad forecasts that are geared for a week or two down the road. Week 1 or 6-10 day forecasts are about twice as accurate as week 2 forecasts 8-14 day. Even the week 1 forecasts can be way off some of the time – it is just the nature of the world we live in.
In a nutshell, here is what I do.
First I look at the precipitation and temperature maps from the NWS. For Canada I often extrapolate these maps – with care and I look at the CMC maps for 8-14 days as well. I ask the questions in my head, what type of weather pattern would cause either above normal or below normal conditions. Is it high pressure or low pressure. I also start asking the question, what kind of storms may we see.
NWS 6-10 day forecast of probability that the temperature will be above or below normal.
Second I pull up the 500 mb heights and anomalies map. Study this map for a while. Look at the long wave ridges and troughs and think about what these means for a specific area. Relate this back to the precipitation and temperature maps you have looked at. Keep your interpretation simple and start thinking about what kinds of storms evolve from these patterns. Also have a look at the anomalies, the difference in predicted heights and the average. It will give an idea of the intensity of a pattern. For example if the isobars, or lines on the map are tight and line up west to east, zonal flow, you can expect a period of quickly moving storms that come in one after another. In other words, it is probably going to snow hard for a number of days. If a large ridge is centered off the BC / WA coast and a trough is over the BC / AB interior, expect cool and relatively dry conditions – maybe an Arctic outbreak.
NWS 6-10 day 500mb heights and anomolies chart.
The third thing I look at and often with the 500mb map is the NWS prognostic discussion. This is a technical discussion by NWS forecasters who specialize in medium range forecasts. You can learn a great deal by taking the time to figure these out and read them. I have. There is often information that is gold embedded in them. Pay attention to the confidence rating – this is key. They use a confidence scale of 1 – 5, with 5 being the best.
After I’ve analyzed and read this information, I start to draw basic conclusions about what I think is going to happen. I don’t try and get specific about certain days, etc. I just try and think about what types of storms or high pressure may happen and what the local effects typically are. Storms or not, avalanche hazard up or down. Simple as that.
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