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One of the major benefits of living in Central Oregon is enjoying all of the outdoor activities that the mountains provide. The Cascades and the mountainous terrain imbedded within the High Desert allow us to go skiing, hiking, biking, camping, and many other things. However, this type of landscape allows for the weather conditions to rapidly and unexpectedly change. This is why it’s important to check the weather forecast on a daily basis, if not more frequently as weather conditions can change very fast for those heading over the passes or heading outdoors.

Scientific Explanation: Central Oregon sees the rain shadow effect on numerous occasions because it is located on the eastern side of the Cascades. Typically storms ride along the jet stream, which mainly runs in a west to east direction. A rain shadowed area is a dry area on the lee (eastern) side of a mountain range. The mountains block the passage of rain-producing weather systems, casting a “shadow” of dryness behind them. When the flow within the atmosphere is out of the west, the air is forced upwards so it can travel over the Cascades. This forced upward motion is called orographic lift. The lifting causes the air to cool, condense into clouds, and precipitate.

Normally, a majority of the rain and snowfall associated with a storm will fall on the western slopes of the Cascades or directly in the Cascades. When that air finally moves over the Cascade crest and begins to sink down on the eastern slopes, it begins to dry and warm. This sinking motion separates the air’s temperature from its dew point temperature. The separation between these two causes the cloud cover to break up and to allow less (if any) precipitation to fall here in Central Oregon.

The rain shadow effect is normally the strongest with a due westerly flow within the atmosphere. When the flow starts to move away from that westerly component, Central Oregon usually will see more precipitation.

Basic Understanding: A straightforward example of the rain shadow effect is to have a storm pass from west to east dropping around an inch of rain in the Willamette Valley, two inches of rain in the Cascades, and only a trace of precipitation in Central Oregon. Even though the Cascades could be very stormy, the weather in the High Desert could be much more tranquil, possibly even with widespread breaks in the cloud cover.

Scientific Explanation: The radar coverage in Central Oregon comes from the National Weather Service offices located in Portland, Pendleton, and Medford. Due to Earth’s curvature and the mountains between Central Oregon and the radar sites, the radar coverage here is not completely reliable. Just due to Earth’s curvature, the radar can not pick up any precipitation in elevations below 10,000 feet in Central Oregon. There are also complete blank areas in the radar coverage since the radar can not penetrate through some of the mountains. During the winter months, a majority of the precipitation will fall in elevations well below 10,000 feet. Thus, it is almost impossible to pinpoint exactly what areas are seeing snow.

Basic Understanding: There is no local National Weather Service radar located in Central Oregon. This doesn’t mean you won’t get an accurate forecast, but this is a reason why it might be raining or snowing in certain areas when the radar isn’t indicating any precipitation.

Scientific Explanation: With the advancement of computer technology over the past few decades, weather forecasting has significantly improved. Super computers can run numerous simulations to predict the weather from just a few hours out to a couple of weeks in the future. Meteorologists rely on these models each and every day. Models begin their forecast based on actual weather reports that are gathered by weather balloons, surface observations, and satellite imagery.

Weather forecasting is much more accurate in areas east of the Rocky Mountains because storms have already moved over areas where weather balloons can calculate the exact weather parameters.

It is rare to have weather balloons released in the Pacific Ocean so the models have to begin their forecast off of satellite derived cloud temperatures for the storms located over water. The exact center and strength of storms are not completely known until they make landfall along the Pacific coastline. Due to this uncertainty in the models, the weather forecast could dramatically change from day to day. This larger forecast uncertainty is found in all states that border the Pacific coastline.

Basic Understanding: We live in a unique area where the forecast can change on a daily, if not hourly, basis. Continue to get the most up to date forecasts during one of our broadcasts, at www.ktvz.com, or on our NewsChannel 21 mobile application.

Scientific Explanation: A majority of the weather reporting stations are found at major or regional airports for two main reasons. First of all, pilots need specific atmospheric conditions found only at the airports. The other reason is to have weather observations that are not influenced by terrain or urbanization. These airport observation sites are maintained and monitored for accuracy by the National Weather Service unlike some of the other observation sites.

Since we live in a rural area, there is only a handful of completely reliable weather reporting stations in Central Oregon. The only observation sites that have all of the valuable weather parameters are at Roberts Field and the Bend Airport. The official weather reporting stations in our most populated cities are at the Bend Airport, Roberts Field, and the Prineville Airport. All temperatures are forecasted at these airports and not for downtown areas.

Basic Understanding: Even though weather reporting stations are nice to have in populated areas, the weather conditions might not truly represent the exact weather conditions. Airport conditions might not represent what’s going on right where you live, but it’s an accurate picture of the weather in general. That’s the reason why it is very nice to receive an email or phone call from you informing us about what exactly is happening in your backyard.

Scientific Explanation: The “Probability of Precipitation” describes the chance of precipitation occurring at any point in a selected area. For example, a 30% chance of precipitation does not mean 30% of Deschutes County will see precipitation. It is often confused that 30% of the entire viewing area will see precipitation. It means that a single location (airport, neighborhood, intersection … a small area) has a 30% chance of seeing precipitation. The Bend Airport could have a 50% chance of precipitation while Roberts Field has a 10% chance.

Due to this “probability of precipitation” confusion, some meteorologists will not use a percentage, but terms like: “slight chance”, “chance”, “isolated”, “scattered”, “numerous”, or “widespread”. In general, each term has a ballpark percentage associated with it. A “slight chance” of precipitation could mean about a 10% chance of seeing precipitation, while “widespread” could mean 80% chance of precipitation. These terms and percentages are somewhat subjective and vary from meteorologist to meteorologist.

Basic Understanding: If there’s a 90% chance of rain, it doesn’t necessarily mean it is going to rain, but it is very probable. This is a great example where your neighbor down the street sees their driveway completely wet, but you don’t see a drop.

Scientific Explanation: The “average” high and low temperatures are based on the high or low temperatures on that particular date over the previous 30 years. The average high on June 1, 2011 would be the averaged highs of each June 1st from 1981-2010. The National Weather Service uses 30 years to normalize the temperatures in case there are a couple of years that are significantly warmer or cooler than others.

Central Oregon’s official reporting station is at Roberts Field. As a result, the temperatures on the 7-day forecast as well as the high and low average temperatures are for Roberts Field. Since Bend is our most populated city, the high temperatures are normally two degrees cooler and the low temperatures are usually two degrees warmer than the temperatures seen on the 7-day forecast.

Basic Understanding: You will constantly hear what is “average” for this time of year, but temperature fluctuations above and below that average should be expected. Elevation changes between Redmond, Bend, La Pine, etc, play a significantly role in which locations are typically warmer or cooler than others. Typically, but not always, Redmond is the warmest location all across Central Oregon, Bend is a couple degrees cooler than Redmond, and La Pine is a couple degrees cooler than Bend.

If you have any other questions about Central Oregon weather, you can reach me via email at bburkel@ktvz.com or feel free to follow me on Twitter @BenBurkel.