The Weather Explained

The Weather Explained: “Why is the Weather in the Mountains so Unpredictable?”

I can’t tell you the number of times I have seen this happen. A strong line of storms pops up somewhere in the Ohio Valley and begins to move closer and closer to the West Virginia/Virginia state border. The weather nerd inside of me always gets so excited to see this happen, strictly for the possibility that we might see a storm that produces some hail or experience some gusty winds (meteorologists are weird people). When it seems that today is the day the storms will make it to my house, all of the excitement quickly fizzles away. A quick glance at the radar shows that the storms have just… vanished. Gone without a trace. It just seems so bizarre, but this phenomenon seems to happen quite often within this area.

As a lifelong resident of the mountainous areas of Southwest Virginia, I can remember this scenario happening so many times throughout my life. I am sure you can probably think of a few times where you’ve thought “How on earth did that happen?”, or “Where did the rain go?”. I’ve got to hand it to the local National Weather Service offices in Blacksburg, VA and Charleston, WV for the great work they do forecasting the weather for our region, but I think they would definitely tell you how difficult of a task it is to deliver an accurate picture of just what will happen over our heads. It just begs the question, “Why is the weather here so unpredictable?”.

Appalachian Mountains

If you could boil it down to just one reason why the forecast in this portion of the state is so tricky, it would be this: elevation. The very reason why the weather forecast for the mountainous region is so difficult is the presence of the mountains themselves. Weather patterns are created by movements of air. When a weather system impacts or is influenced by a chain of mountains (in our case, the Blue Ridge Mountains), all sorts of complications with the forecast arise. Mountains influence the movement of air within weather systems and therefore cause changes on a pretty small scale, making it difficult to provide the accuracy the public demands.

Weather forecasting today is very dependent upon forecast models. While any good meteorologist will tell you that you can’t rely just on the models to make a forecast, they do provide the primary foundation upon which a forecast is built. There are numerous models that are used within the realm of meteorology, and all of them have certain levels of detail (known as the model’s “resolution”). Models like the Global Forecast System (GFS) have lower resolutions, but allow for a look farther ahead into the future. Other models like the North American Model have high resolutions, but can only look a few days into the future. This resolution is very important when it comes to forecast accuracy, especially within mountainous regions. Forecast models also typically have packages built in that represent the local topography, or terrain, of an area. This, however, is where part of the problem lies:

Here's a look at the actual topography of the southwestern portion of Virginia... (NWS Blacksburg)
Here’s a look at the actual topography of the southwestern portion of Virginia… (NWS Blacksburg)
...and this is the topography that the model actually
…and this is the topography that the model actually “sees”. (NWS Blacksburg)

Notice how the model simply doesn’t have the resolution necessary to actually “see” what occurs in reality. You now may be wondering, “Well, why don’t we just make the resolution better?”. The answer to that simply is, that we are! As technology continues to advance, the resolution of these models continues to improve as well. The NAM and HRRR (High Resolution Rapid Refresh) models actually do a pretty good job of painting a picture of how precipitation will act once it enters our region (including that vanishing act I mentioned above). The forecasts that the NWS produce are actually very accurate, and have come a long way from what we had as little as 5 to 10 years ago. While we continue to get better at forecasting, a model that is able to perfectly resemble what occurs in nature and maintains a good handle on the current weather patterns may be a long way off and very well may not be possible. The computing power needed to do that simply does not exist, not to mention the fact that weather patterns can change almost by the second. It’s just incredibly hard to forecast something like summertime, pop-up thunderstorms even a day ahead of time, but especially so in mountainous regions like our own. While accurate forecasting will always be a difficult task, the future is certainly bright.

While model resolution is certainly a key contributor to “unpredictable” weather within mountainous regions, there are also certain meteorological phenomena that occur only within these locations that further complicate the situation. While there are several of these phenomena (such as rain shadows and vorticity stretching), I’ll outline two of the main ones for our region below:

The “Hydraulic Jump”: This phenomenon is typically what’s behind the “vanishing act” I mentioned up above. Weather systems that approach from the northwest (i.e. northwesterly winds; typically from the Ohio Valley) often run into the mountains, which “squeeze” most of the moisture out of the storm. This results in higher precipitation amounts along the WV/VA border and essentially nothing in the New River Valley. The remaining moisture then moves over the Blue Ridge and comes into contact with unstable air and redevelopment occurs (where the “jump” term comes from). This one typically makes the weather nerd inside of me sad…

Before the
Before the “jump”. Notice the higher precipitation amounts along the WV/VA border. (NWS Blacksburg)
After the
After the “jump” where redevelopment has occurred east of the Blue Ridge. (NWS Blacksburg)

Cold Air Damming (or “The Wedge”): This is another very common phenomena in this area, but very rare on a global scale. There are only nine locations in the entire world where CAD occurs, and only two within the United States (east of the Appalachians and east of the Rockies). For the Appalachians, this occurs when high pressure systems form over Eastern Canada, the New England states or slightly off the coast (less often, but even sometimes off the coast of the Mid-Atlantic), which funnel cold, Arctic air down along the spine of the mountains. This cold air then becomes trapped, or “wedged”, near the surface by the mountains. Warm air then moves over the cold air wedged near the surface, leaving us with chilly, damp and cloudy conditions overhead. This is why Blacksburg, VA gets affectionately labeled as “Bleaksburg” by so many Virginia Tech students…

Cold Air Damming locations around the world. (CIA)
Cold Air Damming locations around the world. (CIA)
Graphic explaining the formation of a CAD event. (NWS Sterling)
Graphic explaining the formation of a CAD event. (NWS Sterling)

Being a meteorologist is definitely not the easiest job in the world, and being a good forecaster for a mountainous region is even more difficult. With continued advances in technology, however, forecast model outputs will continue to improve and we will get an even better handle on just exactly what is going on over our heads. We must also continue to take into account the amazing forces of nature that can occur only in mountainous regions. In the meantime, however, cut your local TV weatherperson some slack the next time they get a forecast “wrong”. They are essentially predicting the future and I can bet are just as disappointed as you when the rain mysteriously seems to vanish.

-N

This post is part of an ongoing series describing all things weather. If you have a question you want answered, comment below this post or get in touch with the author on Twitter (@Nickwx92). Thanks for tuning in!

All photos used under Creative Commons licenses unless noted otherwise. 

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