This is the third article in the GMNC weather series. Here are some guidelines on how to add quality to the weather forecast for the GMNC trails.

                                                                                   graphic credit, Mike Baker National Weather Service in Boulder Colorado

This article is for the skier or ski-waxing enthusiast that wants to have the most accurate idea of the weather conditions on the Mesa. To put these rules of thumb into effect, you will have to have at least an amateur understanding of weather, and also be aware of the daily weather pattern. You can look at forecast models yourself. But a better method to stay weather aware is to read the weather discussion written by the on-duty NWS forecaster in Grand Junction.

The National Weather Service forecasts will likely show a systemic bias under certain weather patterns.

Forecast temperature biases:
1) During periods of high pressure, the forecast for the GMNC trails has a cold bias in both morning low and daytime high temperatures.

When Grand Mesa is under a ridge of high pressure, to have a more accurate idea for your ski day, add 5 to 10 degrees to the daytime temperature forecast. If the skies are mostly cloudy, add 5 degrees; when it is sunny add 10. This also means the relative humidity will be a bit lower than forecast during the day.

• High pressure and sunny skies – add 10 degrees to the forecast; subtract a few percent of RH
• High pressure and cloudy skies – add 5 degrees.

2) During stormy/low pressure days, the Grand Mesa forecast typically has a cool bias for morning low temperatures and a warm bias for daytime high temperatures. In other words, stormy days have a smaller diurnal spread in temperature than forecasted. To have a more accurate idea of your ski day, subtract a few degrees from the daytime high temperature forecast. During strong storms, temperatures often are steady or even falling through the day. This also means the relative humidity will be a bit higher than forecast during the day.

• Stormy days with snow falling – subtract 2 degrees from the forecast; add a few percent of RH.

3) During times when the Mesa is neither stormy nor under high pressure, it is difficult to beat the NWS forecast temperatures.  Follow the forecast.

4) Forecasts for the Grand Junction airport (GJT) have less bias, but not zero.  During sunny high-pressure days, GJT forecasts have a warm bias for overnight low temperatures. During stormy/low pressure days, the Grand Junction airport has a cool bias for overnight low temperatures, and an afternoon warm bias, just like on the Mesa.

These temperature biases may be true at other (most?) remote Colorado mountain sites for the winter season.

These biases are based on the cold season and could be different during the spring, summer, and fall seasons.

Microclimates abound in the complex terrain of Colorado. Examples on the trails that we know: open areas are windier than treed areas and are colder overnight. Vista Valley is colder than most other ski trails. These are examples of very localized biases.

Forecast precipitation biases:

These rain/snow biases were not quantified so expect more storm-to-storm variability. Take them with a grain of salt!

1) In the warm sector of the storm (with the cold front and the upper low still out to our west and the winds blowing from the south or southwest), expect the forecast to produce too much mountain snow. Times when the forecast will do well in the storm’s warm sector is when the forecaster discusses extraordinary moisture amounts in the storm. This part of the storm typically produces a heavier, more-dense snowfall.

2) Expect the heaviest snow with the passage of the cold front. Typically, a majority of a storm’s snowfall will occur with the front and afterward. This cold portion of the storm usually produces lighter, less-dense snowfall.

Other ‘watchout’ forecast terms. When you see these terms in the forecast discussion, they should give you an idea of the storm impacts on the GMNC trails.

“Atmospheric river” – expect relatively wet and mild conditions (temperatures in the 20s to low 30s) that produces high snow levels and dense, wet snow. “Pineapple express” is a specific type of atmospheric river, originating near Hawaii.

“Dendritic crystal growth region” – the portion of the atmosphere that has temperatures of 0 to 10F (-12 to -18C). If this layer of the storm is saturated and near the Mesa top it can produce large amounts of fluffy light snowfall, sometimes surprising amounts! Be prepared.

“Splitting storms” – are often weaker storms.

“Slower storm timing” – in the days leading up to a storm, if the storm is forecast to arrive later and later, it often will be a weaker storm with less snowfall.

“The last in a series of storms” – will often be the strongest storm.

Notes about the NWS forecast and the temperature study.

The NWS office in Grand Junction forecasts for all of western Colorado and eastern Utah. That is a huge area. They create a specific forecast for 66,000 grid points for every hour out through seven days. Those grid points are boxes 2.5km x 2.5km (1.55mi x 1.55mi). The forecasters there are highly trained with state-of-the-art tools and local weather-pattern knowledge.

For this study I chose the two grid point boxes directly over the Skyway observation tower and the GJT airport tower. The temperature comparisons were captured during the 2019-2020 ski season from 10 December to 22 February. The biases captured then have been observed to continue into this season.

Study methodology: forecast temperatures for the next day’s morning lows and afternoon highs were chosen in the evening as if one was planning on going skiing the next day. Then the observed temperatures were recorded the next evening. A forecast bias for low and high temperatures were tracked for all recorded days.