Posted: Dec 31, 2012 11:36 AM by Meteorologist Matt Elwell
Updated: Dec 31, 2012 12:13 PM
The mid-range models are showing a modest warm-up for Southwest Montana late this week which will likely mean temperatures topping out in the middle-to-upper 30s for some areas by Thursday and continuing into the weekend.
A large ridge of high pressure will build into the western United States for the late part of the week allowing milder Pacific air to filter into the area. Often during the winter, these ridges of warmer air will be rather shallow, meaning the warm-up can be limited. This system is not a particularly shallow system. By looking at the departures in height in the middle layer of the atmosphere (graphic included here) we see significant increases. When forecasting, meteorologists can equate height change to an increase in temperature since the two are closely related. The models are fairly consistent in bringing a warm-up to the Northwest United States well into the weekend.
This is could be a double edged sword. Since we will be under the influence of high pressure, we will have full sunshine during the day, but clear skies at night. During the day with full sun and warmer temperatures, we will see plenty of snow melting around the area. Clear skies at night will mean a quick cool-down with plenty of moisture. If we couple that with light wind in the forecast, we may also be talking about significant fog developing in part, if not all, of the viewing area.
Keep this in mind as you head out in the mornings this week. While daytime temperatures will be above freezing, our overnight lows will be below freezing, which could spell freezing fog conditions when you start out in the mornings.