The weekly weather forecast for the high country of Rocky Mountain National Park, researched and written by professional meteorologist and mountaineer: Dan "the weather man" Gottas.
Issued Wednesday November 28, 2007
The Week in Review
We’ve had two Wednesday storms affect the area over the last week. After receiving reports and doing some personal observations, it became apparent that last week’s storm produced less snowfall above treeline over the eastern parts of the Park. Unlike most locations below 11 kft, which received upwards of 0.4 inches of snow water equivalence (SWE), the higher terrain only received about 0.2 inches of SWE. Shortly after the storm moved east of the Divide, moderately strong, dry winds scoured, transported, and sublimated much of the newly fallen snow above treeline. Favored terrain-deposition zones became lee and cross loaded with soft and hard slabs, after nearly 24 hours of continuously windy conditions.
The source of the snowfall discrepancy, with respect to altitude, was two fold. First, the forcing associated with the upper-atmospheric disturbance began to dissipate once the storm moved over the Never Summer/ Medicine Bow Ranges (as evidenced in satellite analysis). This translated into a decrease in precipitation production and duration over the eastern portions of the Park above 11 kft. Second, a cold continental airmass, which existed below 11 kft on the east side of the Divide, acted to enhance precipitation through orographic forcing and crystal-growth enhancement. In the former case, east-side cold airmasses, and their associated pressure distributions, force easterly-component flow against the eastern slopes of the Divide. In the latter case, snow generated above 11 kft fell into the supersaturated air (e.g., small cloud-water drops existing below freezing) below 11 kft, where it grew rapidly by the deposition process. Here, at a given subfreezing temperature, the saturation vapor pressure over the water surfaces was greater than that over the ice surfaces. This difference in vapor pressure forced water vapor molecules to move from the liquid drops toward the ice crystals. The most rapid crystal growth occurs between temperatures of -12 C and -18 C, and, during the last storm, these temperatures existed within an appreciable depth below 11 kft.
This Wednesday’s storm (today) was accompanied by more upper-atmospheric forcing than last week’s storm. However, the lifting induced by this forcing worked on a relatively dry airmass (in an absolute sense), which resulted in only light snow accumulations in the Park. Snow pillow observations near 10 kft in the Indian Peaks and eastern RMNP areas reported around 0.1 inches of SWE, and about 1-2 inches of snow. In a similar fashion to last week’s storm, the forcing associated with today’s storm weakened as it passed over the ranges to the west of the Park. This factor combined with the rain-shadow effect that these ranges induce during westerly flow, caused an appreciable difference in snow accumulations from west to east. For example, preliminary reports from the Park Range (near Steamboat) indicate 8-10 inches of snow ( ~ 0.5 inches SWE), and the Never Summer and Medicine Bow Ranges received snow amounts around 6-8 inches.
With respect to wind over the past week, the windiest conditions occurred yesterday (Nov. 27), when gusts in the 70 mph range were reported throughout the day above 11 kft on Niwot Ridge. Low wind speeds occurred with the new snow today; however, as is often the case on the west side of these weather disturbances, the winds will increase. Expect wind gusts up 70 mph once again in the windier alpine locations tonight and tomorrow.
Forecast Discussion
As advertised by last weeks models, the position of the West-Coast ridge has shifted westward, and is amplifying northward over Alaska. This will allow the storm track to deepen farther south over the western US states, however, to a lesser degree than what was previously forecast and for a shorter period of time. The next chance for winter weather in Colorado will occur this weekend, when the deepening storm track ejects a weather disturbance off the coast of Baja northeastward. This will allow deep subtropical moisture to stream into Arizona, New Mexico, Utah, and Colorado. While the San Juan’s of Colorado are forecast to receive heavy amounts of precipitation with this system, the northern mountains are not in a favorable location to receive significant precipitation. Throughout most of this event, the mountain top winds will be from the southwest, which is actually a rain-shadow direction for the northern mountains with respect to the regional-scale terrain. Without orographic enhancement, precipitation production over the Park will be reliant on thermodynamic instability and jet-stream-level forcing. Both of these processes, in particular the former, will be a factor with this storm, and could produce a several inch snowfall, if everything comes together. The NWS forecast link below will provide updated details, as the model forecasts converge near the end of this week.
Beyond the weekend, the ridge of high pressure that is currently over the eastern Pacific Ocean is forecast to break down. In this region, a stormy pattern will become established, which will bring precipitation to the Pacific Northwest throughout the week. This storminess will induce a region of high pressure over the southern intermountain states, including Colorado. Expect dry and relatively low-wind conditions in the Park on Monday and Tuesday. Beyond that time, there is the usual increased uncertainty in the forecast. Long-range forecast indicators show an increased chance for a retrogression of the high-pressure ridge, which would allow some winter weather to return to the northern mountains possibly as soon as next Wednesday.
Detailed 7-Day Forecast
The link above will take you to the National Weather Service forecast for the 12,000 foot level near Longs Peak. These forecasts are derived from computer-generated numerical forecasts, and are updated shortly after 3:30 am, 9:30 am, 3:30 pm, and 9:30 pm local time. In the lower right-hand corner of the page, one can view forecasts for other locations in the Park by clicking on the desired location in the terrain map.
Do-It-Yourself Weather Forecast Links
The link above provides a list of web links to various sites containing a variety of meteorological data and information. Collectively, these resources can be used to monitor and study current weather conditions, as well short-term, medium-range, and climate forecasts.
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