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3/20/08 Weather Forecast for RMNP |
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 Thursday March 20, 2007
The Week in Review
With today’s official arrival of the Spring Equinox, the weather in the mountains of RMNP has been every bit spring-like over the past week. As noted by Eli in this week’s conditions report, the increasingly intense solar radiation has acted, at times, to quickly destabilize the near-surface layers of the mid-morning snowpack. During these times, cloudless skies maximize the amount on incoming radiation, while, in the absence of wind, the absorption of radiative energy is maximized by the surface (whereas windy conditions transport heat away from the surface). While the high reflectivity of the snow surface acts to scatter a majority of the incoming solar energy back to the atmosphere (or towards silly humans when present), very little absorbed energy is needed to change the state and structure of the near-surface snow crystals. In contrast, very large amounts of solar radiation are absorbed by surrounding rock outcroppings, which in turn can quickly add heat to and destabilize the nearby interfacing snowpack. During the next couple of months as the atmosphere continues to warm to higher temperatures for longer periods of time during the daily cycle, these radiation processes will act upon deeper depths of near-surface snow, and produce cycles of wet avalanche activity.
The other common spring-like weather behavior that is entwined amongst the periods of solar baking is the continued occurrence of snow storms. From a climatological perspective, the mountains of RMNP obtain their peak snowpack around the end of April, before the spring runoff begins in earnest in May and June. In fact, nearly 25% of the snow water equivalence in the annual snowpack is obtained from mid March through the end of April. Two storm systems over the past week helped to increase this year’s snowpack, with a total of nearly an inch of water added to the snowpack at Bear Lake (~0.5 inches from each storm).
More snow was observed by Eli in the alpine areas higher and closer to the Divide, which is consistent with the enhanced orographic-forcing effect that the high mountains produce and the precipitation-killing effect that the east-side downsloping winds produce (during westerly flow events). During this last winter, these effects collectively produced 50% - 100% more snow in the neighboring alpine areas compared to what fell at Bear Lake.
The moderate to strong west-to-northwesterly winds cranked up again during the late Mon. through Wed. period, which acted to scour and transport much of the newly fallen snow. With much of the windiest times climatologically behind us now, moderate winds should become more of the norm as the mean north-south pressure gradient continues to slacken over the next couple of months. Temperatures over the last week have been near seasonal norms, with highs 18–28 F and lows 5-20 F near treeline.
The Upcoming Week
Two storm systems will affect the area over the next four days. As seen in the satellite image below, the first system has already moved into the mountains today (Thurs.), and the second, which is currently off the northwest coast, will affect the area on Sat.
On Thurs. afternoon, radar data depicted moderate-to-heavy snowfall rates over the park. The passage of this system during the peak heating of the day, has forced some convective development (embedded cumulus). The visual satellite image below shows the bumpy nature of the cloud tops associated with this convection. Because this system is riding on a swift westerly storm track, widespread precipitation is expected to end by early evening. Total snow accumulations by this evening are expected to be a few inches near the Divide. Most of this new snow is falling with light to moderate winds (sustained in the teens gusting in the 30’s mph). These two factors combined will not greatly increase the avalanche danger. However, if localized drifting does occur, bonding between the old and new snow surface should be examined particularly in areas where crusts previously existed.
Enough moisture and instability will remain behind the system to produce scattered snow showers on Thurs. night and Fri., but any accumulations are expected to be light. The temperature near treeline is expected to be in the 20’s F during the day on Fri. and Sat., with continued light to moderate northwesterly winds.
The next system is forecast to move into the area on Sat. afternoon, with a northwesterly trajectory. Every subsequent model run has produced less impressive forcing with this system. This factor, combined with more forecasted thermodynamic instability, will likely produce scattered convective shower activity, rather than a widespread precipitation event. This type of scenario typically has the greatest concentration and intensity of precipitation along the higher terrain. Model forecasted water equivalence is currently in the 0.1 – 0.2 inch range. Expect temperature to bottom out on Sun., with teens to 20’s during the day at treeline. Winds are forecast to slacken to light speeds, with the possibility of a brief period of upslope northeasterly flow as the system passes to the southeast.
For most of the following week, a ridge of high pressure is forecast to build over the state of Colorado. With this will come a warming and drying trend that will begin on late Sun. Under light westerly winds, temperatures are forecast to reach the freezing mark near treeline on Mon. through Wed. South-facing slopes that receive ample solar radiation may experience wet-snow instability, if the cloudless skies and calm winds verify.
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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