The weekly weather forecast for the high country of Rocky Mountain National Park, Colorado;  researched and written by professional meteorologist and avid mountaineer:  Dan "the weather man" Gottas. 

Issued Friday May 30, 2008

The Week in Review

Hot, cold, sun, snow – it’s spring time in the Rockies in a big way.  Also characteristic of this time of year is the somewhat steady melting of the subalpine and lower alpine snowpack.  Case in point, Bear Lake has lost 3.3 inches of water from the snowpack over the last week bringing the total snow water equivalence down to 11.2 inches over a depth of just 2 feet (ratios indicative of a wet snowpack).  A warming trend has occurred over the past week, during which fluctuations in the daytime high temperature have varied by as mush as 20 F (e.g., highs in the alpine near freezing on May 25th and above 50 F on the 28th).  During the dreaded west–northwesterly flow patterns (on May 23-24 and May 28-30), the winds were gusty in the 40-50 mph range.

How the Weather Affects the Spring Snow Conditions

With the alpine climbing season beginning to kick into high gear (see Eli’s conditions report), all eyes and feet are focused on the surface snow conditions.  During this time of the year, when the snowpack is gradually transforming into a consolidated mass, the daily weather conditions force enough variability in the near-snow-surface conditions to make or break a climbing attempt.  The desired weather conditions are those which force a firm snow surface (neve) for efficient cramponing in the early morning, with slow near-surface softening during the late morning hours (corn) for stable and forgiving descents.

Of the many heat exchange processes which control the surface energy budget at the snow surface (and ultimately the climbing condition of the snow), there are four weather-related processes which significantly affect the snow climbing conditions during the spring season, namely the net flux of incoming solar radiation, the net flux of longwave radiation, the convective flux of heat between the snow and the atmosphere, and the vapor flux between the snow and the atmosphere.

The affect of the sun’s radiation, the most obvious no brainer, is to add heat to the snow during the daylight hours.  However, much of the sun’s radiation is reflected by the snow surface (as high as 90% for new snow but as low as 30% for old snow).  The presence of clouds reflect even more of this radiation back to space (the deeper the cloud the higher the reflectivity).  The typical destabilization of the snow resulting from this process during the spring usually warrants being off the steeper terrain by the late morning.

The process of convective heat flux acts as the communicator of temperature between the atmosphere and the snow.  The tracking of atmospheric temperature trends relative to the freezing point determines whether the near surface snow is being warmed or cooled.  An increase in wind speed will accelerate this process.

The last two processes, longwave radiation and vapor flux, are subtle but very important forces, especially in the alpine.  The vapor flux transports snow to water vapor (sublimation) when the overlying atmosphere is unsaturated.  The dryer the air and the stronger the wind, the greater the vapor flux.  Because sublimation is a cooling process, dry and windy times act to cool the snow surface.  The longwave-radiation balance at the snow surface consists of radiation emitted by the snow toward the sky (cooling effect) offset by downwelling radiation emitted by atmospheric water vapor toward the snow (warming effect).  The former effect is pretty constant and is a strong function of the surface snow temperature, but the latter can be quite variable depending on the vapor content of the overlying atmosphere.  Basically, the more water vapor there is the air the less the snow surface will cool.  In the extreme case, warm and deeply saturated air (clouds) can equate to rotten snow surface conditions.  On the other hand, when the atmosphere dries out, downwelling radiation becomes very small while the snow surface continues to radiate to space.   The result is a strong heat loss at the snow surface, and firm climbing conditions.

From a practical perspective, tracking atmospheric water vapor can offer the snow climber useful information for assessing longwave radiation forcing and identifying favorable climbing windows.  The plot below demonstrates some examples using the Niwot Ridge data.  Integrated water vapor values below 0.15 inches at this elevation in this region indicate some pretty bone dry conditions (and likely favorable climbing conditions).  Real-time plots of these data can be obtained from: http://www.gpsmet.noaa.gov/realtimeview/jsp/rti.jsp

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Detailed 7-Day Forecast

NWS Forecast for the 12,000 foot level near Longs Peak 

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

Weather Observations and Forecasts

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.