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 Thursday June 5, 2008

The Week in Review

Ups and downs dominated the weather mood over the past week.  As seen in the lower panel of the plot below, a warming trend (successive higher highs and higher lows) characterized the first part of the week before a winter-like storm system caused temperatures to plummet 25 F over the past couple of days.

During the warming trend, well defined diurnal cycles in temperature developed (vertical blue and red lines denote daytime highs and lows respectively).  Firm surface-snow conditions favorable for climbing often form during the nighttime hours when the temperature spread in these cycles is large.  During these times, background atmospheric water vapor is relatively low (e.g., below 0.15 inches at this elevation on average) and cloudiness is at a minimum. These conditions allow for a net loss of longwave radiation (and heat) from the snow surface at night.  However, the location of these cycles relative to the freezing point plays an important role in the snow-consolidation process.  In the presence of a warming air mass, a shallower near-surface layer of firm snow forms the higher the overnight low is above the freezing point.

As reported by Eli, snow in the lower alpine zones (e.g. approach slopes) have exhibited firmer climbing conditions relative to the snow on the higher and steeper alpine terrain (e.g., Broadway and the Notch on Longs).  These differences are most directly related to a higher occurrence of freeze-thaw cycles in lower alpine elevations. Here, daytime temperatures are more frequently above freezing, and nighttime lows can still be quite cold, particularly in lower angle terrain and basins where cold air from the surrounding higher terrain pools and cools the surface even more.  In contrast, when the higher steeper terrain/surfaces cool, the cooled air drains down the slope (drainage flows), which exposes the higher slopes to warmer air relative to the draining air.  In addition, these steeper slopes can cool at a slower rate when longwave radiation emitted by neighboring steep terrain is absorbed, thereby offsetting the longwave losses to the atmosphere. Lower angled slopes ‘see’ more sky than terrain, which maximizes the longwave losses and surface cooling.

Evidence of these diurnal mountain-valley circulations (heated upslope flow during the day and cooled drainage downslope flows at night) can be inferred from the diurnal cycles of water vapor (top panel) and surface temperature (bottom panel).  Shortly after the maximum temperature is reached during the day (red line), a peak in water vapor is observed.  When the higher elevations are heated, air is forced to rise up the slopes, and with this, moist air is transported upslope from lower elevations.  At night, air cooled by the higher slopes drains downslope which transports the moisture back to lower elevations. The draining air is replaced by dryer air from above mountain top, which shows up as a minimum in the water vapor observations (blue lines).

The moist cooling trend at the end of the week forced some pretty junky snow climbing conditions.  Extensive cloudiness (0.25 – 0.35 inches of vapor) limited the diurnal temperature cycle, preventing the snow from cooling during the nighttime hours.  Until today, the atmospheric cooling associated with an approaching weather system was not enough to refreeze the snowpack.

With the arrival of today’s weather system came colder temperatures and good forcing.  In the presence of a juicy atmosphere, the result was an extended period snowfall and sub-freezing temperatures.  This has acted to temporarily lock up the underlying snowpack, on top of which several inches of new snow have fallen.  While most of the precipitation was concentrated along the lower foothills, mostly in the form of 1.5 – 2.0 inches of water, the higher elevations may have received around a half foot of wet snow (very crudely estimated from cam obs).

The Upcoming Week

Forecasts are formulated from numerical weather forecast models.  While these models capture and predict the large-scale mass/temperature fields quite well, localized weather phenomena, like the mountain wave, mountain-valley circulations, and convection/thunderstorms, are not accurately predicted.  However, combining knowledge of local weather phenomena with these well predicted environmental features can produce some additional forecasting skill.  For example, when the wind direction is from the west-northwest during post frontal conditions (like the present), wind gusts over the higher terrain are typically observed much higher than model guidance (this is the mountain-wave effect).  At the current time (10 pm 6/5), Niwot ridge is gusting to 55 mph from the northwest, while the model forecast is only in the 20 mph range.  

The models also have a difficult time predicting smaller scale variations in moisture.  For sky-cover forecasts, place more confidence in slowly varying larger-scale trends over short-term variability.  The precipitation potential during this time of the year will be most closely related to the presence of deep convection (thunderstorms).  As we all know from first hand experience, our mountains are magnets for convective initiation, so these numbers should not be taken literally.  Use the trends in model precipitation probability to hedge your daily observations of mountain convection actively.  More detailed observations and analyses will be discussed in the upcoming weekly reports, which will augment the model guidance as we move into the prime convective season. 

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.