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 January 24, 2008

The Week in Review

Chilly temperatures, modest amounts of new snow, and occasional wind gales characterized the weather over the last week in RMNP.  Another surge of arctic air moved in the area on Monday, acting to reinforce the well below freezing temperatures established by the first arctic push from the week prior.  As seen in the plot below, the frontal passage on Jan. 21 (circled region) was associated with a drop in temperature, a rapid increase in humidity, and a southwesterly wind at 11 kft.  The coldest air and the easterly upslope flow were confined to elevations below 8 kft east of the Divide. Deep moisture and dynamical forcing in the southwesterly flow (see last weeks report for a more detailed explanation) produced snowfall rates around an inch per hour, as observed by Eli in the eastern high peaks of the park.

The combination of orographically forced snows on Jan. 18/19 and frontal snows on Jan. 21 precipitated about 0.6 to 1.0 inch of snow water equivalent and up to a foot of new snow over the eastern sections of the park.  However, near and above treeline on the 19th and 23rd, strong winds once again scoured, sublimated, and transported much of this new snow away from the alpine regions.  Lee-side deposition zones, particularly those just below treeline, were likely the greatest recipients of the transported snow.  Here, much like the way in which snow fences function, the trees at the treeline boundary act to disrupt the wind flow (via turbulence effects) and create a down-slope/down-wind region of flow separation, in which snow flakes are able to settle to the surface.  Other types of terrain features, such as transverse ribs/ridges and steep lee-side terrain gradients, also produce similar effects (for example, the cross-loaded slopes along the north side of Thatchtop and the Ptarmigan icefield/couloirs near Flattop Mountain).

The avalanche danger has remained considerable, particularly in areas where recent new snow and transported snow have accumulated to depths of a foot or more.  In addition, the persistently cold temperatures have not allowed for any significant stabilization of the snowpack.

The Upcoming Week

A deep area of low pressure currently off the California coast will translate eastward over the next several days.  Some moisture ahead of this system worked its way into western Colorado on Thurs.  This moisture combined with strengthening west-northwesterly flow will produce orographic snows in the northern mountains on Fri.  As seen in the 4-day forecast graphic below, these winds are expected to be strong, with mean speeds near 50 mph at 10 kft.  These strong winds are expected to persist through Sat., and will likely maintain the considerable avalanche rating, with additional snow loading and slab production.

During the Sat. – Sun. evening period, the atmosphere will warm and dry out.  By early Sun. morning, the wind direction will shift to southwesterly, which will usher in even warmer air.  With this warm up, near to above freezing temperatures are forecast for the 10 kft level by Sunday afternoon (this excludes colder locations like gorges an valleys, but would be more representative for locations like slopes and ridges).  While some strengthening of the snowpack can be expected during this warmer period, the brevity of the warm up would likely confine most of the stabilizing effects to the near-surface snow.  Lastly, expect increasing clouds during the day on Sun.

As the California system moves in on early Mon. morning, cold air and snow will return to RMNP.  Winds will gradually shift from southwesterly to west-northwesterly during the day on Mon.  This will cause loading and slab production to occur on a variety of aspects.  By Mon. night and Tues. morning, temperatures are expected to be in the single digits Fahrenheit near 10 kft, with a brisk north-westerly wind and decreasing snow shower activity.  As for the new snowfall potential, the next five days have the potential for 0.5 – 1.0 inch of SWE in the RMNP area (see graphic below).  The effects of potential new snow up to 12 inches, strong winds transporting additional snow and forming slab, and cold temperatures all equate to a rising avalanche risk on Mon. and Tues.  Local terrain features that are vulnerable to wind loading with winds from the southwest through northwest will be the most suspect.

After the Sun./Mon. system, a northwesterly storm track will once again become established over the western US, with storms coming onshore in the Pacific Northwest and tracking across Colorado as they translate southeastward.  This will bring a chance for storminess to the area on Thurs. and possibly again the following weekend.

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.