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 December 5, 2007

The Week in Review

Welcome to the Front Range, where threading the weather needle is required for respectable snowfall, and forces as insignificant as a sneezing fly induce raging windstorms.  In RMNP, the main weather story for the past week has been the wind, which was particularly strong Sunday through Tuesday (and not forecasted in last weeks discussion).  During that time above 12 kft on Niwot Ridge, sustained winds were measured above 50 mph with gusts occasionally near 70 mph (and as high as 85 mph on Monday).

Although most of the higher terrain lying both west and east of the Divide experienced windy conditions, additional accelerations occurred east of the Divide in association with the formation of a mountain wave.  Here, a near-mountain-top inversion, strong component flow oriented perpendicular to the large-scale terrain, the lack of strong vertical shear above the terrain, and a strong down-slope pressure gradient all contributed to these dive-bomber winds east of the Divide.  As forced by these strong east-side winds, the loss of snow associated with surface sublimation was estimated by real-time remote-sensing techniques to be 0.1 - 0.2 inches of snow-water equivalence (SWE) per day.  Once again, this effect combined with relatively small amounts of new snow over the weekend and significant transport, has resulted in a mostly scoured landscape above treeline east of the Divide (read Eli’s 12/4 Conditions Report for more info).  However, due to transport, there has been snow loading in many of the deposition pockets and zones, where tender soft and hard slabs may now exist.

In terms of precipitation, the weather system that ejected northeastward from Baja last weekend lived up to its hype, and supplied the first major mountain-snowfall event for Colorado.  The San Juan’s and Central Mountains benefited greatly from this storm, receiving 4 – 6 inches of SWE.  Over the Northern Mountains, the northward surge of moist, thermodynamically unstable air from the Southwest US interacted with a weak disturbance in the westerly flow to precipitate 0.4 – 1.0 inches of SWE. Over the western ranges of the Park, about 0.5 inches of SWE was measured in the southern Medicine Bow, and upwards of 1.0 inch in the Never Summer.  Over the eastern parts of the Park, Bear Lake snow pillow received about 0.2 – 0.3 inches of SWE (some of this likely offset by sublimation), where about 6 inches of new snow were observed by Eli over the last week.  Here, the total SWE on the ground is a measly 1.0 inch.  Around the Long’s corner in Wild Basin, about 0.5 inches of new SWE was measured down in Valley near 9.5 kft.

Forecast Discussion

Winter storminess will return to the mountains of Colorado, and the opportunity for significant snowfall in the northern mountains is in the forecast over the next few days.  The windy ridge of higher pressure that was over the area on Monday and Tuesday is breaking down.  In its place, increasing westerly flow is becoming established aloft.  A jet-stream-level disturbance embedded in this flow is forecast to move into the northern-mountain region late tonight.  Deep moisture, west-northwesterly orographic forcing, and upper-atmospheric forcing will accompany this storm.  In particular, the position and structure of the jet stream may force bands of enhanced precipitation that are parallel to the flow.  These bands can persist over localized areas for hours, and, if formed, would give the eastern sections of the Park a better chance for accumulating snow than we’ve seen in recent storms.  The current model guidance is estimating up to 0.5 inches of SWE by Thursday afternoon in the mountains.  Temperatures during this event will be relatively warm, with free-atmosphere freezing levels near 8 kft during the day on Thursday.  Winds will be moderate from the west-northwest.

On Friday, a stronger system will dig its heels into the southwestern US.  In a similar manner to last weekend’s system, this Friday’s system will pump relatively warm, moist air northeastward.  The San Juan’s are forecast once again to be the snowfall winner, with upwards of 3.5 inches of SWE totals by the end of the weekend.  For the northern mountains, a warm front and associated jet-stream-level forcing is forecast to produce modest amounts of precipitation on Friday, with up to an additional 1.0 inch of SWE.  As a side note, the heavier precipitation forecasts for Friday have only recently been advertised in the models for the northern mountains.  Trends in or verification of this forecast can be obtained by following the detailed forecast link listed below.

On Friday night / Saturday morning, energy from the Southwest US is forecast to move northeastward and phase with an east-side cold front.  This will produce upslope flow east of the Divide, and, along with additional upper-level forcing, will maintain additional snowfall accumulations over the higher terrain.  Also expect accumulating snows in the Estes Valley, the foothills east of the Divide, and the low lands along the Front Range.  Current precipitation forecasts are estimating another 0.25 – 0.35 inches of SWE.  Temperature will drop markedly, after the front moves through and the continental Canadian airmass moves in.

On Sunday, another disturbance is forecast to develop off the southern coast of California.  At the time of this posting, most of the weather associated with this system is forecast to remain south of RMNP, as the system moves eastward over Arizona and New Mexico during the early part of next week.  However, a reinforcing shot of colder air from the north is forecast to move down the east side of the Divide, as the California system begins to move eastward.  No strong wind events are currently forecast through next Tuesday.  However, by Tuesday night / Wednesday morning, the northwesterly winds may be on the increase, once the California system moves over the plains. Beyond that time, forecast indicators are suggesting a continued west-northwesterly storm track that would allow weather disturbances to affect the mountains of RMNP on a periodic basis.

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 as short-term, medium-range, and climate forecasts.