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 17, 2007

The Week in Review

Since last Thurs., two rounds of snow occurred over RMNP, one last Fri., and another yesterday (Wed.).  The Fri. system produced 4 – 6 inches of snow (~ 0.4 inches of SWE), and preliminary reports from locations south of the park indicate that yesterday’s system may have produced up to 1-3 inches of snow over the higher terrain.  The change in snow depth over the past week in the subalpine zone across the region is as follows:

Here, the northern mountains of Colorado have received the greatest amounts of new snow, as is often the case when a northwesterly storm track is present.  Since the northwesterly storm track became established about a week and a half ago, about a foot or so of new snow has accumulated in the higher valleys and gorges of the eastern sections of the park (as independently observed by both Eli and myself).

Northwesterly storm tracks are also conducive to windy conditions near and east of the Divide.  Throughout most of the week winds were in the moderate 25 – 35 mph range, with gusts to 45 mph.  On Mon., a period of stronger winds (35-45 mph, gusting in the 50’s) occurred ahead of the arctic cold front that moved into the area on Mon. evening.  The resulting formation of soft and hard slabs was observed near and above treeline in both lee- and cross-loading terrain zones.

With a high amplitude ridge of high pressure recently becoming established along the west coast of North America (see above image), arctic air oozed into the area from Canada over the last couple of days, bringing with it some of the coldest temperatures of the season.  Currently, at 11 kft along the Front Range, overnight temperatures have dropped into -15 to -20 F range.  At these low temperatures, a wind of only 15 mph (which is currently being observed) can drop the wind chill another 20 F.  The vertical and temporal structure of this arctic front is depicted in the following plot:

Here, the upper-level weather wave between 5 – 10 km has southwesterly winds east (right) of the wave axis and northwesterly winds west (left) of the wave axis.  The dynamics associated with this structure produce upward (downward) atmospheric motion east (west) of the wave axis.  Since upward motion is a necessary ingredient for precipitation production, the region east of the wave axis is favorable for precipitation.  Additional forcing of upward motion also occurs in the vicinity of the jet stream, which can greatly enhance precipitation when overhead.  In contrast, the sinking air west (left) of the wave axis combined with a northwesterly wind often produces a mountain wave east of the Divide, which can make for very gusty conditions in the post frontal environment.  In addition, clouds are typically suppressed west (left) of the wave axis, with the exception of low-level moisture (clouds) which can become trapped near and below mountain-top level.

Also interesting to note is the depiction of the arctic airmass.  This airmass has a vertically sloping wedge structure along its leading edge, gradually deepens as it moves over the area, has north-northeasterly low level winds, and has wind shear along the upper boundary.  East-side snow events are often associated with this type of structure.  Precipitation production is maximized when the winds in the low-level airmass are more from the east-northeast (maximizing the orographic lift) and when strong dynamics, associated with a jet stream east of the wave axis, are all coexistent overhead.

The Upcoming Week

The weather for the upcoming week will be affected by the same high-amplitude pattern that became established this last Monday.  However, the huge multi-day purge of arctic air that was initially forecast to occur next week now appears as though it will only be a 1-2 day event and may be a bit colder than the one we are currently experiencing.  The 7-day model forecast of temperature, winds, and moisture as a function of height is shown below.

Strong and moist northwesterly flow is forecast to develop by Thurs. afternoon and extend through Fri./ early Sat. morning.  During this time, expect orographic snows over the higher terrain, strong northwest winds, and cold wind chills ( 0 F + 30 mph = -25 F wind chill) above 10 kft.

Temperatures are forecast to moderate this weekend (20’s F at 10 kft), before the next arctic front moves in on Sun.  During this time, the winds are forecast to blow from a westerly direction and slacken in speed below 12 kft.  Temperatures will drop and snow will develop on Sun. with the arrival of the next arctic cold front.  This will coincide with the development of a deep low over the southwest US and another stronger wave moving over the northern Great Plains (see below).

Because cold air has less capacity for holding moisture than warmer air, snowfall rates are expected to be on the light to moderate side.  In this type of situation, snowfall duration becomes the greatest factor for determining the amount of accumulating snow.  In addition, colder temperatures also produce higher snow/water-equivalent ratios, and I wouldn’t be surprised to see reports of 15-20 inches of snow per inch of SWE in some of the colder mountain locations (like the Park Range).  The model predicted SWE over the next 5 days has varied significantly (with an upward tendency), and amounts are currently forecast to be in excess of an inch of SWE in the northern mountains (see plot below).

Long range forecast indicators show the southwest low moving over the southern part of CO the following week and retrogression (westward movement) of the North America ridge.  The retrogression would still result in a northwesterly storm track and opportunities for more winter storms to affect our area.

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.