Thursday December 11, 2008

The Upcoming Week in Review

It’s been another week of extremes in the mountains of RMNP.  The coldest temperatures of the season were ushered in by two storm systems on Dec. 4^th and 8^th.  Both of these storm systems moved in from the north-northwest in response to a retrograding strong ridge of high pressure over the eastern Pacific Ocean.

Both systems also brought rounds of new snowfall to RMNP, with approximately 6 inches of accumulation with each event. 

Since the second system, and its strong pressure gradient moved in farther west than initially forecasted, very little wind accompanied the new snow.

Eli’s photo of the week is a testament to this, with snow actually sticking to the flanks of Ypsilon Mountain. 

However, over the last day, the west-northwest winds have cranked up once again over the higher terrain, and have acted to scour and transport new snow from wind-exposed terrain features, and created new drifts and wind slabs in deposition zones.

In contrast, these shorter periods of winter weather were moderated by relatively mild conditions associated with the brief eastward extension of the west-coast ridge. 

On Dec. 6^th, relatively warm temperatures, high relative humidity, and solar radiation acted to melt water in near-surface layer of the snowpack.  This created a breakable sun crust on many slope aspects exposed to the sun, which have since been buried by the most recent snows.

One of the key atmospheric parameters that controls whether absorbed solar energy is used to melt or sublimate surface snow is the wet-bulb temperature. 

The wet-bulb temperature is the temperature and air would have if it was adiabatically cooled to saturation.  

The accuracy of one-week forecasts has been steadily improving since the beginning of October.
It is typical to have poor numerical forecast accuracy during the transition seasons (fall and spring). 

As we move into the winter season, the increased circumpolar flow (both in strength and latitudinal coverage), and the weather disturbances embedded within it is usually modeled more accurately for forecast times less than a week.

The Upcoming Week

Big changes are on the way for the western US, as the westward shift and amplification of the mean ridge position over the eastern Pacific Ocean will allow a deep arctic airmass to plunge southward. 

This will induce a longwave trough of low pressure extending from the eastern Pacific Ocean to western Great Plains. 

More specifically for RMNP, winds will be on the increase on Thursday and Friday ahead of the longwave trough formation. 

The passage of a weak disturbance will also occur during this period, but is only expected to produce scattered snow showers over the higher terrain.

During Saturday afternoon and evening, the leading edge of the cold air will move into Colorado.  Strong frontal forcing phasing with a powerful jet stream and deep moisture will produce an active period of widespread snowfall with wind. 

East-side upslope will also accompany the storm, but is currently expected to enhance precipitation processes below 8 kft east of the Divide. 

Wind directions during this event are forecast to be from the west-southwest through west over the higher terrain.

Moderate to heavy snowfall rates combined with wind induced snow-slab production would quickly and dramatically increase the avalanche danger.

After frontal passage on Saturday, expect cold conditions for several days to follow.  Any post-frontal winds are expected to be brief on Sunday, as the next disturbance moving through the longwave trough is expected to turn the winds back to the west-southwest by early Monday. 

Periods of snow will move back into the mountains on Monday through Friday, with the heaviest activity currently expected to be focused on the southern and central mountains of CO.

The NWS numerical forecast for the mountains of RMNP over the upcoming weekend is as follows:

Detailed 7-Day Forecast

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.