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. 

Warning:  Climbing Life and it's contributors accept no liability for your decisions based on the information contained in this site.  Weather conditions change very quickly and unpredictably, especially in this part of the world, and the actual conditions you find may vary wildly from what we have forecast.    

Issued 11-07-07 12:00 MST

La Nina or La NoNo?

As everyone is surely aware of by now, owing to the mass media coverage, a moderate La Nina event is currently in progress.  During La  Nina, which is also known as the cold phase of the Southern Oscillation, the waters in the equatorial Pacific Ocean become colder than average from the west coast of South America to the International Dateline.  These colder waters act to suppress thunderstorm activity in this region.  Also owing to the complex evolutionary processes of La Nina, the waters in the eastern tropical Pacific Ocean (in the vicinity of Indonesia) become warmer than average, which act to fuel organized regions of thunderstorm activity.  These changes in large-scale thunderstorm coverage induce changes in the atmospheric circulations globe wide, including those near the western states of the US.  In particular, the La Nina-forced circulation anomalies over the northeastern Pacific Ocean often increase the frequency of storminess in the Pacific Northwest and northern Rockies.   In contrast, the southern tier of western states are typically less stormy and much drier than average (e.g., southern CA, AR, NM, and southern CO).  For the northern mountains of CO (e.g., the Park Range and the Medicine Bow/Never Summer Range), above average precipitation has occurred during past La Nina events, as storms moving onshore in the Pacific Northwest track across northern CO on a northwesterly storm track.

Got you hopes up for a great backcountry ski season full of above-average powder days?  I know mine have been.  Unfortunately, there may be a fly in the ointment. The oceanic and atmospheric events that have occurred in the last couple of months over the western tropical Pacific have demonstrated that this years La Nina may be anything but typical.  In particular, sea-surface-temperature behaviors in the Indian Ocean have suppressed the larger-scale thunderstorm activity over the eastern Indian Ocean and western Pacific Ocean region.  This configuration is likely playing a significant roll in maintaining the relatively quit weather pattern over the western states of the US in recent weeks.  While there is a strong consensus among forecast models to maintain the current La Nina event through the first quarter of 2008, there is much uncertainty, among the experts, as to how things will evolve in the western tropical Pacific region.  In short, how this winter’s weather pattern will evolve is anyone’s guess, and confident speculations that you may otherwise hear are nothing short of La BlaBla.  Stay tuned.

The Week in Review

Dry and breezy – that pretty much sums it up.  As anticipated, the winds picked up last Saturday, and continued at a moderate clip through the early part of Monday Nov. 5th.  Observations up on some of the windier alpine ridges indicated sustained west-northwesterly winds in the 30-40 mph range with gusts to 50-60 mph.  On Monday and Tuesday, the winds slackened back to the 20 mph range, only to ramp up once again early this morning (Wed. Nov 7th).  After experiencing cooler temperatures last week through last Saturday, the atmosphere has warmed up by nearly 10 degrees F (highs and lows).  Since Sunday and on the ridges near 11,500 feet, daytime highs have been near 40 F, and the overnight lows have been in the upper 20’s.  However, up in some of the alpine basins and higher mountain valleys, temperatures have been some 10 F cooler than the ridge temperatures at equivalent altitudes.  These relatively low-lying terrain features are repositories for colder air that drains from the surrounding higher terrain, and require significantly more heat and/or wind to erode away the resulting temperature inversions.  Knowledge of these types of terrain/temperature behaviors can serve quite useful when trying to decide which ice-climbing routes to visit, during different weather conditions.

Forecast Discussion

A ridge of high atmospheric pressure, located over the western US, will continue to dominate the weather pattern through the end of this week.  During that time frame, dry, relatively warm, and moderately windy conditions will persist over RMNP.  Thursday and Friday will be the warmest days, with Friday being the least windy.  Gradual cooling will begin on Saturday and Sunday, as a Pacific storm system moves onshore and approaches Colorado.  In addition, expect increasing clouds, particularly on Sunday, and a gradual shift in wind direction from westerly to southwesterly, with only light to moderate wind speeds.  At the current time, a Pacific cold front, and its associated band of precipitation, is forecast to move into the mountains of RMNP early Monday morning.  Precipitation is also forecast to occur over a 3-5 hour period, with approximately 0.2-0.3 inches of liquid expected (~ 3-5 inches of snow).  The orographic forcing effect (i.e., precipitation enhancement caused by the force of the wind against the terrain) appears to be minimal with this storm, which is typically a vital ingredient for producing heavier precipitation in the northern mountains.  The computer models have been consistently slower and farther south with this system with each new run, so a watchful eye on the detailed forecast below will help fine tune the timing and intensity.  The storm track is forecast to retreat northward again on Tuesday, as the next system is forecast to move through Montana and Wyoming on Tuesday afternoon and evening.  This system does have the potential to clip the northern mountains of Colorado as it moves by, and would likely have greater orographic forcing associated with it.  At and beyond that time, confidence is quite low in the model forecasts, as they continue to struggle with the complex processes associated with this year’s La Nina and the seasonal transition to winter.

Detailed 7-Day Forecast

The link below 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.

http://forecast.weather.gov/MapClick.php?site=BOU&llon=-106.62&rlon=-105.25&tlat=41.22&blat=39.86&smap=1&mp=1&map.x=184&map.y=179

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