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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 April 24, 2008
The Winter in Review
A strong La-Nina event in the tropical Pacific Ocean forced cold, windy, and moist conditions over the mountains of Colorado and RMNP this past winter season. The anomalous spatial distribution of sea-surface temperature and corresponding atmospheric convection (or lack thereof) across the tropics was communicated to the mid-latitude atmosphere through often times complex physical processes, which most frequently forced storminess in the Pacific Northwest and the Northern/Central Rockies. As a result, colder than normal temperatures persisted over these regions, as depicted in the departure-from-climatology temperature map below, for the 10,000 feet level:
As forced by La-Nina, the mid-latitude atmospheric circulation patterns in the vicinity of the western US were characterized most frequently by strong anticyclonic flow (high pressure) in the eastern Pacific off the southern coast of the US, and accompanying strong west-northwesterly flow on the north side of the anticyclone making landfall in the Pacific Northwest.
The environment associated with the strong jet-stream-level flow off the Pacific Northwest coast (right panel above) was conducive to the growth of smaller scale weather disturbances. These disturbances were guided into Colorado by the northwesterly flow or storm track that became established on the eastern side of the Pacific anticyclone. Near mountain-top level flow (left panel above) was also consistently from the northwest, which persistently produced favorable conditions for mountain-wave wind events to develop near and east of the Divide. These waves forced strong accelerations in the wind, which kept much of the higher alpine landscape (e.g., Longs) inaccessible to underweight humans and scoured free of snow for most of the winter. Forecasting the onset and position of mountain wave development was also a difficult task, which typically required detailed meteorological analysis within 6-12 hours of the event.
With the higher frequency of storms also came more frequent precipitation events for the mountains of Colorado. The central mountains were the big precipitation winners with many sites reporting 140%-160% of normal snow water equivalence (SWE), followed closely by the southern mountains with 120%-140% of normal.
For the northern mountains, including RMNP, it’s been more a split decision, as the western ranges (such as the Park and Never Summer) received 110%-140% of normal SWE and the eastern sites in the park below treeline are 85% - 95% of normal. These lower numbers on the east side of the Divide are likely a reflection of the snow-killing, down-sloping effects of west-northwesterly flow, which occurred quite frequently this winter. Field observations of snowfall depth were consistently doubled near treeline closer to the Divide when compared to the lower east-side sites. In these areas, the orographic forcing is greater, and the down-sloping component is less.
The Week in Review
Over the past week, spring-like weather has prevailed in the mountains of RMNP. The most worthy weather news continues to be a diurnal temperature cycle that has been conducive to snow-crystal metamorphism and a strengthening snowpack. Since a week ago, near-treeline locations have experienced several freeze-thaw cycles.
For most of this week, the winds have backed off to more light/moderate levels, with sustained speeds mostly 10 – 25 mph. No new precipitation was measured in the park since last Thurs/Fri’s event.
The Upcoming Week
Numerical weather forecast skill has dropped significantly during the month of April, as is often the cast during the transition seasons. In particular, the forecasts near and beyond a week should be used with certain degree of skepticism. The deterioration of the forecast verification correlation for one week forecasts is shown below.
With this in mind, the forecast for the next week calls for a flip/flop pattern. The flop will come before the flip, with a storm system moving into the area on Fri. and Sat. With this, the snowpack will become locked up and solid, as temperatures drop back into the 20’s F near treeline. Winds are forecast to increase from the west-northwest up to 35 mph, but higher sustained speeds and gusts are possible given this favorable windy direction, particularly on Sat. Also, there is a chance for some new snow on Sat. morning, but amounts should remain low enough not to raise the avalanche risk.
On the flip side, a warming trend is expected to commence on Sun., and by next Wed./Thurs. we may be seeing daytime temperatures in the 50’s F and above freezing temperatures during the nighttime hours near treeline. Also during this time, the winds are expected to back off to light speeds. These types of weather parameters, if they verify, may be conducive to a large wet slide cycle.
Given the recent track record of the one-week forecasts, stay tuned to the short-term forecast (1-2 day), if weather info is needed for mountain travel during the next week.
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
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.
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