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1/3/08 La Nina Update- RMNP Weather |
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 January 2, 2008
La-Nina Update
After a slow evolutionary start to this year’s La-Nina, sea-surface temperature distributions across the equatorial Pacific Ocean have recently (over the last month and a half) exhibited the classic structure of a moderate La-Nina event (see plot below). In particular, the ocean waters, near Indonesia, have become warmer than climatology (by 1 C), and are contributing to increased large-scale thunderstorm (convection) development on a periodic basis. In addition, the water in the central and eastern Pacific is 2 C colder than climatology, and extends west of the Date Line.
Prior to the mature evolution of La-Nina, the mountains of RMNP experienced an extended Indian summer, with light winds and warm temperatures through the middle of November. As seen in the plot below, a high-pressure circulation over the western US was responsible for the tranquil weather during the early part of November (as depicted by the ‘H’ in the upper right-hand corner of the plot).
By the middle of November, tropical-atmospheric forcing (known as the Madden Julian oscillation) caused significant convection to develop west of the Date Line (for more info on this forcing phenomena see http://en.wikipedia.org/wiki/Madden-Julian_oscillation ). This pattern persisted through the end of November, and forced a change in the storm track along the US west coast. As seen in the plot above, the high-pressure circulation that had been positioned over the western US in early November, shifted (retrograded) westward (as depicted by the black line and arrow in the plot). This allowed a cooler, snowier pattern to develop over Colorado with a northwesterly storm track.
As the month of December commenced, another Madden-Julian forcing event emerged over the Indian Ocean. This, combined with the sea-surface-temperature and atmospheric-circulation patterns associated with a maturing La-Nina, forced intense low-pressure circulations to develop over the western US (see ‘L’s’ positioned near and west of Colorado during December in plot). While most of the mountains in Colorado received beneficial precipitation during this period, the San Juan’s were located in the sweet spot in early December, as17 inches of snow-water equivalence fell in just two storm cycles (see the two ‘L’s’ near 120W at the beginning of December in the plot). During the latter part of December, the position of the low-pressure circulations shifted eastward, and was associated with strong west-northwesterly oriented jet stream. This allowed cold air, strong winds and new snow to invade RMNP, with wind chill temperatures as low as -25 to -50 F and blizzard conditions above treeline.
So what does all of this La-Bla-Bla stuff mean for RMNP? Well, if you’re into ice, the relatively warm November and cold December (with some periodic snow in between) has translated into some of the healthiest ice flows observed for some time. If you enjoy suffering in scouring alpine gales, this could also be your year, as strong northwesterly flow patterns are not uncommon during La-Nina winters in our area (this last December case and point). In terms of snowpack (where it exists), so far it’s nothing to write home about, as we’re pretty close to climatology for this time of year (see snow-water equivalence plot below). However, because of the slow start in November, a number of sites with near-average totals benefited significantly from above average snowfall in December (particularly in the western ranges, like the Park Range).
As for the future, model simulations predict that we will be dealing with La-Nina conditions for the remainder of this winter season. With this, it is probable that a La-Nina climatological storm track will continue to guide storms into the western US on a northwesterly storm track (see plot below). Also, as observed over the past couple of months, occurrences of the Madden-Julian oscillation will also modulate the position and intensity of the La-Nina based storm track.
The Week in Review
It was a week to wear lead-filled shoes and suits of armor, if traveling in the alpine (or even outside of the house). During last Fri. through Mon., wind gusts consistently exceeded 50 mph above treeline. Above 12 kft, wind gusts were clocked on Niwot Ridge above 85 mph. This combined with cold temperatures between -10 to 5 F, created dangerous wind chills of -25 to -50 F.
Strong flow in a northwesterly storm track transported deep moisture and several disturbances into Colorado during the week. Strong orographic forcing in the northern mountains produced 1.5 inches of new SWE in the Park Range and about 1.0 of new SWE throughout RMNP. Snow depth measurements were quite variable, as significant scouring, transport, deposition, and wind compaction of the new snow occurred to different degrees depending on location.
A region of high pressure developed over the state on Tues., bringing welcomed relief from the wind. By Wed. evening, temperatures moderated by as much as 35 F from their lowest values over the weekend (e.g., Loveland and Berthoud Passes). Valleys, basins, and drainages will be the last places to feel these warming effects, and to a much lesser degree. This tranquil weather window will continue on Thurs., but will begin to degrade on Fri., as clouds move in and southwesterly winds increase.
The Upcoming Week
A strong wave of low pressure is developing off the coast of California, and is forecast to precipitate upwards of 10-15 inches of SWE in the Sierra Nevada over the next several days. This current storm-track configuration has more of an El-Nino feel to it than La-Nina, as it deepens southward over the eastern Pacific Ocean and taps into significant moisture from subtropical latitudes (see satellite picture below). Here, the larger scale wave of low pressure (depicted with the two purple lines) is translating slowly eastward, requiring about 2-3 days to move onshore from its current position. Within this framework, smaller-scale disturbances (depicted by the black-colored ‘L’s’ in the image) travel at faster speeds, and propagate through the larger-scale wave of low pressure (approximately following the paths parallel to the purple lines). In this sense, the large-scale low acts as a storm track for these smaller scale weather disturbances. In return, as these smaller-scale disturbances grow and dissipate, they can also alter the structure and speed of the larger-scale wave.
The larger-scale wave is forecast to move into Colorado on Friday afternoon/evening. An initial period of light mountain snow will occur with a plume of subtropical moisture leading the wave. After a respite on early Sat., the main moisture plume and dynamics will move into the region on Sat. afternoon. This will produce mountain snowfall through Sun. evening. During this time, southwesterly orographic forcing will favor greater amounts of precipitation over the San Juan’s and central mountains of Colorado. In addition, the new snow will fall at relatively warm temperatures.
Two more disturbances are forecast to move into our area on Tues. and Wed. These will bring more snow to RMNP, with colder temperatures and a shift in the winds to a northwesterly direction.
Precipitation amounts over the weekend are expected to be in the 2 – 3 inch (SWE) range for the San Juan’s and central mountains of Colorado. For RMNP, where southwesterly flows are less conducive to heavier precipitation, snowfall amounts should be much less. Nevertheless, a strong jet stream and deep moisture will produce the potential for up to an inch of new SWE in the area, particularly in the western sections of the Park.
Much uncertainty lies in the forecast beyond the end of next week. Long term forecast guidance is suggesting that high-pressure ridging will return to the eastern Pacific / west coast of North America. The occurrence and degree of more wind, cold, and snow for RMNP will be dependent on the longitudinal position and amplitude of this ridge. This type of general pattern would again be consistent with the La-Nina climatology.
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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