Issued Thursday May 8, 2008

The Week in Review

Last weekend’s cyclone rapidly deepened over western Nebraska, and ushered in what we hope will be the final big blow of the season for the high country of RMNP.  Sustained winds of 40 – 60 mph, with gusts to 80 mph, raked the high peaks last Fri. and Sat., and made it feel more like mid-winter than peak spring.  What a year for wind it has been.  After a stretch of more seasonable lighter winds earlier this week, the moderate breezes have picked up again today in the wake of another weaker weather disturbance.

While some new snow has fallen over the higher terrain with each passing disturbance, the overall warming temperatures over the past week have acted to significantly reduce the depth and snow water equivalence of the snowpack near 10 kft (see below). 

As seen in the bottom temperature panel, several melt-freeze cycles have occurred, and are contributing to the continuing consolidation of the existing snowpack.  Despite the warming temperatures near the surface, the overlying atmosphere remains relatively cold, due to the existence of a large-scale trough of lower pressure persisting over the western states of the US.  With this, afternoon heating of the surface has produced vertically unstable parcels of air, not too different in concept to the rising bubbles in a pot of boiling water.  The visual result is often an afternoon sky full of cumulus clouds (of the puffy variety), some of which become deep enough to produce showery precipitation and lightening/thunder.  When weather disturbances, associated with the jet stream, pass over during these unstable periods, deep convection often becomes organized into concentrated regions of thunderstorms – much like what was observed along the Front Range this last week.

The Upcoming Week

A northwesterly storm track will persist over northern Colorado through the early part of next week.  Two weather systems will affect the mountains of RMNP during that time.  The first will move into the area on Fri. afternoon/evening, and is forecast to bring cooler air and precipitation with it.  On Fri. afternoon, unstable air ahead of the system will be forced upward to produce scattered thunderstorms.  During the overnight hours, widespread precipitation (snow) is forecast for the higher terrain. Once the wave passes on Sat. morning, northwesterly flow will develop and increase the chances for windy conditions.  The wind direction will rotate to a west to southwesterly direction on Sun., which will likely be accompanied by a decrease in speed.

The next system is forecast to move into the area on Mon. afternoon.  Timing with the afternoon heating will produce showers and thunderstorms over the mountains ahead of this system.  Widespread precipitation will develop overnight, however the amounts are uncertain due to the current model forecast uncertainty associated with western part of the wave.  The plot below shows an ensemble of forecasted storm tracks based on the same initialization time but using slightly different initial conditions (20 different cases).

These types of techniques are used to establish confidence levels in the forecast output.  In areas where the lines converge (e.g., off the BC coast), the confidence is high in the models ability to forecast the feature.  In locations where the lines spread apart (e.g., over Nevada and Utah), model confidence is lower.

The ensembles show the weather system moving east of CO on Tues. (not shown), with post-frontal northwesterly flow, which is favorable for windier conditions up high.  A large ridge of high pressure will develop along the west coast and extend well into Canada for the rest of the week.  Colorado will remain on the eastern side of this ridge, with warming temperatures and north-northwesterly flow at mountain-top level.