The weekly weather forecast for the high country of Rocky Mountain National Park, Colorado;  researched and written by professional meteorologist and avid mountaineer:  Dan "the weather man" Gottas. 

Issued Thursday June 19, 2008

The Week in Review  (click on thumbnails to expand images)

Hello summer.  A ridge of higher pressure built over the region this past week, which effectively guided the polar jet stream, and its associated weather disturbances, well north of Colorado.  The resulting local mountain temperatures significantly warmed and the windy torrent ceased.

An artifact of this warming has left overnight low temperatures well above the freezing mark over the past six days, and daytime highs well into the 50s F.  Such conditions have resulted in significant melting of the remaining alpine snow, and have narrowed the daily time window for favorable firm snow travel to the earliest of morning hours.

With this increase in moisture and warmth, the development of deeper convection and thunderstorms has been forced by daytime heating.  The relatively lower moisture content of the atmosphere west of the Divide (see Grand Junction vapor values relative to Fort Collins) has limited convection over most of the western ranges of Colorado.  However, vapor values east of the Divide have become juicy enough to force some isolated energetic storms near and east of the Divide.

The main forcing mechanism for these storms has recently been the mountain-valley circulation, which is induced by the diurnal heating cycle.  As the terrain is heated during the day, an increasingly deeper layer of moist and thermodynamically unstable air (called the convective boundary layer) moves up the eastern slope of the Front Range from the east.  Convection is typically initiated where the top of this layer intersects the eastern slope, as warm, moist air is vented upwards in this convergence zone.  As the morning progresses, this convergence zone (and convection) propagates westward, as the boundary layer deepens to the height of the Divide and above.

From an observational perspective, this mountain-valley circulation, when acting alone, will force a reversal in the wind direction over the higher elevations.  Ambient westerly component winds shift to an easterly upslope component, when the boundary layer deepens to the height of the observation.  This typically occurs between 10am and 1pm near the Divide during the summer months.  Once the east-slope boundary layer reaches the depth of the Divide, deep convection (supporting precipitation and electricity) can occur within an hour of convective initiation over the high peaks.

During the late spring and early summer, the moisture in the east-slope boundary layer can play a dominant role in the convective patterns over the mountains of RMNP.  As we move into July and August, moist unstable air is also transported into the region from the southwest, as the North America Monsoon develops.  During these times, moisture transports from both sides of the Divide become important.

The Upcoming Weekend   (click on thumbnails to expand images) 

A ridge of high pressure centered in eastern Arizona will build northwestward on Saturday.  Anticyclonic flow around this ridge will produce northwesterly flow over the mountains of RMNP.  This will continue to transport relatively dry air into the mountains of Colorado from the west, and limit the convective development over the western ranges.  East-side moisture transports should still be enough to fire some isolated storms in the afternoon near and east of the Divide.

Consequently, as we move into July and August, this circulation pattern will expand and move northward, allowing deep subtropical moisture to be transported into the region from Mexico (the monsoon pattern).  The numerical model forecast for Saturday is as follows:

On Sunday, the trough of lower pressure off the California coast will allow a weaker disturbance to displace the AZ/UT ridge southward, and potentially increase the convective activity as it moves over the park.  In addition to the mountain-valley circulation forcing, these disturbances, even when weak, can provide enough additional upward forcing to increase the coverage of convective venting.

The current numerical model output for Sunday currently does not reflect this increase in thunderstorm risk:

Tune into your favorite short-term forecast to verify if this change in thunderstorm risk materializes.  Also, given the current levels of heat and moisture, the lightening risk will exist from 11am-12pm onward into the afternoon.  So, the local rule of thumb of being off the high peaks by noon would mitigate most of this risk.

Detailed 7-Day Forecast

NWS Forecast for the 12,000 foot level near Longs Peak

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.