Thanks to the Mount Washington Observatory and particularly Rebecca for putting together this discussion on Upslope Snow. We asked her to do a short piece on the subject after her stellar presentation at the Eastern Snow and Avalanche Workshop. Rebecca has been working on the summit for several years and is very quickly becoming the smartest person I know. I’m sure in about 5-10 minutes you’ll agree. Thanks Rebecca!
The White Mountains sit at the convergence of three major storm tracks, however the range’s prominence and exposure create a prime location for upslope snow development that can greatly increase snowfall amounts from the surrounding areas. As the tallest summit in the northeast and within a mountain range situated perpendicular to prevailing winds, there are few topographical barriers breaking the straight-line atmospheric flow that reach the summit. As winds reach the prominent slopes of the White Mountains they are forced up and over the obstruction.
This forced ascent causes the rapid cooling of an unsaturated air parcel by 5.3°F per 1,000 feet of elevation gained. The air cools until the temperature and the dew point are equal and the parcel is considered saturated. A further forced ascent of the air parcel causes supersaturation and subsequent precipitation. This forced precipitation on the windward side of a topographical obstruction is called upslope precipitation. When temperatures are below freezing this upslope precipitation becomes upslope snow and can greatly increase accumulation amounts in a specific geographic region.
In the White Mountains this is the most common cause of upslope snow and is called stable upslope flow. In this case the air that is being forced over a topographic obstruction is sufficiently moist through a deep layer in the atmosphere. The moisture comes from characteristics of the air mass associated with the upslope event and gains these characteristics based on where it originated. An air mass that formed over a maritime region will have sufficient moisture present throughout, that when it is forced to ascend rapidly over a topographic boundary, it cools causing clouds and precipitation to occur.
When temperatures are below freezing, snow is the result and is further accelerated through accretion and aggregation of ice crystals. Accretion (riming) is the process of ice crystals falling through supercooled water droplets and freezing on collision, while aggregation is when broken ice crystals collide and stick together. Both create larger frozen precipitation and as a result, stable upslope flow creates an area of increased accumulation. The topographic barrier is the main cause.
There are also two other types of conditions that can cause upslope precipitation; seeder-feeder clouds and the upslope release of potential instability. With seeder-feeder clouds, an existing synoptic scale storm higher in the atmosphere produces snow across a wide region, while lower upslope clouds caused by lifting over a topographic boundary feed added moisture. This added moisture is only present on the windward side of the obstruction leading to a snowfall maximum in this geographic location. As snow falls from the synoptic “seeder” cloud and through the lower “feeder” cloud, moisture effectively scavenges more low level moisture. Through the accretion process, the rime forming on collision with snow pulls moisture from the feeder cloud and increases the frozen precipitation amount.
Finally, through upslope release of potential instability, convective forcing combined with existing upslope forcing can greatly amplify snow amounts. If upstream conditions are potentially unstable and convective then further forced to ascend over a topographical obstruction, rapid supersaturation occurs. Due to increased updrafts from the combining of lifting forces, it is possible for snow to remain suspended in the atmosphere for a longer period of time before it becomes large enough to overcome updrafts and fall. This longer period of time allows for increased accretion and aggregation, in effect generating more frozen precipitation compared to stable upslope flow and seeder-feeder clouds alone.
Accumulation from upslope snow events is augmented by the length of the cool season experienced in the White Mountains. This cool season accounts for the time period between October and April of each year. With the White Mountains at the convergence of three major storm tracks and freezing temperatures present for roughly 60% of the year, the time frame for accrual is increased. Lastly, the orientation of the White Mountain range is perpendicular to the prevailing winds and offers the maximum topographic boundary area for windward slope accumulation specific to the geographic area.
Tuckerman Ravine has MODERATE and LOW avalanche danger. The Sluice, Lip, Center Bowl and Chute have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible. Evaluate snow and terrain carefully. All other forecast areas have Low avalanche danger. Natural and human triggered avalanches are unlikely, but watch for unstable snow in isolated terrain features.
The Little Headwall has an open hole on skier’s right side below the steepest part of the route. Several people fell into this hole yesterday, fortunately without injury. Though this is the easiest way to ski out of the bowl, it remains a challenging run. Hiking out is the easiest option.
Huntington Ravine has MODERATE and LOW avalanche danger. Central, Pinnacle, Odell, and South Gullies have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible in those locations. North, Damnation, Yale and Escape Hatch Gullies have Low avalanche danger. Natural and human triggered avalanches are unlikely. Watch for unstable snow in isolated terrain features.
We are starting out this morning with a Low danger rating in all forecast areas. About an inch of new snow has fallen this morning at Hermit Lake. The degree that our avalanche danger increases depends upon the intensity and timing of forecasted snow and wind today. If we receive the upper end of the 1-3″ forecasted on 45-60 mph winds, the potential for avalanches will increase. Moderate rated areas in both ravines are in the lee of WNW winds and as a result could be loaded and crossloaded with windslabs which will develop as winds ramp up and transport the new snow. Furthermore, it is important to bear in mind that the Low danger rating carries with it the possibility of small avalanches in isolated areas so be on the lookout for slabs of unstable snow on steep sections of Low rated areas.
Skiing yesterday defied the forecast and was soft and spring-like, especially on south facing slopes. Last night was a good bit colder than Friday night so surfaces could be icier beneath today’s new snow. Be prepared for the potential for difficult booting. Experienced ski mountaineers typically carry a lightweight ice axe and crampons for easier and safer travel in addition to avalanche safety gear.
The Harvard Cabin is closed for the season. Hermit Lake is the only area in the Cutler River drainage where camping is permitted.
Safe travel in avalanche terrain requires training and experience. This advisory is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
Anticipate a changing avalanche danger when actual weather differs from the higher summits forecast.
For more information contact the Forest Service Snow Rangers, the AMC at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or the Harvard Cabin.
Posted at 7:50 a.m., April 14, 2013. A new advisory will be issued tomorrow.
Frank Carus, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856