Warm Front Weather Conditions
Learning goal 5h. List the weather conditions associated with a
warm
front and their relevance to snow sports.
Fronts are responsible for much of the sensible weather we
experience (precipitation, clouds, winds, temperature changes).
Therefore they are important to your day in the mountains where you are
directly exposed to these effects. To remind you, a warm front indicates the boundary
between a warmer and a cooler air
mass, where the warm air is advancing (introductory info in
Flying Learning Goal 3h).
In general, warm fronts present fewer hazards than cold fronts in
terms of winter weather conditions, but they are certainly not without
hazards.
Temperature trends
In the mountains, warm fronts can often present marginal conditions
where the temperature is close to 0°C. Around this temperature the
precipitation type can change from snow to rain, vice versa, or give
you a mixture of the two, and can also melt and refreeze snow that is
already on the ground. Temperature also changes the density of falling
snow (discussed in Learning Goal 7b).
Fig. 5h.1 Marginal conditions while driving to the
ski hill on I-80 in Colorado. The temperature is fluctuating above and
below 0°C. (Credit: West).
As discussed in Learning Goal 5f, along a front
within a frontal zone, there is a temperature gradient.
In a warm front the warm air is advancing. That means that if you're
experiencing a warm front passage you will experience warming
temperatures. Temperature changes with a warm front are typically
weaker than with a cold front both because the temperature gradient is
weaker and because it's moving more slowly.
Winds
In the Northern Hemisphere midlatitudes, the winds ahead of a warm
front typically come from an easterly direction. Behind the front,
winds
shift to be more southerly.
As with cold fronts, complex topography can change what the wind
direction and speed would be without the terrain. The fastest wind
speeds relative to a warm front will vary from storm to storm.
Sometimes they peak ahead of the front, sometimes with the frontal
passage, and sometimes after the front. Wind forecasting is covered in
detail Learning
Goals 5b, 5j, 5k, 6f, and 6g.
Precipitation, clouds, and visibility
Recall the vertical cross section of a warm front from the Flying Learning Goal 3h. As the warm
front progresses, the warm air mass slowly rises up over the cooler air
mass ahead of it. The vertical motion is generally slower and less
turbulent than with a cold front. This means the clouds are more stratiform
(uniform, slow to change, stratus-type clouds; see Fig. 5h.2),
although some cumuliform clouds can occur.
Fig. 5h.2 Typical weather (clouds and
precipitation) associated with a warm front.
In the real world, this means that as a warm front approaches, the
weather very gradually deteriorates. High clouds, sometimes starting
with a few wisps of cirrus, become thicker, and gradually lower over
several hours, or sometimes even a day or more. If you are on the ski
hill, at some point flat light will likely become an
issue. Flat light is when sunlight is strongly diffused which can make
it difficult to see
the texture of the snow surface. You may be caught off guard by a
mogul, cornice, or cat track (the flat road that a snow grooming
machine travels along). Eventually the cloud base will descend below
mountaintop height. If the cloud descends to your level you will start
to lose visibility due to fog (which is just a cloud that's at or below
ground
level at your location).
Likewise, the onset of precipitation is usually gradual.
Precipitation begins ahead of the surface front, usually in the form of
drizzle or light snow, and covering a large area. Due to the gradual
temperature
increase with the frontal passage, the precipitation may change
characteristics (such as changing from snow to rain, or snow becoming
wetter and more dense).
Fig. 5h.3 - Dense snowfall during a warm frontal
passage. Tree branches sag under the weight of heavy snow and mixed
snow/rain. Visibility is also limited by thick clouds. Mount
Shiribetsu, Niseko, Japan. (Credit: West)
In Learning
Goal 5g, we explain why it is hazardous to ski in bad visibility,
particularly in the backcountry outside of patrolled ski areas. Warm
fronts can also cause bad visibility in the mountains, but the causes
differ slightly as follows:
-
Reduced visibility from blowing snow from the ground and/or air
into your face
Strong winds are more commonly associated with cold fronts,
however it is possible to get some wind gusts in the warm sector.
-
Thicker clouds and fog
Warm air can hold more moisture than cooler air, meaning clouds
and fog associated with warm fronts are typically thicker and denser
than those with cold fronts. Sometimes skiers refer to this as soupy
conditions (Fig. 5h.4).
Fig. 5h.4 - Typical weather (clouds and mixed
precipitation) associated with a warm front. Visibility is likely very
poor above the base of the ski area. Whistler, BC. (Credit: West).
In extreme white out
conditions, you can usually only
see immediately in front of you. This has severe implications for
safety. Navigation is obviously difficult or even impossible in the
fog.
Storytime
One time when hiking by Mount Robson in the northern Canadian
Rockies, my partner and I were on a trail that emerged from the trees
into an open, featureless plain. There was snow on the ground, snow in
the air, and visibility was limited by fog. We figured if we just
walked in a roughly straight line, we should end up at the other side
of the plain where the trail would continue. We walked for probably
20-30 minutes, thinking that the open area was much longer than we
recalled. Eventually, we saw the trees and the trail and were relieved.
However, upon getting to the opening in the trees, we realized that it
was the opening we left from - right back where we started! We had
walked in a giant circle. Two lessons: (1) You can't walk in a straight
line for much of any distance if you can't see, so don't try, and (2)
use a compass bearing and/or GPS to keep you on track in these
situations. Take them out and practice using them before you
get lost.
Another issue that can arise due to a lack of visibility is that
your group can get separated. This can become a dangerous situation if
a skier gets injured or lost, or if the party separates further or gets
lost looking for each other. Lastly, you can't see mountain hazards
without visibility. People have unfortunately walked off cliffs in bad
visibility. More common is crevasse danger on glaciers. Even if you're
following a GPS track that you made in good weather, trying to retrace
it can be dangerous if you deviate a short distance from the path. It's
often best in these situations to stay put and wait for the weather to
improve.
Fig. 5h.5 - Whiteout conditions on Mount Shuksan,
WA, USA. The mountaineer in the picture is joined to the photographer
by a rope for safety while on a glacier (with crevasse danger) in bad
visibility. (Credit: West)
Finally, behind the warm front, some convective (less uniform, more
rapidly changing) clouds and showers are possible due to weak
instabilities within the warm air mass.
Key words: warm front, front, gradient frontal
zone, warm sector, stratiform, flat light, white out
Figure Credits: Howard: Rosie Howard, West:
Greg West, Stull: Roland Stull