Arctic Outflow
Learning Goal 6l: Recognize the large-scale weather pattern
associated with Arctic air and outflow
Skiing hazards
Arctic air (or an Arctic
airmass) presents two major
hazardous weather conditions to skiers: (1) bitterly cold air, and (2) outflow
winds. Remember that when you are not properly dressed for cold
temperatures, frostbite
and hypothermia
can result. Winds can combine with these cold temperatures to make it feel
even colder; this is called wind chill. The coldest
temperatures occur when Arctic air is present. In this section you'll
learn how to recognize Arctic air on weather maps.
Origins and movement
An Arctic airmass originates from the Arctic. It forms in the Arctic
in cold, clear conditions found under a high pressure
system.
This air is blown southward by northerly or northeasterly winds. Arctic
air is very cold, dense, shallow, and stable.
Since it is so dense, it flows similarly to the way water does.
Picture a flood of water moving southward from the Arctic. It moves
quickly and unimpeded across the prairies of northeast BC and Alberta.
This shallow, dense air cannot easily move up over mountains like
the Rockies (recall that stable air doesn't like to move vertically
upwards). However, once it piles up deep enough on the east side of the
Rockies, it starts moving south and west towards the BC South
Coast, through valleys and locally lower mountain passes. Often at the
same time, it surges southward into the northern plains of the USA.
Fig.
6l.1 - A typical pressure pattern set up for Arctic outflow. High and
low pressure and MSLP isobars are shown. The pink arrows indicate
typical pathways of cold, shallow, stable Arctic air as it moves
through the mountain ranges of BC towards the Coast. (Credit: West)
Recognizing Arctic air on forecast maps
Here, we'll apply your knowledge of reading temperature on weather
maps (Learning Goal 5a), identifying fronts (Learning Goal 5f), and recognizing high pressure
patterns (Learning Goal 5e). Take a look at the
85.0-kPa pressure-level temperature map below (Fig. 61.2). In the
85.0-kPa temperature field, very cold
temperatures can be seen over northeast
BC. There is a very strong temperature gradient
(temperature contours packed closely together) associated with a cold
front, stretching from northwest to southeast across northern
BC. Cold fronts that usher in Arctic airmasses from
the north are referred to as Arctic fronts
because of
their Arctic origin. One way to recognize these fronts is by the very
cold temperatures that follow them — typically -15°C and colder. Note
that 85.0 kPa is approximately 1500 m above
sea level, which is ~1000 m above the ground in the prairies of
northeast BC. The relatively shallow Arctic airmass is still visible at
this height.
Fig.
6l.2 - An 85.0-kPa temperature map showing a strong temperature
gradient in northeast BC associated with an Arctic front moving south
and west.
Next, the map of sea-level pressure (Fig. 6l.3) shows strong
high pressure in far northern BC, associated with the arctic airmass.
Also
note the pressure
gradient. Recall that at the surface air often flows across
pressure contours from high to low pressure, which means there is
north-northeasterly flow in this area. This pressure gradient is
approximately co-located with the
temperature gradient; the cold dense air within the Arctic airmass
causes there to be higher pressure at the surface.
Fig.
6l.3 - A mean-sea-level pressure (MSLP) map showing the associated
pressure field.
As we have seen previously in this course, forecasters often like to
look at multiple variables within one map to get a complete sense of
what the weather is doing, by combining all the relevant information.
So now, let's combine the temperature and pressure fields with other
fields to give us a more comprehensive picture (Fig. 6l.4). Take some
time to look at each panel and the variables that are presented.
The wind barbs in both panels indicate that the wind over northeast
BC is blowing from the north and northeast, moving the Arctic airmass
south-southwestward. The 85.0-kPa moisture field (RH, left panel)
indicates that there is probably cloud cover, and the precipitation
field (right panel) indicates that there is precipitation (in the form
of snow) accompanying Arctic fronts. These can sometimes bring blizzard
conditions, even at low elevations, as they move southward.
Fig. 6l.4 - (Left) An 85.0-kPa pressure-level map
from the evening of 30 Jan 2018 showing an Arctic airmass present in
northern BC. Temperature (Temp) is contoured with dashed lines,
relatively humidity (RH) is shaded in colour, and wind barbs display
wind speed and direction. (Right) Mean sea-level pressure map showing
high pressure in far northern BC. Sea-level pressure is contoured in
black, precipitation is shaded in colour, and wind barbs indicate wind
speed and direction.
Figure 6l.5 shows maps from five days later on
the evening of 4 Feb 2018. In the left panel of figure 6l.5, note that
the coldest air remains over far northern BC and to the east of the
Rockies. However, the leading edge of the temperature gradient has
progressed south and westward into central BC, bringing cold air with
it. The associated leading edge of the pressure gradient (right panel,
harder to discern), and the precipitation associated with the Arctic
front, have reached the east side of the Coast Mountains. In the northern
Coast Mountains the very cold, dense, stable air is blocked by the
mountains, and has started to pile up on their eastern side. This is
indicated by the strong temperature gradient on the eastern side of the
northern Coast Mountains (left panel), and to a lesser extent by the
pressure gradient (right panel).
Fig. 6l.5 - As in Fig. 61.4, but five days later on
4 Feb 2018.
If this Arctic front had continued to progress south and westward
(in this case it did not), it would have made it to southwest BC. When
this happens, strong temperature and pressure gradients form on the
eastern side of the southern Coast Mountains.
The pressure gradient forces the cold air out to the coast via the
lower-elevation mountain valleys and passes. This can lead to high
winds in the Coast Mountains, particularly within/near valleys that
span the width of the Coast Mountains. The winds become fastest when
valleys widen. The most familiar example is here in the Fraser
Valley. The lower Fraser Valley widens as it gets closer to the coast
(near Chilliwack, Abbotsford, and Vancouver), and moderate to strong
easterly outflow winds often result in cases like this (see Learning Goal 6m and Fig. 6l.8 below).
Fig. 6l.6 - Wind-affected snow resulting from
outflow winds, near Mount Brew, BC. (Credit: West)
Fig. 6l.7 - Howe Sound (just south of Squamish) on
the same day as in Fig. 6l.6. Northerly outflow winds were blowing
strong, forming whitecaps on the water. Note the blowing road sign.
(Credit: West)
Fig. 6l.8 - Surface (10-m) wind output from an NWP
model showing moderate outflow winds near White Rock and Abbotsford
(yellow arrows). The wind vectors show wind direction (northeasterly)
and the yellow colors indicate speeds of greater than 30 km/h. Also,
note the northerly winds within the Fraser Canyon, north of Hope; and
northwesterly winds blowing down along Harrison lake, north of
Chilliwack. Winds are also funnelling out of Howe Sound, just south of
Squamish.
Surface weather observations
Another way to look for Arctic air moving down from the north in
real-time is using surface observations.
Fig. 6l.9 - Surface observations from the evening of
30 Jan 2018, as an Arctic airmass was moving through northeast BC. Wind
barbs are displayed, along with temperatures (°C).
(Credit:MesoWest/Google)
Note the northerly winds and very cold temperatures over northeast
BC and northwest Alberta in Fig. 6l.9. These are contrasted against
warmer (near freezing) temperatures south of the Arctic front in
Central BC and southern Alberta.
Fig. 6l.10 - Surface observations from just after
midnight local time on 4 Feb 2018, five days after Fig. 6l.9. Wind
barbs and temperatures (°C) are shown. (Credit: MesoWest/Google)
Fig. 6l.10 shows the surface observations five days after Fig. 6l.9.
The Arctic front has progressed south and westwards, indicated by the
surface temperatures over central BC, northeast BC, and western Alberta
range from -15 to -35°C (within the Arctic airmass). A more typical
Arctic front would move this distance in just one day. The cold, dense,
shallow air is just reaching Kamloops at the very bottom of the figure.
It is mostly unable to surmount the Coast Mountains to the west. Strong
northeasterly winds are being reported at one station within the Coast
Mountains. It is possible that Arctic outflow winds
have made it over a low mountain pass out to the coast (see Learning Goal 6m), but the really cold Arctic air
has not.
Keywords: Arctic airmass, outflow winds, wind
chill, high pressure, cold front, temperature gradient, Arctic front,
pressure gradient
Figure Credits: Stull: Roland Stull, West:
Greg West, Howard: Rosie Howard