14 Dec 2000
Cold Air Damming
Along the Cascade East Slopes
Ron Miller, WFO Spokane WA
During the winter of 2000-2001, the
Inland Northwest experienced an extreme drought with precipitation about half
of normal. Coincident with this winter, Spokane set a record for consecutive
days with snow on the ground, 117 days. This string of days started with an
early season snow storm on 9 Nov 2000 and was perpetuated by a large scale ridge
that would significantly weaken storms as they moved into the area. This persistent
snow pack also helped play a role in the winter storm on 14 December. This storm
was one of the few strong storms to move through the area that winter.
The topography of the Columbia Basin
in Eastern Washington is ideal for trapping cold air, with the Cascade Range
to the west and the Selkirk Range across the north (Fig.
1). This is further aided by a snowpack on the lower elevations of the basin.
Post-frontal cold air is often cooled further by long-wave radiational cooling
over the snowpack. In fact, even in the absence of a synoptic-scale cold air
mass, the radiational cooling alone is often sufficient to create a shallow
dome of cold air that can later be dammed.
The damming process typically occurs
as a Pacific low pressure system approaches the Pac NW coast. In Figure
2 a post-frontal cold air mass lies over the Inland Northwest with light
westerly flow at 850 mb. As a low pressure system approaches the coast 12 hours
later (Fig. 3), the low-level flow switches to southerly
or even southeasterly. The result is that the cold air over the Columbia Basin
is pushed northward and westward. But the mountain ranges to the north and west
restrict the movement of this low-level air mass. The result is a dome of cold
air which is banked up against the east slopes of the Cascades.
In addition to the damming of the
cold air, the southerly/southeasterly winds also ride up and over this air mass
isentropically. The result is a lifting of the lower levels of the atmosphere
resulting in further cooling and eventually precipitation.
After the low pressure system passes
by the area, the flow switches to westerly (Fig 4).
If the flow is strong, the westerly wind will push the cold air mass out of
the Columbia Basin. But in many cases, the cold air mass remains although the
"damming" of the air mass is lessened.
It is insightful to examine this
from vertical cross section perspective. Looking along the line shown in Figure
1, we can easily see the dome of cold air in the western half of the Columbia
Basin at the same time as Fig. 3, which is indicated by the tight packing of
the potential temperature contours (Fig. 5). If we
overlay the circulation streamlines at this same time (Fig.
6), we see the gradual upslope flow over the dome of cold air, with stronger
vertical lift in the mid-layers due to the lifting of the synoptic scale system.
Six hours later, the mid-level flow has already switched to westerly and there
is synoptic scale downward motion (Fig. 7). However,
the low-level cold air mass is not removed immediately. Additionally, the low-level
wind flow continues to favor and upslope lift and the continuance of precipitation,
albeit not has heavy as in the preceding panel due to the loss of the mid-level
lift. After another 6 hours, the low-level cold air mass has been almost completely
removed and the flow has switched to westerly at all levels (Fig.
8). The synoptic scale subsidence coupled with the orgraphic downslope should
result in a rapid cessation to the precipitation and cloud cover. However, numerical
models often have a difficult time with this aspect of the forecast. This this
particular instance, a shallow layer of cold air persisted near the surface
and the strong westerly winds
did not mix all the way down to the ground.
shows the snowfall totals for this event across the Inland Northwest. Note the
4.9" of snow at Wenatchee and 4" at nearby Waterville. Farther to
the east and south of the cold air damming, in the central Columbia Basin, snowfall
totals are less.
The topography of the Columbia Basin
is ideally suited to cold air damming events in the winter. The dome of cold
air which is banked up against the eastern slopes of the Cascades is a critical
factor in the weather for this area. The isentropic lift that it generates aides
in the precipitation formation and helps to overcome downslope affects due to
westerly flow across the Cascades. Additional, the type of the precipitation
is often dictated by the depth of this cold dome. Snow levels west of the Cascade
crest will often rise to 5000-8000 feet or more, while on the other side of
the crest snow levels remain at the surface due to the cold air damming. Freezing
rain events are also not uncommon in these situations as the warmer air aloft
from western Washington rides over the cold air dome.