27 Dec 2003
An Example of Conveyor
Belts in a Pacific Northwest Storm
27 December 2002
Ron Miller, WFO Spokane WA
Introduction
The text book "Images in Weather
Forecasting" (Bader et al 1995) put forth an analysis of extratopical cyclones
in terms of the Warm, Cold, and Dry conveyor belts. The storm system on 27 Dec
2002 is an excellent example of these conveyor belts and their impact on the
WFO Spokane forecast areas.
Discussion
On December 27th, 2002 a strong Pacific
storm moved onshore in the northwestern U.S. region. The water vapor image for
this event with the 300mb height contours shows a fairly typical scenario for
these storms (Fig 1). A ridge is located over the
western U.S. with a long-wave trough out in the eastern Pacific. The resulting
southwesterly flow is a favorable pattern for precipitation events. However,
the complex topography in the Inland Northwest (Fig 2)
has a profound affect on the details of the storm. Trapped cold air will result
in snow or ice rather than rain. And the low-level wind flow will modulate the
precipitation distribution due to orographic lifting. With mountains to the
west, north, east, and southeast, just about any low-level wind direction will
result in upslope in some part of the CWA while producing downslope in other
areas.
While examination of an extratropical
cylcone in terms of fronts is informative, a model based on conveyor belts is
sometimes more insightful. The combination of Warm, Cold, and Dry conveyor belts
is shown in a figure from Bader et al (1995) in Figure
3. The Warm conveyor belt (WCB, denoted on Fig 3 as W-W) begins at low levels
in the warm sector of the storm, curves anticyclonicly ahead of the cold front
as it ascends to jet stream level in the northeast part of the storm. The Cold
conveyor belt (CCB, denoted on Fig 3 as C-C) begins in the cold sector north
of the warm front. It ascends gradulally underneath the WCB in relative easterly
flow. The Dry conveyor belt (DCB) is the dry punch of air behind the cold front.
It's drying is due to strong subsidence as well as the possible intrusion of
stratospheric air.
The water vapor imagery in Figure
1 along with infrared imagery (Fig 4)clearly
shows these three conveyor belts. The DCB is easy to identify in water vapor
as the dry (dark) air behind the system. The WCB is the plume of cold clouds
from just off the northern California coast to southern BC. The majority of
the CCB is hidden by the WCB, but it can be seen as the band of enhanced clouds
extending southwestward from Vancouver Island. The 295K isentropic surface from
the Eta model (Fig. 5) also does a good job
of showing these three features. Widespread precipitation from the WCB is due
to the isentropic lift as the WDB ascends over the cold air ahead of the warm
front. This lift is large-scale and provides precipitation to all locations.
The CCB is not as well evident in Figure 4 since it lies underneath the WCB
over eastern Washington. Figure 6 is an 850mb
forecast from the Eta model. It shows the low-level easterly flow over eastern
Washington. This is an upslope flow into the east slopes of the Cascades and
acted to enhance snowfall in this area. Valley snow fall amounts of 10-15"
were common.
Twelve hours later the cold front
has swept through the area (Fig. 7). It is easily
seen as the tightly packed pressure contours from northwest Montana into central
Oregon. Immeadiately behind it is the DCB with dry air in central Washington.
This drying is enhanced by the downslope effects from the Cascades.
Summary
All locations in the Spokane CWA
received measurable precipitation from this event due to the widespread isentropic
upglide from the WCB. The easterly upslope from the CCB enhanced the lift along
the Cascades and maximized the precipitation there.
References
Bader, M.J., G.S. Forbes, J.R. Grant,
R.B.E. Lilley and A.J. Waters: 1995: Images in Weather Forecasting.
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