WESTERN REGION TECHNICAL ATTACHMENT
NO. 97-12
APRIL 1, 1997
CONDITIONAL SYMMETRIC INSTABILITY
CONTRIBUTING TO HEAVY PRECIPITATION
IN THE PACIFIC NORTHWEST
Russell Mann, NWSO Riverton, WY
Introduction
Beginning the evening of February 23rd, 1994, precipitation began falling in west-east oriented
bands across northern Oregon and southern Washington. The precipitation intensity peaked
during the day on the 24th and ended the evening of the 24th. Six-hourly precipitation amounts
during the event are shown in Fig. 1. Record rainfall and snowfall totals were measured at many
locations.
During the event, many rivers in northwest Oregon went above flood stage. At the Portland
NWSFO, the 24-hour rain total of 2.46 inches established a new 24-hour precipitation amount
for February and was the third highest 24-hour total on the all-time list. Record precipitation
was measured at NWSO Pendleton, Oregon, which received an all-time record 24-hour snow
amount of 16.1 inches and record February 24-hour precipitation with 1.41 inches. In south
central Washington, Yakima received 8.5 inches of snow in a 24-hour period, breaking their
February record. In the Washington Cascades, Crystal Mountain Ski Resort near Mt. Rainier
reported 65 inches of new snow in 24 hours, setting a new record for 24-hour snowfall in the
state of Washington.
Traditional synoptic features alone did not appear to be dynamic enough to produce the heavy
precipitation that occurred. However, the contribution of another atmospheric process,
conditional symmetric instability (CSI), could help to better explain the event. This Technical
Attachment will show how CSI contributed to the heavy amounts of rain and snow on February
24th, 1994.
Synoptic Pattern
The typical synoptic pattern for significant precipitation over the Pacific Northwest is
characterized by an upper-level trough (i.e., 500 mb) off the west coast and strong west to
southwest flow aloft. Entrained in this flow is a long fetch of moisture, often extending to the
tropics. This sort of moisture plume is commonly referred to locally as the "pineapple express".
In the event examined here, conventional methods of meteorological analysis did not indicate
prime conditions for a heavy precipitation event. Synoptic-scale features included northwesterly
flow aloft over the Pacific Northwest (Fig. 2). most of
the heavy precipitation occurred on the anticyclonic side of a strong polar jet (Fig. 3). An upper-level short wave was moving through Washington, brushing the northern part of Oregon (Fig.
2) and a warm front producing over running precipitation did not appear to make a strong
contribution (Fig. 4). Overall baroclinicity vaguely depicted by the 850 mb height/temperature
overlay along the Washington/Oregon border (Fig. 5) seemed weak.
Conditional Symmetric Instability
When evaluating for potential CSI on a vertical cross-sectional view, two major environmental
characteristics must be met:
1) On a cross-section taken perpendicular to the thermal wind, theta-e surfaces have
steeper slopes than the momentum surfaces (Fig. 6).
2) The airmass must be saturated or nearly so. The more moisture there is in the
atmosphere, the steeper the slope of the theta-e surfaces. This will increase the
likelihood that the theta-e surfaces will have a steeper slope than the momentum
surfaces.
Figure 7 shows dry theta-e surfaces and moist theta-e surfaces. Above the Lifted Condensation
Level (LCL), theta-e surfaces have a greater slope than the momentum surfaces (necessary for
CSI). Below the LCL, the theta-e surfaces have less slope than the momentum surfaces. Thus,
Fig. 7 illustrates that when motions are unstable with respect to saturated slantwise
displacements, but stable with respect to dry slantwise displacements, the atmosphere has CSI.
Other indicators to look for are (Snook, 1992):
1) Wind speed increasing with height.
2) A veering wind profile in a frontogenetical area (the warm advection in the frontogenetical
Region provides uplift that can release the slantwise instability).
3) A well-mixed layer close to saturation.
4) Multiple bands of cloudiness oriented parallel to the thermal wind (i.e., parallel to 1000 -
700 mb thickness lines).
Analysis
IR satellite imagery shows a single band of clouds stretching across the Washington/Oregon
border (Fig. 8). Close examination of the imagery shows several enhanced cloud tops parallel
to each other across southern Washington. The clouds imply the possibility of a cold front or
frontogenesis in the area contributing to the release of CSI. Visible satellite imagery also
reveals some striations oriented parallel to the thermal wind (NW-SE) (Fig. 9). WSR-88D data
would have been useful in identifying precipitation banding, but this event occurred before
NEXRADs were operational in the Pacific Northwest.
Figure 10 shows a series of vertical cross-sections taken through the area of the heavy rain
accumulation perpendicular to the thermal wind. (Refer to Fig. 1 for location of cross- sections).
Momentum surfaces, equivalent potential temperature surfaces, and relative humidity surfaces
are shown. Weather observing station along (or near) the cross-sections and 6h precipitation
amounts, are shown along the bottom of each cross section. The dark shaded area indicates
a favorable CSI environment, which correlates well with the precipitation maxima location. It
must also be pointed out that on 00Z on the 24th, there is convective instability near the surface
(theta-e decreasing with height), which would overwhelm the CSI.
Slantwise convective available potential energy (SCAPE), a measure of the amount of energy
available in a CSI environment, is proportional to the 2nd power of vertical wind shear (Snook,
1992). SCAPE can be estimated by using a sounding created from data points taken along a
constant momentum surface. From the February 24th event a sounding has been constructed
from data points taken along a momentum surface roughly over the rainfall maxima area (Fig. 11).
The SCAPE result from this is 147 J/KG. This is not substantial compared to CAPE values,
which can reach into the thousands, but it might have been enough to create an unstable
situation in an environment that appeared to be stable to upright convection.
Conclusion
It appears that CSI played an important role in the development of a heavy precipitation event
in the Pacific Northwest on 23-24 February 1994. CSI, along with a trigger mechanism to
release it (cold front, short wave, frontogenesis, etc.), could have contributed to producing the
observed precipitation amounts. Indeed, model gridded data analysis did show a frontogenetical
area near the heavy precipitation (Fig. 12). Forecasters have at their disposal the tools
necessary to ascertain the potential for CSI. Through experience, the forecaster can also begin
to recognize weather patterns that are conducive to CSI, and know when to analyze its potential
further through gridded data.
Acknowledgment
The author wishes to thank Thomas Ainsworth of Western Region Headquarters, Meteorological
Services Division (formerly at NWSFO Portland), who helped with the initial development of this
Technical Attachment.
References
Snook, J.S., 1992: Current techniques for real-time evaluation of conditional
symmetric instability. Wea. Forecasting, 7, 430-439.
Bluestein, H.B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Oxford
University Press, 545-561.