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California State University, Los Angeles: "Topographical and synoptic influences on cold season severe weather events in California"

Final Report


The main objective of this partners project was to identify and develop forecasting aids that would benefit classroom education, research knowledge concerning California severe weather, and operational forecasting of these events.

One of the specific tasks was for the university researchers to compile a 25 - year data set including storm descriptions and synoptic features that will be used to create composite synoptic charts for convective thunderstorms in the different regions of California. The goal was to also identify similarities and differences in synoptic patterns for different regions as well as for different severe weather events, for example, tornadic vs. non-tornadic events.

The university side has compiled a 25 - year severe weather event data set. They are extending the data set to 41 years (1959-1999) currently. From the dataset (only 15 years of the data set was used for the composites due to software restrictions), composite maps were made for tornadoes, funnel clouds, and waterspouts, individually as well as in combination. (The number of large hail and downburst wind cases reported during the 15 years used for composites was too small to be representative).

Another goal was for the NWS and university project partners to develop prototype case studies using representative cases. These cases were to be analyzed in detail to identify the role played by synoptic patterns and regional topography as well as specific dynamic and thermodynamic mechanisms.

Prototypes were developed for our southernmost region. One of the prototype cases was a tornado event. Beyond the synoptic scale pattern, the key feature associated with the tornado was convergence lines created by the offshore islands, known locally as "Island Effect" phenomena. This project allowed us to investigate in some detail the characteristics associated with Island Effect Bands. These bands were shown to produce severe weather over the coastal waters as well as over mainland areas.

Some forecasting aids were identified. Monitoring the development of convergence bands downwind of isolated, elevated terrain features was high on the list of these aids.

We were able to determine the diurnal characteristics of the Island Effect Bands. Multiple bands (usually 2) generally occur downwind of an isolated, elevated terrain feature when the lifted condensation level (LCL) is well below the top of the feature. Over the ocean this usually occurs when the low-level airmass cools and moistens, resulting in a decreased dewpoint depression, increased relative humidity, increased instability, and more widespread convection (especially prevalent during the night through early morning hours). Also, over the ocean, single band structures are more likely to develop when the LCL is near the top of an isolated, elevated terrain feature (usually during the afternoon). Rapid development can occur on these convective bands during the day due to diurnal heating, especially where the bands pass over land areas. This rapid development was a key component of the tornado case. Broken, cellular bands generally develop with wind speeds of 10 m/s (20 knots) or higher, and long, continuous bands are usually associated with lesser winds.

Three other prototype events consisted of convergence downwind of isolated, elevated terrain features. One was a multiple waterspout event, another was a funnel cloud event, and the third was a microburst event. The final prototype event was a golf-ball hail event, most likely an "upslope" type event.

What stood out in the prototypes was the parameters associated with the events were generally rather weak by most Midwestern standards. Forecasters can now look at broad, well organized cells (preferably with distinct updraft/downdraft couplets) associated with VILS as low as 10-20 kg/m2 and composite reflectivities as low as the 50-60 dBZ range as still being good candidates for severe hail and/or microbursts. This is especially true for cold air masses with freezing levels well below about 6000 feet msl (1900 meters). This is critical as severe events can occur well before reaching the more commonly known, higher values associated with Midwestern storms.

As for tornadoes, it was seen that tornadoes rarely occur on consecutive days, therefore on the day after a tornadic event, waterspouts, funnel clouds, and downburst winds should be the focus.

Similarities and differences were found and noted among the composite maps. It was found that upper level low pressure systems associated with tornadoes were stronger than those associated with the other types of severe weather events (for example, the 500 mb geopotential heights were lower and the 850 mb winds were stronger for events when tornadoes occurred). Also events in the southern part of the state were associated with a more "V" shaped trough than those in the north. Although a distinction was made between the synoptic scale patterns, further study is needed to better differentiate between patterns that produced a certain type of severe weather event.

Oftentimes, it seemed that the basic synoptic scale pattern sets the stage for severe weather, and the terrain-forced mesoscale flows were the catalyst to utilize this potential. Based on this partners project, forecasters can first look for the synoptic scale patterns shown in the composites, then concentrate on the mesoscale forcings during such volatile patterns to focus their attention on the most likely "hot spots", such as downwind of isolated peaks. The forecaster can use the forecasting aids and concentrate on the cells approaching those values.

In terms of the educational objectives, these findings are being incorporated both into university classroom lectures, lab exercises and into a Master's thesis.


The project began with an initial meeting of the forecast partners to discuss the path of the project. Afterward, there were 3 other face-to-face meetings to discuss the progress and direction of the project, mostly at the forecast office. Numerous phone calls and emails were a large part of the coordination between meetings. Email was essential in the preparation of an AMS presentation and publication (see section 3 below). Through the friendships, the university partner arranged for the NWS partner to present some early results and other studies at a local AMS chapter meeting.


Small, I. J., G. Martin, S. Ladochy, and J. Brown, 2002: Topographic and Synoptic Influences on California Cold Season Severe Weather. Preprints, 16th Conference on Probability and Statistics, Orlando, FL, Amer. Meteor. Soc., 146-153.

Brown, J. and S. LaDochy, 2002: A Synoptic Climatology of Funnel Clouds in California. Preprint (abstract), Annual Meeting, Association of American Geographers, Los Angeles, CA., Mar. 19-23.

The future of the project is to add the additional supporting information to the AMS conference pre-print in an upcoming paper. Also the graduate student will continue to work on a thesis based on much of the material generated by this partners project.

Findings from this partners project have been incorporated into the winter weather portion of the NWS San Diego training plan.


4.1 Benefits to University

The largest benefit from the project is to our graduate student, Jeff Brown, who is using the results from the collaboration in developing his thesis. Jeff has also presented some preliminary results at conferences, such as mentioned above, which has provided him confidence and direct feedback from a larger audience. CSULA benefits from the partnership in several ways. The results of the collaboration are being used in lectures and lab exercises for meteorology courses. The project provides evidence on the prevalence and severity of extreme weather events in California. The university partner also benefits from learning techniques and practical operational methods used by the NWS. He also benefits from the resources available to the NWS forecast office. The university also benefits from the association with the NWS and the COMET program. There is also the possibility that the project may lead to greater awareness of career opportunities in atmospheric sciences.

The only problem encountered was acquiring hail data from the National Crop Hail Insurance Company. They eventually were only willing to send annual summary reports, rather than detailed hail crop insurance claim data. While this is a setback, there may be indirect ways to obtain the data. The funds allocated for the hail data was transferred to our graduate researcher to extend our database to 41-years and to investigate proxy forms of severe weather information (newspapers, local insurance companies).

4.2 Benefits to NWS
One definite advantage of this partnership was the initial data set for all of the severe weather events, including some initial analysis, supplied by the University side. It was the merger of this database of severe weather events and local analysis experience on the University side with the over 25 years of local forecasting experience on the NWS side that helped make this project a success. Because of the initial data set, the NWS side had more time for data analysis.

One significant challenge was the issue of detailed analysis of severe weather cases throughout the State of California. It was determined that to seek out, acquire data, research synoptic/mesoscale features, and determine the forecast hints for 4 sections of California (for tornado, waterspout, funnel cloud, hail and damaging wind cases) was a huge task. We felt that the time would be better spent on a more comprehensive study of representative events in a more limited area, and the development of forecaster aids that can be used statewide.