James A. Reynolds
NWSO El Paso, TX/Santa Teresa, NM
In this study, composite mapping was used to help identify synoptic patterns associated with past significant snowfall events that had occurred over the El Paso County Warning Area (CWA). An event was considered significant if one inch or more of new snow was analyzed over lower elevations in the El Paso CWA.
The data used to compile the composites of this study were obtained from a CD-ROM entitled "NMC Grid Point Data Set" which was prepared by the University of Washington (Mass 1987).
The NMC Grid Point Data Set contains data for different significant levels beginning in 1946 with data ending in 1989. Additional software used with the CD-ROM, NMCDraw, allows users to retrieve and view gridded data from the CD-ROM, along with producing composites. A 1995 National Weather Service research project using composites to identify synoptic patterns associated with significant rainfall events in San Diego (Atkin and Reynolds) has shown the usefulness of this technique.
To determine which dates had significant snowfall events, NOAA Daily Weather Maps were examined at a local library. Maps were available from 1973 through the present day. However, since the NMC Grid Point Data Set does not contain data beyond 1989, this study was limited to the years 1973-1989. Thirty-one days having significant snowfall events (i.e. one inch or more), between November and April, were identified in this time frame.
The 500 mb and 850 mb charts associated with the selected dates were then viewed using NMCDraw at 1200 UTC the day prior to the snowfall measurement and 1200 UTC the day of the measurement. This process was performed to assure that data for all of the dates could be accessed from the NMC Grid Point Data Set CD-ROM. Maps from two of the dates were inaccessible and these dates were excluded from the study which left a total of twenty-nine cases (Table 1).
Dates of Significant Snowfall
|* Denotes 6 inch or greater snowfall events.|
Further analysis of the twenty-nine individual cases showed that the cases could be sorted into broad synoptic categories for comparative purposes. Cases were grouped so that comparisons could be made between one inch snowfall events versus six inch and greater events, events that involved a 500 mb closed low versus events that involved a 500 mb long wave trough, and events with 850 mb temperatures below 0 C versus events with 850 mb temperatures of 0 C and above. Composite maps were then produced for all of the categories at 1200 UTC the day before each event and at 1200 UTC the day of the event. Additional composites were also produced that included one grouping of all twenty-nine dates in this study.
While composite maps were created for all of the different synoptic categories, numerous similarities were noted in the different composites. Because of this, results are demonstrated using the composites which included the singular grouping of all of the dates used in this study.
Figures 1a and 1b show the 1200 UTC composite maps the day prior to the snowfall measurement at the 500 mb and 850 mb levels, respectively. The 500 mb chart (Fig. 1a) shows a positively tilted trough over the southwest United States with the base of the trough over Arizona. The average height over the El Paso County Warning Area is 558 dam and this is generally found in all of the composites. The most notable difference from this pattern is found in the composite which includes only those events involving a closed low (not shown). In that instance, the positively tilted trough contains a closed low over Arizona with a central height of 549 dam. Heights over the El Paso CWA, however, remain near 558 dam. The 850 mb chart (Fig. 1b) shows a closed low over the southeast Arizona and northwest Mexico border with a central height of 146 dam. A ridge of high pressure also extends over the Pacific Northwest down into the southern Rocky Mountains. The closed low and the ridge of high pressure are evident in all of the individual 850 mb composites. The average 850 mb temperature over the El Paso CWA is around 1 C with a thickness of 410 dam between 850 and 500 mb.
Figures 2a and 2b show the 1200 UTC composite maps the day of the snowfall measurement at the 500 mb and 850 mb levels, respectively. The 500 mb map (Fig. 2a) shows that the base of the trough has moved eastward and is now over southeast Arizona and southwest New Mexico. The trough is exhibiting signs of filling and has a more relaxed height gradient. The 850 mb map (Fig. 2b) shows that the closed low over southern Arizona has opened up, filled somewhat, and moved south-southeast into northwest Mexico. This was also apparent in the majority of the other broad synoptic category composites. As with the low, the ridge of high pressure from the Pacific Northwest has shifted south, but with little, if any, difference in strength. The average 850 mb temperature over the El Paso CWA at this time is near -2 C, again with a thickness of 410 dam between 850 and 500 mb.
Further analysis of the composites indicates that the amount of cold air in the lower levels associated with each system is a major factor in producing snowfall events where six inches or more of snow fell. The composite map at 850 mb for the six inch events (not shown) shows stronger ridging than the other composites which is aided by larger amounts of cold air in the lower levels. Indeed, all of the dates that were used to create the six inch and greater events composites were also included in the composites for systems with 850 mb temperatures less than 0 C. Stronger thermal gradients are also apparent at this level which would indicate the possibility of stronger baroclinicity within these systems which would thereby lend towards greater snowfall production.
Composite mapping has made it possible to identify the synoptic patterns associated with significant snowfall events over the El Paso CWA. The composites in this study show the positioning of a positively tilted trough over the southwest United States. The amount of cold air in the low levels appears to play a major role in determining the amount of snow that will be received for any given event.
The composite maps in this study should not be used for prediction, but should only be used to provide an indication for the potential of receiving significant snowfall. It would be necessary to determine the number of "null" events, if any, with a similar pattern that did not produce significant snowfall events before these composites could be used as a forecasting aid. This was not done for this study.
It should be noted that relative humidity and vertical velocity data are not included on the NMC Grid Point Data Set CD-ROM. As it was, sufficient moisture and atmospheric lift for significant snowfall was assumed to be present.
Software for data retrieval from the CD-ROM and compositing was developed by Mark Albright of the University of Washington. The graphical display and data retrieval software (NMCDraw) was developed by Richard LeBlang, WSFO Bismarck. Maps indicating precipitation amounts were found in "Daily Weather Maps" distributed by the Climate Prediction Center Washington, D.C. Thanks to Val J. MacBlain, NWSO El Paso, for providing suggestions and insights that led to the production and improvement of this paper. Thanks to Timothy S. Brice, NWSO El Paso, for help in the editing and printing of the various composite maps.
Atkin, D. V. and J. A. Reynolds, 1995: Composite Maps of Selected Rainfall Events in San Diego. Western Region Technical Attachment No. 95-06.