A Look Back: 25 December 2011

Note: The following blog post on the multi-day snow event across West Texas last December 2011 was written by NWS Forecaster, Jeff Vitale from the Weather Service Forecast Office in Lubbock, TX. If you have an interesting event to share, please contact the CIPS Analog Guidance developers.

A rare multi-day snowstorm affected the South Plains of West Texas near Christmas time of 2011 in the Lubbock, TX CWA. When wildfire season is usually ramping up at this time, much needed snowfall fell across the region. However, the mean analog guidance products struggled with the evolution of the system because of the highly anomalous nature of this event. Confidence in the analog guidance was not high when first analyzing the mean fields and NWP forecast (GFS212 20111123/1200F048). Below are the 300-mb and 500-mb height fields.

There is a closed low in the NWP forecast at 500 mb and 300 mb while the analog mean shows a broader trough. When looking at the individual 300-mb and 500-mb height members, we can see that one event in particular was most notably different from the analog mean – 19971226/1800. The trough axis is much sharper and digs into the South Plains which is most similar to the NWP forecast.

Since confidence in the analog guidance is not high, there is not much to gather from the snowfall mean and exceedance probability graphics. No snowfall was depicted in these graphics even though what verified was much different. 

We can examine the date of 19971226 further by comparing it to the other analog events as well as analyzing the NARR data from that case. This date was the only analog to depict snowfall within the Lubbock CWA.

When analyzing the NARR, we can see that the 300, 500, and 850-mb height and wind fields were not exactly the same but were fairly similar. The NARR dataset shows a very sharp positively tilted trough extending into northwest Mexico at 300 mb and 500 mb. The GFS depicts a closed low and positively tilted trough roughly in the same location with the closed low directly over the Lubbock CWA. Winds at 850 mb were both out of the north with a trough over the Great Lakes and a ridge over the central Rockies.

The snowfall maps from 1997 showed a swath of snowfall with amounts up to five inches across southwest and central portions of Lubbock’s CWA. Below are the verification graphics for the 24-h period ending 19971225/1200 and 19971226/1200.

Verification maps from both events are shown below. Locations that received the heaviest snowfall were very similar. In fact, measurable snow has fallen at Lubbock, TX on Christmas Day only five times in the 100 year history of the station prior to 2011. Coincidentally, these two similar events occurred on the same day 14 years apart.

A MODIS image taken on the afternoon of 26 December 2011 is shown below. Snow is visible on the left side of the image with clouds on the right side.

The closed low at 500 mb took an unusual and slow track over West Texas which allowed for the large amount of snow to fall.

This particular event using the analog guidance highlights the important need to analyze individual analog dates in addition to the mean fields from the top 15 analogs during operational forecasting situations. Examining individual analogs is especially crucial for highly anomalous events (as in this example) where the mean analog impact guidance fields were not useful.

Storm data entries from both events are shown below.

1997: Heavy wet snow blanketed parts of the Texas South Plains this Christmas Day resulting in a relatively rare white Christmas. A swath of four to five inches of snow was measured generally from Brownfield, Morton and Levelland east-northeast to Abernathy and Floydada. The city of Lubbock generally received one to three inches of snow.

2011: In the throws of a historic drought, old man winter smiled on West Texas and brought a rare white Christmas to much of the South Plains and Rolling Plains. A strong upper-level low pressure system slowly pushed into western Texas Christmas Eve into Christmas Day, bringing snowfall and a rare white Christmas to the entire southern plains region. A solid 1 to 3 inches of snow fell across most of the region, with lighter amounts over the extreme southeastern Panhandle and eastern Rolling Plains. There was also a fairly large snow band that set up early Christmas Eve morning into the mid-afternoon that produced heavier snowfall across the western and northern South Plains. The greatest snowfall amounts occurred in an area stretching from Cochran and southern Bailey counties north and east through northern Hockley, southern Lamb, and southern Hale Counties, where accumulations of 6 inches or greater were common. The heaviest band of snow occurred along a narrow line stretching from Littlefield to Spade to Cotton Center where between 9-10 inches of snow was measured. Additionally, locally heavy snow showers developed across the southern Panhandle on Christmas Day and produced between 2 to 4 inches of snow across Parmer, Castro, Swisher and Briscoe counties, before tapering off later that evening. One final snow band developed early on the 26th and produced an additional 1 to 2 inches over parts of the southwest Panhandle into the northwest South Plains.

Acknowledgements: Mark Conder and Gary Skwira of the National Weather Service in Lubbock, Texas contributed to the verification graphics.