Why was there less snow than predicted at some locations with the winter storm?
19 - 20 January 2007
Above is an image displaying the approximate snowfall totals from Friday, January 19th through Saturday, January 20th (Ron McQueen).
Many people across the South Plains woke up Saturday morning to a snow-covered ground, but not nearly to the depth expected. A number of factors came into play that complicated the “phase” of wintry precipitation throughout the event from early Friday into Saturday. These factors resulted in significantly less snowfall as a persistent and narrow transition zone developed from Plains through Lubbock to Floydada where sleet and freezing rain fell.
The image above shows how the storm system "sat" nearly stationary over the northern Baja Peninsula until Saturday morning, when it finally began moving northeast. Click on the image for a larger picture.
The storm system that was expected to produce all of the snow Friday and Saturday was very slow to move east from the Baja California region. The center of the storm system stalled over Baja for almost 36 hours late Thursday to early Saturday. Influences of the storm were still able to produce widespread precipitation over the South Plains well in advance of the main low pressure center. With the storm’s approach to the region delayed, however, the deep layer of cold air needed to support all snow over the central South Plains was lacking.
|Above is a water vapor satellite image of a large plume of moisture (noted by the large arrow) streaming over West Texas from the eastern Pacific ocean. The abundant moisture was a critical ingredient in the production of widespread heavy precipitation across the area. Also depicted is the upper level storm system (centered at the large red "L") that pumped all the moisture over the region. The image was taken during the afternoon of the 19th. Click HERE or on the image for a larger view.|
The slow storm movement and relatively warm air was accompanied by an intense plume of subtropical moisture streaming from the eastern Pacific northeastward over west Texas. Within this zone, or “moisture plume”, the air’s moisture content was more than 300% of normal for mid January. It was this moisture that led forecasts to conclude that precipitation efficiencies would be very high during the storm, and in fact they were. Lubbock recorded a storm total liquid equivalent precipitation of 1.07 inches. Snow typically measures with a 10 to 1 ratio with water. Therefore had the precipitation that fell in Lubbock been all, or mostly snow, it would have measured around 10 inches.
The depth of the moisture and the slow track of the storm system ultimately combined to allow a shallow layer of above freezing temperatures between 5 and 10 thousand feet above the ground to move into the area. Since warm air is lighter than cold air, this fetch was able to glide up and over the cold air that was already in place. Forecasters actually saw potential for the warm air to enter into the system as early as Thursday morning and some computer forecast models agreed that this would happen. Despite this, all indications suggested that any warming would be short-lived and the atmosphere would quickly cool as a result of strong lift associated with the approaching storm center. In reality, the lift was insufficient to eliminate a warm layer which ended up being stronger, deeper and longer lasting than the forecasters expected.
|Above is a schematic that explains why freezing rain and sleet forms when snow was falling from high in the clouds and temperatures at ground level were below freezing. The scale on the left is the height above ground in kilometers. The scale on the bottom is in degrees Celsius. The red line is a plot of the temperature with height. Note the shallow layer above freezing located slightly above the ground (right of the OºC line). This "warm" layer melted the snow as it fell through it. Then, if there was enough cold air below the "warm" layer, the liquid water droplets would refreeze and fall to ground as sleet. Otherwise, if the cold air below the "warm" layer was insufficient to refreeze the liquid water drops, they would fall to the ground as freezing rain. Click HERE or on the image for a larger view.|
A series of weather balloons were launched from Reese Center late Friday to monitor the storm’s evolution, courtesy of the Texas Tech University West Texas Mesonet. The balloons carried instruments that measured temperatures within the overriding warm layer to be 4 degrees above freezing. Snow flakes that were descending toward the ground melted as they passed through the warm layer. Since temperatures at the ground were below freezing, the predominant form of wintry precipitation was freezing rain from early Friday morning through middle to late morning on Saturday. Some of the rain droplets actually refroze and landed on the ground as sleet. See the above figure for a visual representation of the atmosphere that was in place to produce the freezing rain and sleet. Additionally, the chart below gives a timeline for the National Weather Service forecast and verification of the entire winter storm event.
Above is the National Weather Service (NWS) forecast and verification timeline for the 19-20 January 2007 winter storm in Lubbock, Texas. Click HERE or on the image for a larger view.
|Our final image shows how the snowfall totals compared to the two warnings issued by the Lubbock office Saturday morning. Click on the image for a larger view.|