Freezing rain is a weather occurrence that is not a pleasurable one. Sheets of ice blanket everything the rain hits making the roads we travel almost impossible to drive on. Lives are at an increased risk for the people who dare to travel in these extreme conditions. Freezing rain can also cause downed power lines leaving people without power for weeks as well as downed tree limbs causing millions of dollars worth of property damage. Knowing the atmospheric environment for freezing rain and common scenarios that are prone to producing it will help us better our forecast accuracy to alleviate some of the damage it causes as well as danger to the lives of people. Columbia, SC, on average, endures a freezing rain event once a year and usually hits during the late winter to the early springtime months. This is the time when pressure systems progress across the area creating the perfect environment for producing freezing rain. The common setup starts off when a North Pacific Low or even an Alberta Low pushes in from the Midwest. These systems usually bring in a good push of cold air behind their associated cold fronts that advect across the region. As the systems swing through the southeast region of the United States, surface temperatures drop below or near freezing most of the time. The systems then push out into the Atlantic Ocean and a stationary boundary develops along the gulf coast separating the colder airmass north of it from the warmer airmass to the south. These stationary boundaries can linger in place for days depending on the pattern of the highs and lows in the atmosphere. Next, an upper level shortwave trough follows the upper level flow and dips down into the base of the longwave trough that is normally situated, by this time, over the eastern United States just after a low pressure system exits the United States. As a shortwave trough becomes superimposed over the stationary boundary, cyclogenisis occurs creating what is known as a “gulf low” due to the region it develops in (in this case, the Gulf of Mexico). This is the second stage to the setup of the common freezing rain events that occur over the Carolinas. As the gulf low follows its projected path, the associated warm front pushes north-northeast into the Carolinas advecting in the warm moist air from the Gulf of Mexico. With cold air in place over South Carolina, the warmer air from the gulf rises over the denser air and creates a warm layer in the low levels. As we all know, a warm front is a stable front that creates continuous precipitation. As the precipitation falls, it first starts out as snow in the mid levels (< 0 deg C), then falls through the warm layer (> 0 deg C) which must be thick enough to melt the snow completely. Once through the warm layer, the precipitation falls through a cold layer (< 0 deg C) near the surface which must be shallow enough not to refreeze the precipitation but thick enough to super-cool the water droplets. By definition, freezing rain is super-cooled water droplets that freeze on contact with a surface, thus the surface temperature must be freezing to produce freezing rain. According to Chris Robbins’s and John Cortinas Jr’s journal on freezing rain, there are four conditions that are usually present in the areas that freezing rain occurs. They are, “ 1) nearly saturated air located near the ground, 2) midlevel upward vertical motion, 3) a deep low-level warm layer ( 1.3 km), and 4) a shallow ( 600 m) subfreezing surface layer” (63). A rule of thumb devised by the meteorologist at the Air Force Weather Agency suggest that if the warm layer is greater than or equal to 1200ft and the cold layer at the surface is less than 1500ft, then forecast for freezing rain (1-34). Below is a good example of what a skew-t might look like if freezing rain was a possibility. The Department of Atmospheric Science at the University of Illinois explains, “Freezing rain is one of the most difficult events to forecast”. I agree with that as well as the reasons why they believe it to be. The smallest difference in temperature or the smallest thickness difference in a cold or warm layer can change the outcome of what should be freezing rain to other precipitation such as snow, ice pellets, sleet or rain itself. Also freezing rain occurs in a small band that is usually no wider than 50km making it even harder to accurately forecast (University of Illinois). The specific parameters that are needed to produce freezing rain are what make it so difficult to determine if it will occur or not. Other environmental variables can play a role as well which can affect the outcome of freezing rain events. The two most common known variables that need to be taken into account when forecasting is soil temperature and evaporational cooling. I have seen both variables play a role that has dramatically caused a forecast to be inaccurate for my forecast area. Back in January 2004, a West Gulf Low developed and moved up into Georgia. A cold air wedge was in place over the Carolinas as the warm front, associated with the West Gulf Low, lingered across southern South Carolina. The forecast was for rain as the surface temperature was not expected to reach freezing. Overnight, light rain and drizzle began causing evaporational cooling, thus the surface temperature dropped 4- 5 degrees Celsius across South Carolina as latent heat was absorbed. This drop in temperature was enough to reach the needed 0 degrees Celsius at the surface. By morning, people woke to see an unexpected ¾ to 1 inch layer of ice coating everything. A forecast event that was supposed to be rain turned into freezing rain due to a variable that was not taken into account. Another event occurred in March 2007 where the soil temperature played a role in the forecast. This time, an East Gulf Low developed and pushed into the area. The setup was very conducive to produce freezing rain, but one thing that was not taken into account was the seven consecutive days previously that hit daytime highs of 70 + degrees Fahrenheit. The air temperature was freezing but the soil temperature was not causing the super-cooled water droplets to remain liquid after contact with the surface. These were two events that were lessons learned on my part and conveyed how important it is to evaluate all the atmospheric variables. Forecasting for freezing rain in Columbia, SC may be difficult, but if a forecaster examines and analyzes all the data that is available, a good forecast can be developed. There are many tools available to aid in putting together a forecast where potential freezing rain exists. These tools include: MOS guidance, meteograms, skew-t’s, model forecast charts, METAR data and radar. When forecasting, a good start would be looking at the synoptic-scale environment to determine if the setup is conducive to produce freezing rain. If it is, next, look at the models to see where the warm air advection is taking place. On average, here in Columbia, SC, a warm layer developed by a warm front will be at a height somewhere between 2000 - 3500 feet. By knowing this, a good rule of thumb to use in finding a specific area that is likely to produce the event is to find the 0 deg C line on both the 925mb (2500ft) and 850mb (5000ft) for the forecast times you are expecting the event. Once you have found those, overlay the placement of the two lines on a U.S. map. If the 925mb line runs above or north of the 850mb line, then that is likely the area that will produce freezing rain. The logic behind this allows you to identify the area where the warm layer is. Once the area is determined, analyze the forecasted skew-ts in that region that was identified to see if the parameters associated with temperatures and cold/warm layer thicknesses are conducive to producing freezing rain. Also, utilize radar and upstream observations to help track freezing rain and time the event in the forecast area (if the event had already started upstream). Weather is a science that will never be perfect in forecasting due to the smallest changes that affect the outcome of a given day. Freezing rain itself is the hardest to forecast for due to the small band that it develops in and the many parameters that can affect the type of precipitation that can make forecasting it more complex. By knowing the atmospheric environment and synoptic-scale setups prone to producing freezing rain here in Columbia, SC, the more accurate the forecast will be. Giving a good heads up to the public will save lives, property, and alleviate many inconveniences caused by a busted forecast. Works Cited Robbins, Chris and John Cortinas Jr. “Local and Synoptic Environments Associated with Freezing Rain in the Contiguous United States.” Weather and Forecasting 17.1 (2002): 47-65. 13 Apr. 2008 WW2010. Forecasting Freezing Rain. Depart. of Atmospheric Science. University of Illinois atUrbana-Champaign. 12 Apr.2008. http://ww2010.atmos.uiuc.edu/(Gh)/guides/ mtr/cld/prcp/zr/fcst/fcst.rxml ir Force Weather Agency. “Meteorological Techniques.” Air Force Manual 98/002 (1998): Ch.1 33-34. 13 Apr. 2008 < http://www.novalynx.com/manuals/tn98002.pdf> |