With your head tipped back and mouth wide open, you can catch a few snowflakes on your tongue. These seemingly innocent frozen works of art can be a complex challenge to forecast and produce enough volume to cripple entire cities in a matter of hours.
Pinpointing the exact location of the concentration of snow can be a matter of life and death, especially when it comes to forecasting for aviators. The National Weather Service sites a banding event that cause a plane to skid off a runway, killing one person December 8, 2005 is clear evidence of the potential danger involved in storms. The all-day snow storm spread from northeast Illinois to northwest Indiana, swallowing Midway Airport just outside of Chicago. Up to six inches of snow was dumped in the region, but one narrow band from the airport west into northern Will County received record accumulation of 11 inches during the same storm at about three inches an hour. According to weather reports at that time, a well defined upper level cyclone moved across the Missouri River into southern Iowa with an imbedded jetstreak. Surface low was in the south near Tennessee and inverted trough near eastern Iowa. Satellite loops showed band striations across NE Illinois and cooling cloud tops.
A tongue of enhanced saturated theta-e air was being drawn into northeast Illinois and northwest Indiana. By late afternoon, large snowflakes are falling, indicating some lake effect moisture advecting into the system, but other mechanisms were more influential. Temperatures between 600 to 800 mb were between -12 C to -18C, which is right in the target range for perfect snowflake growth. The southbound plane was heading right into the heaviest snowfall.
The National Weather Service (NWS) leaders came to the conclusion that focused snowfall created by enhanced snowbands over this major airport was a contributing factor to the fatal crash. They found that the three inch per hour snowfall an hour before takeoff may have made it tough for crews to get the snow off the runway.
MAKING SNOW BANDS
John Kowaleski, NWS Meteorologist in the Grand Rapids Michigan office says banding occurs in almost every storm system and can contain rain or snow. “Banding can occur any time of the year. People are more concerned about winter banding because of the heavy snow accumulation. Two inches of snow in an hour are more noticeable than two inches of rain in an hour,” he says.
Snow banding is a concentration of falling snow over a three to six hour period in a 20 to 50 mile area. Usually, he says the systems are oriented west to east, moving north. The convergence of winds cause the precipitation to spread into a band. Since it evolves from a synoptic snow event, Kowaleski says it can happen any where in the Midwest. He says his weather team anticipates banding, but it is tough to pinpoint the exact location of the heaviest snow fall.
NOT LAKE EFFECT SNOW
Mesoscale snowbanding is very different from lake effect snow. Lake effect snow is a local event caused by cold air crossing over warm bodies of water. They are also unique to areas with large lakes and coastal regions. Mesoscale snowbanding is a small part of a synoptic scale snow event. The large event could cross over three states, with small snow bands stretching across one county. Banding isn’t affected by surface features like mountains or hills. Uplift from those local structures create lift, but not enough for banding to occur.
FORCED SET UP
The first signs of snow banding start in the southwest. Kowalski says a synoptic frontal system can start in Texas and move northeast. As it moves across the Midwest, meteorologists are continuing to monitor the mid level layers, paying close attention to the isobars at the 700mb level. “Usually, low pressure with a cold front will take place,” he says. “We often see it occur on the north side of a low pressure system when the low is tracking to the east.”
The isotherms start to tighten over a six hour period. According to Kowalski, the pressure increases as the temperature gradients get closer together. This process of it tightening is called frontal forcing. This pressure gradient becomes stationery over one location and snowfall is intensified.
Check relative humidity numbers. If they are near 90 % and strong lift is present in the mid layers, your next step is to check temperatures. The most important region to focus on for possible snow banding is where peak dendritic growth could occur. Temperatures between -5C and -30C are most conducive to snow crystallization, according to California Institute of Technology Professor Kenneth Libbrecht.
Models will track the temperature gradients throughout the system. Kowaleski says, “The general direction of the banding can be predicted by checking the system’s trajectory and how it is laying. They are usually laying east to west. Propagate that trajectory further to the east, then see where it would line up across the region”.
TEMPERATURE AND TROWAL
Richard Pollman, Lead Meteorologist at the National Weather Service Office in Detroit says strong convection is the basic building block for frontogenesis forcing. “You might have a trough of warm air aloft within and stacked occluded system combining with TROugh of Warm air ALoft (TROWAL). All of these things work together to create instability.” Pollman also notes that it is important to consider Conditional Symmetric Instability where the movement of the parcel slanted, rather than straight up.
“Storms occluding on top of you are in the deepening phase, causing a deeper storm than one that occludes to the left of you,” Pollman says. His forecasting team looks for warm or occluded fronts as the first building block for frontal forcing. He says the extreme forcing occurs because the convergence and lift is over a concentrated area. “Lifting the moist, unstable air with added lift from convection helps the parcel to precipitate out,” he adds.
TROWAL is defined by Professor James T. Moore of the Cooperative Institute for Precipitation Systems at St. Louis University as a 3D sloping intersection of the upper cold front portion of the warm occlusion with the warm frontal zone. It is a wedge of warm air on top of cold air. He points to the exact location of TROWAL is usually along the ridge of high Theta-e values. It is the cyclonic portion of the warm conveyor belt that wraps around to the NW of the cyclone.
Pollman says, “TROWAL is between 700- 500 mb level. Frontal forcing is usually stretched from the surface up through to the 500 mb level, but most commonly found around 850 mb. It is a ridge of theta e extending into the cold sector of the storm. You can find it by looking at the 500-700 mb plan view of theta e. Trowel does enhance the lift, like summertime CAPE. And often, the TROWAL and forcing is accelerated within this winter instability. He says it is hard to forecast. “Usually there are three or four f-gen bands, but only a couple of them really come active.”
This banding can result in a blown forecast. Meteorologists may be able to forecast the large scale synoptic event, but locating the meso bands is more difficult until the storm is about two hours away. Weather balloon measurements may miss the entire event. This is another cause for missing the banding effect. Since balloons are released only every 12 hours, the banding event could have occurred in the middle of the launchings and go undetected until it shows up on radar. Meteorologists may have forecasted two to three inches of accumulation, when the stationary frontal forcing caused over six inches to fall. Pollman says there isn’t one specific thing that can be done to accurately predict where the heaviest snow fall will be. He says, “ You have to keep monitoring them on radar hour by hour.”
4. Professor Kenneth G. Libbrecht, Physics at California Institute of Technology
5. Professor James T. Moore, Cooperative Institute for Precipitation Systems, St. Louis University.