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
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 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
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
“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.