...February 5-6, 2008 Winter Storm Summary...


Yet another powerful upper level storm system plowed through the Missouri River valley this week.  This large storm system produced widespread snow accumulations greater than 6 inches in much of the region. In addition, it was this same powerful upper level storm system which spawned many severe thunderstorms and tornadoes across portions of Arkansas, Tennessee, Kentucky, Illinois, and Indiana Tuesday night. 

This powerful storm began to develop over the southwestern portions of the United States late Tuesday evening, spreading snow and rain over much of the western plains.  Ahead of this storm system, a cold front plunged through the state of Missouri, settling over the Ozarks by Tuesday afternoon.  Behind this cold front the first affects of this powerful storm were felt as a widespread area of showers and thunderstorms developed across Oklahoma and slowly crawled northeast.  This area of rain produced widespread heavy rain reports exceeding one inch over the area, breaking daily record rainfall amounts at St. Joseph (0.62 inches, previous record 0.27 inches in 1987) as well as at Kansas City International (1.04 inches, previous record 0.61 inches in 2004).   In addition, due to the time of the year, the top layers of the ground remained frozen. this heavy rainfall caused flooding of rivers and other flat areas across much of central Missouri. Below is a map of liquid accumulations from this event, which includes both rainfall as well as melted snow accumulations.

Upper Level Storm Track  February 5-6, Rainfall Map
 Click on the Images for Larger Versions


By Tuesday afternoon, as the storm system continued to rapidly deepen and track northeast, a large area of snow started to develop across portions of western and central Kansas.  It would be this large area of snow which would sweep across portions of northeastern Kansas and northern Missouri and bring large amounts of accumulating snowfall to the area.  Further to the east and ahead of  the upper level storm system, a weak secondary cold front arrived from western Iowa and Nebraska late Tuesday afternoon.  This front dropped temperatures which lingered in the lower 30s for much of the day, quickly into the upper 20s the evening.  Snow continued to spread eastward across much of northeastern Kansas and northern Missouri overnight, finally tapering off as the system lifted toward the Great Lakes region by late Wednesday morning.  When all was said and done, a large area of snow accumulations between 6 and 10 inches was reported along a line from St. Joseph, MO to Princeton, MO.  The highest snowfall total in the Pleasant Hill forecast area was found near the town of Ridgeway, MO where 12 inches of snow and drifts close to three feet were reported this morning.  Further to the north, portions of central and eastern Iowa as well as southern Wisconsin experienced even higher snow accumulations, with many areas reporting snowfall totals in excess of 12-15 inches!!!

  February 5-6, 2008 Snowfall Map
Click on the Image for a Larger Version



By looking at the snowfall maps above, you will notice a fairly defined cutoff to the highest snowfall totals.  This contrast in snowfall amounts was created when a large "dry slot" or a large area of dry air aloft worked into the storm system.  By definition a "dry slot" is a zone of dry (sometimes relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a storm system.  In the images below you can see two indications of a "dry slot".  The first image that meteorologists often use is a water vapor image which captures the water content (or amount of water) of the mid-layers of the atmosphere.   As you can see, and as denoted by the brown shaded area of the image, a large area of dry air moved into the system from the southwest and limited the snowfall over central Missouri.   However, enough low level moisture did exist in the atmosphere to create very heavy drizzle or freezing drizzle for much of the night.  Taking a closer look at the water vapor imagery you can see the rich amounts of moisture over the northern portions of Missouri indicating the ongoing large area of snow.  Another interesting feature resides from Louisiana to Ohio where you can see the very high moisture values where thunderstorms where developing and moving east.

Atmospheric Profile Plot for Pleasant Hill, MO
Click on the Images for Larger Versions


A second indicator that forecasters often use to diagnose the moisture content of the atmosphere is called a skew-t diagram, or simply a vertical plot of temperature, dew point, and winds at one point at a given time.  The image on the right hand side depicts a forecast atmospheric profile for Pleasant Hill, MO early Wednesday morning when some of the very heavy drizzle/freezing drizzle was falling.  Right off the bat you can notice a very large separation of the dew point and temperature profile, which is indicative of dry air above the ground.  However, notice where the lines are very tight together towards the bottom of the chart.  This tight gradient shows where the atmosphere is saturated and if enough lift is provided in this layer of the atmosphere precipitation, could be squeezed out. In our case at Pleasant Hill and across much of central Missouri, there was a significant amount of lift and moisture in the lower levels of the atmosphere causing the drizzle.  However, just as the water vapor imagery showed, the large layer of dry air in the remainder of the atmosphere prevented snowfall from developing.  If this atmospheric profile were saturated higher up in the atmosphere (temperature and dew point lines close together) at this time, a much higher probability of snow would have existed.

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