OVERVIEW

The region of Missouri and western Illinois is located in the Midwestern United States near the geographic center of the United States. Being near the center of North America, the region is exposed to large temperature swings across the seasons. The day-to-day weather is often just a matter of which way the wind is blowing. The expansive Great Plains of North America offer little protection from very cold air masses which occasionally rush in from the Arctic during the winter months. Warm, dry air from the deserts of western North America can bring periods of much warmer weather, while moisture from the Gulf of Mexico brings humidity and influences most of the region’s precipitation. Like much of the central United States, thunderstorms are a common occurrence especially in spring and summer. Sometimes these can be severe with large hail, damaging winds, and tornadoes.

Located near 40 degrees North latitude, the region is located near the Westerlies, an area of predominantly westerly wind flow in the upper atmosphere. Within the Westerlies lies the Jet Stream which can meander north and south, driving the movement of pressure systems. In addition to providing sometimes wild swings in temperature, this also helps distribute precipitation relatively evenly throughout the year. The region can experience some prolonged periods of stagnant weather sometimes leading to Heat Waves or drought. According to the Koppen climate classification, this region lies at the border of the Humid Continental and Humid Subtropical climate zones, a reflection of the large seasonal variations in temperature with precipitation relatively frequent throughout the year.

Geographically, the landscape is dominated by large rivers, with the Mississippi River, the largest in the country, setting the boundary between Missouri and Illinois. The Missouri River, the longest in the country, merges with the Mississippi River just north of St Louis. Another large river, the Illinois River, merges with the Mississippi River just upstream of the Missouri confluence near St Louis. These rivers each have large flood plains, expanses of very flat land near the river that can flood during periods of high river flows. The region has some of the lowest elevations in the Midwest with St Louis at about 600 FT above sea level. While much of Illinois and northern Missouri consist of flat prairie and farmland, southern Missouri is dominated by the heavily-forested Ozark Mountains. This is one of the most prominent mountain ranges east of the Rocky Mountains and west of the Appalachians. The Ozarks rise to elevations well above 1,000 feet, with Taum Sauk Mountain, the highest point in Missouri, rising to 1,772 feet. These hills and valleys can lead to local variations in temperature and precipitation, but they can also affect the region more broadly. A wind blowing across these mountains leads to so-called “downslope” winds downstream which act to warm and dry the air. This is most common with southwest winds which can cause the St Louis area to experience the warmest temperatures in the region.

Another geographic feature which affects the climate of the region is the urban heat island. The large metropolitan area around St Louis contains a high concentration of paved areas and buildings which can cause a build up of heat. This is most common during calm or stagnant weather, particularly at night. Temperatures in the urban area can be 5 to 10 degrees warmer than nearby rural areas, sometimes making the difference between a cool night or a freezing night. During summer heat waves it can lead to several days during which the temperature does not fall below 80 degrees at night, exacerbating the effect of the heat in buildings without access to air conditioning. Other local urban heat islands exist, but are not as pronounced as in the St Louis metro.

The National Weather Service has 3 primary climate observing locations in the region at St Louis and Columbia in Missouri and Quincy in Illinois, each with more than 100 years of records available. Weather data is currently collected from automated observing systems at the airports, with supplemental snowfall observations at St Louis and Columbia. The period of record for observations at each location is:

St Louis (1874-present)
Columbia (1889-present)
Quincy (1900-present)

WINTER

Winters are cold overall, but with a high degree of variability from day to day. The frequent passage of pressure systems in the Westerlies brings alternating periods of much colder weather or periods of mild weather, along with occasional wintry precipitation. The coldest air masses can feature temperatures well below zero degrees. Periods of extreme cold are often short-lived, although they can occasionally last for weeks at a time. Alternatively there are typically many days even during the heart of winter which warm to 60 degrees or higher, sometimes even 70 degrees. These alternating warmer and colder periods make the weather very inconsistent with the greatest variability in temperature observed during the winter months. January is the coldest month of the year on average, but this can vary from winter to winter.

Winter is the driest season overall in terms of total precipitation. Precipitation can come in any form, with snow, sleet, and freezing rain occurring in most winters. Along with the wide variation in temperature during the winter, there are also large variations in snowfall from one winter to the next. Most winters see multiple instances of snowfall of at least 1 inch or more, with snowfalls of 6 inches or more occurring once every year or two. Snow may linger on the ground for a few days to a few weeks, although the frequent bouts of more mild weather ensure that snow does not last on the ground through the whole winter.

SPRING

Spring also frequently sees variable weather conditions, but with the overall temperature warming through March and April such that winter cold and snow becomes increasingly rare. Warm, dry air from the desert southwest and moist air from the Gulf of Mexico become more dominant as cold Arctic air masses become less frequent. The last freeze of the season usually occurs in early April, although this can vary greatly from year to year based on weather patterns, and from location to location due to topography and land use differences. The last freeze has occurred as early as the first part of March or as late as the first days of May. Occasional bouts of hot weather become increasingly common as well. The first 90 degree day of the year typically occurs in mid May to early June, but has occurred as early as March or as late as July.

Spring is also the wettest time of year both in terms of total precipitation and in number of rainy days. Normally between a third and a half of days record measurable precipitation in spring. The combination of increasing Gulf of Mexico moisture and continued strong Westerlies occasionally leads to the ingredients for outbreaks of severe thunderstorms. This is the most likely time of year for supercell thunderstorms with large hail and tornadoes. Although “Tornado Alley” is typically defined further west in the Plains, tornado outbreaks also occur in the Mississippi Valley region. The St Louis metro has been hit by damaging tornadoes many times.

For more information on frost/freeze climatology go to our Frost/Freeze Page

SUMMER

Summers are warm to hot, with the most consistent temperatures of the year observed in this season. The Westerlies shift to the north leading to more stagnant weather patterns as strong cold fronts become rare. Most days warm into the 80s and 90s, but the hottest air masses can feature temperatures well above 100 degrees. During periods of high humidity, nighttime lows may not drop below 75 or even 80 degrees, especially in urban areas. While heat waves can bring several days to a week or more of extreme heat and humidity, there are also some periods which see much more pleasant temperatures only warming into the 70s even in the heart of summer. July is the warmest month on average, but this can vary from year to year.

Precipitation in summer most commonly comes in the form of thunderstorms. Due to the nature of these storms, they can drop very heavy rainfall in a short amount of time in localized areas, leaving other areas dry. Flash flooding can occur in the heaviest and most persistent thunderstorms especially when humidity is high. Severe thunderstorms become less common as the summer goes on, with damaging winds becoming the dominant severe weather threat. These severe thunderstorms often take the form of squall lines which can bring damaging winds to large swaths of the region, sometimes even at night. Because weather patterns can become stagnant in the summer, and precipitation falls mostly from hit or miss thunderstorms, periods of drought can occur. While average precipitation through the summer decreases only slightly from spring, it can be much more variable with longer periods of dry weather not uncommon.

FALL

Fall begins the gradual transition away from summer heat as more frequent cold fronts bring in cooler air masses. The last 90 degree day typically occurs in mid September, but has occurred as early as August or as late as October. The first freeze typically occurs in October, but can occur as early as September or as late as mid November. With more frequent cold fronts, the weather tends to become more variable deeper into fall, although bouts of more stagnant weather patterns can occur especially in early fall.

Overall precipitation in fall is typically less than summer, especially in terms of the number of rainy days. September often has the fewest rainy days of the year with rain typically falling on only one quarter of the days. But when rain falls it can occasionally be heavy. Fall is the most common time for Tropical Storms and Hurricanes to batter the coasts of the Gulf of Mexico. The remnants of these systems can move through the region bringing very heavy rainfall. During more stagnant weather patterns especially in early fall, extended periods of dry weather can last for a few weeks.

For more information on frost/freeze climatology go to our Frost/Freeze Page

SUMMARY

Overall the climate of the region is most notable for its variability. Although there are strong differences across the seasons, the coolest summer days can be just as cool as the mildest winter days. The growing season is long, typically around 200 days, with precipitation distributed throughout the year. The following tables provide some yearly statistics based on the most recent climate normals period, 1991-2020.

The following tables display some notable all time records at each location: