Weather

Frost dates

I am planning my first vegetable garden. How do I find the average last spring and first fall frost dates?

Missouri Average Date of Last Spring Frost and Missouri Average Date of First Fall Frost show maps of Missouri with average frost dates

Please note that frost dates can vary significantly from year to year and temperatures can be highly variable within small distances due to topography. Minimum temperatures can vary as much as 10 degrees Fahrenheit from the bottom of a valley to a nearby hilltop. Cool air, being denser than warm air, moves down the slopes of hills, accumulating in the valleys. This is why low lying areas, such as river bottoms, will likely be colder than their surroundings on clear, calm nights. Therefore, while referring to the maps, please consider your local landscape.

Pat Guinan
Missouri state climatologist

Drought history

I've heard a lot about the Southwest Missouri drought lately (2005 and 2006) and want to know more about the current situation as well as drought history for the area. How bad is the situation?

Currently, southwestern Missouri is experiencing the driest conditions in Missouri and many people are asking if the drought will worsen or weaken this spring. The latest outlook, issued by the Climate Prediction Center, reveals a lot of uncertainty in terms of the expectation of rainfall for Missouri. Currently, the outlook for spring calls for equal chances of above, below, and near normal precipitation in Missouri. With this type of forecast, the best forecasting tool to use is climatology.

Climatologically speaking, spring is the wet season for much of Missouri. April precipitation is more than double what usually falls in February and May and June are the wettest two consecutive months for much of the state. In fact, for southwestern Missouri, nearly a third of the normal annual rainfall occurs during April, May and June. So, climatologically, the best opportunity to put a dent in this drought will come during these three months.

This year, as like most years, it is very difficult to know exactly how much rain will fall this spring. Inevitably, warmer temperatures with longer days will arrive which will increase the evaporation rates over bodies of water and soils, but rainfall averages are also highest during this time of year.

The current drought began evolving over southwestern Missouri in March of 2005 and has persisted and worsened over the last 12 months. Since March, many counties in southwestern Missouri are experiencing deficits in excess of 18 inches.

For perspective, 2005 was the driest year in southwestern Missouri since 1980 and, according to climatology, there have been drier periods. The worst drought on record for southwestern Missouri occurred over a five-year period, 1952 to 1956.

Drought is and always will be a recurring phenomenon in Missouri. Over the past few years, Missouri has been dealt a reality check in regard to drought. The decade of the 1990's was a relatively benign period and ranked as the wettest decade on record for the state. So far, in the current decade, parts of Missouri have experienced severe drought conditions during the growing season in 2000, 2002, 2003 and 2005. In my opinion, despite the recent droughty conditions, it is impossible to determine what the rest of the decade has in store for Missouri in regard to drought.

Using climatology, I can only provide a general idea of drought frequencies for the state. In an article for the September 2002 Missouri Ruralist I wrote, "Wayne Decker, professor emeritus, of the University of Missouri 's Atmospheric Science Program, examined the frequency of occurrence of various drought intensities in Missouri using the Palmer Drought Index. He found the probability of extreme drought to be 8 percent, or one year in 13, and the probability of moderate drought to be 15 percent, or one year in six to seven years, and mild drought to be 30 percent, or once in three years.

"Using Columbia as a "typical" mid-state location, 112 years of precipitation observations give us an indication of typical local area meteorological drought probabilities during the heart of the crop growing season. If we consider 60 percent of normal for the three-month precipitation total for June, July, and August as defining a moderate drought, 112 years of precipitation records show that such conditions occurred in 17 of the past 112 years, a probability of 15 percent, or once in seven years.

"Shorter-term drought conditions triggered by less than 60 percent of normal rainfall during the two-month period of June and July occurred during 24 of the 112 years, a probability of 21 percent, or once in five years."

Drought information from the Missouri Department of Natural Resources

Pat Guinan
Missouri state climatologist

Updated 5/15/08