Frost Climatology in Illinois

frost2It is that time of year to discuss the nemesis of every gardener out there – the late spring frost or freeze. We have a full suite of maps showing the earliest, 1 in 10 years, median, 9 in 10 years, and latest frost (32 degrees) and freeze (28 degrees).

One of the things you notice about this map, and all the others in the series, is that even though the date for the last spring frost/freeze gets later the farther north you go, you can get big differences between nearby locations. The reason for that is that most of these last frosts occur during calm, clear, cold nights. As a result, subtle local features can make a big difference. For example, cold air can settle in low-lying areas, causing frost pockets. Or locations out in the countryside may be just a few degrees cooler than in town, due to warm buildings, that may make them vulnerable to a later frost. Also, keep in mind that we measure the air temperature at 5 feet above the ground while the ground and some objects may cool to lower temperatures on calm, clear nights. Therefore, you may see frost even if the temperature is reported as 34 or 35 degrees.

Personally, I use these maps as a general guide. However, I watch the latest weather forecasts closely for a few weeks after I have new plants in the ground for any surprises. The NWS does put out hard freeze warnings (temperatures at or below 28 degrees with or without frost), freeze warning (temperatures at or below 32 degrees with or without frost), and frost advisories for widespread frost.

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New Forecast for the 2014 Growing Season

Today the NWS Climate Prediction Center (CPC) released their forecast for May and beyond. There is nothing much to report for Illinois. We are in the area labeled EC for both temperature and precipitation for May and May-July. EC stands for equal chances of above-, below-, and near-average conditions. I consider that a neutral forecast without any solid evidence for unusual conditions to crop up (heat, drought, etc.).

While not shown here, the August-October maps are just as exciting for Illinois – EC for both temperature and precipitation. At least they have taken out the “increased chances of below-normal temperatures” that have been prevalent in recent months. While there has been some press about the potential arrival of El Niño this summer, the current thinking at CPC is that the effects will be minimal until later in the year.

Meanwhile, the latest 6-10 and 8-14 days forecasts (below) from the Climate Prediction Center show increased odds for both warmer and wetter than average conditions in Illinois. The warmer temperatures are welcome for warming up the soils (see Illinois soil temperatures). However, the increased chances for rain could be a problem for field work.

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6-10 Day Forecast (click to enlarge)


8-14 Day Forecast (click to enlarge)

First Half of April: From Shorts to Snowfall

In one of life’s little ironies, despite temperatures ranging from the 70s to the 20s and a dash of snow, the statewide average temperature for the first half of April was 49.1 degrees, just 0.3 degrees above average.

Snowfall For the 72 Hours Proceeding the morning of April 16.



Precipitation for the first half of April

The statewide average precipitation was 3 inches, which is about 60 percent above average, for the first 15 days of April. The first map below is the April 1-15 precipitation, observed by rain gauges and radar. The second is the precipitation departure as a percent of average.



Thursday Highlights – Drought Monitor and El Nino Forecast

Here are two highlights for today:

U.S. Drought Monitor

After some concerns of dryness over the last several months in parts of Illinois, the conditions across Illinois were much wetter in the last two weeks. Rainfall totals were especially heavy south of Interstate 72 and ranged from 3 to 8 inches or more (see map below).

The U.S. Drought Monitor has removed all areas of drought in Illinois and greatly reduced the region of “abnormally dry” conditions.


Climate Prediction Center El Niño Forecast

The NOAA Climate Prediction Center issued a new El Niño Watch today, saying …

While ENSO-neutral is favored for Northern Hemisphere spring, the chances of El Niño increase during the remainder of the year, exceeding 50% by summer.

This reflects a slightly stronger chance of El Niño arriving this summer than mentioned in their post a month ago. However, they caution that there is still a great deal of uncertainty in the timing and strength of the El Niño and recognize that the skill in forecasting El Niño this early in spring is low. This is a less certain forecast than the one issued by the Australian Bureau of Meteorology earlier this week, indicating that there was a greater than 70 percent of an El Niño event by June.