Fall and Winter Forecast for Illinois

Summary: The NWS Climate Prediction Center has issued their forecasts for September, September-November (Fall), and December-February (Winter). Illinois has an increased chance of being warmer than normal this fall, and wetter than normal this winter.

According to the Climate Prediction Center, the main factors in the forecast are the recent warming trends and the expected La Niña. While the conditions in the Pacific are in the neutral stage between El Niño and La Niña, there is a 55-60 percent chance of a weak La Niña during fall and winter.

Fall

The September forecast (top row) has Illinois and the Midwest with equal chances of being above, below, and near-normal on both temperature and precipitation. I call this a neutral forecast since there are no indications that we will be significantly cooler, warmer, wetter, or drier.

The September-November forecast (bottom row) has Illinois and the US with an increased chance of being warmer than normal. They are neutral on the precipitation forecast.

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Winter

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Latest Seasonal Outlook for IL

Illinois has higher odds of being both warmer and drier than average though June, according to the newly released NWS Climate Prediction Center outlooks.

The main driver of the forecast continues to be the strong El Niño event in the Pacific Ocean. El Niño is expected to go away by late spring or early summer. The increased odds of drier conditions through June could be a concern. However, I am more confident in the temperature forecasts from CPC than I am of the precipitation forecasts.

January:

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Illinois has higher odds of being both warmer and drier than average.

January – March:

Illinois has higher odds of being both warmer and drier than average.

April – June:

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Another Forecast Suggests a Higher Chance of a Wet Fall in Illinois

Last week, the NWS Climate Prediction Center released their official forecast for fall and winter. Their forecasters used a variety of tools and as well their own expertise to develop those forecasts. While their forecast for Illinois this winter was interesting, the one for fall was not.  They had us with equal chances of above-, below-, and near-average temperature and precipitation.

However, there is one forecast tool that showed some results for Illinois this fall. That tool is called the North American Multi-Model Ensemble (NMME). It consist of several forecast models that are run out to 7 months. The advantages of having multiple model runs are that an average of all results tends to be a better forecast than a single model. Also, the spread in the model results gives you an idea of the uncertainty of the forecast. For example, if all the models showed this winter being warmer-than-average, our confidence in the forecast would be much how than if some models showed it being warmer, some models showing colder, and others showing average conditions.

These results below for fall (SON=September, October, November) are considered experimental and not part of the NWS official forecast. However, they do shed some insight on what the models are “thinking” for fall. The first map show the chances on fall precipitation in three categories (wet, dry, average) using all models. They have Illinois and much of the US with an increased chance of above-average precipitation.

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Cool and Wet Forecast for August in Illinois

The forecasts for August and the three-month period of August-October show most of Illinois with an increased chance of being cooler and wetter than average.

A significant El Niño event has developed in the Pacific Ocean and has a 90% chance of remaining throughout 2015 and an 80% chance of remaining until next spring. This is expected to have significant impacts on the weather in the US.

El Niño events occur when the waters in the eastern Pacific along the equator are warmer than usual. This changes the weather patterns over the Pacific and US.

August

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NWS Outlook for July – Cooler and Wetter in Illinois

Illinois has an increased chance of below-average temperatures and above-average precipitation in July, according to the outlook released by the NWS Climate Prediction Center earlier this week.

The July precipitation forecast is amazing for showing such a large area with an increased chance of above-average precipitation. The odds are especially high in southern Illinois. This does not necessarily mean that July will be record-breaking like June, just wetter than average.

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Precipitation forecast for July. Click to enlarge.

The July temperature forecast shows a large area across the central US with an increased chance of below-average temperatures. Historically, it is not unusual in Illinois for wet conditions in May and June to lead to cooler temperatures in July. The wet soils keeps the air temperatures cooler. At least that’s the theory.

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Temperature forecast for July. Click to enlarge.