ECMWF 500 mb Anomaly Showing the High over Low Pressure Pattern which is Responsible for the Deep Upper Level Trough Across the Eastern US.

Fairly tranquil and seasonably cool weather can be expected across Central IL through the end of the week. A blocking high pressure system south of Greenland will help to keep a trough entrenched across the eastern third of the US (Shown in the graphic above). This trough will be nearly stationary through the end of the week with Central IL on the far western periphery of the trough. This will result in slightly cooler than normal temps for the northeast half of the area with near to slightly above normal temps for the far southwest further away from the influence of the upper trough. Wednesday will feature highs in the mid 40s across the eastern half of Central IL with upper 40s to low 50s across the western half of the state. Low Wednesday night will be in the upper 20s to around 30°F for the northern half of the area with low to mid 30s across the southern half of Central IL. Highs on Thursday will be in the upper 40s to around 50°F for northeast portions of the area, with low to mid 50s for the southwest half. The exception may be far Southwest Central IL, where temps may approach 60°F. Expect lows Thursday night to range from the mid 20s across the northeast half of Central IL with low to mid 30s across the southwest half. Highs on Friday will be fairly uniform across the entire area with low 40s for a high and low to mid 30s for the lows. Locations across far northern Central IL may drop to 30°F Friday night. Expect highs in the mid to upper 30s area wide on Saturday.

ECMWF Simulated Precip Type at 7am Saturday, Showing the Potential for a Mixed Bag of Precip Types Across Portions of Central IL.

While the majority of the week will remain dry, the threat for precipitation will increase during the day on Friday (particularly across the southwest portions of Central IL). Precip is then expected to push into the remainder of Central IL overnight Friday into Saturday morning. It is possible that far northeast portions of the area remain dry or only see sprinkles due to the system battling a very dry airmass associated with the lingering upper level trough. The precip will be in the form of all liquid for the first half of the event across the southwest half of Central IL. With both 850 mb and surface temps above freezing during this time, it will promote plain rain as a result. The concern for winter precipitation comes overnight Friday into Saturday morning, at which time 850 mb temps drop to near or just above freezing and surface temps may briefly fall below freezing across northern portions of Central IL, mainly along the I-74 corridor. This may set the stage for at least a brief window for a mix of sleet and freezing rain for the aforementioned area. The precip would likely transition back to all rain by mid morning as surface temps warm above freezing across all of Central IL. There is still plenty of uncertainty regarding thermal profiles as well as precise timing of precip. All of which will play a factor in the ultimate probability of ice accumulation. We will keep a close eye on this situation as it grows closer.

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