The first really widespread frost and freeze conditions of the fall season will be over the Midwest by Tuesday morning. The cold air is a result of the large scale pattern change that has occurred over the past couple of days. A change that was forecast by the long range models last week. A large upper air trough will be over the center of the nation through the Friday. The result will be a continuing dump of Canadian air over about the eastern two thirds of the country.
The graphic shows the temperature readings forecast by the NAM/WRF model valid at 7:00 a.m. CDT Tuesday morning. As can be seen, widespread cold temperatures are the rule. The cyan color shows readings from 30-35°F. The darker shades of blue show readings below 30°. These chilly morning temperatures will be repeated over the next several days, gradually covering more of the Midwest through the week as another lobe of cold air comes down midweek.
For the Chicago metro area, the high in the lower 60s on Sunday will become a quick fade memory. The high of 61 was the exact normal for the date. Highs for the rest of the week will only be in the mid to upper 40s. Lows will be from the mid 20s far out suburbs to the low 30s in the city. Some lake effect rain and/or snow showers are likely over northwest Indiana Tuesday night as low level winds travel down the length of the lake.
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