Yesterday, Midway hit 90° for the fifth time this month, and it looks like three more 90° days will be added to that total by midweek. Humidity will also slowly be on the rise. Showers and thunderstorms appear in the forecast each day starting Wednesday through next weekend. While the core of the jet stream still resides far to the north across southern and central Canada, light westerly winds aloft at this latitude allow low-level southerly flow to feed warm, moist air into northern Illinois. A cool front will sag south through Wisconsin, drifting through Chicago on the heels of a thunderstorm-producing low pressure center Friday. From that point on, the front will become quasi-stationary, with the potential to fluctuate north or south of the metro area. Current computer model projections have the front situated just south of Interstate 80 next weekend, but a slight repositioning to the north could radically change temperature and rainfall forecasts.
Heat and humidity return with a vengeance this week, accompanied by a significantly enhanced probability of thunderstorms.
With the jet stream situated far to the north across southern Canada and weak west-east flow aloft, warm southerly surface winds will return this week, feeding low-level moisture into northern Illinois. Heat indices will rise well into the 90s Monday before crossing the 100° threshold Tuesday and Wednesday. Clouds Thursday may hold the heat index in the 90s, but the highest reading of the week may be reserved for Friday. Any three consecutive days of a 100° heat index will trigger a heat warning for Chicago.
Slow-moving upper-level troughs aloft riding over warm humid air in combination with a nearly stationary front to the north may create periods of showers and thunderstorms Thursday and again late Friday into Saturday. Before the week is up, copious rains could fall over central and northern Wisconsin.
The summer’s tenacious drought has been a disaster for area farmers, and has left lawns and gardens parched and dry. But, in terms of weekend weather which cooperates with outdoor activities, 2005 has few recent meteorological peers. Measurable rain has fallen on only three weekend days—15 (83% of them) have been dry. And the rains which have fallen on the weekend have been negligible—totaling only 0.43”! But heat has been the big story. All but one of the nine weekends currently on the books have been warmer than normal. Summer weekends have produced highs averaging 6.5 degrees above normal, including 2005’s hottest single temperature: 102° on July 24. Readings have exceeded 90° on six of the 18 weekend days of summer to date.
The humidity decline responsible for comfortable conditions Friday continues Saturday. But humidities begin creeping higher Sunday ahead of a new hot spell due next week.