Slow moving upper air low pressure forecast to spin up over Midwest this weekend and linger into first days of next with with cool, frequently cloudy weather and instability showers
Disturbance moving toward the Chicago area
A northeast-bound disturbance, whose cloud shield can be seen on satellite imagery making a move toward the Chicago area from downstate Illinois and Indiana is to usher clouds into the area Thursday overnight. Its northernmost showers are modeled to spread northward reaching Chicago toward morning and into the early and possibly mid-afternoon hours Friday.
A POWERFUL 165 MPH JET STREAM SPEED MAX IS DIVING SOUTHWARD OUT OF BRITISH COLUMBIA IN WESTERN CANADA AND TO CARVE OUT A STUBBORN UPPER AIR LOW
The reservoir of cold, unstable air aloft accompanying it sets the stage for a long advertised new multi-day round of clouds and instability showers beginning Saturday and lingering into Tuesday. By extinguishing the late April sun, temps here will take a hit with daytime highs, which could reach the low 60s Saturday, dropping to 51 Sunday, 47 Monday and 53 Tuesday.
Breaks in the overcast the from Wednesday forward permit warming—but modeling suggest more substantial warming is a good bet in the week which follows next. The early read is ABOVE NORMAL TEMPS are to take up residence here beginning a week from Monday or Tuesday with daytime highs likely to reach the 70s then.
VIEW SLIDESHOW OF DEVELOPMENT
Clouds with the northbound distance expected to bring showers to sections of the Chicago area is shown here Thursday on GOES EAST WEATHER SATELLITE IMAGERY 
FORECAST PRECIP PANELS off the NWS HREF model: This panel for 4am CDT Friday 
5am CDT Fri 
6am CDT Friday 
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Noon CDT Fri 
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Chicago’s Thursday 60s–the warmest temps here in a week–were part of broader warming across the nation’s mid-section 
This 24 hour temp change graphic mid Thursday afternoon shows Chicago and a wide swath of the Midwest part of a temporarily warming weather regime 
24 hour temp change as analyzed at 3pm CDT Thursday afternoon 
Thursday afternoon observed winds. Cooler northerly winds in the northern Rockies and Plains are part of the weather shift which is to include the spin-up of a chilly, unstable upper air low pressure system expected to produce clouds and sporadic instability showers over a multi-day period 
The high dew points on this Thursday afternoon analysis show the humid Gulf air which fueled powerful t-storms on the Gulf coast and into Florida and the Southeast Thursday. 
The strong winds (shaded in orange and red) diving into the northern Rockies and Plains set the stage of the development of a slow-moving cloudy weather system expected to take up residence in the central U.S. this weekend and into next week. This panel shows 34,000 ft. winds early Thursday 
This Friday forecast shows the strong upper winds digging southward into the central Plains 
This Environment Canada JET STREAM LEVEL upper air analysis shows conditions around 34,000 ft. Thursday morning. a strong 165 mph pocket of upper winds is shown sweeping off the Gulf of Alaska into western Canada and the northern Rockies 
The SEQUENCE OF UPPER AIR MAPS which follow offer a graphic illustration of the spin-up of our weather dominating upper low pressure as we approach and move into the weekend. This was the flow pattern at 10am CDT Thursday 
7am CDT Fri 
7am CDT Sat forecast 
7am CDT Sunday forecast 
1pm CDT Sunday forecast 
7pm CDT Sunday forecast 
7am CDT Monday forecast 
7pm CDT Monday forecast 
High temps Saturday 
Forecast Sat high temp departures 
Sunday high temp forecast 
Sunday high temp departure forecast 
Monday high temp forecast showing the cool air under the cloudy, showery upper air low pressure 
Monday high temp departure forecast 
Circled on the GOES WEST Thursday weather satellite image is the region of powerful upper winds initiating upper low pressure development in the central U.S. 
Waves of satellite inferred precip are evident as powerful upper winds aiding in the development of the big weekend upper air low pressure sweep into the airspace above North America. Courtesy: CIMSS at the Univ of Wisconsin-Madison 
Satellite inferred precip Thursday from CIMSS (the Cooperative Institute for Meteorological Satellite Studies) at the University of Wisconsin-Madison 
You can see the strong upper winds–what meteorologists refer to as a JET STREAM–diving into the northern Rockies Thursday 
Closer look at the large, slow moving pool of cool, unstable air aloft predicted at 1pm CDT Sunday 
Sub-freezing 5,000 ft. level temps suggest small hail which forms in some cloud towers within the cold air aloft could make it to the surface where temps will be well above freezing. This is the Sunday temp forecast at 5,000 ft. 
Monday 5,000 ft. level predicted temps. The light blue/green area is the region in which temps will be below freezing 
Forecast cloud coverage 1pm CDT Sat 
Clouds at 1pm CDT Sun 
Clouds at 1pm CDT Mon 
Clouds at 1pm CDT Tue 
PRECIPITATION PROBABILITIES: Saturday 
PRECIP PROBS Sat night 
PRECIP PROBS Sunday 
PRECIP PROBS Sun night 
PRECIP PROBS Mon 
5-DAY AVERAGE TEMP DEPARTURES–Day 1-5 per the National Weather Service GFS ensemble model 
Days 6 -10 TEMP DEPARTURES 
DAY 11-15 TEMP DEPARTURE FORECAST
May Temperatures in Chicago
AS POINTED OUT YESTERDAY, a survey of May temp records here shows May to be a month still capable of cool periods–but with a MUCH GREATER POTENTIAL FOR WARM SPELLS. Chances for 80-degree temps surge to 99%, to 90% for one or more 85-degree temps and even a 49% chance for the year’s first 90-degree temps.
