AFTERNOON – OVERNIGHT: An area of low pressure will continue to spin-off to the forecast zones east and northeast this afternoon. With this low in the area it will keep the chance at some rain in the forecast through the afternoon, primarily across southern zone 7 and zone 6.
By 4-5 PM you can see some showers still lingering across Illinois.
However, I do also think there will be some sunshine this afternoon and evening as the low moves farther away from the forecast region.
By 7-8 PM tonight most of the rain showers will be out of the region or fading out. So we will be dry across the region later tonight.
Overnight conditions look to remain dry across the forecast area as we will be between two weather systems. However, I will not rule out areas of fog once again overnight into Tuesday morning.
Lows will be in the upper 40’s north to mid 50’s south.
TUESDAY: A strong, negatively tilted area of low pressure will lift to the north of the region on Tuesday. As it does it will allow for a cold front to sweep eastwards across Illinois through the afternoon and evening hours.
Along this front rain and thunderstorms will develop. It currently looks like initiation of rain and storms will occur in western Illinois around 4-5 PM Tuesday afternoon as the cold front moves in.
By 7-8 PM the cold front will be moving into central Illinois. At this time a line of thunderstorms looks to be in place along it. These storms will be moving off to the east with the cold front late Tuesday afternoon into the evening.
It does look like this line of thunderstorms will exit the forecast area around 9 PM or so.
I do want to mention it does look like the environment will be conducive for severe weather Tuesday afternoon and evening.
Along and ahead of the cold front daytime heating will allow for instability to build quickly across the forecast areas. CAPE values or instability values are forecast to rise around 1500-2500 J/kg. If you are unfamiliar with this, all you need to know is this time of year 800-1000 J/kg is plenty of support for severe weather.
Aloft a strong westerly 500 mb jet stream will be surging in at 55-60+ mph and a southerly 850 mb jet stream will be moving through at 35-45 mph. The will create wind shear or rotation with height. This will also aid in severe storm development and maintenance.
The supercell composite, which takes both of the previous two factors into the equation shows some relatively high values Tuesday afternoon and evening across Illinois.
Overall I think the best timing for severe weather will be between 3 PM and 9 PM. The main areas that will be impacted by severe weather look to be zone 7 and the northern half of zone 6. However, I won’t totally rule out the chance of a severe storm in southern Illinois either.
The main risks from these storms will be damaging winds, hail, and a few tornadoes. I do think if storms can remain discrete along the cold front the tornado threat would be greater. Overall it’s a forecast we will have to fine-tune over the next 24 hours as the small details can be hard to forecast more than 6-12 hours out.
Highs on Tuesday will be in the upper 60’s north to upper 70’s south.
Tuesday night into Wednesday most of the rain will move out of the forecast zones with perhaps a few showers remaining across zone 7.
Lows will be in the mid 40’s west to mid 50’s east.
Regarding rainfall over the next 48 hours, .25-.60″ will be more common. Areas that get under a strong to severe storm could see .90-1.25″ locally.
To the north rainfall will accumulate with the round of storms coming through Tuesday afternoon. While .25-.50” will be more likely with thunderstorms, I won’t be shocked to see some locations get 1-1.25” of rain under some of those strong/severe storms.
WEDNESDAY: An area of low pressure will be in the place to the regions north on Wednesday. While we will start the day with mostly sunny skies clouds will be on the increase through the afternoon as an upper level disturbance moves through the forecast zones.
While no major rain is anticipated, some light afternoon and evening showers will be possible.
Highs will be cooler topping out in the upper 50’s to mid 60’s.
However, the bigger story will be the winds on Wednesday. As the low spins to the regions north, the pressure gradient will tighten up and strong winds will be possible. Gusts of 35-45 mph are possible through the day.
THURSDAY – FRIDAY: The area of low pressure will start to move off to the northeast Thursday. However, it will still allow for very windy conditions to remain for at least the first half of the day. Sustained winds of 10-20 mph will be likely, with wind gusts up to 40 mph!
While most of the day will be dry I won’t rule out a chance of some isolated showers in the afternoon.
Highs will be in the mid 50’s to mid 60’s.
Friday the low will be well out of the region and an area of high pressure will very briefly ridge in from the north and northwest, but there will be another area of low pressure developing to the southwest. Overall, it will be a dry day with partly to mostly cloudy skies.
With the northern area of high pressure moving in temperatures will be cool to end the week with highs in the mid 40’s to mid 50’s.
7 DAY FORECAST
TUESDAY: MIX OF CLOUDS AND SUN WITH PM T’STORMS, SOME STRONG/SEVERE. HIGHS IN THE UPPER 60’S NORTH TO UPPER 70’S SOUTH. WINDS FROM THE SOUTHEAST AT 5-15 MPH, GUSTING TO 25-35 MPH.
TUESDAY NIGHT: MOSTLY CLOUDY WITH T’STORMS EARLY, SOME STRONG TO SEVERE. LOWS IN THE MID 40’S WEST TO MID 50’S EAST.
WEDNESDAY: STARTING MOSTLY SUNNY, BUT INCREASING CLOUDS, 10-20% CHANCE OF AFTERNOON RAIN SHOWER. HIGHS IN THE UPPER 50’S NORTH TO MID 60’S SOUTH. WINDS FROM THE WEST AT 5-15 MPH, GUSTS OF 35-45 MPH POSSIBLE.
WEDNESDAY NIGHT: DECREASING CLOUDS WITH LOWS IN THE LOW 40’S WEST TO NEAR 50 EAST.
THURSDAY: MOSTLY SUNNY TO PARTLY CLOUDY WITH 20% CHANCE OF PM SHOWERS. HIGHS IN THE MID 50’S NORTH TO MID 60’S SOUTH. WINDS FROM THE WEST AT 10-20 MPH, WITH GUSTS UP TO 40 MPH.
THURSDAY NIGHT: PARTLY CLOUDY WITH A 20% CHANCE OF RAIN AND/OR SNOW SHOWERS. LOWS IN THE LOW 30’S NORTH TO AROUND 40 SOUTH.
FRIDAY: PARTLY TO MOSTLY CLOUDY, COOLER. HIGHS IN THE MID 40’S NORTH TO MID 50’S SOUTH. WEST TO NORTHWEST WINDS AT 6-12 MPH, GUSTING TO 20 MPH.
FRIDAY NIGHT: INCREASING CLOUDS CHANCE OF RAIN AND/OR SNOW WEST. LOWS IN THE LOW TO UPPER 30’S.
SATURDAY: MOSTLY CLOUDY 40% CHANCE OF RAIN. HIGHS IN THE LOW 40’S TO NEAR 50.
SATURDAY NIGHT: CLOUDY WITH RAIN CHANGING OVER TO SNOW? LOWS IN THE LOW TO MID 30’S.
SUNDAY: DECREASING CLOUDS WITH AM SHOWERS. HIGHS IN THE 40’S.
SUNDAY NIGHT: PARTLY CLOUDY, 20% CHANCE OF SOME SNOW SHOWERS. LOWS IN THE UPPER 20’S TO MID 30’S.
MONDAY: MOSTLY SUNNY WITH HIGHS IN THE MID 40’S EAST TO MID 50’S WEST.
MONDAY NIGHT: PARTLY CLOUDY WITH A SLIGHT CHANCE FOR SOME RAIN SHOWERS WEST. LOWS IN THE LOW 30’S EAST TO MID 40’S WEST.