AFTERNOON-OVERNIGHT: High pressure continues to build into the forecast area from the southwest this morning. This has created drier conditions with clear to mostly clear skies across the forecast zones this morning.
The area of surface high pressure will continue to control the weather across zones 6 and 7 today. This will allow for sunny to mostly sunny skies to continue across the forecast zones.
As an upper-level ridge will be building in across the Great Plains, the Midwest will get in on the eastern portions of the ridge. Not only will that ridge help reinforce the nice weather, but it will also allow the warmer air bottled to the south to spread northward.
Through the overnight into Saturday morning skies will remain mostly clear across the region.
Lows will be in the low 50’s to around 60.
SATURDAY: A cold front will be in place to the north of the region Saturday morning. However, that front will start to sag southward through the day increasing both clouds and rain/storm chances.
While morning into early/mid-afternoon will remain dry, precipitation chances will be on the increase by at least late afternoon. By 3-4 PM on Saturday the cold front will be in place across north central Illinois. Rain and storms will develop primarily along the front by 3-4 PM Saturday afternoon. However, more isolated rain/storms will be possible across both zones ahead of the main line.
Through the afternoon and into the evening the cold front will move southward across Illinois. As it does, rain and storms will continue to develop along the frontal boundary and move south-east along it. By 6-7 PM it does look like there will be a MCS (Mesoscale Convective System) or storm cluster developing across north central Illinois, possibly across zone 7.
Through the evening rain and thunderstorms will remain possible along the front. By 12-1 AM Sunday rain and some ongoing storms will remain possible along and just north of the slowing front across northern zone 6 and zone 7. However, by this time a lot of storms will be losing their intensity.
While I am not expecting a major severe weather event tomorrow across the zone forecast area, some strong to severe storms can’t be ruled out. Strong northwesterly flow at 500 mb at 65- 75 mph combined with westerly low-level winds of 25-40 mph should create some wind shear supportive of multi-cells or brief supercellular structures.
CAPE values of 1000-2000 J/Kg will be possible along the front. This will be plenty of instability for storms to get going in the afternoon and evening hours along said front.
Temperatures aloft will be very cold. With low-level lapse rates of 8.5-9.5°, hail (some large?) would be the main severe threat. However, some damaging winds and an isolated tornado can’t be totally ruled out.
Highs on Saturday will be in the mid 70’s north to mid 80’s south. Winds will be strong at times on Saturday with gusts of 30-40 mph possible around the front.
Overnight Saturday into Sunday the cold front will start to slow and possibly even become stationary. The forecast models keep the rain/storms north of the front overnight. However, I’m not 100% sold on this theory as generally the rain and storms will move south/southeastward along the front and with southeast 700 mb flow I really see no reason that these storms will stay in place. For that reason, I will call for a better chance of rain and storms across zone 6 Saturday night into Sunday.
Lows Saturday night will be in the upper 40’s north of the front to right around 60 south of the front.
SUNDAY: By Sunday morning the front will either be moving very slowly or will be completely stationary across southern Illinois or northwestern Kentucky. Regardless, this will provide enough lift across the area for isolated to widely scattered rain showers to continue Sunday morning across Illinois.
Through the late morning into the afternoon on Sunday the front will remain mostly stationary across southern portions of the zone. While this will mostly likely create partly to mostly cloudy skies throughout the day, only some isolated rain chances will remain through mid-day. So Sunday will not be a total wash out.
By 4-5 PM Sunday afternoon some scattered rain and storms will develop along the stationary front and move east and northeast across the forecast area. However, I do think these will be hit and miss.
While some storms could be on the strong to low-end severe side Sunday afternoon, the overall severe weather threat doesn’t appear to be all that great. However, with some instability and a surface front in place some strong to severe storms can’t totally be ruled out.
Highs on Sunday will be dependent on the exact location of the front. Along and north of the front temperatures will be in the low to mid 60’s. However, south of the front highs will be in the 70’s to around 80°.
Sunday night into Monday and area of low pressure will be lifting northward across the Great Plains. As it does, the stationary front will begin to lift northward as a warm front. This will likely lift the atmosphere enough through the overnight that more scattered rain and some storms will develop across Illinois.
Regarding rainfall totals across the forecast area an average of .25-.50” looks likely where rain does occur through late Sunday evening. However, models are trying to pick up on a couple of things with the heavier totals. The first being the developing MCS, the second being where the better chances for rain will set up with the stalling front. Right now the NAM 4km shows the heaviest rain to the north of zone 7. However, with the front in place across southern Illinois, don’t be surprised if models do trend southward with some of the heavier totals over the next 12-24 hours.
MONDAY: The warm front will continue lifting northward through the daytime hours on Monday as the area of low pressure moves closer to the area.
If the warm front is far enough north it could actually allow Monday to be mostly dry through the daytime hours with scattered afternoon storm development. Overall, I would favor that scenario for the late morning/afternoon hours on Monday.
Highs on Monday will be in the low to upper 70’s. Winds will be strong at times on Monday with gusts of 30-35 mph possible.
MONDAY NIGHT – WEDNESDAY: A bit of a complicated scenario is setting up by next Monday night. This will be as the low start to slow as it heads eastward into the region.
Models are really having issues with timing of the cold front next week. The EURO and CMC suggest the front passes Tuesday afternoon and into the overnight from west to east across the forecast region.
However, the GFS is nearly 24 hours slower with the frontal passage, brining into though on Wednesday.
Overall, I do tend to favor the faster EURO/CMC solution. The current pattern, while slow with exiting the initial sagging cold fronts, has been quicker on eastward propagating cold fronts.
Regarding severe weather next week, many things will need fine-tuned to forecast the severe weather threat, if there is any. Right now models are pretty messy with this system, so we will have to see the progression of the models/forecast over the next several days.
LONG RANGE: An area of high pressure looks to move into the region mid to late next week as the system exits the region. While this will give us a short break from the active pattern more rain and storms will return to the forecast.
By late next week we will be watching another developing system that will have the potential to spread rain and storms back into the region next THU/FRI.