AFTERNOON-OVERNIGHT: High pressure remains in control of the forecast area today. This keeping warm, dry air in place across zones 6 and 7, making for yet another very nice day!
Through the afternoon the high will move slightly to the east, but will still remain well in control of the weather. By later this afternoon clouds from the large storm system ongoing out west will start to work into western Illinois. While initially some of these could be thicker, the dry air will cause them to erode some this afternoon/evening.
Overnight some clouds may stick around, especially some high, thin clouds. However, I do think it will be a mostly clear night across the forecast area.
Lows overnight will be in the upper 50’s east to mid 60’s west.
TUESDAY: Tuesday morning the area of high pressure will be located farther to the east of the region. Through the early morning hours on Tuesday an area of rain and storms will be entering western Illinois. By 7-8 AM these storms looks to be impacting western zone 7 and points south/southwest across western zone 6.
Through the morning hours Tuesday scattered storms will continue to move southeast across western Illinois. By 12 PM some isolated to scattered storms will be continuing across western zone 6 and possibly western zone 7.
Through the afternoon rain and storms will lose intensity as they move across zone 6.
While initially storms could be strong as they cross into Illinois, the overall environment will not be overly conducive for strong to severe weather. Main risks will be strong winds and hail.
Through the afternoon the storms will fade out with an isolated area of rain or t’storm across southern Illinois in zone 6. By 6-7 PM many places will be dry and calm across the forecast area.
Highs on Tuesday will be warm once again with temperatures in the low to mid 80’s Depending on overall cloud cover and precipitation, highs in the upper 80’s could be achieved.
Tuesday night will start off dry and calm across the forecast area. However, to the west a 500 mb disturbance will be moving through the atmosphere. This will cause storms to develop in association with the disturbance. The short-range, high-definition models are suggestive of a cluster of storms entering southwestern Illinois around 4-5 AM Wednesday.
Lows overnight Tuesday night will be in the mid to upper 60’s.
WEDNESDAY: Early in the day Wednesday morning an area or possibly cluster of rain and storms will continue moving across southern Illinois. By 6-7 AM the NAM 4km shows those t’storms.
While instability will be minimal, the 850 mb winds will be strong, so some damaging winds and perhaps hail can’t be ruled out.
Through the day on Wednesday a warm front will be lifting northward through the state. By mid-day scattered rain and storms will be moving across Illinois in association with the frontal boundary.
During the afternoon hours an area off 500 mb vorticity will move through the forecast area. As this does, it may not only allow for more development of storms, but also to enhance the intensity of those that are ongoing.
Through the evening the 500 mb vorticity max will move off to the forecast areas east. This will take a lot of the rain and thunderstorm activity with it.
Highs on Wednesday will be in the mid 70’s to low 80’s.
While a few storms could be on the strong to low-end severe side on Wednesday as the upper-level vorticity max passes, overall dynamics are not impressive for widespread severe weather. With that being said, the main problem with severe potential is weak instability because of ongoing rain/storms. If storms should hold off until afternoon or exit earlier, then instability could build and severe weather would be more likely.
Wednesday evening into the overnight looks to start dry. However, yet another area of upper-level energy will move through the forecast area. This will once again spark rain and thunderstorms in the warm, humid environment.
Right now it looks like best timing and location for these will be around 3-4 AM across zone 7 and points north. However, timing and placement will need fine-tuned over the next couple days in regards to where the upper-level energy will actually track.
Lows Wednesday night will be in the mid to upper 60’s.
THURSDAY – FRIDAY: Thursday remains to be a bit of a complicated scenario that will need fine-tuned over the next couple of days. However, it appears likely that a MCS will impact portions of the forecast area on Thursday. The EURO model shows is more suggestive of an early morning complex of storms moving across northern Illinois and into central Illinois and Indiana.
However, the NAM suggests a few smaller clusters of storms to start the day on Thursday across Illinois and Indiana.
Then throughout the day upscale growth into a larger MCS late in the morning into early afternoon would occur.
Whenever storms do occur, strong/severe storms will be possible. Main threats will be damaging winds, hail, and perhaps a couple of tornadoes. Impacts, timing, and other details will have to be ironed out as we get closer to the forecast period.
Highs on Thursday will be in the upper 70’s to mid 80’s.
A continuing active pattern will allow rain and t’storms to continue across the forecast area through the end of the week.
It does appear better upper-level support will arrive Friday across the forecast area. This coupled with the moist, unstable air that will be in place will be conducive for a better chance of severe weather. For now, we will not go into much detail since it is still several days out, but we will bring you more as we get closer to the forecast time period.
Another serious concern will be rainfall and flooding potential. The environment will be saturated this week and precipitable water values will be between 1.75-2” at times!
While average rainfall totals through Friday night will likely average out around 1-1.25”, locally much more could occur.
The latest NAM 4km shows areas of 4-7” of new rainfall through Wednesday afternoon in localized areas. For this reason, I do think significant rainfall accumulations and at least localized flash flooding should be anticipated this week.