Marine Weather and Tide Forecast for Jacksonville, IL

DISCUSSION
Issued at 128 PM CDT Sat Jun 21 2025

12z RAOBs sampled a highly amplified upper pattern across the CONUS, with a closed low over the Pacific NW and a building ridge over the eastern half of the CONUS. 500mb heights of 590 dam were sampled at ILX, with peak values of 593 dam over the southern US.
Height rises are expected to continue into the day on Sun, by which time an expansive area of 594+ dam 500mb heights are anticipated. Presently, hot and muggy conditions are in place across central IL, with temps in the upper 80s as of 17z/12pm, dewpoints in the mid 70s, and heat indices around 100 degF.
Perhaps the one saving grace right now is the breezy winds, with SW gusts of 25-35 mph, which should offer minor relief compared to a stagnant airmass. Skies are mostly clear aside from sparse diurnal Cu east of the IL River, and temps should climb a few more degrees this afternoon, into the low/mid 90s. Conditions on Sunday will be similar to today. Winds remain breezy, but will be slightly weaker than today, with gusts forecast to stay below 30 mph.

On Mon, the sfc front shifts a bit closer to the region, but will still be positioned well to the northwest, extending from W KS to the UP of Michigan. While I noted yesterday that this approaching boundary led to some uncertainty about cloud cover on Mon, all signs right now point towards another day of clear skies, continuing the heat/humidity. Can't rule out a pop-up storm across SE IL on Mon, as forecast soundings show uncapped sfc-based profiles as the sfc warms towards the convective temp. The GFS has consistently produced light QPF across the region Mon, but this seems overdone given the synoptic scale subsidence and lack of sfc convergence. If any isolated storms do manage to form, upper level winds are very weak, resulting in minimal wind shear, so an organized severe threat is unlikely, but a few strong downdrafts can't be ruled out as cells collapse. The Grand Ensemble probability of precip is around 20% south of I-72, but this is largely driven by the seemingly overzealous GEFS (50% chance). Our current forecast keeps PoPs below 15% Mon afternoon, thus precip is not mentioned in the gridded forecast.

Tues remains the lowest confidence day of the current heat advisory, as the potential for MCS activity to nudge in from the northwest increases Mon night into Tues. If any such activity does occur, it could be strong/severe owing to strong instability, although wind shear will decrease with southeastward extent as upper level winds weaken in proximity to the ridge. There is a 20-30% chance of precip during the day Tues, but deterministic guidance still suggests the front itself stays NW of the ILX CWA
Despite the potential for additional cloud cover and/or precip, the Grand Ensemble still has a 60-80% chance for highs over 90 degF on Tues.

Despite the anomalously strong ridge, record high temps (which are in the upper 90s/low 100s) are not expected. This is not a surprise given the high humidity, as we typically need a drier airmass for high temps to approach records. Conversely, the high humidity will limit nocturnal cooling, and there is potential for record warm low temps to be set during this heat event. In fact, it appears both Peoria and Springfield have set record warm lows for today, June 21st, barring something unforeseen happening between now and EOD. Peoria (observed low: 77F) has tied their record warm low, previously set in 1890, while Springfield (observed low: 76F) has set a record warm low, breaking the previous record of 75F set in 2006. With both forecast lows and record lows in the mid 70s over the next few days, additional record warm lows could occur. From the perspective of duration, this type of June heat wave is uncommon, but not unheard of. The last time central Illinois experienced four consecutive days of high temps over 93F in June was in 2022.

Around the middle of next week, the strong upper ridge begins to gradually flatten, with 500mb heights falling below 594 dam on Wed. As the upper ridge weakens, the synoptic pattern becomes more zonal, with potential for shortwaves across the north-central Plains to initiate precip locally, especially as the synoptic scale subsidence weakens. The front north of the ILX CWA never seems to completely push into our area, allowing the hot conditions to persist (50-60% chance for highs over 90 degF on Wed- Thurs, lower probability by Fri). The presence of this front in the region combined with the increasingly zonal flow pattern leads to unsettled conditions, with at least some potential for precip (20-50%) each day Wed into next Sat. Despite the daily chances, total precip amounts still appear light, with ens mean values through next Sat generally between 0.25- 0.75". The probability of exceeding 1" of rain at any one location over the next week is just 20-40%. If the current forecast holds, the heat advisory may need to be extended later into the week, but at this range forecast confidence was too low to warrant an extension.

Erwin

AVIATION
(For the 00z TAFs through 00z Sunday Evening)
Issued at 600 PM CDT Sat Jun 21 2025

Another set of VFR TAFs with just a few diurnal CU Sunday afternoon. Southwest wind gusts will diminish over the next couple hours, then increase again by late morning Sunday with 20-25 kt gusts common through afternoon.

25

ILX WATCHES/WARNINGS/ADVISORIES
Heat Advisory until 7 PM CDT Tuesday for ILZ027>031-036>038- 040>057-061>063-066>068-071>073.