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Minooka, Illinois Weather Forecast Discussion

850
FXUS63 KLOT 201131
AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 631 AM CDT Sat Sep 20 2025

.KEY MESSAGES...

- Scattered showers and embedded storms through early this morning, some with brief heavy rain and gusty winds. Additional threat for scattered gusty storms later this afternoon.

- Increasing signal for more widespread showers and embedded storms tonight/early Sunday morning, mainly north of I-80.

- Periodic chances for showers and some thunderstorms through early next week, though there will continue to be plenty of dry hours too.

- Afternoon temperatures will remain above normal through the Autumn Equinox (Monday at 1:19 PM CDT), though not quite as warm as they`ve been the past few to several days.

&&

.DISCUSSION... Issued at 328 AM CDT Sat Sep 20 2025

Through Sunday:

Messy/tricky forecast continues today with a main upper low to our northwest and several embedded lower-amplitude vorticity maxima set to pivot through the general vicinity today. Model guidance generally has exhibited poor run-to-run consistency as well, and appears to be having a difficult time handling the degree of mid- level saturation and precise placement and magnitude of embedded vort maxes. All of this casts some extra layers of uncertainty on the forecast for today and tonight, although some trends are beginning to emerge.

Through the rest of this morning, a north-south axis of increased 800-600 mb moisture coincident with broad, modest warm isentropic upglide is resulting in the development of elevated showers and embedded storms. Through daybreak, have focused the highest precipitation chances east of about a Pontiac to Woodstock line, and particularly focused across Chicago and the south/western burbs, although isolated convection will be possible farther west. Some showers and storms may linger through mid-morning across far northeast Illinois and over the lake prior upglide weakening and deeper moisture pushing off to the east. Thereafter, expecting a brief lull in activity during the late-morning.

Towards midday, the main 500 mb shortwave axis should be pressing across the southwestern third of the forecast area as the primary upper low shifts into Wisconsin. An additional subtle 700 mb trough axis currently moving across central Missouri will commensurately lift northward across central Illinois. As all of this occurs, a zone of modestly-increased mid/upper flow--presently scooting across southern Iowa--will start to impinge on the region. The Des Moines, Iowa VWP has recently indicated flow near 40-45 knots at 500 mb associated with this westerly jet streak, which is a bit stronger than current model guidance would suggest. While it`s possible these readings are a bit high and contaminated a bit by ongoing precipitation, this could play a role in the evolution of this afternoon`s weather.

While it remains a bit unclear how much insolation we`ll see today following the morning activity, trends suggest at least filtered sunshine will develop, allowing temperatures to push into the upper 70s/near 80 degrees. With dewpoints possibly rising through the mid 60s and 500 mb temperatures near -15 C, MLCAPE values around 1500 J/kg may materialize through early afternoon, along with diurnally-eroding CIN. The arrival of the aforementioned shortwave will likely serve as the main large scale driving mechanism today, and if we destabilize as currently expected, scattered showers and thunderstorms may re- develop. At this time, the main focus area appears to be about along/east of I-55, where the strongest DCVA is expected to encounter the greatest instability. Accounting for perhaps a little more mid-level flow than guidance currently indicates (based on upstream radar observations), deep-layer shear values near around 30 kts may be sufficient for at least ephemeral updraft organization. Suspect the HRRR is too aggressively mixing dewpoints out again, and resulting in deeper inverted-V soundings than we`ll see in reality, but regardless, stronger cores today may be capable of strong/gusty precip-loaded downbursts.

This evening and tonight, a trailing shortwave currently over Nebraska will slide into the region. At the same time upper divergence is forecast to increase under the right entrance region of an anticyclonically-arcing jet streak. Given this recent trend, while model guidance does not universally show increasing precipitation coverage, based on upstream observations and ongoing model trends, elected to boost PoPs late this evening and overnight, mainly near and north of I-80, with the expectation of increasing shower and embedded thunderstorm coverage. Low-level flow again does not look super concerning from a backbuilding/training perspective, but some very localized training could materialize.

Precipitation may persist into Sunday morning, although expect a gradual diminishing trend through the morning once again. Model guidance has been all over the place regarding the handling of follow-up shortwaves/convectively-augmented MCVs in the vicinity, so forecast confidence on Sunday remains notably low. Increased cloud cover could also end up muting insolation to some degree as well, all of which suggests capping PoPs in the 30-50 percent range for now. Can`t rule out the potential for a few stronger storms given indications of some stronger 700-500 mb flow riding in overhead, but there are too many other uncertainties to pin this threat down at this point.

Carlaw

Sunday Night through Friday:

Intermittent shower and thunderstorm chances will persist Sunday night through Tuesday as additional disturbances ripple through the west to northwesterly flow aloft. A chaotic upper pattern into the middle and end of next week will likely continue to result in lower-than-typical forecast confidence. Some signal remains in the extended guidance for at least a brief return to drier conditions (especially near/north of I-80) in the wake of a cold front towards midweek, but depending on the influence of another upper low towards the end of the week, this could end up being short-lived. The latest GFS in particular would indicate little/no break in the action with a Fujiwhara dance taking place between several robust disturbances across the central CONUS.

Temperatures this upcoming week will start out solidly above normal (along with fairly humid conditions as well), with highs well into the 70s to near 80F through Tuesday, then trending closer to normal and a bit less humid in the mid to late week.

Carlaw/Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/... Issued at 630 AM CDT Sat Sep 20 2025

Key Aviation Messages for the current TAF period:

- Multiple rounds of showers and thunderstorms

The first round of showers and storms is slowly eroding and pushing east out over Lake Michigan and northern Indiana. However, another cluster of storms is moving over northern Illinois. While the RFD TAF will start out dry as these storms are already east, these storms are expected over Chicago area between 14Z and 16Z. There is some uncertainty in how much they will maintain their current strength as the move northeast, but it warranted a TEMPO at DPA and ORD, with a VCTS at MDW. A break in the rain is expected after they pass.

Winds are expected to start out light and out of the southeast today before switching to the southwest around midday for the remainder of the TAF. The next round of showers and storms are expected to move into northern Illinois and northwest Indiana this afternoon. These storms will have the potential to have localized downpours that could reduce vis down to MVFR levels and wind gusts around 25 knots. There is some uncertainty with wind directions this afternoon. In the wake of the storms, there is a chance that they could become locally southeast or east for a short window. However, there was higher confidence in a southwest wind remaining for a longer period.

There is uncertainty with how much showers linger over the area past 00Z after the main wave of storms pushes through, so VCSH was kept prevailing through the end of the TAF. A PROB30 was added as well for the potential for yet another round of showers and storms, though it is expected to remain near and north of a KVYS to KMDW line.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None.

&&

$$

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NWS LOT Office Area Forecast Discussion

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