textproduct: Central Illinois

KEY MESSAGES

- A fast-moving storm system will bring another round of snow to central Illinois today and tonight. Snow accumulations will generally range between 2 and 5 inches. Plan on travel impacts this evening into the overnight where the more intense snow bands develop.

- Much below normal temperatures will prevail through next weekend...with the coldest readings arriving Thursday night when 12z NBM guidance suggests a 50-70% chance of lows dipping below zero along and north of a Macomb to Tuscola line.

DISCUSSION

(through next Sunday) Issued at 329 AM CST Mon Dec 1 2025

A broad mid-level trough sweeping across the Upper Midwest continues to tighten the height gradient over the region, with the latest QG diagnostics indicating a corridor of pronounced 700-mb Q-vector convergence centered directly over central and southeast Illinois between 18z and 06z. This supports a period of enhanced large-scale ascent, maximized late this afternoon into early evening. While moisture remains modest, the combination of synoptic lift and increasing mid-level FGEN (700-600 mb) should be sufficient to generate moderate to occasionally heavy snowfall, mainly north of I-70.

The latest 00z HREF/LPMM QPF guidance exhibited a slight shift southward with the axis of heaviest QPF, and this trend was reaffirmed by the 06z HRRR/NAMnest guidance, which now focuses the axis along the I-72/Danville corridor. For this reason, we have expanded the Winter Weather Advisory further south.

In general, the QPF amounts did not change much, with most of the region lying somewhere in between 0.1-0.3 inches. Cobb/NAM mesoscale output continues to suggest a SLR closer to 15:1 throughout the duration of this event, and even that might be considered modest given the deep/saturated DGZ. This suggests snowfall totals in the 2 to 5 inch range. And, where the HREF/RRFS LPMM QPF hints toward 0.4 to 0.5 inches within mesoscale banding, totals could exceed 6 inches locally.

Confidence is high for an advisory-level snow because the Q-vector convergence and strengthening FGEN provide a robust dynamical forcing mechanism. However, actual totals will be sensitive to the mesoscale placement of banding. A shift of the band by 30-60 miles changes which counties get the higher totals.

Forecast uncertainty remains an issue for areas near and south of I-70. Coarser guidance, such as the RAP13 model, suggests a second band of moderate snowfall could develop across far southeastern Illinois late tonight. This is associated with an area of sharpening mid-level FGEN (700-600 mb) and a compact shortwave pivoting across the Missouri Bootheel, which will augment forcing. However, the broad synoptic ascent appears weaker than the initial snow band that moves through central Illinois earlier in the day.

This uncertainty, which is well-represented in recent CAMs (Convective-Allowing Models), is why we have paused on expanding the Winter Weather Advisory south of I-70. Since the potential second push of moderate snow across our southeast counties is not expected until after 00Z, we have time to monitor the trends and make adjustments as needed.

Temperatures in central Illinois will oscillate between very cold and just plain cold this week due to shifting high pressure. A brief warming trend is forecast for Wednesday afternoon, offering the best chance for much of the area to finally climb above the freezing mark. This temporary warm-up is driven by a southwest wind shift ahead of a weak cold front approaching from the north.

The passage of this front may bring a few snow showers Wednesday evening, but any accumulation is expected to be limited to a dusting. Following the front, a period of sharply colder air will settle in. There is high confidence that overnight lows will dip into the single digits on both Wednesday and Thursday nights. While a Cold Weather Advisory is currently unlikely, wind chill values are still anticipated to fall below zero.

One notable trend in the extended forecast is a southward shift of the late week winter storm system. It is hard to tell if this is indeed the trend, or just a one-off with the latest round of model guidance. Nevertheless, the chance for a 1 inch snowfall across our area is now limited to a 1 in 7 chance, which is considerably lower than what we were seeing yesterday. Consequently, the freezing rain potential also diminished.

MJA

AVIATION

(For the 12z TAFs through 12z Tuesday Morning) Issued at 519 AM CST Mon Dec 1 2025

Mainly MVFR to begin this TAF period, conditions will quickly deteriorate near or just after 18z as snow moves across the regional terminals from west to east. Snow may be heavy at times, likely lowering flight categories to LIFR. Visibility will greatly improve prior to 06z as the snow exits, but ceilings will largely remain in the IFR to MVFR range through the remainder of this TAF period.

MJA

ILX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from noon today to 6 AM CST Tuesday for ILZ027>031-036>038-040>057-061.