FXUS63 KILX 040811 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 211 AM CST Wed Dec 4 2024 .KEY MESSAGES... - There is a low (20%) chance that a band of briefly moderate- heavy snow accompanies a cold front this evening north of a roughly Macomb to Robinson line. The combination of gusty winds and snow would result in diminished visibilities and slick roads. - There is a 60% chance west-northwest winds gust over 45 mph this evening, resulting in difficult travel conditions for high profile vehicles. A Wind Advisory is in effect for areas along and north of roughly I-70. - There is an 80% chance minimum wind chills tonight are between roughly -11 and -3 north of I-70 and between -5 and 3 further south. This would increase the risk of hypothermia and frostbite to those outdoors who are inadequately dressed. && .DISCUSSION... Issued at 204 AM CST Wed Dec 4 2024 Nighttime microphysics satellite imagery shows an area of clouds around 3-5 kft AGL shifting across eastern IL with a warm front, which has resulted in gradually rising temps and dewpoints. Highres guidance suggests temps will continue to rise through early to mid afternoon when highs will surpass 40 degF in most places along and south of a roughly Macomb to Paris line; in fact, HREF suggests a 20-40% chance highs will reach 50 south of the I-70 corridor. This reprieve from largely below normal temps will be short-lived, however, as a potent storm system across the Great Lakes pushes a cold front southeast across the region this afternoon and evening. This system will be responsible for our three forecast concerns: (1) Snow squall potential this afternoon and evening, (2) gusty northwest winds this evening, and (3) bitterly cold wind chills tonight into tomorrow. ***Snow Squall Potential*** Timing: 4-8pm This Evening Confidence: Low Along and immediately ahead of the aforementioned cold front, there will be around a 30% chance for a line of rain/snow showers to develop north of a Macomb to Robinson line. If (big if) this develops, the precip (while brief) could become moderate-heavy given strong frontal forcing and the presence of instability; in fact, HREF mean/max for SBCAPE is 10/50 J/kg, and in the same area the RAP's snow squall parameter exceeds 2. This briefly heavy precip would lead to a quick changeover from rain (or rain/snow mix) to snow due to evaporational cooling in an initially mid 30s degF airmass, and could also aid in mixing strong winds aloft to the sfc. While a low confidence scenario, the potential for sudden visibility reduction in a burst of moderate-heavy snow and wind gusts surpassing 40 mph around or just after the evening commute time is something we want to bring attention to. We'll continue to monitor forecast trends from highres guidance and sfc obs to see how the potential shifts today. ***Gusty Northwest Winds*** Timing: 5-10pm This Evening Confidence: High Along and immediately behind the cold front, pressure rises of ~6mb in 3h will favor efficient mixing of strong winds aloft to the surface. Bufkit soundings from the NAM and GFS continue to advertise winds at the top of the mixed layer (nearly 850mb) around 40-45 kt, while both HREF and NBM suggest a 40-60% chance gusts surpass 45 mph (Wind Advisory criteria). We generally try to wait until confidence is nearer 80% to issue an Advisory, however given the meteorological factors (tight pressure gradient, strong CAA favoring efficient mixing, etc.) and in collaboration with adjacent offices we decided to go ahead and issue one. Whether we hit criteria or not, confidence is high it will be windy this evening with a 95+% chance for gusts over 35 mph. ***Bitterly Cold Wind Chills*** Timing: Tonight-Thursday Morning Confidence: High While the timing of strongest winds and coldest temps will be offset, there will be some overlap tonight into early tomorrow morning as sustained winds even by sunrise have an 80% chance of falling somewhere between 14 and 20 mph according to HREF. At that time, forecast temps range from around 10 degF in Galesburg to 15 in Lawrenceville. This combination would result in wind chills between roughly -10 and -3 north of I-70 and -5 and 3 further south. This isn't quite hitting Cold Weather Advisory criteria, however it is dangerously cold to anyone inadequately dressed. We'd encourage folks to make sure they're bundled up, carry an extra layer in the car, and ensure kids are appropriately dressed (especially if they have to wait at the bus stop). Temps will rebound over the weekend as heights rise across the Ohio Valley and Great Lakes behind the departing trough. The deterministic models also depict a cut off low getting swept up by the jet stream and lifted across the Midwest at some point late Sunday-Tuesday, which could further increase warm advection across the area. While there's considerable spread between the GFS, EC, and CMC on the evolution of this wave, precip chances come up with its approach late Sunday, by which time NBM gives the region a 50-80% chance for highs over 50 degF. Fortunately temps should be warm enough to keep this all rain, even overnight into Monday morning given forecast lows in the low to mid 40s. NBM/LREF mean suggest temps start to trend cooler Tuesday into Wednesday, though there is considerable spread on when the next deep trough encroaches on the area; for example, the 04.00z EC has the aforementioned cut off low deepening over the Great Lakes and blasting the Midwest with cool advection late Monday-early Tuesday, while the GFS maintains more of an open wave-type pattern and has the stronger trough/cold air push arriving mid week. For what it's worth, NBM has a 40-70% chance morning lows are back in the teens Wednesday morning. In addition, a subset of about 20-30% of LREF membership has measurable snow falling sometime Tuesday-Wednesday - something we'll keep one eye on over the next several days. Bumgardner && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1122 PM CST Tue Dec 3 2024 Strengthening southerly winds and VFR conditions are expected overnight ahead of a an area of low pressure. A prefrontal trough will move across central Illinois late morning and afternoon turning winds southwesterly to westerly with gusts increasing into the 25-30kt range. MVFR ceilings may accompany this trough, especially along the I-74 corridor. A cold front will sweep across the region Wednesday evening with winds turning northwesterly and gusts further increasing above 30kt. A band of snow showers may briefly move across the terminals. Confidence in snow on station is low but if it does occur could briefly produce very low visibility under a half mile. Deubelbeiss && .ILX WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 3 PM this afternoon to midnight CST tonight for ILZ027>031-036>038-040>057-061>063. && $$