736 FXUS63 KLOT 271633 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1033 AM CST Tue Feb 27 2024 .KEY MESSAGES... - Today will likely be the warmest February and meteorological winter day ever recorded in our area - Thunderstorms, possibly severe, will probably impact at least portions of our forecast area this evening - Power-house cold front will rip across the area late this evening with a truly mind-boggling apparent temperature drop of 60-75 degrees in less than 12 hours - Chance (40-60%) for quick hitting accumulating snow late tonight with threat of hazardous travel - Briefly turning sharply colder Wednesday into early Thursday before another likely significant warm-up into the weekend && .UPDATE... Issued at 1028 AM CST Tue Feb 27 2024 As of around 9 AM, a mature low pressure system is spinning over Minnesota bringing blizzard conditions to northern portions of the state. Farther south over northwest Missouri, a secondary surface low is taking shape along the storm's cold front. Wind obs also suggest that a warm front may be trying to develop east of the low extending toward the river. This secondary low is our primary feature of interest for severe convection later today. Some unexpected low clouds and fog across interior parts of the CWA have kept a chunk of the area cooler and with higher dewpoints this morning than previously forecast. However, it looks as though values are leveling out with the forecast as conditions clear up. Otherwise, the forecast for today remains in good shape. Additional low clouds tucked beneath a capping inversion are spreading northward over the southern half of the CWA. We see instability building atop this inversion with latest RAP guidance suggesting as much as 1,500 J/kg of MUCAPE have built up in our south. Instability will continue to build across the area through the day, although will remain capped to surface-based convection through at least 3-4 pm. Guidance is in good agreement that the low will track right through the heart of the CWA. Strong convergence near the low pressure center puts areas near the track at the seemingly highest risk for severe convection. Namely, this would likely mean the I-80 corridor and points north. Hail remains the primary concern owing to large instability, terribly steep mid level lapse rates, and strong low-level inflow and 50+ knots of effective deep shear, though damaging wind is a secondary concern as well. The tornado threat is largely conditional on whether storms can beat out the reinforcement of the cap with nocturnal cooling and go surface based. It'll be a tight window of opportunity, but late afternoon (from the time of CI) through around 6-8 PM looks like the best bet. In our south and southeast, confidence is rebounding, in some aspects at least, with the severe potential. A key feature that we'll be eyeing in this corridor is this pseudo dryline feature that could aid in parcels punching through the cap and firing up surface based convection. This would also enhance the tornado risk, but again the potential lies in a seemingly narrow window early this evening. Recent model soundings are resolving weaker and weaker capping later today. The HRRR actually just recently reintroduced convection in our south this evening with its 14Z run. We'll have to keep a close eye on obs and CAM soundings as the low approaches and this environment comes together. Doom && .DISCUSSION... Issued at 238 AM CST Tue Feb 27 2024 Today: It will feel more like June than February, in fact there are frequently days in June that aren't as warm as today will be! Once again Monday, temperatures exceeded the warmest available guidance and that will likely be the case again today. The boundary layer will not be as dry as Monday, however temperatures will be starting more than 20 degrees warmer than Monday morning. There will be a gradual increase in mostly high cloudiness today, but it looks like the morning into the early afternoon should see plenty of sunshine. Given 925mb temps are progged to be 2-3C warmer than Monday, a good starting point for highs today would be about 4-6F warmer than Monday. This is close to the NBM 90th percentile which has been consistently too low. Going to run with those temps which is mid-upper 70s across the CWA, which would also be the warmest February and winter day ever recorded. An impressive elevated mixed layer over the region should provide a stout capping inversion. It looks increasingly likely that convection will be hard pressed to develop prior to 00z. Have maintained some small PoPs over the southern CWA in advance of a dry line in the 22-00z time frame, but stronger forcing doesn't arrive until this evening, so it seems like the cap will hold. Tonight-Wednesday: A meteorologically fascinating 12 hour period of weather is set to unfold across our region tonight. There remains a unusually large spread in guidance regarding where, when, and even if convection will develop. It appears that a bulging dryline will be surging across areas roughly south of I-80, with a relatively moist warm sector arcing back west across northern IL toward a triple point over northwest IL. Strong synoptic ascent ahead of the advancing trough along with strong convergence near the triple point seems likely to spark at least scattered thunderstorm development early this evening north of I-80 across northern IL. Some threat exists that additional storms could develop farther south along the bulging dry line. However, confidence is lower in this area as there are questions about how much moisture will survive afternoon mixing and pool ahead of the dry line and if it will be enough to allow for convective initiation as synoptic ascent begins to weaken the cap. Instability and shear look sufficient to support supercells this evening, the question will be whether convection will be north of the warm front and clearly rooted above the boundary layer or if some storms can form in the warm sector. If there is warm sector storms, it's unclear if they will be rooted in the boundary layer. Forecast soundings show increasing BL inhibition this evening, though it is possible that this inhibition may be overdone if low level moisture doesn't mix out today. Ultimately, large hail looks to be the most prominent potential severe weather hazard this evening. That said, should a supercell or two become rooted in the boundary layer, then low level shear profiles suggest there would be a tornado threat during the early to mid evening hours. This is very conditional risk and we'll be closely monitoring the evolution of the mesoscale environment through the day today. A truly power-house cold front will come ripping across the CWA late this evening, clearing the eastern CWA not too long after midnight. This should end the severe weather threat quickly and result in temperatures crashing at a break neck pace. Hard to put this level of detail into the grids, but with a rapidly deepening surface low and very strong pressure rises behind the front, it is possible there could be a quick hit of very strong winds behind the front, with gusts potentially briefly peaking at or above 50 mph. This type of brief, intense wind burst is something that will be better handled in the "nowcast" time frame of the forecast and have not included winds of this magnitude in the grids. Even if this intense wind burst doesn't occur, frequent gusts of 30-40 mph are expected behind the front overnight into the first half of Wednesday with at least occasional gusts to 45 mph possible. Later shifts may need to consider a Wind Advisory for at least portions of our area during this time frame. There remains considerable spread in the guidance regarding the potential for a post frontal band of snow whisking rapidly across the area late tonight. It is going to be a very dynamic and quickly changing environment overnight, so it isn't too surprising to see the spread in guidance. It is concerning to see models like the NAM, ARW, and HRRR squeezing out enough QPF to support an inch or two of snow for some areas. The GFS is similar, but confined to mostly our far northwestern CWA, which the ECMWF suggests any post front precip will occur before temps cool enough for snow. It's really difficult to handle this threat in deterministic grids, so will likely try to message this threat with some probabilistic graphics this morning. Unfortunately, the difference between the 25th percentile snow amount and the 75th percentile snow accumulation is the difference between a potentially hazardous and difficult Wednesday morning commute and little or no problems. Hopefully morning guidance will come to a better consensus on the snow potential tonight. Finally, can't stress enough how dramatic the temperature change will be from this evening to Wednesday morning. We are literally going to go from June-like temperatures this evening to frigid January level temperatures by sunrise Wednesday morning. Have trended temps colder in the grids late tonight into Wednesday morning and am now explicitly forecast wind chills in the single digits above zero to several degrees below zero over the northwestern CWA. While skies will clear out Wednesday, it will be deceptive sunshine Wednesday as strong northwest winds and only a very modest rebound in temps will result in wind chills struggling to get any warmer than 10F above zero Wed afternoon. - Izzi Wednesday Night through Monday: While the below normal temps Wednesday and Wednesday night will feel harsh coming off the exceptional, record breaking warmth of Monday and Tuesday, the pendulum will swing right back to spring through the weekend. A brief reinforcing surge of cold air advection Wednesday evening will keep breezy/blustery conditions going for a few hours, with west-northwest winds gusting up to 25-30 mph. Wind chills will bottom out in the single digits to lower teens. Ridging from 1030-1035 mb surface high pressure passing to our south will cause winds to quickly diminish and turn light southerly overnight. Good radiational cooling will support lows in the mid teens to around 20F. The southerly pressure gradient will quickly tighten as the surface high slides east Thursday afternoon. Southerly winds gusting to 25-30 mph, warm advection, and full sunshine will allow afternoon high temperatures to recover to the lower to mid 40s, or a bit above normal for leap day of what will go down as one of, if not the warmest Februarys on record. A stout low-level jet overhead Thursday night should result in breezy conditions at times, limiting the temperature drop. A minority of model guidance brings a short-wave far enough north on Friday to spread clouds and showers into the area, so our official forecast has a sliver of slight (15-20%) chance PoPs over the far southern CWA. If the minority solution does end up coming to fruition, temps may under-perform. On the other hand, if the short-wave remains to our south, the mid 40s to lower 50s highs in the forecast may be a bit conservative. The weekend will offer another taste of early spring fever thanks to well above normal mid-level heights downstream of deep western troughing. Persistent warm advection, breezy south-southeast winds, and plenty of daytime sun suggests highs well into the 60s if not warmer on Saturday and upper 60s to mid 70s on Sunday, if not warmer in spots. The official forecast is above most of the raw guidance, though below the 90th percentile of the NBM distribution, which has performed better (even slightly underdone at times) on our recent mild days. With the ground drying out again and breezy/gusty southerly surface flow, we'll have to watch for elevated fire weather concerns if dew points and RHs end up lower and temps a bit warmer than forecast, as appears possible. The only minor exception to the weekend warmth is along the Illinois shore where any east of south component to the wind will keep daytime temps several degrees cooler. Western troughing should eject eastward, at least piece-meal, early next work week. This will result in another fairly sharp cold front passage Monday PM/night or Tuesday, though unlikely to the magnitude of tonight's per medium range guidance. Showers and even thunderstorms are plausible out ahead of the cold frontal to start the work week. The likely (55-70%) PoPs on Monday and Monday evening in the official blended forecast appear reasonable enough at this range given good ensemble support. Castro && .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/... Issued at 559 AM CST Tue Feb 27 2024 Main Concerns: - Brief lower CIGs possible early this morning - Scattered TS this evening, possibly severe - Powerful cold front passage late this evening with strong and gusty northwest winds through Wednesday morning - A period of snow/snow showers possible late tonight into early Wednesday, with pavement accumulation and BLSN possible Patchy lower CIGs and some BR developed overnight, including at DPA. Brief high IFR or low MVFR CIGs may occur at ORD, MDW, GYY if the patchy CIGs happen to move overhead. The rest of the daylight hours today expect breezy south-southwest winds gusting to 20 kt or a bit higher at times. A surface low pressure system and very sharp cold front will approach this evening, with scattered TSRA expected to develop somewhere over northern/north central Illinois and track northeast. Maintained TEMPO mention for this. Ingredients favor the potential for any TS this evening to be severe with large hail the main threat. There's a low chance for a tornado near a terminal. Strong northwest winds will develop behind the cold front passage late this evening and it's possible a brief burst of 40-45 kt winds may occur immediately with the front. Gusts should average in the 30-35 kt range through Wednesday morning. Finally, as sharply colder air rushes in late tonight, a disturbance may produce a period of accumulating snow that would accumulate on runways if it moves directly over terminals given well below freezing temps by then. The strong winds may cause BLSN issues as well. Any lingering snow showers/flurries should end by 12-14z Wednesday, with lower to mid MVFR CIGs expected to develop likely to linger through the morning. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 2 PM CST this afternoon for the IL and IN nearshore waters. Gale Warning from midnight tonight to 6 PM CST Wednesday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago