textproduct: Chicago

KEY MESSAGES

- Dense freezing fog this morning will affect the commute in the Chicago metro area.

- Today will be unseasonably warm and breezy with a threat for rapid brush fire spread.

- A pair of storm systems may bring accumulating snow to parts of the area this weekend, favoring areas near and north of the Wisconsin state line Saturday afternoon, and areas south of Interstate 80 Sunday night. - There is a growing signal for an active stretch featuring periods of showers and possibly thunderstorms mid to late next week and beyond, likely accompanied by much above average temperatures at times.

UPDATE

Issued at 619 AM CST Fri Feb 27 2026

Opted for a short fused Dense Fog Advisory in the Chicago metro. Strengthening southwest flow aloft has advected the overnight fog bank northeastward and it's gotten noticeably thicker on webcams (and here outside the NWS office in Romeoville). Considering that a moving shallow dense fog bank can be quite hazardous for drivers, with the Friday morning rush upon us, decided on the advisory (vs. continued special weather statement issuance). The advisory is in effect until 9 AM, though it's possible the worst conditions improve earlier in the 8 AM hour. Within the city of Chicago, warming conditions downtown may help keep the lowest visibility a few miles inland of the lakeshore.

Castro/Petr

RC

DISCUSSION

Issued at 350 AM CST Fri Feb 27 2026

Through Sunday Night:

Light winds and clear skies have allowed shallow fog to develop mainly southwest of the Chicago metro in Illinois early this morning. Below freezing temperatures temperatures could also lead to patchy slide spots developing where fog has become locally dense. Given the shallow and patchy nature of the fog, have opted to handle this with an SPS through 7 AM CST but a consideration for a Dense Fog Advisory may become warranted if the dense areal coverage increases toward daybreak. Expect any lingering fog to erode within an hour or two of sunrise.

We are headed for an unseasonably warm day today with strong southwest winds developing in response to a sub-1000 mb low moving across Ontario. Strong warm advection paired with deep mixing will allow temperatures to warm into the 60s across the area. In fact, some areas southwest of the Chicago metro could potentially approach or even reach 70F if the most deeply mixed guidance (HRRR/RAP) verifies. The depth of mixing will also play a crucial role in the upper bounds of wind gusts and lower bounds for the dewpoint/relative humidity, all important factors when assessing the potential for rapid spread of brush fires (more details on this can be found in the Fire Weather section below). HRRR and RAP remain fairly consistent in a brief window when winds could gust to 35-40+ mph for a few hours late in the afternoon before easing around sunset as winds turn west to northwest behind a cold front.

Attention then turns to a low amplitude shortwave expected to dive out of Canada toward the Upper Midwest on Saturday. This clipper-system may bring a period of f-gen driven accumulating snow somewhere in the region Saturday afternoon into the early evening; however, big question marks remain as to where this band ends up tracking. The latest forecast leans toward the more northerly guidance which keeps the center of the snow band north of the WI-IL stateline. There remain a few solutions with a farther south track which would take this band across northern Illinois. Forecast vertical profiles also suggest that there could be convective snow shower elements south of the main band of snow, which would include northern Illinois into northwest Indiana even with a more northerly track provided that there is sufficient saturation. Thus, opted to maintain snow chances across the area (20-50%), highest near the WI-IL state line. The temperature forecast on Saturday will also largely driven by the position of this band of snow which could result in a sharp gradient in temperatures across the local area, from as low as 30s north to 50s to around 60 well south of I-80.

Yet another system will follow on the heels of the first bringing the potential for accumulating snow to the region Sunday night. Similar to the first system there remains a lot of uncertainty in the system track. The latest blended guidance continues to have highest snow chances for areas south of I-80 toward central IL/IN. An expansive surface high that sets up across the Upper Midwest and broader Great Lakes region could help shift the northern periphery of the snow farther south, which some of the global guidance has been starting to catch onto, though the ECMWF remains the farthest north. With temperatures in the 20s any snow that does fall would readily accumulate on untreated surfaces. Will continue to monitor trends over the coming days. Stay tuned!

Petr

Monday through Thursday (and beyond):

While the month of February will finish up solidly above normal temperature wise, it still featured predominantly northwest flow aloft, resulting in well below average precipitation (and snowfall). This will change in a big way during the upcoming first week of March following a seasonably chilly first couple days of the month (possibly accompanied by some snow in spots). The pattern will transition to the negative phase of the Pacific North American (PNA) teleconnection pattern, characterized by western CONUS troughing and eastern CONUS ridging at the mid and upper levels. Such a pattern is commonly active locally, which should entail periods of beneficial precipitation as we head into the growing season.

Also notable in the upcoming pattern is the lack of any corresponding high latitude blocking in the Arctic and North Atlantic (positive Arctic Oscillation/AO and North Atlantic Oscillation/NAO). This will allow for the eastern ridging to become particularly pronounced, suggesting a generally above to well above average temperature regime possibly persisting all the way through mid March. However, in the early spring, even in a mild pattern, individual storm tracks play a large role in temperature distributions, as the chilly waters of Lake Michigan result in notably cooler temps in onshore flow north of storm tracks and/or reinforced warm fronts.

Turning from the big picture teleconnection pattern to the day to day, specific details are rather murky given a large range of solutions offered by the various ensemble solutions. The northward extent of an active baroclinic zone will be modulated by the proximity of surface high pressure to the region, which is quite variable between the ensemble systems and across ensemble members. An initial round of precipitation could reach into at least portions of the area in the Monday night-Tuesday timeframe, with the overall highest PoPs south of I-80. This could feature some threat for wintry precip as surface wetbulb temps may still be flirting with freezing.

Variance only increases into the mid to late workweek timeframe, lowering confidence in most of the specific details. The main exception to this is for rain being the predominant ptype with high confidence in the Wednesday-Friday period next week. In general, the GEFS is faster in bringing rain back to the area during the day on Wednesday and into Wednesday night, while the the foreign produced ensemble systems are more focused on the Wednesday night and Thursday timeframe. Regardless of exactly how the mid to late week period evolves, precipitable water of 250-300% of average/normal for early March could certainly spell corridors of heavy rainfall and an associated flooding threat. The extent (or lack thereof) of embedded thunderstorms will likely play a role as well, though for now, the forecast does not yet feature any explicit mention for thunderstorm chances. Finally, the exceptionally mild and moist cyclone warm sectors in the upcoming pattern may also feature a threat for organized strong to severe convection within in the general region.

Castro

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 606 AM CST Fri Feb 27 2026

Main Concerns:

- A period of LIFR conditions appears probable at ORD and MDW this morning, though it's uncertain how low VSBY will get.

- Strengthening southwest winds this afternoon, with gusts near to possibly above 35 kt at times late this afternoon.

- Winds will likely shift to northeasterly Saturday morning.

A bank of LIFR stratus and dense fog just southwest of ORD and MDW as of this writing should move across the Chicago terminals (and DPA) in some fashion between about 12-14z, with TEMPO LIFR conditions indicated. Once the low conditions improve later this morning, southwest winds will steadily increase. There looks to be a short period just ahead of an approaching cold front where gusts could reach as high as 35-40 kt. The direction will likely veer to west-southwest during the strongest winds. Early this evening, winds will shift to west-northwest with gusts in the 20-25 kt range behind the cold front passage. Winds should ease to around 10 kt from the northwest overnight and then shift to northeasterly Saturday morning. Expect thickening and gradually lowering VFR CIGs into Saturday morning.

Castro

FIRE WEATHER

Issued at 350 AM CST Fri Feb 27 2026

Threat for brush fire spread this afternoon...

Strong southwesterly winds are expected to develop across the region this afternoon. At this point, our forecast favors sustained winds increasing toward 15 mph and gusts ranging from 30 to 35 mph late morning through early afternoon. If more aggressive mixing scenarios pan out, sustained winds could exceed 20 mph and gusts could exceed 40 mph for a brief 2 to 3 hour window late in the afternoon. When paired with unseasonably warm temperatures in the low to mid 60s and afternoon minimum relative humidity values of 25 to 30 percent, conditions may approach Red Flag Warning criteria on a localized basis, particularly southwest of the Chicago metro area.

Borchardt/Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 9 AM CST this morning for ILZ012- ILZ013-ILZ020-ILZ021-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...None. LM...Small Craft Advisory until 9 PM CST this evening for Winthrop Harbor IL to Gary IN.

Small Craft Advisory from noon today to 9 PM CST this evening for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.