textproduct: Chicago

KEY MESSAGES

- Low clouds, drizzle and areas of dense fog persist this morning.

- Next period of widespread rain expected Friday morning through Friday evening.

- Chance of rain Saturday night/Sunday Morning and again Monday night into Tuesday.

DISCUSSION

Issued at 156 AM CST Thu Dec 26 2024

Through Friday:

The threat of dense fog continues to be the primary forecast concern this morning, particularly across far northern IL (generally north of I-80). Surface observations and webcams across this area have continued to show deteriorating visibilities over the past couple hours as the lowering stratus deck continues to thicken the fog deck. However, as of this writing the fog has not yet become dense enough (VSBY < 1/4) to justify the issuance of a widespread dense fog advisory. This may likely change over the next couple hours, however. One potential fly in the ointment is an area of developing shower activity recently noted across southwestern sections of the area. While these may not necessarily persist very long early this morning, they may (at least briefly) improve the local visibilities in the areas they impact. We will thus continue to monitor observational trends over the next couple hours to determine if a dense fog advisory will be required across northern IL this morning.

The fog will persist through the morning in most areas, before some gradual improvement begins to occur from south-to-north through the afternoon. However, some areas of fog may persist for much of the day, particularly for areas north of I-88 along and west of the Fox Valley. While improved visibility is likely into this afternoon, cloud cover will persist area-wide through the day. This will thus make for another dreary day with some occasional patchy drizzle at times through the day. In spite of all the cloud cover, temperatures will be warming through the 40s today as we continue to advect a warmer airmass northward into the area. This turn towards milder weather will continue through the weekend.

Rain chances for tonight have diminished as we await the arrival of the next Pacific impulse (and an associated surface low) on Friday. While I did maintain a small chance (20%) for light rain and drizzle tonight, much of the area may end up largely precipitation free until rain chances quick ramp up during the daylight hours of Friday. Periods of rain look to be a good bet for the area from late morning through the evening as the weather system lifts northeastward into the Western Great Lakes.

KJB

Friday Night through Wednesday:

Primary forecast concern is precip chances with, one over the weekend and another next Monday night into Tuesday.

As the first system departs from the area Saturday morning, there may be a lull in the precipitation Saturday afternoon into Saturday evening, before the next system moves across the Ohio Valley Saturday night and across the eastern Great Lakes Sunday. There remains reasonable ensemble support for rain spreading back across at least the southeast half of the cwa Saturday night but this will be dependent on the track of the system. Though high temps will reach well into the 50s on Saturday, cooler air will spread into the area Saturday night but precip type still appears to be all rain with temps too warm to support any mix with snow. The rain should be ending sometime during the day on Sunday with Sunday night and Monday looking dry.

Another system for early next week has a bit less support in the ensembles with location/track differences but rain would potentially spread back across the area Monday night and continue into Tuesday. With a bit cooler air spreading into the area on the back side of this system, there is a low chance for some mix with snow as precip ends Tuesday afternoon/evening, as timing looks currently. A mix was included in the blended guidance and made no changes.

Models then suggest a possible pattern shift with colder air spreading back into the region for the middle and end of next week though temps may still be near or just below normal for early January. cms

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 536 AM CST Thu Dec 26 2024

Forecast concerns include...

Vlifr/lifr cigs/vis with areas of dense fog this morning. Ifr cigs this afternoon lifting to mvfr this evening.

Areas of dense fog will continue through mid morning mainly west of ORD/MDW and north of a VYS/LOT line. Confidence is low for how long the lowest visibilities will persist as there will be slow improvement from southeast to northwest. Fog may persist through much of the afternoon across northwest IL and at RFD. Once the fog lifts later this afternoon or possibly early this evening across far northern IL, no additional fog is expected tonight. Cigs will also slowly lift later this morning and likely into ifr this afternoon and then low mvfr this evening. However, cig trends are quite a bit uncertain as they are lifting into high mvfr/low vfr across parts of eastern IL and western IN. Guidance is aggressive showing these higher cigs moving across the Chicago terminals this afternoon into this evening and that is possible, but confidence is low. Have trended slower and more pessimistic with cigs into this evening but eventually, at least mvfr cigs are expected tonight.

Patchy drizzle and a few showers are possible this morning with another chance of showers this evening. Rain is expected to spread across the area late Friday morning into Friday afternoon, around or just after the end of the new 30 hour ORD and MDW taf period. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Dense Fog Advisory until 9 AM CST this morning for the IL and IN nearshore waters.