Marine Weather and Tide Forecast for Chicago, IL

DISCUSSION
Issued at 309 PM CDT Thu May 15 2025

Late This Evening Through Friday Night:

The threat for severe thunderstorms will come to an end from west-to-east this evening as a cold front/dry line shifts eastward across the area. A much drier airmass will move in across the area in the wake of this feature tonight, and this will set up a rather quiet night across the area.

This very dry airmass will persist across the area into Friday and will act to foster deep diurnal mixing of the boundary layer as temperatures warm into the mid to upper 80s. With mixing heights expected to top 6,000 ft AGL into a layer characterized by 40-45 kt of flow, increasingly gusty surface winds are expected across the area late Friday morning and afternoon. We also would not be surprised to see some surface gusts reach these magnitudes, which would be right around wind advisory criteria (45 mph). If this occurs, it looks to be the most favored across my northern IL counties. Given the expectation of these very windy, dry and warm conditions a Fire Weather Watch was issued for my northern IL counties. See the fire weather discussion below for more on this.

The other concern from these strong gusty winds on Friday is the potential for blowing dust, particularly in areas of north central and northwestern IN that misses out of storms this evening. Very dry soils and recent farm field work will make for favorable conditions for dust to be picked up and blown in the wide open areas of northern IL. At this point no headline is planned for this, but a wind advisory and/or a blowing dust advisory could be needed for parts of northern IL west of the Fox Valley.

Later Friday afternoon we should see a better northward return of low-level moisture (low 60s dew points) across eastern IL into northwestern IN in advance of another approaching cold front. At this time it appears the best low-level moisture return will be mainly along and east of I-55 late in the afternoon and evening. This thus appears to be the best potential area for scattered late day and early evening thunderstorm development. The SPC highlights this general area in a level 2 of 5 severe risk. Conversely, the activity looks to remain more isolated farther to the northwest of I-55.
Regardless, any storms that develop will be capable of producing damaging wind gusts given the inverted-V type soundings with strong low-to-mid level lapse rates. Instances of very large hail would also be plausible, particularly with any supercells.
Expect the storm threat to come to an end by mid to late Friday evening following the passage of this second cold front.

KJB

Saturday through Thursday:

A strong, slow-moving Hudson Bay high will gradually shift eastward this weekend into early next week as western CONUS troughing settles toward the central Great Plains. Breezy conditions with seasonable temps in the upper 60s to low 70s are expected Saturday. A backdoor cold front crossing the area from the northeast early Sunday will result in high temps ranging from the mid 50s near the lake to the mid 70s well inland.

The trough will then slowly cross the Ohio River Valley and bring a a period of unsettled with with period of showers and some storms amid cooler conditions late Monday through midweek.

Kluber

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/
Issued at 713 PM CDT Thu May 15 2025

Key Messages:

- Scattered thunderstorms affecting the terminals early this evening. Severe wind gusts and large hail remain possible.

- Strong non-thunderstorm winds expected through mid-evening.

- Another period of strong southerly/south-southwesterly wind gusts expected during the daytime tomorrow.

- Westerly wind shifts expected behind cold fronts this evening and again tomorrow evening.

- Chance for gusty thunderstorms tomorrow during the late afternoon/early evening.

A broken line of storms near a cold front is tracking across northern and central Illinois at press time. While there is a chance that these storms may miss the Chicago metro terminals, the strongest of them will contain severe wind gusts (in excess of 50 kts) and large to very large hail. The threat for storms in the Chicago metro should end from west to east by 02Z-03Z.

Strong non-thunderstorm winds are also currently being observed with gusts in excess of 40 kts being reported at some locations.
These stronger gusts may continue for a few more hours before subsiding later this evening. A westerly wind shift should also occur this evening as the cold front passes through. Should also mention that there has been some blowing dust observed at some locations behind this front. Did not include a mention of this in the outgoing TAFs, but couldn't rule out some brief visibility reductions if this dust happens to pass over a TAF site.

Winds will pick back up again tomorrow morning out of the south to south-southwest with gusts in the 30-40 kt range likely to occur during the afternoon tomorrow. Another westerly wind shift is then expected behind another cold front tomorrow evening.

Lastly, scattered high-based thunderstorms may develop ahead of the aforementioned cold front and affect the terminals late tomorrow afternoon/early tomorrow evening. Given the abundance of dry sub-cloud air, these storms would be capable of producing strong to severe wind gusts to near and in excess of 50 kts. The going PROB30 groups highlight this potential well.
Have omitted a PROB30 group for TSRA from the RFD TAF for now as the probability of storms occurring there is currently closer to 20-25%, but it's possible that a PROB30 or TEMPO group for TSRA may eventually need to be added to the RFD TAF as well.

Ogorek

MARINE
Issued at 309 PM CDT Thu May 15 2025

Key Messages:

- Severe thunderstorms possible late afternoon into early evening - Gale Warning in effect Friday afternoon/evening for IL nearshore to Gary, IN

Thunderstorms appear poised to develop just west of if Lake Michigan late this afternoon into the evening. Any storm may produce gale to storm force winds and damaging hail. A brief period of southwesterly gale force winds cannot be ruled out behind the storms, as well. A small craft advisory is in effect this afternoon into the evening.

Finally, a period of southwesterly gale force winds are expected Friday afternoon, particularly northwest of Gary, Indiana. A Gale Warning has been issued to account for this.

Team LOT

FIRE WEATHER
Issued at 1138 AM CDT Thu May 15 2025

We have issued a Fire Weather Watch for our northern IL counties for Friday.

A much drier airmass is expected to shift over the area in the wake of a cold frontal passage later this afternoon. The dry airmass will remain in place across most of northern IL through most of Friday afternoon, before some better moisture recovery occurs late in the day across parts of east central IL and northwestern IN. As temperatures warm into the mid to upper 80s by early Friday afternoon, very low RH's in the 20 to 25 percent range are expected, particularly across northern IL. These very dry conditions are likely to combine with strong and gusty south winds (gusting up to 45 mph), thus creating a period of critical fire weather conditions.

A period of showers and thunderstorms are expected across parts of northeastern IL and northwestern IN late this afternoon into the early evening. Rainfall from these storms could help mitigate the degree of the fire weather threat to some extent for Friday, but areas that receive little to no rainfall will be primed for rapid wildfire spread on Friday as dry and windy conditions develop during the mid to late morning hours.

Very late in the day on Friday (after 4 or 5 pm), some better low-level moisture return is anticipated from the south in advance of another approaching cold front. At this time, it appears this better moisture return (near 60 degree dewpoints)
will occur generally along and east of the I-55 corridor. This may thus result in quickly improving conditions for these areas late in the afternoon, though this moisture may foster some very gusty late afternoon and evening storms. Farther northwest of I-55, continued dry and windy weather through the afternoon will likely continue the threat of wild fire spread through around sunset before abating winds and improving RH's end the threat.

KJB

LOT WATCHES/WARNINGS/ADVISORIES
IL...Fire Weather Watch from Friday morning through Friday evening for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023- ILZ032-ILZ033-ILZ039.

IN...None.
LM...Small Craft Advisory until 1 AM CDT Friday for the IL and IN nearshore waters.

Gale Warning from noon to 10 PM CDT Friday for Winthrop Harbor IL to Gary IN.