Marine Weather and Tide Forecast for Fairview Heights, IL

SHORT TERM /THROUGH MONDAY/
Issued at 315 AM CDT Sun Sep 20 2026

Showers and thunderstorms are currently moving northeast through northern MO and southeastern IA along a cold front and along the nose of the low-level jet. As they progress, the convection in northern MO is moving into a region with less favorable conditions and a weakening low-level jet to continue to support their development. As a result, these storms don’t pose a threat to our area aside from a stray shower in our northern counties during the early morning hours.

An upper level ridge, which has been the driver behind the heat and humidity during the last several days, is beginning to be suppressed by an upper level trough as it moves east. This trough will push the cold front into our area starting today.
Temperatures today will still be hot and humid to the south of the front as it begins its slow push through the region. This will lower high temperatures into the 70s and 80s in northern MO, while southeastern MO will remain well into the 90s. Thanks to slightly more moderate humidity values in the region, widespread 100+ heat index values are not expected. Monday’s high temperatures are slated to be even lower as the front makes its trek south through the region. However, there still remains some uncertainty in the exact high temperatures for Monday.
Latest NBM guidance’s interquartile range for the high temperatures is 6-10 degrees across the region due to the uncertainty in the front’s exact position and speed. That being said, behind the front we can expect high temperatures in the 70s and 80s with some areas in northern MO experiencing upper 60s for high temperatures.

Showers and thunderstorms are expected along and behind the cold front tonight into Monday morning. While the confidence is low on the exact timing and location of this convection, HRRR guidance shows there is a 80-100% chance across the region of measurable rainfall during this period, and 30-70% chance of seeing 0.25 inches of rain. These showers and thunderstorms will be multi-cellular with the strongest thunderstorms possibly producing heavy localized rainfall and gusty winds in regions where CAPE is sufficient (~1500-2000 J/kg). Due to the limited nature of the severe threat, the severe potential is not being messaged for this event.

LONG TERM /MONDAY NIGHT THROUGH SATURDAY/
Issued at 315 AM CDT Sun Sep 20 2026

The end of the relentless heat is in sight. Behind the front temperatures are expected to be much cooler. For Tuesday and Wednesday, NBM guidance shows that high temperatures will be in the low to mid 70s, with the northern portions of the area possibly staying in the upper 60s for their high temperatures (60-80% chance). Low temperatures will also dip into the 50s.
This air mass will continue to dominate for the rest of the week with a gradual warming through the 70s, reaching the low 80s by next weekend.

The GFS and ECMWF both show a mid-level ridge building over the Great Lakes and into the southeastern Canada region with a center of low pressure over the Ohio Valley by mid week. The ridge should remain fairly stationary over southeastern Canada, while the low pressure center will slowly move to the east coast of the CONUS. This pattern will allow for continued westerly and northwesterly flow to help moderate our temperatures into next weekend and limit the moisture transported into the region, preventing another return of the heat and humidity this week.

AVIATION /00Z TAFS THROUGH 06Z MONDAY/
Issued at 1047 PM CDT Sat Sep 19 2026

Area of showers and a few thunderstorms over north central Missouri is expected to move just north of UIN over the next several hours, however still expect the chances for showers will increase at UIN late tonight into Sunday with a chance (30-40%) between 09-14Z. There is an additional chance at UIN, after that but confidence is too low in timing to include in the TAF at this time. Expect MVFR ceilings to move into UIN by mid-morning remain through the rest of the period with a chance (30-40%) for IFR ceilings after 00Z. There are also low chances (20-30%) for showers at COU/JEF between 12-15Z. Chances for showers remain low at the St. Louis metro terminals until after 00Z on Sunday evening when I have continued a PROB30 group starting at 02Z. I have only included showers at this point as the coverage of thunderstorms is expected to be limited, though I cannot rule out thunderstorms at any of the sites during the time I have included rain chances. Ceilings will drop to MVFR after 06Z on Saturday night. Winds will generally be light, particularly outside of thunderstorms.

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.