Chicago O'Hare Weather Area Forecast Discussion Thursday June 15, 2023

Area Forecast Discussion 
National Weather Service Chicago/Romeoville, IL 
1259 AM CDT Thu Jun 15 2023

.UPDATE... Issued at 809 PM CDT Wed Jun 14 2023

Just a couple of minor adjustments to the going forecast this evening, primarily to increase sky cover somewhat to account for wildfire smoke aloft, to shift the back door cold front an hour or two earlier on Thursday, and to add haze mention mainly to portions of the IL forecast area Thursday based on HRRR near- surface smoke progs and upstream obs where low-level smoke has been affecting surface visibility.

Weather conditions were fairly quiet across the forecast area this evening, with light mainly easterly low level winds and generally partly cloudy skies. Visible satellite imagery as well as our out the window view does show considerable amount of wildfire smoke aloft however, and upstream surface obs across parts of western WI and central/southern MN indicating visibilities in the 1-4 mile range indicative of smoke closer to the ground. RAP/HRRR near surface smoke forecasts depict this spreading southeast into northern IL by Thursday morning, which is expected to result in hazy/smoky conditions at the surface for a time. This low-level smoke is expected to be pushed out of the region during the afternoon however, as a cold front currently stretched across the upper Midwest accelerates south-southwestward and allows breezy northeast winds to develop. Based on latest high-res model runs, have moved the timing of the cold front up an hour or two across the Chicago metro area into the late- morning/midday period, and have also added areas of haze to our IL counties (front largely prevents if from making it into IN) based on the 18Z HRRR near-surface smoke progs. Have also bumped sky cover grids through the period to account for higher-level smoke as well.

With the slightly earlier frontal passage along the IL Lake Michigan shore, will also move the start time to our Beach Hazard 2-3 hours earlier, due to expected building wave heights with the breezy northeast winds behind the front.

Otherwise, going forecast is in good shape, with falling temps across northeast IL and northwest IN behind the front Thursday, along with a slight chance of a few stray showers/isolated thunderstorms possible ahead of the front. No other changes needed at this time.

Ratzer/NWS Chicago

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.SHORT TERM... Issued at 327 PM CDT Wed Jun 14 2023

Through Thursday night...

Low clouds remained hugged to the Lake Michigan shoreline Wednesday morning, but have started to clear. Meanwhile, light fields of cumulus clouds have bubbled up farther inland. There is a light haze that is featured on satellite imagery, most likely still some of the smoke from the Canadian wildfires moving over the area. Luckily, the concentrations have been fairly light and aloft offering little impacts to Chicagoland. Winds have been generally light and onshore. Continued warming under the clearer skies will raise temperatures inland into the low 80s, which is fairly typical for mid-June. And yet, areas along the immediate Lake Michigan shoreline, including parts of downtown Chicago will be in the low 70s if not struggling to leave the upper 60s, which is around 10 degrees below normal for this time of year. Precipitation chances for Wednesday afternoon look fairly weak. Limited levels of CAPE are present in the north-central portions of Illinois, so under diurnal heating there is around a 15 to 20 percent chance that, this afternoon, a stray shower may pop u Pleaving a trace to a couple hundredths. However, it will be mostly focused on the interior portions of the CWA as the clouds over the lake will shield the shoreline.

Wednesday`s clouds over the lake are on the far western tail of a low pressure system centered around Western New York that continues to move east. Ensemble models are in agreement with the higher-res NAM alluding to a short wave trough at 500 mb developing and extending westward from the center of the low over Lake Superior, and dropping southward toward NW Indiana. At the surface, a stark baroclinic zone along a cold front will likely develop and dro Psouthward. With the vorticity enhancement from the trough aloft and the front adding a lifting mechanism, a renewed slight interest in showers and the potential for thunder look to be in store for Thursday afternoon. However, model dew points ahead of the front do not look overly impressive in the upper 50s and low 60s, so the probability for the rain was kept below a 25 percent chance with the expectation of a mostly dry front with any precip that falls being between a trace and a couple tenths of an inch. As dry conditions persist with the worsening drought concerns, this system does not look to be potent enough to break that.

The main impacts with this system will be from the winds and temperatures. Winds will turn to the north Thursday morning and increase, particularly over Lake Michigan, eventually turning northeast and providing a lake enhanced breeze that can reach farther inland Thursday afternoon. Timing will be watched closely. For now projecting winds to start moving inland around noon, but that can be adjusted if the front develops and moves slower then guidance is currently anticipating. With northerly fronts such as this that have plenty of fetch associated with it, the strong winds will likely produce fresh swell generating conditions on the lake that will be hazardous for smaller vessels. Additionally, dangerous swimming conditions will be present with a heightened risk for rip currents on Thursday evening and into Friday morning.

The speed of the front will also determine how well the temperature forecast verifies. Inland high temperatures are forecast to be in the mid to upper 80s, while temperatures behind the front will struggle to climb out of the low 70s. For now, confidence remains such that the southern part of the CWA should still reach the 80s despite the afternoon forecast package nudging the deterministic numbers down lightly. The northern part of the CWA, including the lake shoreline will likely remain below normal in the 70s and 60s, respectively.


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