NWS FORECAST: KORD | KPWK | Forecast Graph O’Hare
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O’HARE FORECAST …
Thursday/Veterans Day: Showers, mainly before 11am. Temperature falling to around 48 by 5pm. Breezy, with a south southeast wind 15 to 20 mph becoming west southwest in the morning. Winds could gust as high as 35 mph. Chance of precipitation is 100%. New precipitation amounts between a quarter and half of an inch possible.
Tonight: A 20 percent chance of showers after 1am. Mostly cloudy, with a low around 38. Southwest wind around 15 mph, with gusts as high as 25 mph.
Friday: Scattered rain and snow showers before noon, then snow showers likely. The snow could be heavy at times. Cloudy, with a temperature falling to around 37 by 2pm. Breezy, with a southwest wind 15 to 20 mph, with gusts as high as 30 mph. Chance of precipitation is 70%. New snow accumulation of less than a half inch possible.
Friday Night: Scattered snow showers, mainly before 9pm. Mostly cloudy, with a low around 34. West northwest wind 10 to 15 mph. Chance of precipitation is 40%.
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Busy weather for Veterans Day morning. Main forecast messages and any changes are:
* A 3-4 hour period of soaking rain showers this morning remains on track to likely interfere with some community veteran tributes, though a sharp end to the rain late this morning
* Basically no change in regular 30-35 mph gusts today and Friday; spotty 40 mph gusts possible this afternoon
* Boosted likelihood of snow showers on Friday with a more consistent signal of periodic heavier rates; if materializing would result in brief very low visibilities and dustings
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The upper trough the NWS Chicago Office has been watching in forecast model solutions for a week is now digging into the Upper Midwest as a 145 kt upper jet (as measured last evening over the Pacific Northwest) carves into the country’s midsection. This all means continued deepening of the surface low today of about 8 mb over northwest Wisconsin with its progressive cold front swinging eastward over the forecast area mid-late morning. In advance of this, a corridor of showers is approaching early this morning. A lead short wave on water vapor imagery in southern Missouri is forecast to move northeast across the eastern Chicagoland helping to keep shower coverage high. This also brings a narrow plume of elevated instability into the far southern/southeastern forecast area between 6-9 A.M. We could see isolated embedded storms with this in south and southeast Chicagoland. Rain ends quickly from west-to-east from 10 A.M. to 1 P.M. as the developing dry slot advances over. Some spotty sprinkles are possible in lingering stratocumulus this afternoon into evening.
Southerly winds in advance of the cold front have been under-performing on gusts early this morning. Within the rain showers upstream, there have been a couple 30+ mph gusts but that’s been about it, according to NWS Chicago. So really not expecting the main widespread wind gust push until after the rain ends and the low-level cold advection and steepening low-level lapse rates ensue. Some models do show ephemeral 40 mph winds at the top of the channel. Do not see gusts overperforming from that given some clouds expected in the dry slot and peak pressure rises of only 2-3 mb/3 hr due to the slow overall movement of the surface low. Temperatures will take a sharp drop of five degrees or so with the frontal passage late this morning and then hold nearly steady the rest of the day.
The upper low will continue to dig into the Minnesota/Wisconsin border region by tonight, with the developing synoptic scale comma-head of deeper moisture set to wrap back into northern Illinois this evening. A re-increase in cold advection at/near 850-925 mb (top of the mixed layer) will support cloud depth increasing and likely some scattered showers developing north of I-80 later tonight. Any of these would probably be rain at first due to a lack of cloud ice, but with continued cooling through daybreak some snow could start to mix in any showers. Overall there is low confidence in the shower coverage during this time including into the Friday morning commute.
Confidence has increased for Friday to bring the first frozen precipitation of the season to many Chicagoland communities in the form of scattered/numerous snow showers (50-80 percent coverage). It’s a generally classic setup on all the primary models with …
1.) a strong upper vort/cold trough,
2.) cold advection in the lowest 2 km producing instability (50-90 J/kg) from the surface that taps into the ice nucleation layer, and
3.) sufficient wet bulb cooling for snow at the surface. Additional support to that are CAM (Community Atmosphere Model) solutions that go through Friday and depict scattered 35-45 dBZ late morning through early evening. Given the current forecast vort max path, much of Chicagoland looks like it could be in this footprint, with mesoscale foci driving where snow showers may be a bit more regular. A failure mode would be that the ice nucleation layer is not tapped as greatly due to either less CAPE (Convective Available Potential Energy) depth or more marginal saturation with respect to ice. Precipitation then may be more graupel or “low quality” snow.
In terms of impacts, sharply varying visibility would be the case if the heavier snow showers indeed do setup late Friday morning through early Friday evening. The 30-35 mph gusty west-southwest winds on Friday would only add to that. The HRRR (High-Resolution Rapid Refresh) depicts some sub 1 1/2 mile visibility, which for a CAM is not bad for this type of setup. As for accumulation, it seems likely at this time some places would get a dusting, but what type of coverage of that is hard to say. Warm, early season ground temperatures could be overcome by heavier rates, but again those rates should be brief/transient due to quick cell motions.
DEW
Light dew on vehicle metal only, grass damp. Dew point depression of 5 to 9°F, cloudy overnight last night.
Weather Radar shows a long band of shower in western Wisconsin, eastern Iowa and the entire length of Illinois from northwest Illinois to southeast Illinois. The precipitation is moving northeast with some precipitation reaching toward the northwest suburbs as close as DuPage County at 6:40 a.m.
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