MORNING PREVIEW: Weather, Traffic Monday, December 27, 2021 Dense Fog Advisory

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NWS FORECAST:

KORD | KPWK | Forecast Graph O’Hare

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 O’HARE FORECAST … 

Today: Widespread dense fog, mainly before noon. Otherwise, cloudy, then gradually becoming mostly sunny, with a high near 46. Breezy, with an east southeast wind 5 to 10 mph becoming west 15 to 20 mph in the afternoon. Winds could gust as high as 30 mph.

Tonight: Mostly cloudy, with a low around 31. West wind 10 to 15 mph becoming north 5 to 10 mph after midnight. Winds could gust as high as 25 mph.

Tuesday: Snow before 1pm, then rain and snow between 1pm and 3pm, then rain likely after 3pm. High near 38. Southeast wind 5 to 10 mph increasing to 10 to 15 mph in the afternoon. Winds could gust as high as 30 mph. Chance of precipitation is 100%. New snow accumulation of less than one inch possible.

Tuesday Night: A 20 percent chance of rain before midnight. Mostly cloudy, with a low around 29. Southeast wind around 10 mph becoming west after midnight. Winds could gust as high as 20 mph.

Wednesday: Partly sunny, with a high near 35. West northwest wind 5 to 10 mph, with gusts as high as 15 mph.

Wednesday Night: A 50 percent chance of snow. Mostly cloudy, with a low around 22.

Dense Fog Advisory URGENT – WEATHER MESSAGE National Weather Service Chicago IL 908 AM CST Mon Dec 27 2021

ILZ006-013-103-104-271800- /O.EXA.KLOT.FG.Y.0017.000000T0000Z-211227T1800Z/ Lake IL-DuPage-Northern Cook-Central Cook- Including the cities of Waukegan, Buffalo Grove, Mundelein, Gurnee, Naperville, Wheaton, Downers Grove, Lombard, Carol Stream, Evanston, Des Plaines, Schaumburg, Palatine, Northbrook, Chicago, Cicero, Oak Lawn, Oak Park, and La Grange 908 AM CST Mon Dec 27 2021

…DENSE FOG ADVISORY IN EFFECT UNTIL NOON CST TODAY…

* WHAT…Visibility less than one quarter of a mile in dense fog expected.

* WHERE…Lake IL, DuPage, Northern Cook and Central Cook Counties.

* WHEN…Until noon CST today.

* IMPACTS…Hazardous driving conditions due to low visibility.

PRECAUTIONARY/PREPAREDNESS ACTIONS…

If driving, slow down, use your headlights, and leave plenty of distance ahead of you.




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WEATHER DETAILS

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 924 AM CST Mon Dec 27 2021

.UPDATE… Issued at 923 AM CST Mon Dec 27 2021

Regional visibility has dropped into the “pea soup” category across much of our area as a surface low pressure system slides overhead. In fact, visibility outside our window can`t be much more than an eighth of a mile! The exception is roughly along and south of a line from Fairbury to Morocco in the warm sector of the aforementioned low where mostly sunny skies are being observed.

As the low pulls away this morning, visibility is expected to increase from west to east. The current Dense Fog Advisory expires at 11 AM west of I-39, and 12 PM east of I-39. Updated products have been sent.

Borchardt

&&

.SHORT TERM… Issued at 335 AM CST Mon Dec 27 2021

Through Tuesday…

The main forecast concerns are fog early this morning and then the expectation of a 1-3 hour burst of wet snow changing to rain on Tuesday between 9:00 a.m. and 2:00 p.m. While significant snowfall amounts are not anticipated, a brief period of moderate to heavy rates may cause travel impacts due to slush on some roads and poor visibility. Some locations, especially north of I-80 and near and west of I-355 in northern Illinois could pick up a quick inch or two of snow.

Early this morning, 1000 mb low pressure is centered near KEOK. Impressive for late December convection is exiting off to the east, with areas of drizzle in its wake. With slackening winds near and north of the surface low path over the area this morning and a saturated boundary layer, we’ll be monitoring for dense fog development and expansion, such as has occurred in the southeast 1/3 of Iowa. It’s still uncertain whether a dense fog advisory will be needed or if we can handle any visibility impacts with a SPS. The low will track near or just south of Lake Michigan by the late morning and to lower Michigan by the late afternoon.

Winds will shift to westerly from west to east and then increase as low cloud cover erodes and cold advection ensues. Looking at gusts up to 30-35 mph and a sporadic gust up to 40 mph can’t be ruled out during the afternoon prior to sunset. Unseasonably mild temps, especially south of the low path (50s with 40s north) will peak in the early afternoon and then steadily fall toward sunset.

In the very active jet pattern, our next system will be waiting in the wings for Tuesday, so high clouds will quickly increase and thicken tonight, limiting temperature drop despite a much drier air mass advecting in. More importantly, a pronounced dry wedge centered around 850 mb will advect over the area and be in place on Tuesday morning. Temperatures early Tuesday will range from the upper 20s northwest to the mid 30s points south and along the Chicago shore.

Tuesdays synoptic set-up has some similarity to the event almost exactly 1-year ago (Dec 29-30), with main exception being that the antecedent air mass will be not nearly as cold and surface high pressure a good deal weaker. Big picture-wise, from the morning to the early-mid afternoon period, the local area will be in the favored right entrance region of a strong anti-cyclonically curved jet streak, providing good large scale ascent. Main mid-level short-wave trough and associated vorticity maxima will emerge from the southern High Plains area by 12z (6 AM) Tuesday morning. There is overall better model agreement than this time yesterday as would be expected, though some notable differences in magnitude of evaporative/dynamic cooling Tuesday morning into the early to perhaps mid afternoon. The operational ECMWF (European Center for Medium-Range Weather Forecasts) continues to be on the colder/snowier side of the envelope. [CARDINAL NOTE: Some TV forecasters are pushing the ECMWF outlook with some areas getting 2-3 inches of snow.]

Surface low pressure in upper 990s mb near Kansas City mid day Tuesday to 1000-1005 mb over southeast Wisconsin by late Tuesday evening. This average forecast surface low track is unfavorable to sustain snow as a precipitation type by late day Tuesday, though how we get there aloft will determine the magnitude of the potential thump of snow Tuesday morning. As has been mentioned the past few days, in favor of a potential period of moderate to heavy snow rates and travel impacts from poor visibility and snow/slush on some roads are: high PWATs of 0.6 to 0.9 for a winter system; good synoptic ascent from favored jet entrance region; strong isentropic ascent; very steep lapse rates above 600 mb (with even some upright instability possible); and strong, but transient mesoscale banding from lower and mid-level frontogenesis.

These above ingredients currently have the best chance of aligning for a few hours to several hours on the high end with a sufficiently cold column for snow for locales mainly north of I-80. Evaporative cooling and top-down saturation will likely produce a period of mainly snow at onset even for southern sections, followed by south to north warming of the column as 925-850 mb WAA (Warm Air Advection) intensifies and wind trajectories shift from southeast to south-southwest. In the official forecast, the NWS Chicago office says it will message the best chance for 1+ amounts to be I-88 and north for interior northern IL, and tapering southward from there, with associated slippery travel/low visibility type travel impacts. While a warmer outcome with a very brief period of snow can’t be ruled out, confidence is higher in most areas getting at least an hour or two burst of wet snow. Forecast soundings favor low ratio aggregates with a deep isothermal layer.

The ingredients for heavy snow rates being there, assuming a cold enough column for long enough, can sometimes allow for a period of over-performing snow, as happened to occur on 12/29/20 even down south of I-80. The NBM (National Blend of Models) 90th to 95th percentile shows this “worst-case scenario”, not too dissimilar to the recent ECMWF solutions. So on the high end, if heavier rates can last 3+ hours, could see localized amounts north of I-80 up to 2-3″ (even down into extreme NW IN). One thing that we can continue to say with more confidence this go around is that temperatures are likely to warm safely above freezing after p-type (precipitation type) transition Tuesday afternoon given current favored surface low track, so were not looking at an icing threat like what occurred the evening of 12/29/20 into early 12/30. Finally, for general weather/climate data interest, Chicago’s remarkable measurable snow drought has an increasingly good chance of ending with this event.

— NWS Chicago with CARDINAL NEWS edits

DEW

Wet, condensation, low temperature above freezing.

Weather Radar in northeast Illinois was clear at 10:00 a.m.

Is it cloudy over Chicago?

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