MORNING PREVIEW: Weather, Traffic Wednesday, December 15, 2021


NWS FORECAST: KORD | KPWK | Forecast Graph O’Hare




Today: A slight chance of drizzle before noon, then a slight chance of showers after noon. Cloudy, with a high near 65. Breezy, with a south southwest wind 15 to 20 mph, with gusts as high as 35 mph. Chance of precipitation is 20%.

Tonight: Showers and possibly a thunderstorm before 2am, then a slight chance of showers between 2am and 3am. Low around 41. Windy, with a south wind 20 to 25 mph increasing to 25 to 30 mph in the evening. Winds could gust as high as 55 mph. Chance of precipitation is 90%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Thursday: Sunny, with a temperature falling to around 40 by 5pm. Breezy, with a west wind 15 to 20 mph, with gusts as high as 40 mph.

Thursday Night: Mostly clear, with a low around 29. West wind 5 to 15 mph, with gusts as high as 20 mph.

Friday: Partly sunny, with a high near 41. West northwest wind around 5 mph becoming east northeast in the afternoon. Winds could gust as high as 15 mph.

Friday Night: A slight chance of rain and snow before midnight. Mostly cloudy, with a low around 31. Chance of precipitation is 20%.


An active short term forecast highlighted by the following key messages:

* Increased confidence in damaging winds tonight into early Thursday morning, when a High Wind Warning is in effect for generally north of I-80, while an Advisory roughly along/south of there. The peak winds look to be between 8 PM and 3 AM.

* Sandwiched within that high wind time will be a line of rapidly moving convective showers crossing northern Illinois late this evening and gradually fading in intensity during the overnight.

* Cannot forget about daily record high temperatures today that may close in on the all-time December record high at Rockford.

Early this morning, broad but stout warm advection covers much of the Midwest ahead of ongoing cyclogenesis in the immediate lee- side of the central Rockies. There has been spotty drizzle/light showers within the isentropic ascent across the area and do not see much change in that the rest of this morning as the warm front takes shape over the area. Stratus bases should lift enough this afternoon as the front progresses out of the area, but some spotty light showers cannot be ruled out in the north. With a fairly mild ground, do not see advection fog being an issue.

Temperatures are in the upper 40s and lower 50s early this morning and they’re only going to get warmer from here. Strong WAA throughout our atmosphere will keep temperatures climbing through the rest of the day and even through a good chunk of the night. In fact, temperatures shouldn’t really cool, at least not noticeably, until the cold front passes over late tonight (or very early tomorrow morning?). Prior to then, we’ll see highs likely breaking daily temperature records across the area as they climb into the middle and even upper 60s.

Winds early this morning are already beginning to gust 15-20+ mph across parts of the area with our pressure gradient tightening u Pand steady pressure falls taking place. Gusts to near 35 mph are likely by mid afternoon today. The sub-985 mb low pressure center will continue to deepen rapidly as it cuts across the Plains throughout the day continually increasing our wind speeds through frontal passage which will occur late tonight. Additionally, we have an unusually strong low level jet moving over only about 1-4 kft above the surface. Guidance suggests that we may not quite be mixing into the bulk of the LLJ, but with temperatures expected to be in the lower 60s, the boundary layer will be at least somewhat mixed. South to southwest winds should gust upwards of 50 mph for a better part of the mid to late evening ahead of the cold front passage around midnight. The frequency of gusts around 60 mph (High Wind Warning criteria and where some damage is more likely) is somewhat uncertain, as this is not the typical setup for synoptic high winds, which is normally being on the dry/cold advection side of the boundary. However, there is enough of a signal of synoptic anomalies and in the forecast vertical profiles that gusts of a higher end impact will at least briefly materialize, especially in the northern half of the CWA. We cannot rule out a couple gusts getting to 60 mph southeast of the current High Wind Warning either, and at least enough there for some downed branches. While the main timing of the highest winds looks to be just ahead of the front, during the cold advection and still tight gradient in the several hours after frontal passage, at least Wind Advisory criteria should be met for several hours include after daybreak. So have included all this in the High Wind Warning (until 9 A.M. Thursday) for simplicity. Heading into the afternoon tomorrow, conditions begin to quiet down as a high pressure center moves in the Midwest.

Within the synoptic high wind period, there will be a strongly- forced line of convective showers and possibly storms moving eastward into northern Illinois late this evening ahead of the system cold front. This is an exceptionally well agreed upon evolution in model guidance, and conceptually makes sense within the narrow corridor where lapse rates steepen in the low-levels and very briefly connect up to the advancing dry slot/steeper lapse rates in the mid-levels. Because of the highly anomalous wind fields and rapid propagation of the showers (>45 kt motion), it would not take much for organized cells to user down gusts higher than the ambient surface winds. That organized mode of quasi-linear or at least good coverage of a line of broken cells, is most favored over north central Illinois before activity eases with less instability and broadening of the surface convergence along the front. The SPC Marginal Risk (level 1 of 5) for this risk in mainly north central Illinois looks good, and again the activity may have little to no lightning with it as far east as our County Warning Area (CWA).

As the front pushes through the CWA early Thursday morning, our winds will take a turn to northwesterly and immediately begin pumping in a cold, dry air mass. This will cause temperatures to fall or at most remain rather steady through the day on Thursday before diving following sundown tomorrow. The high temperature for Thursday will officially occur at midnight tonight or shortly thereafter, and is also likely to be at or very close to record levels.


No dew observed.

Weather Radar in northeast Illinois was clear at 9:30 a.m.

Is it cloudy over Chicago? |




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