O’HARE FORECAST …
Today: Increasing clouds, with a high near 52. South southeast wind 10 to 15 mph, with gusts as high as 20 mph.
Tonight: Scattered showers, mainly between 7pm and 3am. Cloudy, with a temperature rising to around 53 by 5am. South wind 10 to 15 mph, with gusts as high as 20 mph. Chance of precipitation is 50%.
Wednesday: A 20 percent chance of showers. Cloudy, with a high near 65. Breezy, with a south wind 15 to 20 mph, with gusts as high as 40 mph.
Wednesday Night: Showers likely before midnight, then a chance of showers and thunderstorms between midnight and 3am, then a chance of showers after 3am. Cloudy, then gradually becoming partly cloudy, with a low around 46. Breezy, with a south wind 20 to 25 mph, with gusts as high as 50 mph. Chance of precipitation is 60%.
Thursday: Sunny, with a high near 48. Breezy, with a west southwest wind 15 to 20 mph, with gusts as high as 35 mph.
Thursday Night: Partly cloudy, with a low around 30.
The active pattern across the middle of the country continues this week. In the short term, the main points of note are scattered light rain showers tonight and then the record warmth on a windy Wednesday, and this still looks every bit on track.
Some amplification to the upper pattern has occurred over the Continental United States (CONUS) these past 24 hours, mainly in response to the digging longwave into the west. As a result, some building heights have moved over the area, and warm air advection (WAA) will gradually ensue today. This advection will pull from 925 mb temperatures of 8 to 10C sampled upstream on last evening’s Universal RAwinsonde OBservation program (RAOBs). These values for mid-December support low to mid 50s, and even with increasing clouds into this afternoon, see those values been attained.
Warm Air Advection (WAA) this time of year typically spawns low clouds and that will be the case by late Tuesday into Tuesday evening. This moisture transport from the south — which is already seen on upstream GOES-16 imagery — will be following isentropic ascent. Saturation of several thousand feet is predicted by mid evening within this, so confidence has increase in scattered light rain showers, with 50 to 70 percent-ish coverage expected along/east of I-55. This high Probability of Precipitation/Quantitative Precipitation Forecast (PoP/low QPF) event should not be of much impact, and temperatures should gradually climb especially overnight. While fog can commonly be an issue this type of year in nocturnal moist advection, the predicted low-level Dew Point Depression (T/Td) spreads are kept large enough to prevent fog. Visibility guidance indicates any signal well northwest of the area toward the true developing warm front.
For Wednesday, stratus will prevail through the day, but the strengthening low-level warm advection coupled with the very high starting point in temperatures (mid 50s) will set the area up for record highs. Stepping back to the synoptic signal, a rapidly deepening low will be moving from the Colorado Rockies to near Sioux Falls, South Dakota during Wednesday afternoon, pumping the low-level thermal ridge over the Mississippi Valley. No matter which height or temperature variable looked at, they are all highly anomalous and at or a smidge above record levels for December. This includes 500 mb heights of near 576 dam (250m above normal), and 925 mb temperatures of near 15C (near record for ILX sounding climo). Highs should materialize well into the 60s Wednesday afternoon and cannot rule out 70 creeping into north central Illinois.
Southerly winds during the day Wednesday will be increasing within strengthening pressure falls. Limited mixing heights should keep gusts from accelerating too quickly yet during the day, but up to 35 mph looks reasonable in the afternoon.
As for any rain potential during the day Wednesday, there could be spotty drizzle in the morning given the forecast cloud bases to or a little below 1,000 ft. Any showers should be isolated. So while becoming windy and not a sunny day, the 60s and generally rain free look to be a good day to get fresh air into the house!
Moderate to heavy frost was observed on vehicles and grass. Roads were slick in some areas. The Dew Point Depression was within 4 to 5°F at times overnight and early Tuesday morning with partly cloudy skies (32/28, 32/27, 34/29, 34/29, 35/31°F hourly to 6:51 a.m.).
Weather Radar in northeast Illinois was clear at TIME.
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