Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 201734 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1234 PM CDT Tue Apr 20 2021 .UPDATE... 1041 AM CDT An impressive band of snow has developed near the I-80 corridor in association with the upward branch of a strengthening low to mid-level frontogenetic circulation along the baroclinic zone. Snow in this band has been falling at a decent rate, with visibilities falling under a mile. This has lead to some minor accumulations up to around a half inch on grassy and elevated surfaces, but with warm pavement temperatures roads largely remain wet. This band of heavier snow will gradually sag to the southeast into this afternoon, likely impacting areas south of I-80 over parts of east central IL and northwestern IN. To the north of this band, the activity will become more showery in nature into this afternoon as the main upper impulse approaches. While snow rates will general be light with these showers, a few localized bursts of briefly more moderate snow will be possible due to their convective nature. However, the overall low residence time of any heavier showers will limit additional accumulations, especially as surface temperatures remain just above freezing. For this reason, the main potential for accumulations into this afternoon will primarily be tied to heavier rates occurring with the better organized band of snow. Some localized areas could experience an inch or two of snow with this band, mainly on grassy surfaces. Expect the main band of snow to end across my southeastern areas by early this evening, though some lingering scattered snow showers do look to continue during the evening. KJB && .SHORT TERM... 324 AM CDT Through Wednesday... No substantial changes in the forecast today, with slushy snow accumulations still expected for portions of the area, primarily east of I-55 and well south of I-80. A mid-level wave currently exiting the central Rockies will cross northern Illinois late this afternoon under the right entrance of a 110kt upper jet streak. Widespread precip currently blanketing a larger portion of Kansas will spread ENE along an existing low- level baroclinic zone over and just south of the far southern CWA. Based on the current latitude of precip develop and recent trends in model guidance, have made a slight southward shift in the main precip axis today. This favors the best chance for at least 0.25" of precip to the south of a line from Peoria to Morris to Valparaiso. Encroaching low-level dry air from the north will also make a dent in precip north of I-80 this morning and likely keep precip totals there at or under 0.10". Where precip is lighter across northern Illinois, low- level diurnal warming under increasingly colder temps aloft will support a transition from broad light synoptic precip to more showery activity this afternoon. The greatest snow accumulations are still expected across the southern CWA where precip rates will be low enough to keep temps in the mid 30s for most of the day. Slushy accumulations of 1-2" almost entirely on non-paved surfaces are possible south of the aforementioned line, with a narrow corridor of >2" not out of the question across the far southern CWA. Even with lower precip rates farther north, the continued influx of drier low-level air will maintain low-level wet bulb temps close to freezing, allowing for snow as the dominate precip type. While snow will struggle to accumulate this morning, the more convective nature of the afternoon showers may result in heavier bursts of snow capable of producing quick localized accumulations of a few tenths of an inch. Note that the record April 20th snowfall for Chicago is 0.2" (1943) and for Rockford is a Trace (five times, last in 1959). Tonight: Broader synoptic-scale precip will end by mid-evening with the loss of mid-level moisture. However, steepening low to mid-level lapse rates will support some continued snow shower chances into the overnight hours, during which a couple additional tenths of accumulation will be possible. With a trend to more cloud cover under the approaching upper low, the risk a temps falling well below freezing may be decreasing. However, there should be enough breaks in the clouds in conjunctions with continued low-level CAA to allow temps to fall into the upper 20s for most of the area, with mid 30s in the core of the Chicago metro. Wednesday: Unseasonably cold air aloft highlighted by a core of -35C 500 hPa temps with a -3SD anomaly will cross northern Illinois midday. This will support a well-mixed PBL below steep lapse rates of >8C/km up through at least 700 hPa. Meanwhile, mid- level vort max will cross the CWA during the late morning through mid-afternoon. Scattered showers are expected to develop across the area, but especially over northeast Illinois where lapse rates are steepest. Precip type possibly favors more snow than rain due to low-level wet- bulb temps only slightly above freezing. Additionally, MLCAPE values surpassing 200J/kg combined with EL values nearing 12kft/-25C could pose the risk of isolated lightning strikes. Given the convective nature of the precip, graupel will likely occur with any deeper cells. Low-level inverted-V soundings also suggest the potential for gusty winds with any showers. Kluber && .LONG TERM... 324 AM CDT Wednesday night through Monday... Large scale upper trough is progged to be moving off to the east of the area Wednesday night, though with one last smaller scale short wave rotating east-southeast into the departing trough axis. Cold mid-level temps associated with this wave/departing upper trough (around -22C depicted at 700 mb) combined with a deeply mixed boundary layer (adiabatic lapse rates from the surface up to 700 mb) will likely continue to support scattered convective rain/snow showers early Wednesday evening. Loss of diurnal low level instability and passage of the short wave trough axis should allow these showers to dissipate by late evening, with subsidence developing across the region after midnight and inducing clearing skies. Clearing within the cold airmass is expected to aid in producing widespread freezing temperatures across the forecast area, with lows in the upper 20s outside of the core of the Chicago metro area by sunrise Thursday. We`ll likely need freeze warning headlines again for Wednesday night, but for simplicity at this time we`re continuing to mention the Wednesday night impacts in the additional information section of the current headline in effect for Tuesday night. Mid-level short wave ridging then develops across the area Thursday into Thursday night in the wake of the departing upper trough. Strong 500 mb height rises in excess of 80 meters is progged across the area initially Thursday, indicative of deep large-scale subsidence. Along with very dry thermodynamic profiles, subsidence should promote plenty of sunshine across the area and allow a quick rebound of temperatures back into the mid to upper 50s, and somewhat breezy west winds limiting lake breeze potential. Guidance remains in general agreement in depicting a low-amplitude mid-level short wave approaching the mid/upper Mississippi River valley later Thursday night, which would likely bring increasing high cloud cover into the region after midnight. Lows will likely dip into the 30s in the dry air mass prior to the arrival of high clouds however, possibly producing some patchy frost. This mid-level wave is then progged to dampen across the region Friday, as it propagates into the upper ridge position. Low level return flow develops across the Plains in association with this feature, but becomes weaker and less-focused as the amplitude of the disturbance decreases across our area. Lower and mid level southwest flow does provide warm/moist advection into the region by Friday afternoon however, with most guidance depicting the potential for some showers. Global models continue to show some differences with a couple of additional short waves within developing northern and southern streams Friday night into Saturday, though the general consensus remains for shower chances to persist into the day Saturday as these waves move east and a cold front associated with the northern stream wave pushes through the area. Temperatures do continue to moderate Friday into Saturday with southwest low level flow, though this will likely be modulated by increased cloud cover and a potentially stronger southern stream low as noted in the ECMWF and ECS ensemble runs. Highs both days should be near 60, and possibly into the mid-60s if cloud/shower coverage is lower. A stronger southern stream low per the ECMWF would also result in a greater potential for northeast surface winds and cooler temps off of Lake Michigan however on Saturday. Both of these short waves and their developing larger scale upper trough are progged to move off to the east of the area Saturday night with a broad, flat upper ridge depicted across much of the central CONUS by Sunday. Associated surface high pressure moves across the forecast area during the day, allowing dry weather and likely some lake cooling with temps in the 50s to near 60 expected to round out the weekend. Upper ridge is then progged to amplify across the central CONUS/Midwest early next week, in response to an upstream long wave trough moving into the West Coast. Developing southerly flow into the region should support warmer temps (perhaps 70s) by Monday, with the best potential for warm advection showers/storms currently looking to remain off to our northwest. Ratzer && .AVIATION... For the 18Z TAFs... As "system snow" departs to the south, scattered snow showers will increase in coverage this afternoon across northern Illinois. We currently believe the greatest coverage should be south and west of the Chicago terminals due to low-level dry air drifting in from the east, though a flurry or two cannot be ruled out through sunset. At RFD, occasional drops in visibility to as low as a mile will be in play through early evening. Winds will remain light and variable through the overnight hours. As a potent upper-level waves passes through the western Great Lakes tomorrow, numerous showers are expected to develop across the area by mid-morning and last through the evening hours. Based on forecast instability parameters, the synoptic regime, and overall similarities to previous events (4/2/2016 and 10/20/2018), we believe the showers will have thunderstorms-like characteristics with cumulus towers to 20 kft, gusty winds and related turbulence, visibility drops to a mile or lower, a mix of rain, snow, and graupel (small hail), and occasional lightning strikes. With collaboration with CWSU ZAU, opted to aggressively and offer SNRAGS as early as 15Z and vicinity thunder as early as 17Z, with the highest coverage expected between 18-22Z. Confidence is high that the showers will occur and medium that they will impact the Chicago terminals. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Freeze Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010- ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032- ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108...midnight Wednesday to 9 AM Wednesday. IN...Freeze Warning...INZ001-INZ002-INZ010-INZ011-INZ019...midnight Wednesday to 9 AM Wednesday. LM...None. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: is the U.S. government's official web portal to all federal, state and local government web resources and services.