Area Forecast Discussion
Issued by NWS Chicago, IL

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888 FXUS63 KLOT 191600 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1100 AM CDT Mon Apr 19 2021 .UPDATE... 1053 AM CDT No major changes to the forecast through this afternoon. The cold front continues to move southward and is moving across northwest Indiana and Iroquois and Ford Counties at this time. West-east corridor of rain continues between the I-88 and I-80 corridors and will spread southward with time this afternoon. May see some redevelopment along and just north of the I-88 corridor early this afternoon as a shortwave is expected to quickly move east across that area. Otherwise he bulk of the precip will be to the south. Areas behind the front have seen their highs for the day while temps of around 60 ahead of the front will fall into the 40s. MDB && .SHORT TERM... 335 AM CDT Through Tuesday... All eyes remain on the potential for a late snowfall across the area beginning late tonight and continuing into Tuesday night. The southeast trend noted in guidance 24 hours ago continued with the 00Z guidance, with a bulk of the precipitation across the CWA now expected to occur during the day Tuesday. These changes have further diminished the risk of any substantial snowfall in the forecast area (though any accumulation this late could be considered substantial) primarily due to lower precip rates and a more diurnally-unfavorable period for snow to accumulate. Today and Tonight: The cold front continues to progress southeastward toward the CWA, stretching from near Green Bay to Dubuque and into north-central Missouri as of 3am. As guidance has noted for a couple days, a faster trend in this frontal passage across especially the northern CWA continues to favor a colder day today than was anticipated early this past weekend. Temps will struggle to reach 50F north of I-80 where cloud cover will dominate through the day. A fairly prominent swath of low-level f-gen developing along the front will foster a narrow band of rain late this morning through this afternoon. The initial focus will be centered from Mendota to Aurora to Chicago around mid-morning, then translate south to an area centered between I-88 and slightly south of the Kankakee/Illinois River Valley for much of the afternoon and into this evening. An overall lull in precip is then expected late this evening into the overnight hours north of a Pontiac to Valparaiso line as the favored axis of low-level fgen drifts farther south. However, increasingly colder low-level thermo profiles will support a transition over to snow where precip does continue. Tuesday: An axis of precip within the moist conveyor belt of a developing surface low will quickly spread northeast into the area beginning around daybreak. A neutral to slightly negatively- tilted trough rotating into the mid-Mississippi River Valley under the right-entrance of a departing upper jet streak will provide continued ample forcing for widespread precipitation across much of the CWA through the day. However, the bulk of the deepening of the surface low over the Ohio River Valley is now progged to occur further east, resulting in a broader and lighter precip shield across the area. Thermo profiles will continue to support mainly snow mixed with some rain during this time, but accumulations will be suppressed by lower snowfall rates and the increasingly strong late April sun providing enough insolation to maintain melting at the surface. With storm total precip trending toward under 0.5" and effective SLRs likely staying well below 10:1 for a bulk of the event, slushy accumulations of 1-2" on primarily non-paved surfaces appears likely for areas south of I-80 and especially east of I-55. Tuesday night: Mid-level moisture will quickly advect northeast of the area by mid-evening, effectively ending any appreciable widespread synoptically forced precip. Guidance has continued to indicate the potential for a secondary piece of mid-level energy closely following the primary wave across the area Tuesday night. Decreasing mid-level temps will begin to increase low-level lapse rates amid residual moisture below 700 hPa. This will support the potential for lingering snow showers through the night for portions of northeast/east-central Illinois into northwest Indiana. while cloud cover may be more abundant than previous expected due to the factors noted above, confidence was high enough to prompt the issuance of a Freeze Watch for the entire area. With temps starting in the 30s for much of the area in the early evening, it will not be hard to cool into the low 30s and upper 20s. With the potential high impact of a hard freeze on budding vegetation after 2+ weeks without sub-freezing temps for most of the area, felt it was prudent to issue the watch on this shift to provide two full days notice to complete any necessary preparation. This is especially the case with the expected snow falling on the vegetation. Kluber && .LONG TERM... 306 AM CDT Wednesday through Sunday... Rain/snow showers will likely linger Wednesday beneath cold upper trough, before dry and somewhat milder weather develops Thursday. Medium range guidance depicts the potential for some additional rain showers Friday into Saturday as another frontal system approaches and transits the forecast area. Sunday looks to be largely dry at this distance. With the exception of cool temperatures Wednesday, readings through the period are expected to moderate back to near average levels for late April. Surface low pressure will be east of the area by Wednesday morning, with associated mid-level short wave and more widespread precipitation also moving away from the region. The larger scale upper trough is progged to hang back across the western Lakes and upper Midwest however, with a couple of weaker vorts expected to translate through the flow. Cold mid-level air aloft within the trough (-17 to -19C at 700 mb) is expected to result in a layer of fairly steep lapse rates up through about 675 mb, which will likely support the development of convective snow/rain showers during the day and especially with diurnal "heating". The thermal column appears cold enough to support snow showers during the morning, which would eventually transition to a rain/snow mix then all rain as surface temps warm by midday/afternoon. Some minor snow accumulations can`t be ruled out during the morning when surface temps are expected to be near the freezing mark. Given the expected thermodynamic profiles, have leaned toward the cooler end of the guidance spectrum for surface temps and undercut blended NBM highs by a few degrees into the low-mid 40s. Scattered showers may linger into Wednesday evening, before they dissipate Wednesday night as the upper trough and its cold pool begin to move off to the east of the region. Mid-level height rises on the order of 60-70 meters at 500 mb are noted later Wednesday night into Thursday as short wave ridging develops aloft in the wake of the departing trough. Surface high pressure, centered over the lower Missouri and Ohio valleys, spreads east on Thursday with low level winds becoming more westerly. Sunshine and moderating temps aloft should allow daytime temps to warm back into the 50s, with the breezy west flow limiting lake cooling to the immediate northwest Indiana shore areas. Guidance is in reasonable agreement in bringing another deamplifying mid-level short wave across the region Friday, with low level return flow focused mainly along and west of the Mississippi River during the day. ECS and GEFS ensemble means maintain some potential for precipitation to spread into the forecast area on Friday, though with a bit of a slowing trend noted and with the 00Z operational runs of the ECMWF and GFS focusing lower chance pops for showers mainly west and later in the day/Friday night. Guidance shows more spread into Saturday with the degree of phasing of two additional mid-level waves. Despite these differences, the general consensus is that a frontal system will move through the region Saturday with an accompanying chance of additional showers. Ensemble temps continue to moderate ahead of the front Saturday, thought still in the average range for late April. Global guidance depicts surface high pressure building in Saturday night into Sunday behind the departing upper trough, supporting generally dry conditions to end the weekend. Broad ridging is depicted aloft across the central CONUS, with the fast/flatter flow however potentially brining additional low-amplitude short wave energy toward the region later Sunday or Sunday night. This could support some precip chance as warm/moist advection returns into the region on the back side of the departing surface high. For now though, the bulk of Sunday looks to be dry if not the entire day. Ratzer && .AVIATION... For the 12Z TAFs... Aviation Forecast Concerns: * Cold front moves through terminals early this morning with a northwest wind shift and gusts around 20kt. * Period of light rain/showers develops behind the front. MVFR ceilings possible in precipitation. * Precip sags south of terminals later this afternoon/evening, with period of VFR conditions expected into overnight. * Precip redevelops into area early Tuesday morning, in the form of a snow/rain mix. MVFR ceilings and some IFR visibility possible. Surface cold front was pushing into northern IL at 5 AM/10Z, and will move southeast into the Chicago metro terminals through 14-15Z. Southwest winds ahead of front have started to gust near 20 kts, and this should continue for a time behind the front as well after winds shift northwest. Scattered showers behind the front are expected to fill in across the terminals for a few hours behind the front this morning, with MVFR ceilings developing. Rain should sag south of KRFD, KDPA and KORD by midday, lingering a little longer at KMDW and longer still into the afternoon at KGYY. Precip eventually pushes south of all terminals this evening, with VFR conditions and lighter northwest winds overnight, eventually shifting north and then northeast early Tuesday morning. Precipitation is expected to spread back into the area early Tuesday morning, likely in the form of a snow/rain mix. Ceilings again lower to MVFR, with visibility into the IFR range possible as precip increases. Precip may change over to all snow at times, with some minor wet accums on grassy surfaces possible. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Freeze Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033- ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108...9 PM Tuesday to 9 AM Wednesday. IN...Freeze Watch...INZ001-INZ002-INZ010-INZ011-INZ019...9 PM Tuesday to 9 AM Wednesday. LM...None. && $$ Visit us at weather.gov/chicago Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/NWSChicago www.twitter.com/NWSChicago www.youtube.com/NWSChicago

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