Chicago, Illinois

• Periods of showers or sprinkles today, with highest coverage early this morning.
• Additional periods of wet weather are likely especially mid-week.
• Warmer than average temperatures early in the week are expected to eventually trend slightly below average by next weekend.

Scattered small clusters of convective showers and isolated thunderstorms were lifting across eastern portions of the forecast area early this morning. This weak convection was occurring in association with a sheared northwest-southeast area of mid-level vorticity, coincident with a layer of steep lapse rates roughly in the 800-500 mb layer, as noted in current RAP soundings. Though model sounding ELs are just shy of -20C, MRMS isothermal reflectivity does indicate the tallest updrafts reaching the -20C level with some 40-45 dBZ cores. The expectation is that these scattered showers and isolated thunderstorms will continue to lift across far northeast IL and northwest IN (mainly east of the I-55 and I-355 corridors) prior to sunrise before the sheared vort lobe moves northeast of the forecast area.

Another weak mid-level short wave was noted across the mid-Missouri Valley early this morning, and is progged to track east across the area later this morning and afternoon. The strongest forcing and juxtaposition of the low-level baroclinic zone to our south suggests greater potential for measurable rain remains largely south of our forecast area across central IL/IN, the presence of the mid-level trough, weak warm advection and reasonably moist profiles should support occasional spotty light showers or sprinkles across the CWA during the day. Have maintained a 15-20% chance mention, highest generally along/south of I-80. Short-wave mid-level ridging is then indicated across the area tonight, in the wake of today`s short wave passage.

Attention then turns to our west, focused on the evolution and eastward progression of a complex long-wave upper trough and it`s associated deep surface low. Guidance has come into better agreement with the upper trough axis (and embedded closed upper low) propagating east of the Rockies and onto the Plains late Monday, with a deepening sub-995 mb surface low developing into eastern South Dakota by early evening. This is forecast to induce strong southerly return flow into the Midwest within a tightening surface pressure gradient. For northern IL/IN, the main impacts Monday look to be the development of somewhat breezy southerly winds (though forecast soundings depict fairly shallow mixing, supporting gusts around 20 mph during the afternoon), the potential for low stratus to advect in from the southwest, and perhaps some scattered showers as isentropic ascent increases with an elevated warm frontal zone lifting across the area. While warmer temperatures are expected, the shallow mixing may limit this somewhat with forecast highs Monday are in the mid-upper 60s.

A strong mid-level short wave (currently off the northern Baja coast) is progged to wrap around the south/southeastern periphery of the larger scale upper trough late Monday/Monday night, further deepening the closed upper low over the Central Plains. This results in new surface low development (or redevelopment) southeastward across IA, eventually slowing as the system begins to become more vertically stacked on Tuesday. Forecast sounding and time/height sections indicate development of a ~45 kt low-level jet into the forecast area Monday night into Tuesday/Tuesday night ahead of the deepening system. Strengthening south winds will likely keep Monday night temps very mild with lows in the 55-60 degree range. Winds may gust in excess of 30 mph during the day Tuesday, with temperatures potentially topping out in the low-mid 70s.

Deeper moisture feed during this process is initially focused west of the Mississippi River, due to the southwestern periphery of high pressure ridging from the Gulf Coast to the mid-Atlantic region. This moisture eventually looks to reach our area late Tuesday/Tuesday night however, with the greatest potential for showers and perhaps a few thunderstorms in this moisture axis ahead of the system`s occluding cold front. The extent of thunderstorm coverage is a little uncertain, with models indicating a pinching off of the MUCAPE axis to the north of the surface frontal occlusion Tuesday night.

Confidence in forecast details then decreases mid-week, due to model differences in the further evolution of the slow-moving upper low across the region - complicated somewhat by the degree of phasing with a northern stream short wave across the upper Midwest/northern Lakes. Overall, the Wednesday-Friday period continues to look unsettled as the closed upper low and occluded surface system track slowly east across the region, though some timing differences linger in the guidance at this distance. While no strong intrusion of colder air is evident at this time, temperatures do look to eventually cool to/slightly below average on the back side of this departing system.

Ratzer

Forecast concerns for the terminals

• Non-zero (10-20%) chance for light showers/sprinkles this afternoon.
• MVFR ceilings develop Monday morning.

The ongoing showers across northern IL will continue to lift out of the area over the next 2-3 hours leaving us with mostly dry conditions for the TAF period. However, the disturbance centered over the mid-Mississippi Valley will move overhead this afternoon and may result in a period of intermittent light showers/sprinkles. Given that forecast soundings continue to show a decent amount of dry air in the sub-cloud layer confidence on shower/sprinkle coverage remains too low to warrant a TAF mention at this time. Regardless, any showers/sprinkles that do develop should taper by 00z this evening.

Otherwise, expect winds to become more south-southeasterly this morning with speeds generally around 6-7 kts before speeds increase closer to 10 kts on Monday. Mostly cloudy VFR skies are expected to prevail through tonight with cloud bases around 8000-10000 ft. Though, MVFR ceilings will begin to move into the terminals Monday morning as low-level moisture increases ahead of our next system.

Yack

IL

None.

IN

None.

LM

None.