Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
373 FXUS63 KLOT 222337 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 637 PM CDT Sun Sep 22 2024 .KEY MESSAGES... - Showers will gradually come to an end this evening leaving mostly dry conditions for tonight and Monday - Another period of showers and possibly a few thunderstorms is expected Monday night and Tuesday especially along and east of I-55 && .DISCUSSION... Issued at 237 PM CDT Sun Sep 22 2024 Through Tuesday: Widespread showers continue to pivot across northern IL and northwest IN in association with a mesolow that has developed ahead of the cold front that is now over northwest IL. These showers are expected to persist into the early evening hours before gradually tapering from northwest to southeast as the cold front advances through the area. So far the lightning coverage across IL and IN has been virtually non-existent likely due to the lack of instability. Therefore, the thinking is that the thunder potential is near zero at this point. However, have decided to maintain a slight (15-20%) chance for thunder in the forecast mainly for areas south of I-80 in case the aforementioned mesolow is able to overcome the weak instability and generate a rogue lightning strike or two. As showers come to an end this evening, winds will quickly increase behind the front with gusts in the 15 to 25 mph range expected especially over the open waters of Lake Michigan. These wind speeds in combination with the northerly direction will also build waves on the lake into the 6 to 10 ft range this evening and overnight making for dangerous swimming and hazardous boating conditions. While the strongest winds speeds will taper Monday morning, the persist onshore flow should allow waves to remain elevated through Monday afternoon and possibly into the evening. Therefore, a Beach Hazard Statement remains in effect for all northern IL and northwest IN beaches through Monday evening for this threat. Outside of the winds and waves, Monday looks to be a decent day weather wise with mostly dry weather expected as the front is expected to be stalled across central IL and IN. However, a weak disturbance and lingering mid-level moisture overhead does look to keep a fair bit of cloud cover around. Regardless, more typical temperatures for late September will be in place with highs Monday afternoon in the 70s with overnight lows in the 50s both tonight and Monday night. Unfortunately, the break in the weather will come to a close Monday night into Tuesday as the upper-trough over the central Plains pivots into the Great Lakes and lifts the stalled front back north. While this will once again bring some meaningful rainfall to northern IL and northwest IN, the greatest coverage of showers looks to be along and east of I-55 where better moisture and instability are forecast. Given that forecast soundings show a similar CAPE profile to what was seen today, am not overly impressed with the coverage of thunderstorms Monday night and Tuesday. Though, did maintain a 20% chance of thunder in the forecast, mainly for areas south of the Kankakee River Valley, in the off chance the dynamics can overcome the modest instability. The showers and any storms that do develop are expected to conclude Tuesday evening as the trough moves east and drier air filters back into the area. Yack Tuesday Night through Sunday: Medium range forecast guidance continues to build a high amplitude mid/upper-level ridge across the Midwest into the western Great Lakes during the mid to late week period. This help foster a blocking pattern across the central CONUS later in the week as a cut off upper low meanders south of this ridge across the central/southern Plains eastward towards the Mid-Mississippi Valley. Unfortunately, the predictability associated with these of these types of patterns tends to be lower than normal, particularly with smaller scale details. Accordingly, this lends to lower forecast confidence in specific forecast details beyond Thursday. The weather for Wednesday and Thursday looks to largely be dry and seasonable across our area as surface high pressure dominates across the western Great Lakes. This pattern should support high temperatures in the mid to upper 70s both days (warmest on Thursday) under mainly sunny skies. Onshore flow may keep areas along the Lake Michigan lakeshore a couple of degrees cooler, however. The forecast beyond Thursday will largely be dependent upon the interaction that occurs between the cut off low and a northward shifting tropical system expected to lift northward from the Gulf late in the week. If they interact favorably, heavy rainfall associated with the remnant tropical wave could potentially develop northward into our area late in the week as the tropical wave slingshots north-northwestward. However, confidence with this occurring into our area remains rather low at this point. Given the low predictability and overall low confidence with the forecast beyond Thursday, I made no changes to the NBM initialization, which does include some very low pops Friday into the Weekend. KJB && .AVIATION /00Z TAFS THROUGH 06Z TUESDAY/... Issued at 637 PM CDT Sun Sep 22 2024 Key Messages: - N winds become gusty around/above 20 kts this evening behind cold front. Eventually winds ease and turn NE late tonight/Monday. - MVFR/patchy IFR ceilings this evening gradually rise to VFR by or prior to midday Monday. Surface low pressure was over south-central lower MI early this evening, with a slow-moving cold front trailing through northwest IN and central IL. Tightening pressure gradient lags the front a bit, though we`re starting to see increasing NNW winds spread from WI into northern IL, with gusts above 20 kts at times. These breezy N winds will likely persist across the terminals this evening before speeds/gusts ease, and eventually turn more NE late tonight and Monday. Spotty -SHRA/-RA lingers behind the front, as well as MVFR and patchy IFR ceilings, though the breezy north winds will gradually bring drier air into the area and allow showers to dissipate and cloud bases to rise later this evening. MVFR ceilings may linger through Monday morning, especially near Lake Michigan as winds turn onshore, though confidence is somewhat low in how quickly conditions improve to VFR. It`s possible lower clouds scatter out or rise to VFR later tonight and we see only patchy MVFR ceilings across the Chicago terminals Monday morning before all sites go VFR by midday. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement through Monday evening for ILZ006- ILZ103-ILZ104. IN...Beach Hazards Statement through late Monday night for INZ001- INZ002. LM...Small Craft Advisory until 7 PM CDT Monday for the IL nearshore waters. Small Craft Advisory until 10 PM CDT Monday for the IN nearshore waters. && $$ Visit us at weather.gov/chicago