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
49
50
194 FXUS63 KLOT 140504 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1204 AM CDT Fri Jun 14 2024 .KEY MESSAGES... - Hot and humid conditions early next week. - Periodic chances for showers and storms next week as well. && .UPDATE... Issued at 555 PM CDT Thu Jun 13 2024 The initial round of convection that developed across the south half of the Chicago metro earlier this afternoon has sagged southeast across the Kankakee River Valley. These storms continue to feed into an increasingly narrow corridor of higher MLCAPE as low-level moisture profiles become less favorable with southeast extent. With low-level flow and deep-layer shear vectors also becoming generally parallel to existing low-level confluent axes, expectations are that thunderstorms will maintain pulse-like characteristics. The strongest cores may produce large hail, while an antecedent DCAPE reservoir supports the potential for damaging wind. Eyes then turn to a cluster of impressive severe storm cores around the IL/IA/MO tri-state region and its downstream evolution toward the southern CWA in the next few hours. By the time any upscale MCS growth occurs in the vicinity of the southern CWA, the southward-moving cold front (analyzed almost just north of and parallel to I-80 as of 5pm) will have shifted south to roughly a Pontiac to Rensselaer line. The downstream environment into our area will remain favorable for maintenance of strong to severe storm clusters with primarily damaging wind swaths in the 8-11pm window. Also of note is a west to east axis of 800-650 hPa moisture across northern Iowa ahead of the main broad mid-level trough. Marginal mid-level lapse rates to around 6.5C/km could support some a corridor of isolated to scattered showers and storms south of I-88 and especially into the IL/Kankakee River Valleys from 9pm-midnight. Steep lapse rates below the LPL/cloud base could support gusty sub-severe winds with any more formidable cores. Kluber && .DISCUSSION... Issued at 236 PM CDT Thu Jun 13 2024 Through Friday Night: Recent satellite imagery depicts an axis of agitated cumulus clouds extending from waukegan to Davenport within a broad zone of low-level confluence across the Lower Great Lakes. Lingering capping has thus far kept a lid on early attempts for sustained convection. However, continued surface heating (temperatures rising toward 90F), low-level moistening/pooling via southwesterly flow and evapotranspiration (dew points rising toward 70F), and mid-level cooling/moistening via evaporation of convective turrets into the base of the cap should eventually win the battle and lead to explosive thunderstorm development within the next few hours. Extrapolating the position of the axis of agitated cumulus clouds, such explosive development is poised to occur between the I-88 and I-80 corridor sometime between 2 and 4 PM. With MLCAPE >2500 J/kg (including some 1000 J/kg in the hail growth layer) and convective-layer shear >45kt (largely focused from 2-6 km), sustained thunderstorms will be poised to become supercells with a threat for destructive hail locally greater than 2" in diameter through the afternoon. With time, clustering of cells may lead to an increasing damaging wind threat including localized gusts as high as 75 mph. Coverage of thunderstorms this afternoon remains an item of lower than average confidence, with anything from just an isolated cell or two to literally an entire line of supercells plausible. Meanwhile, a second area of explosive thunderstorm development is expected in southern Iowa this afternoon near an area of focused low-level convergence along a cold front (e.g. the Level 3/5 threat area in the SPC outlook). Upscale growth into one or more clusters appears plausible as they move east-southeastward into western and central Illinois this evening. If coverage of thunderstorms this afternoon is relatively sparse, incoming convection from Iowa may provide a "round 2" of sorts along and south of I-80 where the receiver of MLCAPE >2500 J/kg remains. Similar to this afternoon, any storm this evening will carry a threat for destructive hail >2" and, if clusters can become established, winds >75 mph. Moreover, if a west-to-east axis of thunderstorms were to materialize within a mesoscale zone, PWATS >2" would certainly support efficient downpours with a threat for localized flash flooding. The WPC threat level 2/5 area for flash flooding highlights the area well. In all, "round 2" may occur from 7 PM to as late as 3 AM. Tomorrow will be decidedly quiet with seasonable temperatures (highs in the low to mid 80s) and a northwesterly breeze. A lake breeze should surge inland during the afternoon leading to cooling temperatures along the lakeshore. Borchardt Saturday and beyond: An upper-level ridge is expected to begin to drift into the southern CONUS on Saturday which will continue to support dry conditions. While winds will begin to turn southerly on Saturday, the strongest warm advection is expected to be west of our area which will likely keep temperatures near more seasonable readings in the low to mid- 80s. Though, a lake breeze is forecast to develop and keep highs in the 70s near Lake Michigan, particularly the IL lakeshore. Heading into Sunday, the aforementioned ridge is forecast to shift into the Mid-Atlantic region as a large upper-level trough establishes over the northwest CONUS. As this occurs southwest flow is expected to develop and advect in warmer and much more humid air. While guidance remains in decent agreement on the upper-level pattern, there continues to be uncertainty on how far east the ridge will drift on Sunday into the early part of next week. Therefore, the exact magnitude of heat remains a bit of a question mark. Regardless, there continues to be a strong signal (around 70-80% chance) for highs Sunday through at least Tuesday to be within the low to mid-90 range with the potential for readings to overperform, especially on Monday. Couple these temperatures with the forecasted increase in humidity (dew points in the mid to upper 60s to near 70) and there is a growing threat for heat indices to top out in the upper 90s to near 100 degrees early next week. Depending on where exactly the aforementioned ridge sets up will also dictate whether or not the troughing over the western CONUS will be able to eject shortwaves into northern IL and generate periods scattered showers and thunderstorms. Given the previously mentioned uncertainty in the ridge position, the confidence on whether or not we will see any storms remains on the lower side. That said, any days that do see showers and storms may also see highs verify cooler than forecast which would in turn lower the threat for heat related concerns. To encompass this potential in the forecast I have maintained a 20% chance for showers and thunderstorms for most of the area Sunday through the middle of next week, but specifics on timing and coverage will likely need to be adjusted with future forecasts. Though, latest guidance is eyeballing the late Saturday night into Sunday timeframe as a favored period for showers and storms. As for when this period of heat and humidity will break, guidance continues to hint at a cold front moving towards the region towards the later portion of next week. However, timing on when exactly the front, and associated relief, will move through continues to vary amongst the guidance. Couple this with the fact that guidance typically struggles with blocking patterns and confidence to say when a break in the heat will occur remains low at this time. Therefore, we recommend keeping updated on the forecast as we head into next week for any changes. Yack && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1204 CDT Fri Jun 14 2024 Aviation weather concerns are: - Northeast wind shift this afternoon. A diffuse/subtle outflow boundary is near a downtown Chicago to Aurora line and slowly meandering northward. Don`t currently expect this to make it to ORD, but will monitor. FEW-SCT cloud layers around 020-030 exist near this boundary, so have included a mention in the MDW and GYY TAF, but don`t currently expect this to turn into a ceiling. Winds will become northwesterly and perhaps intermittently gusty later this morning and afternoon. A lake breeze and synoptic cold front will shift southward out of Wisconsin and off the lake, resulting in a northeast wind shift at all the TAF sites later today. Winds may briefly gust towards 20 knots as the front arrives before diminishing through the evening. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago