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
421 FXUS63 KLOT 130532 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1232 AM CDT Thu Jun 13 2024 .KEY MESSAGES... - Weakening showers/storms may move into the northern portion of the CWA late tonight or early tomorrow morning. - Level 2 to 3 out 5 risk for severe weather Thursday afternoon/evening for large hail and strong winds. There is also a localized flooding threat. - Hot and humid conditions likely to develop early next week, particularly on Monday. - Periodic chances for showers and storms next week as well. && .DISCUSSION... Issued at 313 PM CDT Wed Jun 12 2024 Through Thursday Night: It will continue to be hot (but with thankfully tolerable dew points/humidity levels) through the remainder of the day with temperatures holding in the upper 80s to lower 90s until near sunset. The lake breeze hugging the Illinois shore will get pushed back offshore and result in rising temps there into this evening. Zonal northwest flow across the northern Plains has already allowed a cluster of storms to get going. As these storms potentially approach the far northwestern and far northern CWA this evening, they will be in a weakening phase with isolated showers and perhaps an embedded rumble of thunder. Another round of storms will likely form later tonight and advance southeast through MN and WI towards the northwestern and far northern CWA. Dry low to mid-level air and a capping inversion across the area will result in these storms weakening as they reach the CWA. Although unlikely, there is a chance that a few of these storms both tonight and into early tomorrow morning could pose a strong to locally severe wind threat if outflow is able to maintain itself (best chance far NW CWA). Outflow may maintain long enough for a breezy/variable wind shift as far southeast as the Chicago metro around dawn. Kept PoPs in the 15-35% range through the late morning with the best chance for showers and storms around sunrise Thursday morning across the northwest 1/3 to 1/4 of the CWA. Thursday afternoon, models are in good agreement that southwesterly flow will allow moisture to build into the area with dew points in the upper 60s (more aggressive models reach the low 70s). With temperatures rising to around 90 degrees, MLCAPE values in the 2000-3000 J/kg range are expected. A surface cold front will be moving southeast towards the CWA Thursday afternoon. Initially weak surface convergence, a cap, and weak upper-level support will inhibit storms early in the afternoon, however, as frontal convergence increases and a modestly strengthened mid-level jet moves overhead, storms will become more likely through the late afternoon. The large CAPE values and 35-45 kts of shear will cause storms to become supercellular in nature early on. Steep mid-level lapse rates (7-8 degrees C/km) will present a threat for large to locally destructive (2"+ diameter) hail with large DCAPE values (>1000 J/kg) bringing a threat for strong to severe winds (70-80 mph gusts possible). These storms will also pose a localized flooding threat with heavy rainfall rates and PWATs forecasted to be around or potentially over 2 inches. The threat for severe weather has warranted a level 3 out of 5 risk for the southwestern portion of the CWA (where confidence in higher coverage of severe weather is highest) and a level 2 out of 5 risk for the remainder of the CWA from SPC. Depending how the convection plays out Thursday morning could impact how things unfold Thursday afternoon/evening. If showers/storms hold on through the late morning across the area, then temperatures will take longer to rebound and instability will struggle to build early in the afternoon. This stability would limit storm initiation chances until later Thursday allowing the cold front to advance further south and thus moving the highest threat for storms and severe weather further south as well. However, if showers dissipate before they reach the CWA, then temperatures and instability will be able to build quickly and storms may be able to form further north during the afternoon. The 12Z HRRR seems to be playing more into this scenario, which would be a "worst case" scenario for this setup, featuring convective initiation and the anticipated level 3 type threats currently farther southwest to likely include more of the Chicago metro. This will be monitored closely. Initial supercell development will likely grow upscale into clusters or a more linear mode as the evening progresses. Once the convection moves out of the CWA Thursday night, things will quiet down into Friday morning. Carothers/Castro Friday through Wednesday: An upper trough is expected to be pivoting through the Great Lakes on Friday as broad upper-level ridging begins to establish across the southwestern CONUS. The position of the upper trough will advect in cooler air into the region resulting in warm but comfortable temperatures and humidity levels for Friday and Saturday with readings in the low to mid-80s. Though, daily lake breezes will likely keep temperatures cooler (in the 70s) for locations closer to Lake Michigan. Otherwise, dry conditions are expected during this period courtesy of the aforementioned ridge. 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 across the Midwest resulting in increasing heat and humidity through at least the early portion of next week. While guidance is in good agreement on the upper- level pattern, there continues to be a lot of variance on how far east the ridge will drift. Therefore, the exact magnitude of heat remains uncertain but there is a notable signal amongst both deterministic and ensemble guidance that high temperatures in the 90s are likely with overnight lows possibly in the 70s Sunday through Tuesday. Additionally, there is also the threat for showers and thunderstorms Sunday and Monday which may further add a failure mode to the magnitude of the aforementioned heat. If the ridge does establish further east then our area would be in the favored area for shortwave troughs to pivot through and generate at least scattered showers and thunderstorms and in turn keep high temperatures lower. However, if the ridge remains further west then showers and storms could miss our area resulting in highs over performing from the current forecast and possibly even nearing record values (upper 90s) particularly on Monday. Given these uncertainties I have decided to maintain the 20% POPs offered by the NBM for Sunday and Monday, but did lower high temperatures into the low to mid-90s to account for the cooler temperature potential with storms. Obviously this will be a period to watch especially for those with heat sensitivities so stay tuned. Guidance is hinting that cold front may try to push through the region during the Wednesday into Thursday timeframe which looks to bring a better potential for showers and storms in addition to a break from the heat. Though, details on when and where this possible front will track remains uncertain at this range therefore stay tuned. Yack && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 1232 AM CDT Thu Jun 13 2024 Forecast concerns include... Scattered strong/severe thunderstorms this afternoon. Gusty southwest winds later this morning/early afternoon. Possible east winds after storms. Various areas of thunderstorms nearby this morning. Storms over southeast Lake Michigan will move away from the terminals while storms over eastern IA will move southeast into west central IL and remain southwest of the terminals. But the area of storms across eastern MN into central WI will continue moving southeast and is expected to slowly weaken and dissipate. However, if this area of convection does not dissipate fully, showers may reach into far northern IL and RFD by daybreak this morning. The focus will then shift to new thunderstorm development which is expected to develop just north or potentially right over the terminals this afternoon. There is still some uncertainty for timing and location and there is a small chance thunderstorms will develop south of the terminals. However, the consensus among the latest guidance is that thunderstorms will develop over northern IL ahead of an approaching cold front and then push to the southeast this evening with some isolated thunder potential remaining through sunset. Opted to trim tempo mention to just 2 hours, which may still need some refinement as trends emerge and then include vicinity mention for a longer time period. Any of the thunderstorms that form today may become severe with locally strong/erratic winds/gusts. Ifr/lifr cigs and vis will be possible along with very heavy rain. Southwest winds will slowly increase early this morning with some gusts possible by daybreak. Then gusts into the 20kt range are expected through early afternoon. Winds will turn westerly ahead of the front and then possibly northerly though will be impacted by thunderstorms. Models are now showing an easterly wind for a few hours behind the storms, that settles into a light northerly wind for the rest of tonight. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago