


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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410 FXUS63 KLOT 182129 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 429 PM CDT Wed Jun 18 2025 .KEY MESSAGES... - Severe weather threat has ended, though the threat for isolated thunderstorms and localized flooding continues in portions of the area into early this evening. - Stretch of hot and humid conditions with peak afternoon heat indices possibly over 100 degrees this weekend into early next week. && .MESOSCALE... Issued at 429 PM CDT Wed Jun 18 2025 The band of intense convection has shifted well east of the area into eastern Indiana. Center of the MCV is still spinning slowly eastward across our CWA late this afternoon. Some weak instability in advance of this MCV has allowed for some additional isolated storms to develop over northwest Indiana, but weaker instability and weaker low level shear is resulting in minimal severe threat from this activity and Tornado Watch 434 has been cancelled. On the northwestern side of this MCV`s circulation there is an area of moderate to borderline heavy rain will continue to spread east-northeastward across mainly southern portions of the Chicago metro area and into Lake and Porter Counties in Indiana through sunset. Rainfall rates with this activity seem pretty unlikely to result in a significant flood threat, so unless radar trends reverse course, we will likely be able to cancel the flood watch prior to the 8 PM scheduled expiration time. Finally, a narrow band of convection across our northwestern CWA could persist as it pivots slightly eastward over the next couple of hours. There have been numerous reports of funnel clouds and a couple of brief landspouts with this line closer to the Mississippi River where conditions appear more favorable. Given the developing updrafts on/near the boundary up there, can`t rule out funnel clouds and perhaps a brief landspout with this activity for the next hour or two. Have issued an SPS to cover this threat in the near term. These showers have been producing narrow corridors of heavy rainfall rates. The very small footprint of the heavier rates should generally limit the flooding threat vs. what has occurred just over the WI border in portions of Rock and Walworth Counties. - Izzi/Castro && .DISCUSSION... Issued at 403 AM CDT Wed Jun 18 2025 Through Thursday: Main forecast concerns in the near term are thunderstorm trends and severe weather potential across the area today, along with the potential for locally heavy rainfall amounts. SPC`s Day 1 outlook has expanded the enhanced risk a little farther north into the southern parts of the Chicago metro area, generally south of where last evenings convection has tracked. Early morning water vapor imagery depicts a synoptic mid-level short wave trough upstream (roughly through the Missouri River valley), likely convectively-augmented by the MCS/MCV over far southeastern KS. A couple of smaller MCVs were evident in regional radar data as well, one over central NE, another over northeast IA, and one lifting northeast out of southeast WI which brought our evening storms. Expectation is that there will be a relative lull in shower and isolated thunderstorm activity here early this morning in the wake of this departing MCV, before thunderstorm chances increase again mid-late morning ahead of the approaching short wave and embedded KS MCV. While still depicting more spread than desired, guidance is in general agreement in developing an associated surface low pressure wave across the forecast area this afternoon, backing low-level flow. Meanwhile, mid-level winds are forecast to increase to 40-50 kts along the southern periphery of the MCV, across central IL/IN and the southeast portion of the WFO LOT cwa. Warm and humid low- level air mass (low-mid 80s temps upper 60s dew points) is forecast to support 1500-2000 J/kg of MLCAPE, with 50 kt deep layer bulk shear expected to support organized convection in the form of clusters and line segments, and upscale growth into a more solid squall line appears possible across the southeastern parts of the forecast area this afternoon. A little bit of a warm layer in the 500-400 mb layer per forecast soundings weakens mid-level lapse rates somewhat aloft, with damaging wind gusts the primary expected severe weather threat. Backed low-level winds near/south of the low track will also support a tornado threat as well.Precipitable water values around 2" will also support the potential for torrential rainfall with storms during the midday and afternoon hours, with locally heavy rainfall amounts in excess of 2" possible. Storm coverage and severe threat should decrease late this afternoon as the low moves east of the area, though the upper trough axis will be slow to move across the region overnight into early Thursday morning. This will likely maintain scattered showers and isolated thunderstorms through the overnight hours and into early Thursday before strong subsidence develops by afternoon. SPS has maintained a marginal (level 1 of 5) risk for Day 2 across our far northern counties, though the potential appears to be higher north of the IL/WI border. Ratzer Thursday Night through Tuesday: Main focus through the extended period continues to be building heat and humidity this weekend into at least early next week. Broad upper level ridging will be building across the Rockies and high Plains Thursday as upper troughing shifts east across the Great Lakes. This will place the local area under a northwesterly flow aloft ahead of the building ridge to the west. This set up will support the passage of subtle shortwaves through the northwest flow which may result in some isolated shower/thunder chances, especially across far northern Illinois later Thursday or Friday. Better chances look to be to the north so most if not all areas may remain dry. A closed upper low upstream over the west coast is progged to further amplify the downstream ridge and push it eastward into the weekend. This will bring in much warmer air by Saturday with a breezy southwesterly low level flow setting up across the area. With such an amplified pattern, the ridge axis will extend well north of the area by Sunday, which combined with the fairly northerly position of the base of the western trough would keep the favored track for showers and thunderstorms north of the area through the weekend. Given that we`re still a few days away from this pattern shift, we`ll have to monitor how the amplitude of the upper features evolves over the next few days to see if the signal for convection to be favored to the north persists. The question going into early next week is how quickly the amplitude decreases and when a favored track for thunderstorms returns to the area as well as how quickly the heat abates. As far as temperatures, 850 mb temps are expected to warm into the 21-23C range for the weekend. Breezy south to southwest surface winds will support strong warming with highs expected to reach the mid 90s for most areas both Saturday and Sunday, and likely into Monday and perhaps Tuesday as well. Dewpoints are forecast to reach the lower 70s but this will be another element to monitor as drought currently persists across the area. Rain from today`s thunderstorms may impact this, but Thursday and Friday will likely allow for drying out of the top layers of the ground. Given this, and the early stages of crop growth (as opposed to mature stages in July and August which allows for crops to add a notable amount of water vapor to the air) there is some concern that current dewpoint forecasts could be a bit too high. A drier airmass would allow for higher temperatures in the upper 90s. Ultimately, a hotter/drier scenario could net a similar heat index as a slightly less hot/more humid scenario with the resultant messaging of taking heat precautions being the same. At this point, the bigger concern is the duration of the heat as opposed to heat reaching an extreme or unusual magnitude. MDB/Ratzer && .AVIATION /18Z TAFS THROUGH 00Z FRIDAY/... Issued at 1256 PM CDT Wed Jun 18 2025 Concerns: - Thunderstorms with heavy rain this afternoon - Messy winds through the afternoon into the early evening - MVFR ceilings and IFR/LIFR visibility in the heavy rain Low pressure will continue to strengthen and lift to the northeast this afternoon. An area of showers and thunderstorms will spread north out ahead of the low. The low will move overhead just to the south and east of the Chicago terminals, and likely passing over KGYY. The strongest storms will be out ahead of the low to the east, where gusty southwest winds will accompany the storms. For the Chicago area, storms may remain elevated. However, the storms will have heavy rain and reduce visibility to LIFR even at KORD and KMDW. The main window of storms will be 19z-22z, before becoming showers with embedded thunder. The other challenge will be the winds. Winds will remain quite variable given the low track moving close by, but should maintain some easterly component. ORD and MDW also have a lake breeze to contend with, that will also aid winds from transition from a SE to NE, though they will likely bounce around. If any westerly component occurs, they should be on the lighter side or brief, with some easterly component prevailing during the storms. Winds will shift to north and then northwest as the low pulls away this evening. Some showers and MVFR ceilings will remain in place this evening, with a 40% chance of IFR. KMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch until 8 PM CDT this evening for ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108. IN...None. LM...None. && $$ Visit us at weather.gov/chicago