Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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896 FXUS63 KLOT 251747 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1247 PM CDT Tue Jun 25 2024 .KEY MESSAGES... - Two potential rounds of severe thunderstorms capable of producing damaging winds and flash flooding today. The first window mid-morning across northern Illinois and the second window late afternoon into mid-evening across much of the forecast area. - Another period of active weather with strong to severe thunderstorms and flash flooding is possible Friday into Saturday. && .DISCUSSION... Issued at 413 AM CDT Tue Jun 25 2024 Through Tonight: A substantial pool of high theta-e air in eastern Iowa owing to low to mid 70s dew points will gradually advect eastward into the forecast area this morning below a growing EML plume. The result is an impressive CAPE profile primed for explosive thunderstorm at any point through this evening. The major caveat is the corresponding cap that will also be advecting into the area this morning. At this time, we are focused on two areas of potential severe convection in our area: 1) A mature MCS across central/east-central Wisconsin has started to turn more south along the existing MUCAPE gradient as the veering low-level jet impinges on the western flank of the line. Recent elevated convection developing over southwest Wisconsin indicates that the incoming EML has not capped the environment east of the Mississippi River. So, there is growing concern that the gap between this convection and the MCS begins to fill in over the next hour or two and grow upscale into northern Illinois in the 6-10am window. Will be closely watching this activity through sunrise as the downstream thermodynamic environment supports a damaging wind threat with any convection this morning. 2) Conditional on the absence of any substantial lingering cold pool across northern Illinois through mid-morning, recent CAM guidance suggests convective temps in the mid to upper 90s may be reached by mid afternoon. With a mid-level wave over eastern NE/SD and resultant convective enhancement of a decaying MCS early this morning, this feature may provide the needed forcing to allow convection to rapidly grow roughly around the I-80/88 corridors by 3- 5pm. Though deep-layer shear will be quite modest, MLCAPE values over 4,000 J/kg and DCAPE values surpassing 1,500 J/kg will support initial discrete cells/clusters with damaging hail/winds congealing into a slow-moving line with damaging winds and flooding from torrential rainfall. Ultimately, the convective line should drift south through the remainder of the CWA through this evening, continuing to produce a notable flood threat as low-level flow becomes only slightly oblique to the convection (and backward propagation vectors begin to turn to the southwest). Kluber Wednesday through Monday: A cold front will press across the region on Wednesday. Depending on how things evolve Tuesday evening and overnight, some convection may be ongoing across the area Wednesday morning, mainly south of I-80, but the trend should be towards a drier one through the day. Some guidance does show some fairly deep boundary layer mixing developing through the afternoon, perhaps enough to pop a few high-based showers, but this doesn`t seem like a significant enough signal to justify PoPs at this point. A brief period of dry and quiet conditions will then prevail through Thursday as a transient region of high pressure drifts overhead. By Thursday night into Friday morning, the surface high will be departing and shuttling across the northeast, yielding a return of moist southerly trajectories in the region. A series of low- amplitude/subtle shortwaves emanating from a robust upper low pinwheeling across Montana and into Saskatchewan will likely get swept up within freshening mid and upper level west-southwesterly flow. This in turn will lead a commensurate increase in low-level warm advection across Iowa, which will eventually spill east of the Mississippi River during the day on Friday. Latest model guidance suggests that warm advection/isentropic upglide-induced convection will probably develop late Thursday night into Friday morning across Iowa within the plume of returning elevated instability. However, latest indications are that this activity may outpace the slowly-returning elevated mixed layer leading to a gradual weakening trend through the day. However, Friday night continues to look like a period we`ll have to watch closely, as another area of low pressure and associated cold front will be impinging on the region from the north. Ahead of this, intensifying warm advection acting on any remnant outflow from morning activity will also finally be able to impinge on the returning warm/moist sector as the kinematic parameter space also becomes more supportive of storm organization. Still pretty far out with plenty of spread and uncertainties, but this is a period we`ll continue to closely eye for a severe and also flash flood potential with PWATs forecast to surge into the 2 to 2.5 inch range. We`ll get into another brief quiet period over the weekend as high pressure returns. From a synoptic perspective, winds may surge a bit down the lake as the high arrives which could yield a building wave/rip current potential over the weekend. Carlaw && .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/... Issued at 1239 PM CDT Tue Jun 25 2024 Key messages... - Confidence in the timing of storms/wind direction changes is lower than usual. - Thunderstorms will redevelop across the airspace late this afternoon, but confidence on whether they hit the Chicago terminals is low. They may remain south of the terminals. - Gusty winds will accompany thunderstorms. If TS hits the terminals, winds will veer around to E, SE before shifting back to SW overnight. - Showers and thunderstorms will move through the region again overnight, again with low confidence on coverage and location. Higher confidence exists in these getting close to or remaining south of the terminals. The main forcing mechanism for storms later today/this evening will be along a front which is currently focused in IA and extends along I-88 into IL. This will be focus for isolated to scattered TS later this afternoon and evening. Right now it appears a few storms will develop, but coverage does not look to be high enough late this afternoon and evening to include a formal TAF mention just yet. Higher confidence exists in these becoming focused along and south of I-80 with time this evening. Showers and storms may also continue into the overnight hours ahead along and ahead of an approaching cold front. This appears to be a bit more favorable period for showers or storms in the area, though again it is not clear how far north these will be. Winds will become variable in and around storms. Winds will shift to north-northeast on Wednesday. Timing may need to be tweaked on this for ORD/MDW. KMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 1 PM CDT this afternoon for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago