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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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944 FXUS63 KLOT 141713 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1213 PM CDT Sun Jul 14 2024 .KEY MESSAGES... - Additional clusters of storms are expected through this evening, with continued threats for severe weather and flash flooding. - Very warm to hot and very humid this afternoon and hot and very humid on Monday. Tomorrow (Monday) will likely be the hottest day of the pair, and will likely need a Heat Advisory. - Additional rounds of thunderstorms are expected Monday evening through Tuesday with continued threats for severe weather and flash flooding. - The pattern finally breaks Tuesday night with a return of tranquil conditions Wednesday onward. && .UPDATE... Issued at 1213 PM CDT Sun Jul 14 2024 A complex convective scenario will unfold later this afternoon through the evening as a well defined MCV from overnight severe convection tracks across the region. We`re finally seeing the organized morning MCS exit southeast into central and northern Indiana. In the near term, monitoring the elevated convection that redeveloped from southwest Wisconsin into northwest Illinois. With lingering cold pool effects over our area, it`s unclear if these storms will maintain or intensify (leaning toward weakening), but did add in PoPs for isolated to widely scattered coverage over portions of the northwest CWA through early afternoon. Aside from the clouds associated with that cluster, cloud cover will steadily erode through the early to mid afternoon. Lingering cold pool effects from morning convection greatly delayed the heating process and also brought dew point temperatures lower than previously forecast to be at mid day today. Given the cooler temps (70s) and dew points (upper 60s to around 70F) as of this writing, lowered the high temperature forecast to the upper 80s to low 90s. Still expecting dew point temperatures to recover to well into the 70s later this afternoon, supporting peak heat indices in the mid 90s to low 100s, highest for the southwest 1/2 or so of the CWA. Turning back to the thunderstorm forecast, the presence of the MCV over Iowa moving into the region during peak heating and large instability certainly adds to concern for a scattered severe weather threat, along with flash flooding. Given the current location of the MCV, suspect that scattered thunderstorms will initiate over the northwest CWA as early as 4-5 PM. Will need higher PoPs than in the official gridded forecast this evening as the MCV translates eastward. Deep layer shear will be on the increase (up to 35 kt if not more), so initial storm mode may be mixed supercellular and multi- cellular clusters. There should be a tendency for cold pool development and transition to bowing segments with time this evening. Hazards wise, initial supercells may be capable of all hazards, though extent of any tornado threat would be driven by how much low-level shear is increased by the enhanced flow aloft associated with the MCV, as LCLs will certainly low enough. Despite the high moisture and freezing levels, initial supercells will also be capable of producing large hail due to steep mid-level lapse rates. Overall greatest threat, especially into the evening should be for pockets of damaging winds. With the LLJ ramping up in concert with MCV flow enhancement, may also need to monitor for mesovortex and QLCS tornado development. Finally, last but not least, given the significant flooding in Rockford and nearby across far northern Illinois last night, plus very high moisture, flash flooding continues to be a noteworthy concern. We opted to cancel the previous Flood Watch but will likely need to issue a new one this afternoon for a chunk of the area. The potential for significant flooding may be somewhat less if propagation tied to cold pools and the progression of the MCV clears deep convection east of our area quicker, by midnight or a bit after. Castro && .DISCUSSION... Issued at 322 AM CDT Sun Jul 14 2024 Through Tuesday: On the heels of a robust cluster of overnight thunderstorms, yet another cluster of storms is surging down the Mississippi River along the edge of the EML plume. While the next cluster of storms is moving into a relative minima of instability, a steady feed of instability via a 925/850mb low-level jet should allow it to reach at least the Wisconsin state line and likely into the northern-most row or two of counties of northern Illinois around or after daybreak. Given the areas of heavy rainfall that fell overnight, including significant flash flooding that continues around Rockford at press time, opted to issue a Flood Watch for counties along and north of I-88 through 1 PM. Behind the morning round of storms, a period of dry weather is anticipated into the early afternoon. Attention turns firmly toward yet another round of thunderstorms this evening. Regional radar imagery shows a derecho racing across southeastern South Dakota. The latest radar trends indicate that the gust front is now well ahead of the reflectivity, which is not surprising given the system is well south of the upper-level jet and source of deep-layer shear displaced to the north. As a result, the derecho should begin to weaken as it surges into northwestern Iowa later this morning. With that said, there appears to be a highly organized MCV within the core of the system, which will approach northwestern Illinois by mid- afternoon and ignite the next round of storms. With low- to mid- level shear augmented in the vicinity of the circulation, any storm may be severe with damaging winds, damaging hail, and even a tornado. In addition, flash flooding may occur as well especially if thunderstorms favor far northern Illinois once again. With the threat for two rounds of storms today, confidence in the temperature and heat index forecast is quite low. In scenarios with minimal convective coverage and a footprint of outflow, highs should climb into the lower 90s. When combined with dew points in the low to mid 70s, afternoon heat indices would top off in the 100-105F range. I gave serious consideration to issuing a Heat Advisory, but simply didn`t have high enough confidence in widespread 105F+ readings today. Now, it does appear that there will be a break in the waves of thunderstorms during the day on Monday, which gives higher confidence in heat- headline criteria being archived. Indeed, highs in the low to mid 90s and dew points in the mid to upper 70s will support peak heat indices of 105 to 110 areawide. Additional rounds of storms are expected Monday night through Tuesday, which will include the threats for severe weather and flash flooding. Borchardt Tuesday night through Sunday... Following the cold front clearing our southern CWA Tuesday night into Wednesday morning, mean troughing will set up over northeastern North America while powerhouse 500 mb ridging rebuilds over the western US. This should bring an extended period of dry, cooler and less humid conditions Wednesday PM onward through next weekend. Castro && .AVIATION /18Z TAFS THROUGH 00Z TUESDAY/... Issued at 1213 PM CDT Sun Jul 14 2024 Key Aviation Messages: - Widespread thunderstorms are expected late this afternoon into the evening. These storms will have an accompanying threat for damaging winds, in addition to reduced VSBY and CIGs. - Another round of thunderstorms are expected tomorrow afternoon and evening. Discussion: The primary concerns through the TAF period are the multiple chances for thunderstorms through tomorrow afternoon. Radar and satellite imagery show the convection from this morning moving out of the area. There are some showers and storms in northwest IL that are headed towards KRFD and will be arriving shortly. Current thinking is that this convection will dissipate before it reaches the terminals as it interacts with the remnant cold pool from this morning`s convection, however, can`t rule out the chance for some VCTS this afternoon. Another round of strong to severe storms is expected this evening across the whole area, with the main hazard expected to be damaging winds. Most guidance suggests a start time of around 23Z at KRFD with an hour lag for KORD, KMDW, and KDPA and another hour lag for KGYY. These storms will decrease visibility and CIGs through the evening and into the overnight hours. Winds will likely get a little squirrelly as the storms pass through as well. Similar to this morning`s convection, it is not out of the question that winds turn easterly/northeasterly behind the storms, however, they should turn back to a southwesterly component shortly after the storms move move out of the area. Things should be quiet through the remainder of the night with southwesterly winds prevailing through the end of the TAF period. Tomorrow afternoon, there is yet another chance for widespread storms. Best timing looks to be around 00Z, however, some guidance has storms initiating in the mid-afternoon. Thus, confidence is currently low in the timing, but have covered the possibility of afternoon initiation with a PROB30 at KORD and KMDW. Carothers && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103- ILZ104-ILZ105. IN...None. LM...None. && $$ Visit us at weather.gov/chicago