Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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087 FXUS63 KLOT 161739 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1239 PM CDT Sun Jun 16 2024 .KEY MESSAGES... - Severe thunderstorms will sweep across northern Illinois this afternoon and evening, mainly along and north of I-80. - Damaging winds and a brief tornado are the main threats. Flash flooding may occur if thunderstorms regenerate this evening over the Chicago metropolitan area. - Hot and muggy conditions will prevail through the end of the week. Periodic chances for thunderstorms will continue, though many hours will remain dry. && .DISCUSSION... Issued at 1239 PM CDT Sun Jun 16 2024 Through tonight: An impressively organized mesoscale convective vortex anchored by a semi-circular outer-core of convection is currently lifting from northeastern Missouri and into southeastern Iowa. Low-level moisture continues to increase ahead of the MCV, with dew points in the lower 70s now spreading toward the I-39 corridor and Wisconsin state line. Mostly sunny skies have allowed for highs to build into the upper 80s to around 90, which when combined with the steepening mid-level lapse rates and increasing low- level moisture, is contributing to some 2000-2500 J/kg of MLCAPE. As broad southwesterly steering flow continues guiding the MCV northeastward into the destabilizing airmass, coverage of ongoing thunderstorms near the MCV will increase with perhaps a rapid uptick near or west of I-39 sometime in the 2 to 4 pm timeframe. Thereafter, convection will sweep across northern Illinois, primarily along and north of I-80. Locally augmented flow on the eastern side of the circulation (KDVN radar is sampling some 50+kt of flow at 12kft) will provide a mesoscale kinematic environment for the development of supercells and bowing structures as convection matures across northern Illinois this afternoon, with a threat for "upscale growth" into a compact MCS as it crosses northern Illinois this afternoon and evening. For this reason, the threat for a swath of damaging winds (60 to 75 mph) and even brief tornadoes continues to increase, focused near and north of I-80. Accordingly, the Storm Prediction Center has introduced a Level 2/5 threat level for severe weather this afternoon, mainly along and north of I-80. Given the highly organized structure of the MCV, we remain concerned for regenerating convection along the backside of the circulation across northeastern Illinois after sunset. Such a threat will come down to mesoscale details of whether a focused zone of low-level confluence will develop at the intersection of the western side of the cyclonic circulation and any continued southwesterly low-level synoptic flow. Forecast confidence in such a band of regenerating convection remains low, though it`s something we can`t rule out. If radar and satellite trends begin to hint at a mesoscale band of regenerating convection materializing, we will consider issuing a short-fused Flash Flood Watch for part of our area (likely just a few counties). Borchardt Monday through Saturday: Going into Monday, the upper-level ridge will remain locked in place, allowing for ample sunshine to further warm the newly arrived air mass. High temperatures are thus likely to end up being a few degrees warmer on Monday than they will be today. There`s still some uncertainty as to how much dew points will mix out on Monday, with HRRR forecast soundings suggesting that the 850-700 mb layer will be quite a bit drier than it will be today, possibly allowing for more efficient mixing to take place and scour out dew points more readily. Nevertheless, continued warm/moist advection off of southerly winds and evapotranspiration should offset this mixing to some degree and keep dew points from plummeting lower than the mid 60s, affording us another humid day. The degree of mixing that occurs shouldn`t have a substantial effect on heat indices as lower dew points would mean that air temperatures would likely end up being a little higher, but our latest forecast grids have peak heat indices getting to about as high as 100 degrees, with some potential for them to end up being as high as 105 degrees if dew points struggle to mix out. Either way, it looks like we will fall shy of meeting Heat Advisory criteria. It remains possible that isolated to scattered pulse-type thunderstorms could be seen tomorrow afternoon. Most CAMs are quite bullish on thunderstorms developing and expanding in coverage as new convection bubbles along the outflow that they spit out. However, large-scale forcing for ascent looks pretty nebulous tomorrow, and it appears that we will see slight geopotential height rises as well, so the thinking is that the CAMs are a little overcooked with their output, and have continued to carry just slight chance PoPs for now with the expectation that their coverage should remain isolated to widely scattered at best in our CWA. Nevertheless, wherever storms do develop, their rain-cooled outflow will provide temporary local relief from the heat and humidity, but there will also be some potential for strong to perhaps locally damaging downburst winds to occur as deep updraft cores collapse in an environment characterized by meager deep layer shear. The northern fringes of a deeper moist plume advecting off the Gulf of Mexico will arrive in the area on Tuesday, and PWATs look to build towards 1.75-2 inches through the day. At least some increase in mid-level cloud cover looks to accompany this advancing moist plume, and this may curtail mixing a bit compared to Monday. This should result in slightly higher dewpoints and lower high temperatures, driving peak heat indices in the mid to upper 90s. Somewhat gustier southerly winds may help take some of the edge off though, and this is reflected with slightly lower wet bulb globe values compared to Monday. Large scale forcing still won`t be significant, but low-amplitude vort lobes within the southwesterly steering flow will likely continue parading around the periphery of the sprawling east coast high. Isolated to widely scattered mainly afternoon shower/storm chances will continue, as a result. Through Tuesday night, a southward-advancing cold front to our north looks to be pretty active across Iowa, Minnesota, and parts of Wisconsin with widespread convection. While it`s possible some of this activity tries to ooze into our northwest into Wednesday morning, thickness gradients and LLJ orientations seem to point to any upwind propagating portions of MCSs building preferentially into southern Iowa, while the forward propagating components scoot across central/northern Wisconsin. On Wednesday, the aforementioned cold front will have sagged further southward into southern Wisconsin as outflow from overnight activity pushes closer to our region. With diurnal thunderstorm activity igniting in a bit closer proximity, higher- end chance PoPs across northwest Illinois seem appropriate during the afternoon. The thermodynamic/kinematic parameter space suggests pulse convection will be the norm, with heavy rain (due to slow cell motions) and gusty winds as the main threats, if activity manages to sneak south of the Wisconsin state line. With the core of the parent upper ridge expected to be situated near Long Island, highs on Wednesday will probably be near where they were on Tuesday although there is a decent spread across the guidance suite. The latest gridded forecast shows heat indices generally in the mid and upper 90s again. On Thursday, guidance suggests the east coast high may attempt to build farther west which would increase thicknesses locally and support a higher ceiling on air temperatures compared to the previous two days. However, it`s possible the portion of the quasistationary boundary/cold front across southern Wisconsin starts to move southward, with some acceleration as a lake breeze/lake-enhanced boundary. Latest indications are this probably wouldn`t occur until later in the day and for a small part of our forecast area across northeast and northern Illinois, but there`s quite a bit of uncertainty regarding this evolution. Ahead of wherever the front ends up during the afternoon, eyeing a potential for temperatures to surge a bit within the thickness bump ahead of the synoptic front, along with dewpoints to locally pool which could result in a corridor of notably higher peak heat indices. Pinpointing where (and if) this will set up remains too unclear to try to paint this in the grids for Thursday, but something we`ll be keeping an eye on. Looks like another toasty day on Friday as the front lifts back northward, but we`ll eventually push a reinforcing front through here on Saturday or Sunday. This will bring increased chances for thunderstorms to the area along with a cool down into early next week. Regarding heat headlines: while this continues to look like an extended stretch of anomalously warm and humid conditions, it still looks like we`ll end up largely under local Heat Advisory criteria with peak heat indices generally near or under 100 degrees. However, if these values creep up a bit more, particularly Wednesday and Thursday, there could be an argument for an advisory due to the extended nature of 100+ values. We`re not seeing a strong enough signal for this right now, however. Ogorek/Carlaw && .AVIATION /18Z TAFS THROUGH 00Z TUESDAY/... Issued at 1226 PM CDT Sun Jun 16 2024 Key Aviation Impacts for the current TAF period: - Gusty pre-storm southwest winds this afternoon - Increasing confidence for thunderstorms to be expected at the Chicago terminals this evening/tonight. Main threat will be stronger, potentially severe, winds and heavy rain - After lighter winds overnight, another chance for gusty southwest winds Monday afternoon Skies are VFR with a some higher clouds moving overhead from the storm system that is approaching tonight. Winds remain out of the southwest. Gusts have generally been between 20 to 25 knots, though an occasional rouge gust up just less than 30 cannot be ruled out before the storm arrives. As mentioned above, the threat for thunderstorms arriving at the terminals has increased to the point that it is now expected. The storm that is currently over Northern Missouri has maintained itself and is tracking to the northeast. While the TAF reflects the expected timing, it can arrive around KRFD as early as 20Z, and the Chicago terminals around 22Z. While small hail and a brief tornado cannot be completely ruled out, the main threat will be strong and potentially severe winds, as well as heavy rain. It is a large complex that will be seen coming on radar and will move through the airspace through the night and moving out over Lake Michigan away from the city after midnight. Winds will diminish behind the system as conditions dry out. Temperatures will heat up tomorrow and with lingering moisture around, a pulse shower or storm is possible, but the probability is around 20 percent or less at this point therefore not currently in the TAFs. Otherwise VFR conditions are expected on Monday with chances for southwesterly wind gusts to return in the afternoon. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Air Quality Alert until midnight CDT Monday night for ILZ005- ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105- ILZ106-ILZ107-ILZ108. IN...Air Quality Alert until 1 AM EDT Monday for INZ019. LM...None. && $$ Visit us at weather.gov/chicago