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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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250 FXUS63 KLOT 112014 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 314 PM CDT Thu Jul 11 2024 .KEY MESSAGES... - Isolated to scattered showers/storms this afternoon and early evening, with the greatest coverage south of I-80. Gusty winds and locally heavy rainfall the main threats. - Patchy fog development possible south of I-80 tonight. - Hot and humid conditions expected early next week. Heat indices will likely approach or exceed 100 degrees. - Periodic thunderstorm chances Saturday through mid next week, including the potential for a couple nocturnal/early AM t-storm complexes. && .DISCUSSION... Issued at 223 PM CDT Thu Jul 11 2024 Through Friday: Isolated to scattered showers and thunderstorms will develop this afternoon in the presence of broad upper troughing interacting with several surface boundaries. The main area is expected to focus generally south of I-80 in the vicinity of a diffuse quasistationary boundary, along which dewpoints have pooled in the mid and upper 60s. Instability looks like it wanes with northward extent owing to fairly extensive morning cloud cover and shower activity, and it seems like some suppressive effects from ongoing subsidence may also curtail additional thunderstorm activity, at least on a widespread basis. North of the Wisconsin state line, instability increases once more where mid-level temperatures are a bit colder, so suspect that additional widely scattered activity will percolate this afternoon. Slow southwesterly cell motions could bring some of this activity into our far northwest locales later this afternoon. Severe weather is not expected, but gusty winds and brief torrential downpours will be possible with any activity. Slow cell motions (under about 10 mph) may lead to very localized pockets of minor flooding. Otherwise, activity will slowly diminish with time this evening although we may not entirely get rid of isolated, festering showers as broad ascent on the southern flanks of a well-defined vort max and modest divergence along the cyclonic shear side of an upper jet linger in the region. Patchy fog development is possible as well, with soundings looking most supportive south of I-80 where low-level flow looks weakest amidst slightly deeper near- surface moisture. Generally quiet conditions are expected on Friday as subsidence on the backside of a departing upper wave/trough spreads overhead. Can`t totally rule out a few showers developing with heating near the lake breeze, but soundings look pretty dry aloft with some notable 500-600 mb warming which should further act to suppress afternoon activity. Carlaw Friday Night through Thursday: Following a quiet and seasonably mild Friday night, the first potential episode of convection in the humid and at times stormy pattern expected this weekend through early next week may arrive Saturday morning in parts of the area. This will occur in concert with a warm front lifting north, and a large instability reservoir along with a mid-level impulse advancing eastward across the mid and upper MS Valley. With very dry mid- level air in place early Saturday being shunted eastward with the arrival of the MUCAPE reservoir, have chance (30%) PoPs for elevated convection in the western 1/2 or southwest 2/3 of the CWA, tapering quickly to the east and northeast due to the west to east instability gradient. Large scale forcing mechanisms will be lacking through the afternoon following the dissipation of morning elevated convection, with a warming EML base increasing capping. For these reasons, the afternoon may end up with isolated thunderstorms (slightly better chance south of I-80?) at most with a potential for mainly dry conditions. Highs on Saturday will be in the mid to upper 80s, except slightly cooler lower 80s along the Illinois shore due to lake influence backing winds east- southeasterly. Dew points in the upper 60s to lower 70s at peak heating will yield peak heat indices in the low-mid 90s, a harbinger of things to come Sunday through Tuesday. Expansive mid-upper 500s DAM 500 mb ridging centered over the inter-mountain west and mean troughing centered over Hudson Bay will set up a hot and humid northwest flow aloft locally in the Saturday night-Tuesday morning timeframe. Unlike previous occurrences this warm season of overnight arriving convection weakening quickly as instability diminishes, the impressive instability reservoir (3000-4000+ J/kg of MUCAPE) will largely remain in place through the period. This entails a period of "ring of fire" conditions, where our position on the northeast flank of the ridge puts us in the threat zone for southeastward moving convection, including potentially well-organized severe MCSs, and episodes of flash flooding due to PWATs up in the ~1.75-2" range at times (150-200% of normal). Also concerning regarding flooding potential is a west-southwesterly low-level jet feeding into southeastward moving convection, which is a pattern that can favor backbuilding and training. Aside from operational and ensemble members hinting at the likelihood of this type of activity, specific timing predictability in these types of patterns are commonly low confidence. This maxes out PoPs only in the chance (30-40% range) during windows of MCS favorability at this lead time. The first window may be Saturday night, with moderate deep layer shear up to 30-35 kt present, followed by Sunday night into early Monday when 40+ kt of deep layer bulk shear is forecast. Monday night into early Tuesday could end up being another window for parts of the area as well. Classic in these type of patterns, there will be many dry hours, and a relative daytime lull/capping Sunday and Monday PM will support highs in the lower to mid 90s with dew points in the 70s yielding peak heat indices flirting with or exceeding heat advisory (105+F) criteria. Maturing cropland will likely contribute an evapotranspiration component to the dew point temperatures on Sunday and Monday, so wouldn`t be surprised to see upper 70s (locally higher) Td realized at observation sites in farming areas. While Tuesday will again be humid (upper 60s-mid 70s Td), overnight-early Tuesday convection may shunt the effective boundary south and force the zone of any afternoon thunderstorm development primarily south of I-80. Temperatures will be a notch below Sunday and Monday, in the upper 80s to lower 90s, yielding heat indices in the 90s to around 100F, highest southern CWA. The last few model cycles have pointed toward the instability axis being shunted well south by a true cold front passage Tuesday night into Wednesday to bring a cooler, less humid, and mainly dry day on Wednesday, with these conditions continuing into Thursday. Castro && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1254 PM CDT Thu Jul 11 2024 There are no significant aviation weather concerns for the current TAF period. A couple of spotty showers could redevelop near RFD this afternoon, but confidence in this occurring was too low at TAF issuance to warrant having a formal shower mention of some sort in the RFD TAF. A stray shower or two near GYY also couldn`t be ruled out through tomorrow morning, but confidence in that is similarly low. Additionally, patchy fog may develop in the area overnight, but the likelihood of notable visibility reductions occurring at any of our TAF sites is low. Otherwise, expect VFR conditions and light winds under 10 kts out of a predominantly easterly to northeasterly direction throughout the TAF period. Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago