Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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666 FXUS63 KLOT 311144 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 644 AM CDT Fri May 31 2024 .KEY MESSAGES... - Rain and embedded thunderstorms Saturday. - Periodic shower and thunderstorm chances Monday-Wednesday. && .DISCUSSION... Issued at 329 AM CDT Fri May 31 2024 Through Saturday Night: Early morning surface analysis depicts weak high pressure across the Great Lakes region. The high will drift slowly off to the east today, allowing low-level winds to turn southerly and associated warm advection to help boost temperatures into the upper 70s/near 80 in many spots today. Locations along the IL Lake Michigan shore will be a little cooler (low-mid 70s) thanks to an east-southeasterly wind component there and an afternoon lake-breeze push. A persistent but thin cirrus layer aloft should still allow for partly to mostly sunny skies through the day. Attention then turns to a slow-moving mid-level short wave trough taking shape across the central/southern Plains and lower Missouri Valley region. This disturbance is progged to lift northeast across the mid-Mississippi Valley and western Great Lakes region through Saturday, with an accompanying weak surface low tracking across downstate IL Saturday and central IN Saturday night. Guidance spreads deeper moisture northward into the forecast area Saturday morning, setting the stage for a rainy/showery day as the wave moves across the area. Various models continue to show some spread with respect to the greatest areal coverage of rain and QPF amounts, though the EPS and CMC ensembles have been fairly consistent in favoring the southeast half of the cwa, generally along and southeast of the I-55 corridor for highest pop and QPF focus. Precipitable water values increase to 1.75" (165% of normal) across these areas Saturday, suggesting the potential for some 1.00"+ amounts (especially noted in some CAM and deterministic model QPF) though the EPS/CMC ensemble and the 06Z HRRR generally place the axis of heaviest rain amounts off to the south and southeast of the forecast area where convection looks to be more favorable. Mid level lapse rates are not very impressive per model forecast soundings, generally in the 5.5-6.5 C/km range, suggesting mainly an embedded thunder threat within the broader area of rain/showers. Best chances for thunder looks to be Saturday afternoon/early evening where some weak (<500 J/kg) MUCAPE is depicted. WPC has the southeast 2/3 of the cwa in a marginal threat (level 1 out of 4) for excessive rainfall for Saturday and can`t argue with that given the higher pwats. Global ensemble QPF is generally in the 0.50- 0.80" range across our southeast cwa, which may be more representative for rainfall amounts where convection is more limited. Rain/showers should end Saturday evening across our western counties, and gradually end from the west across the remainder of the area Saturday night as the mid-level short wave axis shifts east of the area. After low temperatures in the 40s in spots this morning, increasing clouds and south-southeast flow tonight will keep overnight lows in the mid-50s in most areas. Considerable cloud cover and rain on Saturday looks to limit daytime highs to around 70 degrees, with lows in the 50s expected again Saturday night. Can`t rule out some patchy fog or low stratus development Saturday night due to moist ground conditions, light winds and late clearing. Ratzer Sunday through Thursday: Weak high pressure centered over Lake Michigan will bring a quiet and seasonable day on Sunday. Some pockets of stratus and/or fog may hold on past sunrise near Lake Michigan and in our southeast CWA. Expectation for the afternoon is partly cloudy skies for most of the area from fair weather Cu. Highs will reach the upper 70s to around 80F inland and upper 60s to lower 70s near the lake (mid 70s a bit farther inland) due to onshore flow and then lake breeze reinforcement. Fairly widespread convection will get going Sunday evening well to our west and northwest (MO Valley to the northern Plains and upper Midwest) ahead of a cold front trailing from low pressure tracking toward central Canada. This will occur as a short-wave intercepts the frontal zone. A relatively sharp west to east MUCAPE gradient from the mid-upper MS Valley to the Great Lakes, plus the strongest large scale forcing peeling north of the region will likely mean whatever convection pushes into areas near and west of I-39 Monday morning should be in a weakening phase as it tracks southeast. It`s plausible that the farther east you go in the CWA, any decaying earlier convection could miss to the west/southwest. The presence of the decaying convection, the effects of any outflow associated with it, and lingering debris cloud cover, render the Monday forecast increasingly uncertain into the afternoon. A warm front should be draped across the area, conceivably merging with retreating outflow from the morning. Given neutral height tendencies in the PM hours (modest at best larger scale forcing from the weakening short-wave lifting northeast), and items above that may limit destabilization across at least portions of the area, the exact focus or trigger mechanism for afternoon convective development is a bit unclear. Pockets of heating near the warm frontal zone may be enough for 1000-2000 J/kg of MLCAPE to develop with minimal capping as temps warm to the lower 80s with dew points well into the 60s. Our forecast has likely (~60%) PoPs for the Illinois portion of the CWA and chance PoPs for our Indiana counties Monday afternoon, though this may be a bit broad-brushed in reality. Capped thunderstorm mention at chance/scattered type coverage. Marginal deep layer shear will result in mostly non-severe storms, but can`t rule out a few feistier storms with downbursts and small hail. Anticipating a relatively quick diurnal decay of showers and thunderstorms lingering into the evening given lacking deep layer shear and large scale forcing. The next period of interest will be Tuesday PM and night, possibly into Wednesday. As things stand now, our area may be yet again caught in the middle timing wise for a higher chance of organized convection. Isolated to widely scattered diurnal pulse-type convection may be able to develop Tuesday afternoon as capping erodes amidst mid 80s temps and dew points pushing 70F. A seasonably strong short-wave trough and associated surface low ejecting from the northern Plains into the Canadian Prairies will drag its cold front eastward Tuesday night. This looks to be yet another favorable synoptic pattern for extensive severe thunderstorms to initiate late day Tuesday into Tuesday evening out ahead of the cold front and near the lifting warm front, from the Missouri Valley to the northern Plains. For our area, if this sort of timing and evolution comes to pass within the realm of the recent guidance consensus, decaying convection may push into our area from the west in the evening and overnight. While it`s too early for a firm prediction, it doesn`t currently appear that the ingredients will be in place to maintain a strong nocturnal MCS well eastward across the local area Tuesday night. The system cold front may also sweep across the area at a less favorable time diurnally (Wednesday AM-mid day) on the heels of the weakening overnight convection. Thus prospects for afternoon redevelopment will likely hinge upon a slowing of the cold frontal approach and passage. For now, maintained 40-50% chances of showers and t-storms Wednesday PM, but a scenario of a primarily dry afternoon and evening is firmly in the realm of possibility. The upper level low from Wednesday`s frontal passage may become trapped near or north of Lake Superior to close out the work week, resulting in a cooler (how much cooler is uncertain) and drier air mass into the following weekend. Castro && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 644 AM CDT Fri May 31 2024 Main Concerns: - Timing of likely lake breeze wind shift to easterly this afternoon at ORD, MDW, and possibly GYY - SHRA developing Saturday morning Light southerly winds will increase to near/around 10 kt by the mid to late morning. Direction will probably vary between 160 and 200 degrees at ORD and MDW. As the lake breeze takes shape this afternoon and slowly pushes inland, direction may favor due south to just west of south. We`re still forecasting the wind shift to easterly near 10 kt during from about 22z (MDW) to 23z (ORD), though confidence remains only medium. Some of the guidance continues to be a bit stronger with the opposing southerly winds and delays the shift to a light southeast/east- southeast direction until after 00z at ORD and MDW. While this isn`t the current most likely scenario, it`s something to monitor for subsequent updates. VFR conditions will likely prevail through Saturday morning. Expect generally light showers to spread northeastward by mid to late Saturday morning, though there is timing uncertainty. Went with a 15z rain onset for now in the ORD and MDW TAFs, with any sub-VFR VSBY and CIGs not expected until 18z Saturday onward. It appears most if not all of the shower activity will hold off until after 12z Saturday at RFD, so withheld precip. mention in the latest RFD TAF. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago