Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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610 FXUS63 KLOT 011750 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1250 PM CDT Sat Jun 1 2024 .KEY MESSAGES... - Rain/showers persist through the day. Some localized rainfall amounts of an inch or more are possible, which could cause some ponding of water in typical low spots this afternoon. - Periodic chances for showers and thunderstorms Monday into Wednesday morning. && .DISCUSSION... Issued at 327 AM CDT Sat Jun 1 2024 Through Sunday: Surface low pressure was analyzed over central Missouri early this morning, associated with a slow-moving mid-level short wave trough which was evident in GOES vapor imagery across Missouri/Arkansas. The surface low will gradually fill while lifting east-northeast across downstate Illinois and central Indiana through tonight, as the mid-level trough tracks across the region. Despite dry air in the 850-500 mb layer noted in DVN and ILX 00Z RAOBs from last evening, southerly 35-40 kt winds above the surface (925-700 mb) ahead of the approaching mid-level trough will help to transport deeper moisture (precipitable water values approaching 1.70" or 160 percent of normal) into the area later this morning. This will set the stage for a rainy/showery Saturday. HREF and EPS ensembles depict average rain amounts in the 0.35-0.75" range through this evening, with highest amounts generally forecast across the eastern 2/3 of the cwa. Deterministic (HRRR/RAP) and ensemble (HREF max QPF) CAM solutions continue to indicate the potential for some higher (1-2 inch) amounts however, roughly in the Peru/Pontiac to Chicago corridor. While confidence in these higher amounts and their placement is somewhat low, it has been a fairly consistent signal and can`t be ruled out given the available moisture and the relatively slow movement of the mid-level wave. Guidance trend toward less thunderstorm potential (poor lapse rates and MUCAPES only <250 J/kg mainly across the south this afternoon) may help to keep 1"+ amounts isolated. Ponding of water in low spots would be possible where these heavier rains occur this afternoon. Rain/showers should gradually taper off from west to east across the area this evening, though moist low levels may maintain some spotty shower/drizzle potential through midnight before the mid-level trough axis passes. Have maintained a patchy fog mention overnight (perhaps as early as later this afternoon over Lake Michigan) with moist ground and moist low- level conditions persisting. Some indication that low stratus may persist much of the night however, which could limit how thick fog becomes. Dry weather returns to the area Sunday, as weak surface high pressure drifts in from the west. Clouds may linger especially during the morning, though at least partly sunny conditions should develop. Temperatures today will likely be limited to the upper 60s/near 70 in most spots due to thickening overcast and the arrival of rain. Highs Sunday look to rebound to the upper 70s/around 80, though with onshore winds off of Lake Michigan limiting shore temps to the mid-60s. Ratzer Sunday Night through Friday: Quiet weather will persist locally on Sunday night due to the continued influence of weak high pressure centered over Lake Michigan. Meanwhile, 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 east-southeast. It`s likely that the farther east you go in the CWA, particularly the eastern half, 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 remains unclear. Pockets of heating near the warm frontal zone may be enough for 1000-1500 J/kg of MLCAPE to develop with minimal capping as temps away from the immediate Illinois shore warm to the lower to mid 80s with dew points well into the 60s. Given the uncertainty regarding convective coverage Monday afternoon, we`ve lowered our forecast PoPs into the ~30-50% range, highest western half of the CWA. The best chance for scattered thunderstorms will probably be in the mid-late afternoon into the early evening when capping may be eroded. Marginal deep layer shear will result in mostly non-severe storms, but can`t rule out a few gusty storms with 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 only element that might keep pockets of elevated convection going (and possibly focused farther south than implied by the official PoPs) beyond typical evening diurnal decay would be another weak mid-level short-wave translating from central Illinois into Indiana. The next period of interest will be Tuesday PM and night, possibly into Wednesday morning. As things stand now, our area may be yet again caught in the middle timing wise for a higher chance of organized convection. Despite the presence of dry air at the mid-levels (h7-h5 RH 25% or less) and likely at least some capping and a lacking large scale trigger mechanism, isolated to widely scattered diurnal pulse-type convection may be able to develop Tuesday afternoon as heating erodes the capping amidst mid 80s temps and dew points pushing 70F. Suspect our Tuesday afternoon PoPs in the 40-60% range may be a bit aggressive. 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 another potentially favorable synoptic pattern for 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 still 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`s cold front will likely 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 continue to hinge upon a slowing of the cold frontal approach and passage. Maintained 30-50% chances of showers and slight chance of t-storms Wednesday PM, but a scenario of a primarily dry afternoon and early 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 and drier air mass into next weekend. There may be a few showers with the digging short-wave and secondary cold front Wednesday evening and night, likely followed by primarily dry conditions thereafter through day 7 (Friday) and the start of the weekend. Castro && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 1251 PM CDT Sat Jun 1 2024 Key Aviation Messages: - IFR ceilings this afternoon through tonight, with periods of LIFR ceilings possible tonight. Periods of RA/SHRA will continue this afternoon as ceilings steadily lower from low-end VFR levels into IFR levels. While precip coverage and intensity will greatly diminish this evening, IFR ceilings will lower into LIFR levels at times late this evening into the overnight hours. Expected FG over Lake Michigan should remain east of ORD/MDW overnight given NNW winds, but may begin to advect inland around daybreak as winds veer NNE. While diurnal trends favor rising ceiling heights through the morning, it is possible that IFR ceilings persist through at least mid-morning if upstream FG over the lake becomes more widespread. Ceilings will ultimately lift to VFR levels and/or SCT by Sunday afternoon. Winds should remain E/SE around or under 10 knots through the afternoon, then become VRB at 5 knots or less this evening as a surface trough drifts over the area. Winds behind the trough will settle NNW for several hours overnight before veering NNE/NE Sunday morning. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago