Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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086 FXUS63 KILX 040516 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1216 AM CDT Sat May 4 2024 .KEY MESSAGES... - Near daily chances for showers and thunderstorms over the next week as a series of fronts advance across the region. The Tuesday through Wednesday period looks particularly active, with medium confidence in thunderstorms capable of producing all severe weather hazards. && .UPDATE... Issued at 917 PM CDT Thu May 2 2024 The surface ridge axis has made eastward progress into Illinois this evening, helping to nudge high clouds eastward into Indiana as well. The back edge of the clouds is mainly covering counties along the Indiana border as of 9 pm, with additional clearing expected in the next few hours. Light and variable winds are expected overnight, under the weak pressure gradient. The main concern is for a possible need to expand the patchy fog across more of our counties than just our far southeast 6 counties south of I-70. There doesn`t appear to be enough support in the high res guidance at this point to do that, so will leave the fog coverage as is. Only minor updates were done to sky and wind grids. The remainder of the forecast database remains on track. Shimon && .DISCUSSION... Issued at 127 PM CDT Fri May 3 2024 Current GOES WV imagery depicts a strong upper-level low positioned over western Ontario with more subtle shortwave energy ejecting out of the Great Basin. Model guidance remains in excellent agreement that shortwave energy lifts into the Northern Plains and Upper Midwest regions overnight, ultimately helping to force a synoptic cold front into western Illinois Saturday afternoon. A plume of low-to-mid 60s dewpoints is progged to advect northward ahead of the cold front, and SBCAPE values should be able to achieve 1000-1500 J/kg with CINH eroding in the presence of steep 0-3 km lapse rates (> 7.5 C/km). This all adds up to likely (> 60% chance) thunderstorm development Saturday afternoon along and ahead of the cold front. Coverage of Saturday storms could be limited by somewhat underwhelming deep-layer shear profiles, with much of the area at or below 30 kts in the vicinity of the cold front. Updrafts may struggle to become organized or struggle to stay organized, especially some of the early updrafts, and recent CAMs hint at this as well. Nevertheless, CAPE/Shear profiles are just good enough to support the potential for an isolated severe hail and wind threat. As storms move toward and east of I-55 Saturday evening, the severe potential should gradually fade. This will largely be due to the loss of daytime heating and the lack of a LLJ to help augment instability. This thinking lines up well with SPC`s Day 2 Marginal Risk, which ends the threat near the I-57 corridor. Flooding should not be a concern with the Saturday storm event. NBM Mean QPF is around 0.25", and this guidance only offers a 10% chance that rainfall exceeds 0.75", which is well below 3-hour flash flood guidance. The rest of the weekend through Monday should be fairly low drama, with the main frontal zones displaced just to our south. Attention quickly turns to the Tuesday - Wednesday timeframe as persistent, deep southwest flow envelops the region ahead of an elongated upper-level low. While we`re running lean on details at this timeframe, the synoptic pattern (kinematics/thermodynamics) being modeled supports a multi- day severe weather threat across portions of the Plains and Midwest. This strong signal in the global models is also reflected in the analogs and machine learning tools for severe weather potential. MJA && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1216 AM CDT Sat May 4 2024 VFR conditions will prevail in the near term through mid afternoon on Saturday, under surface high pressure. A non-zero chance of light fog at our eastern TAF sites of DEC/CMI, but better chances of MVFR/IFR fog should remain south of I-70. A weakening line of thunderstorms is projected to impact (70% chance) the western TAF sites (PIA/SPI/BMI/DEC) during the late afternoon through mid- evening time frame, with primarily a 2-3 hour window of thunder. More isolated thunder chances (20%) at CMI. Shimon && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$