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Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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025 FXUS63 KILX 080109 AFDILX Area Forecast Discussion...Updated National Weather Service Lincoln IL 809 PM CDT Sun Jul 7 2024 .KEY MESSAGES... - Scattered thunderstorms are expected west of the Illinois River this evening into tonight. There is a low chance (level 1 of 5) of severe wind gusts or hail from these storms. - The remnants of Tropical Storm Beryl are expected to bring widespread rainfall, including a corridor of 2"+ totals (30-60% chance) to the area Tuesday into Wednesday. The exact placement of the axis of heaviest rainfall totals remains uncertain. && .UPDATE... Issued at 808 PM CDT Sun Jul 7 2024 Radar trends over the last 1-2 hours have shown a decrease in coverage and intensity of showers and storms along a weak convergence zone over western IL. At 8 pm the outflow boundary has surged east to I-55, while the decaying area of showers lingers along and west of the IL River. Would expect these trends to continue this evening as diurnal instability wanes. However with the moisture convergence in place overnight, scattered showers and a few storms (around 20-30% coverage) will remain in the forecast west of I-55. Lows should only fall into a 65-70 degree range, thanks to increase cloud cover and dewpoints. 25 && .SHORT TERM... (Through Late Monday Night) Issued at 238 PM CDT Sun Jul 7 2024 Sunday afternoon, a long wave trough was present over the central Plains/upper Midwest, while Tropical Storm Beryl continued it`s approach towards the TX Coast. Closer to home, thunderstorms have blossomed along a weak convergence boundary that extends from the northern Chicago suburbs to the far NE corner of MO. The storm coverage is already surpassing that shown in any of the available CAMs, and RAP-based mesoanalysis fields show weak near-sfc fgen and moisture convergence all along this corridor, suggesting continued support for the ongoing t-storm activity into the evening. While this weak boundary is expected to be slow-moving, the storm evolution may become dependent on specific storm outflow interactions. Between this and the poor CAM depiction, there is some uncertainty as to how quickly storms will progress SE (meaning there is some potential for storms to push SE faster than what is currently forecast). At any rate, the expectation is that scattered to widespread thunderstorms are likely to continue along this zone, shifting E/SE in time. These storms could occasional pulse to near severe limits, as there is modest instability of 1000-1500 J/kg of SBCAPE present, but deep layer shear values below 30 knots will limit organization. Tonight into Monday, as perturbations continue to progress through the upper trough, additional scattered storm development will be possible (20-50% chance), with the precip chances gradually shifting east in time. Severe storms are unlikely during this time frame. Skies will stay mostly cloudy tonight, keeping lows in the upper 60s. Erwin .LONG TERM... (Tuesday through Next Sunday) Issued at 238 PM CDT Sun Jul 7 2024 The forecast for the remnants of Beryl remains fluid, with ensemble guidance trending towards a considerably faster system and greater impacts across central IL compared to just 24 hours ago. The center line of the latest NHC forecast track (issued 1 PM Sun) now has the remnants of Beryl reaching the Great Lakes by Thurs morning, as opposed to yesterday`s forecast which had it reaching the Ohio Valley Thurs morning. This means the associated rains will arrive in central IL on Tues-Wed, rather than late Wed into Thurs. Additionally, ensemble mean precipitable water (PWAT) values now depict values over 2" (rather than the 1.5-1.75" shown yesterday). The latest ECMWF shift of tails (based on the 00z run of the ECMWF) highlights the potential for an outlier heavy rain event (focused mainly east of I-55). All three of the GEFS, EC Ens, and GEPS have a swath where the probability of 2"+ of rainfall is 30-60%, although there is still spread in the placement of this swath, with the GEFS being further north (near the IL River) than the other two ensembles. Run-to-run consistency of the models remains poor, for example, the deterministic GFS has oscillated the rainfall axis from near the I-70 corridor, to near the IL River, then back south to the I-70 corridor over its last several iterations. Wherever that rainfall swath does occur, higher amounts aren`t out of the question given the tropical moisture combined with broad forcing from both the remnant circulation of Beryl and the synoptic trough over the Midwest. The Grand Ensemble shows 90th to 95th percentile precip amounts (5-10% chance of exceedance) in the 3-4" range, which if realized would likely lead to some hydrologic issues given 6-hour flash flood guidance in the 2.5-4" range. There are still some factors that give me pause regarding the rainfall totals, such as the fact that Beryl continues to struggle to organize prior to landfall (although conditions are expected to be more favorable over the next 24 hours and Beryl`s vis satellite appearance has recently improved) and the antecedent dry conditions (much of the area was classified as abnormally dry or moderate drought in the last Drought Monitor). The remnants of Beryl could also bring some blustery winds to the region. The exact speeds will depend on how well an organized sfc low is able to maintain its structure as the remnants lift northeast, but current guidance suggests gusts of 20-30 mph are most likely east of I-55 on Wed., and a few outlier members of the EC Ens have gusts over 40 mph. With increased cloud cover and rainfall, temperature forecasts for Tues-Wed have lowered, with highs now forecast to be in the upper 70s/low 80s. The rains associated with Beryl are expected to depart the ILX CWA before Wed night. Into the latter half of the work week, guidance has a shortwave digging into WI, keeping upper level troughing in place over the Midwest despite the remnants of Beryl lifting to the northeast, and the presence of that upper wave could result in some light, scattered precip on Thurs. Upper ridging will persist over the intermountain west into the weekend, and while PoPs are below mentionable thresholds after Friday we`ll need to keep an eye on the potential for MCS activity as disturbances round the upper ridge. For now, the favored track of any such systems appears to be north of the ILX CWA. Persistent southerly flow Fri into the weekend will result in increasing temps and moisture content, with highs forecast to climb into the mid 90s by next Sun, and heat indices approaching the triple digits. Erwin && .AVIATION... (For the 00z TAFs through 00z Monday Evening) Issued at 631 PM CDT Sun Jul 7 2024 A frontal boundary was pushing into western IL this evening, bringing scattered TSRA west of KPIA. These could affect KPIA over the next couple hours with brief reductions to IFR. With the loss of diurnal heating the storms are expected to dissipate and currently not expecting them to impact other terminals this evening, though scattered showers could. The front will be slow to move across the region Monday with a few showers and storms around, but confidence in coverage and timing is too low to include in TAFs. Besides an isolated storm, conditions should be VFR through 00z with south to southwest winds at 5-10 kt. 25 && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$