Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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426 FXUS63 KILX 041741 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1241 PM CDT Sat May 4 2024 .KEY MESSAGES... - Level 1 (Marginal) Risk of severe storms from late Saturday afternoon into evening for all of central and southeast IL. - Periodic chances of showers and storms Monday through Friday, with severe storm potential elevated on Tuesday and Wednesday during the late afternoon into evening each day. && .UPDATE... Issued at 1018 AM CDT Sat May 4 2024 Have opted to increase PoPs and speed-up the time of arrival of afternoon convection based on latest 12z hi-res trends. Up until this point, two limiting factors for afternoon convective coverage has been the idea that deep-layer shear is somewhat underwhelming (30 kts or less), and that synoptic forcing atop the cold front is somewhat nebulous. If true, then updrafts may struggle to get organized or stay organized ahead of the cold front. However, we have some concern about a decaying MCV currently lifting across north central Missouri at this hour. MCVs notoriously augment forcing and vertical wind profiles, and it bears watching over the next couple hours. If this feature can hold together as it approaches the mid-Mississippi Valley, then convective coverage could become greater than previously anticipated. A quick glance at the RAP-derived parameter space suggests the mesoscale environment this afternoon will be sufficiently unstable as steepening 0-3 km lapse rates and good solar insolation work to erode convection inhibition. SBCAPE values are now progged to be 2000-2500 J/kg ahead of the cold front; which is a notable increase from yesterday`s modeled instability. This all adds up to increased confidence in severe weather coverage this afternoon and early evening in areas west of I-55. The primary severe weather hazards are still hail and wind, and recent HRRR guidance suggests that the latter could be the more formidable feature as cold pools strengthen ahead of the front. The expectation is still for severe convection to fade after sunset, as the lack of an appreciable LLJ will make it difficult for storms to maintain their punch. MJA && .DISCUSSION... Issued at 310 AM CDT Sat May 4 2024 The latest IR satellite images show Illinois in the coll zone between two systems, with high pressure providing mostly clear skies and light winds across the CWA. Ongoing storms across eastern Nebraska and Kansas will continue to make slow progress toward the mid-Mississippi River Valley as a cold front approaches. Low level flow will increase ahead of that feature from the southwest, boosting low level dewpoints into the low to mid 60s this afternoon. That will support instability parameters climbing into the 1000-1500 J/kg MLCAPE range ahead of the front, despite thickening anvil cirrus and mid clouds this afternoon. Marginal shear values around 30kts will be a limiting factor in severe storm potential, as a broken line of storms progresses into central Illinois between 3 pm and 5 pm. Chances for any rainfall will peak at 60-70% west of I-55, and remain in the chance category east of I-55. A marginal threat for large hail and damaging wind gusts will be present 22z/5pm through around 01z-02z before diurnal stabilization limits updrafts. By the time the line reaches I-57 the lightning threat will be diminishing/ending. The next period of concern will start on Tuesday as a stationary front across southern Illinois lifts north as a warm front in response to surface cyclogenesis in the Plains. Warm sector shear and instability parameter space will be more favorable for severe weather, as 0-6km shear climbs over 60kts along with MUCAPE climbing toward 2500 J/kg under the support of 65+F deg SFC dewpoints. All severe hazards will be in play as supercells develop during the evening across central Illinois. The upper trough will evolve eastward on Wednesday, dragging a cold front across Illinois during the late afternoon and evening time frame. There are some differences in the model guidance timing of the trough evolution, but Grand Ensemble guidance is pointing toward 40-50% chance for bulk shear of at least 50kts and CAPE of at least 2000 J/kg both Tuesday and Wednesday for areas south of I-72. The best shear is along an axis northwest of the best instability, but enough overlap exists for a wave of severe storms each late afternoon and evening. Additional storm chances will linger through Friday, but at a much lower coverage and intensity per long range guidance. Total rainfall over the next 7 days looks to climb into the 1 to 2 inch range, especially south of I-72, where Grand Ensemble probs show 50-60% chance of over 2 inches. Some rivers may climb back toward minor flood stage due to saturated soils allowing for higher runoff into streams. Shimon && .AVIATION... (For the 18z TAFs through 18z Sunday Afternoon) Issued at 1241 PM CDT Sat May 4 2024 There is a high chance (>80%) a line of storms will work across the area later this afternoon, impacting TAF sites from west to east for a few hours through early evening. South-southwest winds will swing around to the northwest behind the storms as a cold front sweeps through. Winds could be gusty with storms, but otherwise should range from 10-15 kts through tonight. MVFR ceilings become common late tonight and stick around at most airfields through at least late Sunday morning. NMA && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$