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FXUS63 KILX 180135 AFDILX

Area Forecast Discussion...Updated National Weather Service Lincoln IL 835 PM CDT Fri May 17 2024

KEY MESSAGES

- Isolated showers and storms (15-30% chance) east of the Illinois River through sunset this evening. Severe thunderstorms are not expected.

- Fog development is likely tonight across eastern IL, with a 40-70% chance of visibility falling below 1 mile east of I-55.

- Severe thunderstorms are possible next week on Tuesday and/or Wednesday. The exact timing remains uncertain at this range.

UPDATE
Issued at 835 PM CDT Fri May 17 2024

Isolated thunderstorms continue along a narrow line just south of IL Rte 10 in Mason/Menard, Logan, DeWitt, Piatt, and Champaign counties. Based on how these look on radar and in satellite loops, thinking is they will diminish in next couple of hours. Then still expecting clear skies through the night with fog developing in the east and southeast parts of the CWA Besides the isolated storms, current forecast still looks good for the rest of the area. Update should be forthcoming.

Auten

DISCUSSION
Issued at 231 PM CDT Fri May 17 2024

** THROUGH SUNDAY **

A weakly forced pattern exists across IL this afternoon, with the state sitting between lower sfc pressure in the northern Plains and weak sfc high pressure over the mid-Atlantic. Vis sat shows two corridors of more agitated Cu development at 140pm/1840z, one from Quincy to Bloomington, and another from Shelbyville to Paris.
Forecast soundings suggested 1500-2000 J/kg of SBCAPE developing, but with very weak shear (less than 20 knots), and that is well aligned with the latest RAP-based mesoanalysis. Given the weak forcing those two aforementioned corridors of enhanced Cu will be the favored area for any storm development over the next few hours. Given the weak shear, these will be short-lived, and that matches what we've seen with the few cells that have developed thus far. As we progress later into the afternoon, the forcing could become a little more nebulous, with a collapsing cell producing outflow that kicks off new cells, and so on. With the potential for that kind of messy evolution, maintained a 15-30% PoP east of the IL River through sunset, after which point any lingering cells will quickly diminish.

Another round of fog development is possible tonight, including some dense fog, owing to a combination of light winds and mostly clear skies. Guidance has been consistent in showing the chance of fog increasing with eastward extent. 12z HRRR guidance had a 40-70% chance of vis falling below 1 mile east of I-55 tonight, while the available runs of the HRRR that go out far enough show dense fog development near/east of I-57.

For the weekend, central IL will be positioned between a sfc low over south-central Canada and a weak shortwave moving through the lower TN Valley. In between these two systems, upper level height rises occur over IL, resulting in mostly dry conditions and steadily warming temps. CAMs do show storm development Sat afternoon along the cold front associated with that Canadian sfc low, but it will be positioned across WI and IA, and the activity fades in the evening with the loss of heating before reaching the ILX CWA

Previously, there had been high confidence that Sunday would be dry area-wide, but now guidance is showing storms developing over east CO Sat night and progressing east along the instability gradient, which puts it on a track towards west-central IL by Sun afternoon, if it holds together. For now, the NBM introduced 15-20% PoPs which seemed reasonable given the uncertainty in the evolution of a convective complex at this range. Shear values look quite weak across our area Sunday, with deep layer shear values again below 20 knots.

Temperatures do become quite warm on Sat-Sun, with highs in the mid to upper 80s which is about 10-15F above normal. Record highs for Sunday (May 19) are around 92-93F, and at this point it looks unlikely that we'll breach those thresholds.

** EARLY NEXT WEEK **

A trough is set to amplify over the western US and send a low racing towards from the central Plains to the upper Midwest on Tuesday. Multiple days of gulf moisture advection in advance of this system will help push dewpoints near or into the low 70s. The warm-sector associated with this system continues to appear quite volatile, but mesoscale details and timing uncertainties remaining regrading the svr threat. From an ensemble perspective, the latest GEFS shows a 70-90% chance of exceeding 2000 J/kg of SBCAPE. CWASP (a parameter which combines numerous environmental factors related to severe weather into one index) is depicting values over 70 west of I-55 Tues afternoon. For reference, most EF2+ tornadoes occur in environments with CWASP values over 70.

It needs to be emphasized that CWASP is just a summary of the environment, it does not factor in whether or not storms will be present in that environment. In that regard, it does look like the latest guidance has trended a bit slower with the cold front, such that the higher threat may be focused in areas west of the ILX CWA Tues afternoon. The previous forecast from the NBM had PoPs Tuesday afternoon around 60%, but the latest forecast ranges from 30% along I-70 to 60% NW of the IL River. PoPs across the ILX CWA peak during the Tues night time frame (60-80%). It's too early to say how the severe threat could evolve into the Tues night timeframe, but perhaps the diurnal timing could be our saving grace from a higher-end severe weather outbreak. The flip side to a slower system is that it could mean a greater portion of the area is at risk for severe storms on Wednesday. With several days of convective activity needing to evolve between now and then, confidence in placement of storms on Wednesday is low, and SPC did not introduce a Day 6 Outlook area in this morning's forecast.

In summary, there is a chance for severe thunderstorms both Tuesday and Wednesday - with the latest guidance suggesting precip/severe chances increase with northwestward extent Tues afternoon, then storms push into the ILX CWA Tues evening/night.

High temps remain in the 80s through Wed, then cool into the 70s for the latter half of the work week following FROPA.

Erwin

AVIATION
(For the 00z TAFs through 00z Saturday Evening)
Issued at 640 PM CDT Fri May 17 2024

VFR conditions will prevail at PIA, SPI, and BMI for the next 24hrs with clear skies or scattered mid clouds at BMI due to some evening convection that is south of the site. DEC and CMI will also see clear skies or scattered mid clouds for the same reason as BMI. Some CU is expected tomorrow around 2.5kft at BMI, SPI, DEC, and CMI, but only scattered. The main concern will be fog at DEC and CMI during the morning hours with CMI seeing the lowest possible vis of 1.5 miles. Winds will be light and variable through the night and then be southerly tomorrow but still below 7 kts.

Auten

ILX WATCHES/WARNINGS/ADVISORIES
None.

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