Marine Weather and Tide Forecast for Lake Petersburg, IL

UPDATE
Issued at 1052 AM CDT Tue Apr 29 2025

15z/10am surface analysis shows a cold front advancing eastward across the I-55 corridor. Clusters of thunderstorms are ongoing well ahead of the boundary along the I-70 corridor...with one cluster now east of the KILX CWA across central Indiana. Meanwhile upstream, the next more significant cluster is noted on regional radar imagery across southern Missouri where a bowing segment has developed. Given favorable instability/shear parameters, this convection is expected to maintain its intensity as it tracks E/NE across southern Missouri/Illinois. Based on radar timing tools, it appears the apex of the bow (and thus the greatest risk for damaging wind gusts) will remain just south of the KILX CWA
However, the northern edge of the feature will sweep across Clay, Richland, and Lawrence counties between 2:30pm and 4:30mp with an attendant risk for damaging wind gusts and perhaps an isolated tornado
Have updated PoPs to better reflect current trends
and to focus highest PoPs south of I-70 this afternoon. Further north, stratiform showers may reach as far north as I-72.

Barnes

DISCUSSION
Issued at 103 AM CDT Tue Apr 29 2025

***** Severe Weather Risk Today *****

At 1am, widely isolated showers were drifting across central IL, with an area of scattered storms upstream across northern and west- central MO. Ahead of those storms, strong warm air advection (WAA)
is maintaining surface temperatures in the mid 70s and dewpoints in the low to mid 60s across central and southeast IL, with south winds continuing to gust to around 25-30 mph despite the developing stable layer/weak inversion beneath the LLJ. KDVN is sampling winds as high as 75 kt in the core of that LLJ, which is around 4-5 kft AGL (~875mb). Upstream across southern and central IA, a burst of intense southerly winds gusting 40-60 mph has been observed on the northern edge of some light showers, likely due to evaporational cooling transferring momentum from the LLJ to the surface. HRRR ensemble max wind gusts highlighted this relatively well with pockets of 60+mph, and for what it's worth it decreases as that area of light reflectivities (along with the core of the LLJ) shifts east into our neck of the woods over the next couple hours; in addition, we're already beginning to observe a weakening trend in those winds.
Nonetheless, we'll be watching surface observations closely, and if gusts start to exceed 40-45 mph we may issue an SPS.

The surface cold front, draped from south central MN to southeast NE, is forecast to progress southeast towards our area the next several hours, reaching roughly the IL River by 8-9am. Ahead of it, strong moisture advection by, and enhanced directional wind shear from, the LLJ will provide a favorable environment to sustain any elevated storms that manage to fire, though getting convection to poke through the capping inversion might just prove too difficult early this morning. Given the ~8.5 C/km 700-500mb lapse rates and resultant 2600 J/kg MUCAPE observed on our 00z raob, any of the stronger updrafts could support some marginally severe hail, and since we've already seen gusty to marginally severe winds upstream it seems reasonable that there's at least a small risk for that as well.

Unfortunately, the cold front is slated to slow down as it approaches the I-70 corridor mid to late this morning, and as a result the severe threat will linger there. Global deterministic models suggest a shortwave rippling northeast along the front mid morning into early afternoon, and various CAMs suggest convective development will ensue. Increasing surface based instability due to moisture pooling along the front (where dewpoints may touch 70) will be ample to spark some robust updrafts capable of severe hail, with precip-laden downdrafts also generating strong wind gusts - especially if storms organize into a more linear structure which a few CAM solutions have depicted.

***** Showery Midweek *****

Global deterministic models suggest a southern stream shortwave trough will lift out of the Southern Plains and into the Prairie State Wednesday into Thursday. In response, the frontal boundary, which will stall across southern IL tonight into tomorrow morning, is expected to lift back northward into central IL late tomorrow.
Isentropic upglide focused along the frontal boundary will promote scattered showers and perhaps a few thunderstorms. Some of the more bullish guidance even supports sufficient instability near and south of the warm front to support a severe storm or two tomorrow evening- night as wind shear increases beneath the upper trough, for which reason we remain in a level 1 of 5 (marginal) risk for severe storms from the SPC. We'll have to keep an eye on precip amounts as well, especially tomorrow night into Thursday. Right now, NBM suggests only a 5-15% chance for more than 2 inches of rain totals (just under our current 6h FFG), highest along the IL River; however, convection tends to not be well resolved by the global models, which alone comprise NBM at this forecast time range. We'll be interested to see if and where CAMs depict any localized pockets of higher totals. For what it's worth, there remains an SOT for precip totals in the ECMWF ensemble north of roughly I-72; this suggests the top 10% of its members portray rain totals higher than the 99th percentile of its model climatology ("M-climate," or all the previous forecasts made by its ensemble between 2004 and 2025 for the 5-week period centered on May 1st). WPC maintains a level 1 of 4 (marginal) risk for excessive rainfall for both Wednesday and Thursday.

***** Turning Dry for the Weekend *****

As the upper trough shifts east into the Great Lakes, the deterministic global models suggest a vort max on its southwest flank may spark a couple additional showers on Friday, though precisely where the best overlap between cool temps aloft and breaks in cloud cover (allowing for steep lapse rates off the surface, and hence low level instability) occurs remains unclear. The GFS would suggest that happens in central/northern IL, while the ECMWF portrays those showers over IA and northern MO. In any case, Friday is looking like a more seasonably cool day, while ridging expanding into the region from the west over the weekend should promote gradual warming and also drier conditions. NBM gives a 50-60% chance high temperatures exceed 75 degF once again by Sunday.

Bumgardner

AVIATION
(For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1247 PM CDT Tue Apr 29 2025

Based on the forward speed of a cold front pushing eastward across central Illinois early this afternoon, it appears the boundary will pass KCMI by around or shortly after 18z...resulting in NW winds gusting 20-25kt across the board this afternoon. The gusts will gradually diminish after 21z, then will back to N/NE and drop to around 5kt from late evening through the overnight.
Winds will then become E/SE at 10-15kt by Wednesday morning as the boundary begins to lift back northward as a warm front. A thick blanket of mid/high clouds associated with convective blow-off to the southwest will obscure the sky this afternoon before temporarily clearing out this evening. Clouds will once again be on the increase from south to north across the area Wednesday morning, with scattered showers developing before midday. Have introduced VCSH at the I-74 sites after 15z/16z...with predominant showers further south.

Barnes

ILX WATCHES/WARNINGS/ADVISORIES
None.