Marine Weather and Tide Forecast for Sleepy Hollow, IL

UPDATE
Issued at 915 PM CDT Mon Apr 28 2025

Conditions remain quiet here from a storm perspective as of around 9 PM. An initial push of showers with a couple of embedded thunderstorms developed along a strengthening low level jet out along the Mississippi River late this afternoon. But aside from a few lone lightning strikes over Ogle County, these never amounted to anything more than some isolated pockets of light rain and virga showers as they're now moving out of the Chicago metro.

A reservoir of 1,500 to 2,000 joules of MUCAPE sits atop the area currently, although a capping inversion and general lack of forcing for ascent in the open warm sector here kept us from utilizing much of that with these recent showers. The 00Z RAOBs out of DVN and ILX as well as supplemental launches from Argonne National Lab out of Morris and Humboldt Park show an inversion at between 850 and 700mb. Now, conditions are expected to remain rather quiet until the cold front approaches overnight tonight.
The front is now approaching central IA and a couple of separate lines of thunderstorms have developed near the boundary extending from western WI down into eastern KS.

Central and southern IA is getting largely gapped between two more robust lines: the first draped from western WI into northern IA. This line coincides with where the greatest synoptic forcing is focused and deep layer moisture is pooled out ahead of the surface low in MN. This line, and its better environment, are on a trajectory to pass to our north across WI tonight. The second line is moving from eastern KS into NW MO.
These storms fired up along a dryline but are expected to be on a general weakening trend as they move across central IL later tonight.

It's likely that we'll see some additional showers and storms go up along the front as it moves across IA later this evening, but it's not clear how strong they'll get and how well they'll be able to maintain themselves as they work into IL overnight and toward morning. Forcing looks all around pretty meager, including along the face of the front later tonight. There are disagreements among guidance in how much CINH we maintain just ahead of the front, but between the weak cap and a general lack of low level moisture, it appears that storms could really struggle tonight. There's a good chance too that whatever storms do move over our area tonight will be elevated. In that case, storms should struggle to become organized as shear above 1 km is very poor. Steep mid level lapse rates of about 7.5-8.5 K/km could promote some hail with these storms, but severe hail looks like a bit of a stretch out of any elevated, disorganized storms. A few model camps do resolve a brief window of opportunity ahead of the front for storms to possibly form off the surface. While this does not appear to be the likely scenario, any surface-based activity will have the best chance at going severe with damaging hail and winds possible. But at this point, a rather uneventful night with only a few to no storms may be the more likely option. Overall, the forecast in the near term remains very much on track with no noteworthy changes made.

Doom

DISCUSSION
Issued at 242 PM CDT Mon Apr 28 2025

Through Tuesday Night:

A recent hand surface analysis placed a 996 mb surface low pressure system near the North Dakota/South Dakota/Minnesota borders, a hybrid cold front/dry line extending southward through eastern Nebraska and into central Kansas, and a warm front (delineator of higher-quality moisture) arcing southeastward through eastern Iowa into far western Tennessee. Meanwhile, GOES-19 water vapor imagery depicts an elongated positively-tilted upper-level trough over the northern Plains with a broad region of implied southwester low- to upper-level flow across the Mississippi River Valley. Aggressive warm-air advection within the broad warm sector of the cyclone facilitated by gusty south to southwest winds in tandem with mostly sunny skies have allowed for temperatures to rocket into the upper 70s to lower 80s across the region, including up to the Lake Michigan shoreline.

There appears to be two low-probability windows for thunderstorms in the next 12 to 18 hours. The first is this evening, associated with a subtle wave embedded in the southwesterly flow propagating from northern Missouri toward southern Wisconsin. At this point, still think that low-level capping will be too hard to overcome for surface-based convection given continued low-level warm air advection and neutral mid-level height tendencies (which are acting to maintain the cap). So any storms that do develop may remain elevated with threats of damaging hail and winds. If any activity were to develop with the wave, it may sneak into northwestern Illinois toward sunset (perhaps within the Level 3/5 threat level area advertised by SPC), though the trajectory of the wave appears to favor southern to central Wisconsin.

The second window for thunderstorms will be overnight. In spite of weak synoptic-scale forcing and confluence along the hybrid dryline/cold front, the warming boundary layer across eastern Kansas should allow for convective temperatures to be met in the coming hours. As a result, confidence is growing that a broken band of supercells will develop near the Missouri River (west of Kansas City, MO) by sunset. With weak synoptic-scale forcing atop and deep-layer shear vectors maintaining semi-oblieque orientations to the dryline/cold front, the supercells may struggle to grow upscale into a cold-pool driven MCS. As a result, they will become increasingly susceptible to succumbing to increasing MLCIN as they approach the Mississippi River overnight (basically need a giant cold pool to plow into the cap to activate the MUCAPE). Meanwhile, ongoing supercellular development along the front in the Minnesota/Iowa border region should continue this afternoon and evening, given slightly better confluence and synoptic-scale support. With stronger linear forcing, "upscale growth" should allow for the line of storms to make it into Wisconsin during the overnight hours.

What happens in central Iowa between the "northern" and "southern" convective zones this evening and overnight remains an item of low confidence, as widespread clouds and nebulous forcing may prove ineffective to breach the cap. In fact, subsidence behind the wave due to move into Wisconsin this evening may very well suppress any attempts for convection to develop in central Iowa altogether. With these factors in mind, do think there is a very realistic scenario where convection fails to develop and move into our area during the second window, leading to a completely dry evening/overnight period.

With all of that said, if a storm were to somehow develop and move into our area overnight, the envionrment characterized by MUCAPE >2000 J/kg and effective deep-layer shear >40kt would support a threat for severe weather. All things considered, will advertise a 20 to 30% chances for isolated to scattered storms from this evening through the overnight.

Outside of the threat for thunderstorms, the main concerns in the next 12 hours or so are gusty south to southwest winds.
While the surface low won't be deepening as it moves into the UP of Michigan tonight, the surface pressure gradient across the Lower Great Lakes will tighten remarkably from late afternoon through the overnight hours ahead of the approaching cold front.
Increasing boundary layer moisture ahead of the front may delay surface decoupling, allowing for an increasingly strong south to southwesterly low-level jet to mix downward to the ground.
The inherited forecast for 35 to 45 mph wind gusts thus remains on track from late afternoon through the overnight hours. If decoupling fails, can certainly see a scenario where gusts overachieve with widespread 45-50 mph readings by midnight. For now, will withhold issuing a Wind Advisory and let the evening shift watch trends. Otherwise, tonight looks unseasonably warm with overnight lows in the 60s.

The cold front will swing across the area tomorrow morning, perhaps accompanied by a few showers and storms. Without a real push of cold air behind the front, temperatures tomorrow may be able to climb into the upper 60s to even lower 70s, especially southeast of I-55. Southwest winds will turn west and remain gusty through the morning, and then gradually ease and even turn northeasterly off the lake during the afternoon as a surface high pressure system moves into the Great Lakes. Given the baroclinic zone should be shunted well south of our area by tomorrow, think the odds favor a dry afternoon.

Borchardt

Wednesday through Monday:

A shortwave trough is expected to be traversing across the southern Plains Wednesday morning before ejecting into the Ohio River Valley and Great Lakes Wednesday night through Thursday.
At the surface, a stalled frontal boundary stretching from OH to KS is forecast to serve as the breading ground for a broad area of low pressure that should track into northeast IL and adjacent areas of IN on Thursday. As this occurs, the front will pivot north as a warm front across our area on Wednesday bringing with it a broad area of showers and perhaps even a few embedded thunderstorms. Once the rain begins late Wednesday, it is forecast to persist through the day on Friday as the surface low and aforementioned trough pivot through the region. While forecast soundings do show sufficient instability (especially during the Wednesday night through Thursday timeframe) for thunderstorms, deep layer shear is forecast to be only around 20- 30 kts which should keep the threat for any severe weather low.

However, the high moisture content (PWATs in the 1.3-1.4 range)
and stout forcing does look to generate a good soaking rainfall across northern IL and northwest IN. In fact, ensemble rainfall probabilities indicate a 50-70% chance for a swath of at least 1.0 inch of rain to occur during this time frame with a 20-30% chance for amounts to exceed 2.0 inches in spots. Obviously there is still some uncertainty as to where exactly the highest totals will end up, but the dry antecedent conditions does look to limit the threat for any flooding at this point.

Precipitation is expected to taper Friday evening as a northern stream shortwave begins to phase with the aforementioned trough over MI and pushes the entire system east of our area.
Therefore, dry conditions are expected for the upcoming weekend but persistent northwest flow aloft will keep temperatures more on the seasonable side with highs in the 60s and lows in the upper 30s and 40s. Heading into next week guidance is in remarkable agreement that a broad upper ridge will begin to build across the Great Lakes and Ohio River Valley and get blocked in place by the phasing trough to our east and another Pacific trough/upper low forecast to establish across the southwest CONUS. As a result an extended period of above normal temperatures and dry conditions looks to be in play for the first week of May with highs back into the 70s and possibly 80s.

Yack

AVIATION /00Z TAFS THROUGH 06Z TUESDAY/
Issued at 657 PM CDT Mon Apr 28 2025

Aviation forecast concerns for 00Z TAFs:

- Strong SSW winds with gusts 35-40 kts this evening, only gradually diminishing overnight and Tuesday morning.

- TS threat overnight continues to be of low confidence, but any storms that may occur could be strong/severe.

- Wind shift to NE likely late Tuesday/Tuesday evening.

Surface low pressure was centered over north-central MN early this evening, with the forecast area across northern IL/IN within a wind, mild warm sector air mass. Surface winds from 190-200 deg have been sustained around 20 kts with frequent gusts around 35 kts, and this is expected to continue this evening with a few sporadic 40 kt gusts possible as a 50-55 kt low level jet spreads overhead. Winds should begin to gradually ease by the predawn hours, eventually becoming west/northwest Tuesday while continuing to slowly decrease. A "back door" cold front looks to push south across the area late Tuesday/Tuesday evening, resulting in a shift to northeast winds at that time.

Current radar depicts a line of elevated SHRA/isolated TS from west of KRFD south toward KPIA, with a couple of recent lightning strikes WNW of KRPJ. Latest AMDAR soundings continue to depict a capping inversion in the 700-650 mb range preventing surface-based development however, thus not expecting greater than isolated TS coverage and a brief isolated SHRA prior to about 0130Z with a further weakening trend is possible farther to the east toward the Chicago area. It continues to appear as if the cap will remain difficult to erode overnight, and may suppress further TS development later tonight over the terminals. However, any TS that does develop could be strong/severe with hail and strong winds. Have maintained PROB30 from 03-07Z at KRFD and 06-09Z at KORD/KMDW given potential impacts of severe convection even if coverage remains quite low.

Ratzer

MARINE
Issued at 242 PM CDT Mon Apr 28 2025

- Gale Warning Issued for Lake Michigan nearshore waters between Winthrop Harbor, IL and Gary IN this afternoon through Tuesday morning.

South to southwest winds will continue increasing in magnitude this afternoon and evening as a low pressure system moves into the Upper Peninsula of Michigan. The increasingly tight pressure gradient will hence support a period of gale force winds to 35 kt, mainly northwest of Gary, Indiana.

A cold front will move across the Lower Great Lakes tomorrow morning, causing winds to turn westerly. Gale-force gusts may continue through mid-morning, before subsiding into the afternoon as a surface high pressure system moves into the region.

A Gale Warning remains in effect for the nearshore waters of Lake Michigan between Winthrop Harbor and Gary, Indiana from this afternoon through early Tuesday afternoon.

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Gale Warning until 1 PM CDT Tuesday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 4 PM CDT Tuesday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.