Marine Weather and Tide Forecast for Sleepy Hollow, IL

DISCUSSION
Issued at 355 AM CDT Wed May 14 2025

Through Thursday Morning:

Dense fog and low stratus continues to drift inland across northeast Illinois early this morning. This will lead to further deterioration of visibility as it continues its slow march inland, with further expansions of the Dense Fog Advisory possible. Outside of the marine-based fog influence, patchy fog (some locally dense) has developed further inland across east central IL and northwest IN due to light winds and residual high low-level moisture. Visibility will improve quickly by 8-9 AM CDT for inland areas.

Expect another round of mainly low coverage showers and isolated thunderstorms this afternoon southwest of the Chicago metro along a narrow confluence axis. As has been the case the past couple of days the main hazards with any storms that develop will be lightning strikes, locally heavy downpours, and gusty winds up to around 30-40 mph upon the storm collapsing (if they develop). Will note that any outflow boundaries from these afternoon storms could lead to additional development during the late afternoon/early evening period. Have included some low-end slight chances (15-20%) into the Chicago metro to account for this potential.

It will be another warm and humid day today, with high temperatures in the upper 70s to mid 80s. Continued onshore flow will keep temperatures cooler, in the mid 60s to lower 70s near the lakeshore with fog over the lake potentially drifting inland along the immediate lakeshore at times through the day. This may push a bit farther inland across far northeast Illinois again tonight.

Steepening lapse rates ahead of deepening low pressure across the north central Plains, along with strong warm advection and isentropic ascent could support an axis of elevated thunderstorms developing. Given this is a new trend in only a few models, opted to introduce slight chances (15-20%) for thunderstorms overnight through mid Thursday morning. While confidence is low in this potential, for what it's worth the 00z operational ECMWF was on the more aggressive side with the elevated convection footprint and its ensemble came in with 15-30% of members forecasting QPF. If storms were to form they would be capable of large hail and lightning strikes.

Petr

Thursday afternoon through Thursday Night:

Hot, near to record high temps still appear on track for Thursday. If elevated convection does occur in the morning as mentioned in previous section, its associated debris cloud cover could slow the warming trend for a couple hours and leave some residual boundaries in place. Cloud cover should thin out for the midday-mid afternoon timeframe, when temps should be off to the races away from the IL shore (held cooler by southeast winds until towards evening). Exceptionally warm low-level temps are expected at peak heating. In fact, progged 925 mb temps in the mid to perhaps upper 20s Celsius are above the 90th percentile of DVN and ILX mid May sounding climatology. With these very warm temps aloft (and pushing +20C at 850 mb!), southerly winds gusting to 30-40 mph, and plenty of sunshine should translate to highs from around 90F to the lower 90s.
It's even not out of the question that a few 94F readings occur in portions of the southwest CWA The heat indices, while not necessarily problematic for most for mid summer, will be unusually hot for mid May, peaking in the mid-upper 90s F in spots.

The hot and moderately humid conditions amidst very steep mid-level lapse rates around 8C/km will likely yield 2-3kg of MLCAPE. In addition to the large instability, strong mid-level shear (effective deep layer shear pushing 50 kt) from seasonably robust southwesterly mid-level flow veering to westerly above 500 mb will set up a conditionally volatile air mass regarding the threat for severe weather. The aforementioned ingredients suggest supercell mode with large to destructive hail being the most notable threat (and a persistent feature on sounding analogs). A cold front/dry-line trailing from the powerhouse 980s mb cyclone over MN will be the source of lower-level convergence.
Meanwhile, the strongest forcing from the deep, negatively tilted mid-upper trough will remain off to our northwest, though modest 500 mb height falls will overspread the mid-upper MS Valley late in the afternoon.

As already alluded to above, this looks to be another conditional high intensity severe weather threat scenario (conditional upon storms actually forming). Some of the guidance veers the southerly winds to southwesterly out ahead of the front, which would greatly curtail low-level convergence. This and the dryline surging eastward more quickly would scour the air mass out more quickly. Even if a pre-frontal trough or surface wave keeps flow more backed (similar to the uncannily consistent operational ECMWF depictions the last several cycles), the large scale forcing is rather meager given the very warm EML base and associated capping from it. Versus the April 28th failed conditional potential, this setup appears much more likely to have strong heating through insolation, helping to erode the capping to an extent. It's worth noting that the ECMWF/EPS suite has been very consistent showing CI (~75% of 00z members showing QPF between 18-00z), even without dew points into the 70s as some guidance is depicting.

Yet another competing factor appears to be very dry air at the mid-levels, especially with westward extent that could provide deleterious dry air entrainment into some updraft attempts.
Given the competing and limiting factors, PoPs in the 30-40% range from the mid afternoon through the early evening (~2pm-9pm CDT) appear sufficient. Even if CI (convective initiation)
occurs, coverage may only end up isolated to perhaps widely scattered. Given the impressive ingredients for higher end severe weather, especially hail, and also downburst winds, SPC introduced a level 3 of 5 threat into a portion of the CWA (level 2 of 5 for the rest of the area) on its day 2 outlook.
Threats wise, in addition to damaging to potentially destructive hail, and strong downburst winds (from steep low-level lapse rates), elevated LCL heights and comparably more modest low-level shear forecast may limit the tornado threat somewhat vs the other hazards, if a few supercells do indeed occur. We'll continue to message the uncertainty/conditionality of the setup in our graphics.

It's worth a brief mention that the faster dryline scenarios could bring a short-fused elevated fire danger scenario into play from the late afternoon through a bit after sunset, especially for the western half or third of the CWA This will be due to southwest winds gusting to near 40 mph, and dew points dropping into the 50s if not 40s with temps still around 90F/low 90s. The threat for thunderstorms and severe weather should end fairly quickly by the mid evening as the dryline/front sweeps across the area.

Friday:

The main forecast concern is the potential for high fire danger due to strong west-southwest winds and very warm temps (well into the 80s) plus dew points in the 40s (or lower) yielding RH in the 20-30% range (or lower). Friday's forecast admittedly is still a bit lower confidence due to overall considerable variance among guidance members. With that said, the most consistent suite has continued to be the ECMWF/EPS, which also (unfortunately)
happens to feature the most concerning scenario from a fire weather perspective. One of the key items to watch for the forecast will be how fast and how far south the upper trough associated with the strong low pressure to the north closes off, which would act to keep the 980s mb surface low farther south.
Should this occur, a very tight pressure gradient will remain in place over our area with dew points mixing out into the low-mid 40s or even lower. In addition, stronger southwesterly 850 mb flow would be in place to tap into with deep mixing.

Wind gusts were nudged upwards a bit from the NBM initialization into the 35-40 mph range, strongest I-80 and north. Also slightly nudged Td lower, with some explicit mid 40s values in spots. These steps were taken with the good run to run consistency of the ECMWF/EPS in mind. We'll need to closely monitor guidance trends for this period, needless to say. A Fire Weather Watch may need to be considered with the afternoon update for at least portions of the area after consultation with local fire weather/land management partners.

Later Friday and especially into Friday evening, despite the warm, dry, and windy conditions during the day, another impulse tracking from the mid-MS Valley to the OH Valley and eastern Great Lakes should induce some moisture return back into the CWA The late moisture return should generally be most effective into areas generally southeast of I-55. With steep mid-level lapse rates still in place, high based showers and isolated to widely scattered thunderstorms are possible, especially southeast of I-55. Supportive deep layer shear could even support an isolated severe threat in our south/southeast CWA, but the better chances look to be farther south. Suspect the NCEP (NAM/GFS/GEFS) depictions remain a too wet outlier for Friday, though PoPs in the 20-50% range (highest southeast of I-55) appear sufficient.

Once the evening convective threat ends, the rest of the night will be breezy and cooler in the wake of another cold front passage. Forecast lows are in the mid-upper 50s.

Castro

Saturday through Tuesday:

Generally cooler and dry conditions are forecast for the weekend.
The main weather item of note over the weekend is yet another day of breezy/windy conditions on Saturday, albeit not quite as windy as Friday. Westerly winds will gust up to 30-35 mph, with gusts as high as 35-40 mph possible based on the more aggressive guidance. The cooler temps (70s) will limit the fire danger versus Friday, however. Looking ahead to next work week, while confidence in the details remain low this far out, long-range guidance supports a return to a more active weather period as multiple impulses ejecting off the Rockies next week which could bring additional shower and thunderstorm chances to the region.

Petr

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
Issued at 1234 PM CDT Wed May 14 2025

The main aviation weather concerns for the 18Z TAF period include:

* Fog and low ceiling potential this evening and tonight, mainly at the Chicago sites and GYY.

* Thunderstorms possible across all sites Thursday afternoon and early evening.

Easterly winds will remain largely below 10 kt for the remainder of the day and will subside even further tonight.
Expect SE-to-SSE winds during the day on Thursday gusting to around 20 mph during the back half of the morning, and closer to 25 to 30 kt for the afternoon.

A dense fog bank resides over southern Lake Michigan this afternoon. This fog is expected to spread inland later this evening bringing reduced vsbys and possibly lower cigs to the Chicago sites and GYY. Guidance is not handling the potential well with quite the spread of possible outcomes, but IFR to even LIFR cigs and vsbys appear attainable during the night. In the TAF, categories bottom out at MVFR for now given the lack of confidence.

A potential exists for a broken line of thunderstorms late Thursday afternoon into early evening. It remains unclear whether storms will be able to develop, but if they do, it's likely that at least a few will become strong to severe. Any storms that move over the airfields may bring vsby reductions and locally gusty winds.

Doom

CLIMATE
Issued at 613 PM CDT Tue May 13 2025

Record high temperatures are possible on Thursday May 15.

Here are the current record highs for May 15:

Chicago: 91 in 1962

Rockford: 90 in 1944

Castro

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Dense Fog Advisory until noon CDT Thursday for Winthrop Harbor IL to Gary IN.

Dense Fog Advisory until 4 PM CDT this afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.