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LMZ744 Expires:202404270945;;438582 Fzus53 Klot 270207 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 907 pm cdt Fri apr 26 2024
for waters within five nautical miles of shore on lake michigan
waves are provided as a range of significant wave heights, which is the average of the highest 1/3 of the waves, along with the average height of the highest 10 percent of the waves which will occasionally be encountered.
lmz742>745-270945- northerly island to calumet harbor il- calumet harbor il to gary in-gary to burns harbor in- burns harbor to michigan city in- 907 pm cdt Fri apr 26 2024
.small craft advisory in effect through Saturday evening - .
Rest of tonight - Southeast winds 15 to 25 kt becoming south after midnight. Chance of showers and Thunderstorms early, then showers likely and a chance of Thunderstorms after midnight. Waves 2 to 4 ft. Saturday - Southwest winds to 30 kt. Slight chance of showers and Thunderstorms. Waves 2 to 4 ft. Saturday night - South winds 15 to 25 kt diminishing to 10 to 20 kt. Chance of showers and Thunderstorms. Waves 1 to 3 ft. Sunday - South winds 10 to 20 kt. Chance of showers and Thunderstorms in the morning, then slight chance of showers in the afternoon. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 270554 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1254 AM CDT Sat Apr 27 2024

KEY MESSAGES

- Showers and thunderstorms will redevelop over the area late this evening into the overnight hours.

- Saturday looks summer-like with highs in the upper 70s to lower 80s, increasing humidity levels, and eventually partly cloudy skies.

- Showers and thunderstorms will develop toward and after sunset Saturday night and continue through Sunday morning. A threat for flash flooding will exist where thunderstorms train.

- Periods of showers and storms will continue through midweek, a few of which could be strong to possibly severe on Sunday.

UPDATE
Issued at 827 PM CDT Fri Apr 26 2024

Minor adjustments to going forecast tonight, primarily to add detail for a period of dry weather early-mid evening, then to increase thunder chances somewhat overnight. Overall however, going forecast is in good shape.

Evening surface analysis depicts 992 mb low pressure over eastern Nebraska, with a warm front extending east-southeastward across southern IA into central IL. The low is forecast to lift northeast into the upper Mississippi Valley by morning, in response to a strong negative-tilt shortwave trough propagating across the region. As this occurs, the warm front over central IL/IN will lift north across the forecast area overnight, pulling warmer and more humid summer-like air into the area by morning.

While earlier rains had lifted northeast of the area late this afternoon, another round of rain and embedded thunderstorms is expected to develop northeastward across the area later this evening and overnight. Strong ascent associated with the negative-tilt short wave to our northwest will help to steepen mid-level lapse rates, while lower-level south-southwesterly flow increases (approaching 60 kts at 850 mb) transports moist/unstable air into the region. Thus, the current area of rain/embedded thunder currently noted in regional radar plots across eastern MO/downstate IL is expected to spread northeastward into the forecast area late this evening, likely with some expansion of elevated convection to the north. RAP forecast soundings depict ~600 J/kg of elevated MUCAPE across northern IL after midnight, which along with strong unidirectional deep layer shear may support hail up to nickel size as well as some gusty surface winds. This would likely be more likely if convection over central MO were to expand and result in some upscale growth. Otherwise, the severe threat appears generally low and SPC's level 1/5 (marginal) risk across our IL portion of the cwa is reasonable. Convective coverage looks to decrease quickly toward sunrise, as the warm front lifts north along with larger scale forcing.

Going forecast has all of this handled nicely. Did make some adjustments to pop trends this evening for the current break in precip, before rain spreads back into the area after 9-10 pm.
Also did increase thunder chances somewhat overnight with forecast soundings looking more supportive. Otherwise, no significant changes made for the overnight hours.

Ratzer

DISCUSSION
Issued at 248 PM CDT Fri Apr 26 2024

Through Saturday Night:

Regional water vapor and visible satellite imagery, surface observations, and radar data depict a surface low pressure system centered along the Kansas and Nebraska borders underneath an upper level shortwave trough lifting northeastward into the Upper Mississippi River Valley. Bands of showers and a few thunderstorms are evident east of the center of the low, including severe convection across the Plains and warm-air advection driven showers along and east of the Mississippi River. A diffuse surface warm front is draped across southern Illinois, and delineates summer-like moisture to the south and more seasonable (and wet) conditions to the north.

Over the next 18 hours or so, waves of showers will parade over northern Illinois and northwestern Indiana as broad warm air advection continues atop the northward-surging warm front. While coverage of thunderstorms will initially be isolated this afternoon and evening, gradually increasing instability (by virtual of low- level moistening and steepening mid-level lapse rates) will support embedded thunderstorms particularly after sunset and especially midnight. While the threat for severe weather remains low, a localized threat for pea to nickel size hail or gusty winds exists across much of the area through the overnight hours, particularly if any upscale growth of any cluster originating out of Missouri occurs overnight. The SPC Level 1/5 threat level for the overnight hours covers the potential well.

After daybreak, the warm front is expected to stall across central Wisconsin as the low-level jet wanes. At the same time, a secondary upper-level trough moving into the Southern Plains will induce gradual mid-level height rises across the Great Lakes. The net result should be the erosion of any remaining shower or thunderstorm after daybreak affording a dry period from mid-morning to mid- afternoon. As low-level stratus thins from mid-morning onward, a "pop" of southwesterly winds gusting 40-45 mph may occur before noon as mixing builds into the waning low-level jet. Thereafter, southwesterly gusts of 30-35 mph will continue through the afternoon. With 850mb temperatures climbing to +13 to +14C and prospects for even filtered sunshine, temperatures tomorrow are poised to shoot into the upper 70s to even lower 80s. When combined with a humid airmass (surface dew points in the low to mid 60s and PWATs nearing 1.5"), it will feel more like late June than late April by tomorrow afternoon!

Tomorrow afternoon, neutral height tendencies and boundary layer heating may prove sufficient to erode capping. As a result, isolated showers and thunderstorms may develop after 2 or 3 PM, though confidence in anything developing so early is pretty low (equating to a 20% chance for thunderstorms).
Tomorrow evening, coverage of showers and storms will increase markedly first across Missouri and Iowa, and later across northern Illinois, as the next upper-level shortwave pivots northeastward and supports the redevelopment of an expansive low-level jet. With a steady feed of low-level moisture and convergence along the nose of the low-level jet overnight, showers and storms may continue more or less continuously through daybreak Sunday along a southwest to northeast, or even west to east, oriented axis.

While the pattern doesn't conceptually match classic flash- flood set- ups (as the upper-level jet will be displaced too far to our north and west, and cells may be more broken than within a continuous, training line), the ability for the low-level jet to continuously feed replenishing instability and moisture into a relatively confined band of thunderstorms over a prolonged period of time does raise concern for a localized threat for flash flooding mainly in urban areas. At this point, CAM guidance (including an experimental FV3 core extended CAM ensemble) appears to favor the axis aligning near the Illinois and Wisconsin state line which is reflected in our forecast.
However, we would be remiss to ignore the deterministic HRRR, RAP, and experimental RRFS, which all favor the thunderstorms aligning somewhere near I-80 or I-88. For now, felt the course of least regret was to collaborate a southeastward expansion of the D2 WPC Flash Flooding Threat level 1/5 area to encompass much of northern Illinois.

Not to be forgotten, overnight lows Saturday night will be incredibly (record?) warm and in the low to mid 60s.

Borchardt

Sunday through Friday:

The secondary upper trough, currently over the Great Basin, is expected to be traversing the central Plains into the upper Midwest as a closed upper low on Sunday. This trajectory will place the Mississippi River Valley and western Great Lakes in the diffluent region of the trough and therefore continue to support periods of showers and thunderstorms through Sunday night. However, there is a notable signal amongst the mid-range forecast guidance that a modest cap may develop Sunday morning which could limit (if not inhibit) shower and storm development until better forcing and destabilization arrives Sunday evening into the overnight hours as the aforementioned trough is closest and a low-level jet forms overhead. While it continues to look as if the better forcing and subsequent instability should remain near and west of I-39, 30-35 kts of effective shear are forecast to be in place which could allow a few storms to become better organized and be capable of producing severe weather, primarily during the late afternoon and evening hours Sunday.
Therefore, a marginal (level 1 of 5) severe weather risk remains for the western half of our forecast area.

As the aforementioned upper low lifts into the upper Midwest Sunday night into Monday, a cold front is expected to move through northeast IL and northwest IN bringing yet another period of showers and thunderstorms. Though, the instability on Monday does look to be notably weaker than on Sunday which should keep the threat for any strong to severe storms to a minimum. While temperatures are expected to cool a bit in the wake of the front, high temperatures are forecast to remain in the mid to upper 70s through the middle of next week.

A brief break in the active weather is expected for Monday night and Tuesday as the upper-level pattern transitions into a more zonal (east-west oriented) pattern. However, additional shortwave troughs are forecast to traverse through the pattern Tuesday night through Thursday which looks to return the threat for periodic showers and thunderstorms. It does look as if high pressure may try to establish over the central CONUS towards the end of the week and into next weekend with more seasonable temperatures as well.

Yack

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
Issued at 1250 AM CDT Sat Apr 27 2024

Key Aviation Messages: - Scattered SHRA with isolated TS early this morning - SW wind gusts over 30kt expected after daybreak - Periods of MVFR ceilings through the period - SHRA/TSRA redevelops late evening through early Sunday AM

A narrow line of elevated convection will lift across the Chicago area terminals near the start of the TAF period and are accounted for with a TEMPO for TS from 6-7Z. Confidence in whether additional showers/storms develop overnight remains on the lower side, and accordingly opted to convert to a VCTS mention through ~10Z to account for the back edge of a cluster of storms west of Peoria that are lifting northeast toward the area. Any lingering showers will diminish in coverage toward daybreak.

Winds early this morning are breezy out of the southeast around 25kt with sporadic gusts near 30kt in the metro. Directions will gradually veer S to SW with time overnight. More persistent gusts in excess of 30kt are expected to develop after daybreak and continue into the afternoon before easing toward sunset.

Additional isolated showers can't be ruled out during the afternoon, though confidence remains too low for a formal TAF mention. Confidence does increase in showers developing by mid- late evening along with the potential for embedded TS. The timing for this period will continue to be refined with later updates.

Petr

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.

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