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LMZ744 Expires:202404270345;;421335 Fzus53 Klot 261949 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 249 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>745-270345- northerly island to calumet harbor il- calumet harbor il to gary in-gary to burns harbor in- burns harbor to michigan city in- 249 pm cdt Fri apr 26 2024
.small craft advisory in effect from 10 pm cdt this evening through Saturday evening - .
Tonight - Southeast winds 15 to 25 kt becoming south after midnight. Showers likely and a chance of Thunderstorms. 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 261948 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 248 PM CDT Fri Apr 26 2024

KEY MESSAGES

- Waves of showers and a few thunderstorms will move over the area this afternoon and again overnight.

- Tomorrow 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.

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 /18Z TAFS THROUGH 00Z SUNDAY/
Issued at 1236 PM CDT Fri Apr 26 2024

Key Aviation Impacts for the current TAF period:

- Waves of showers and isolated embedded thunderstorms through the overnight period.

- Stronger wind gusts around 30 knots expected late Saturday morning

As a low pressures system in Eastern Nebraska drifts to the north, waves of showers are expected at area terminals through the day and the overnight period. Lightning has been detected in Western Illinois and is drifting northward. Given the track of the storm, there was enough confidence to add VCTS to the KRFD TAF for the next two hours. But given the more northerly track and the diminishing instability, VCTS was not added in TAFs for the Chicago terminals. While isolated embedded strikes may occasionally occur, it was left out of the TAF but will be monitored very closely. If lightning were to occur over KORD/KMDW, it is expected to happen between 19Z and 21Z.

But that is just the first wave of rain. While there may be breaks in the rain, the next chance for stronger rain showers that would have MVFR cigs/vis associated with them will be after 00Z (closer to 03Z for Chicago terminals). Once again, there will be a risk for embedded thunder with the rain tonight, but the probability is around 30 percent (below the threshold for a TEMPO for thunder). Therefore the PROB30 group was removed from the TAF. There was a TEMPO added for the risk of lower MVFR cigs. There have been isolated obs of IFR level cigs to the west, but confidence was not there to add cigs that low. If thunder would develop over terminals, it would likely be in this window. Showers will gradually diminish into the Saturday morning hours.

Winds are out of the southeast with occasional gusts around and just over 20 knots. Those winds are expected to increase through the afternoon and evening to be more prevailing around 25 knots.
By Saturday morning, the winds will gradually pivot to the southwest and increase as a low level jet increases. There is the potential for gusts up to 35 knots late Saturday morning.
Currently left it through the end of the TAF period, but the duration for the strongest wind gusts will depend on cloud cover tomorrow and the placement and strength of the jet aloft.

DK

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Saturday for Winthrop Harbor to Northerly Is. IL.

Small Craft Advisory from 10 PM this evening to 10 PM CDT Saturday for Northerly Is. IL to Michigan City IN.

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