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LMZ741 Expires:202311301000;;631397 Fzus53 Klot 300316 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 916 pm cst Wed nov 29 2023
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.
lmz740>742-301000- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 916 pm cst Wed nov 29 2023
Rest of tonight..Southwest winds 15 to 20 kt. Partly cloudy. Waves 1 to 3 ft.
Thursday..Southwest winds 15 to 25 kt becoming west. Mostly cloudy. Waves 1 to 3 ft.
Thursday night..Northwest winds 10 to 20 kt becoming northeast overnight. Slight chance of rain in the evening, then rain overnight. Waves 1 to 3 ft.
Friday..East winds 15 to 25 kt becoming northeast. Rain in the morning, then chance of rain in the afternoon. Waves 3 to 5 ft building to 4 to 6 ft occasionally to 8 ft. A small craft advisory will likely be needed.

LMZ700
No data

FXUS63 KLOT 300521 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1121 PM CST Wed Nov 29 2023

SHORT TERM
Issued at 310 PM CST Wed Nov 29 2023

Through Thursday...

Key Message:

- Gradual warming conditions ahead of the next system

High temperatures this afternoon ran 10 to 15 degrees warmer than Tuesday in response to the warm air being advected into the region from the prevailing southwesterly winds.

A strengthening low level jet out of the southwest will increase overnight. While it will pull in additional moisture for low to mid level cloud development, the main effect of the departing LLJ will be after sunrise Thursday morning, allowing stronger winds to mix down to the surface. Southwesterly wind gusts between 20 to 25 mph are expected through much of Thursday afternoon until mixing ceases with sunset. High temperatures will once again increase, into the mid to upper 40s. Cloud cover will continue to increase ahead of the next system developing in the Southern Plains. There is a low chance (20-30% for rain showers developing into locales near and south of US-24 before 6 PM, but otherwise the main impacts to the system are not expected until Thursday night. For more information on that system, reference the long term discussion below.

DK

LONG TERM
Issued at 310 PM CST Wed Nov 29 2023

Thursday Night through Wednesday...

Key Messages for Thursday Night-Friday:

- Widespread precipitation (80-100% probability) will develop Thursday night into Friday morning, with a rapid northward expansion from pre-dawn through mid morning Friday

- Most of the appreciable precipitation will be in the form of rain and non-accumulating snow mixed with rain, however a fairly narrow swath of some wet/slushy/low ratio snow accums are likely (60%+) over portions of interior northern Illinois, primarily near/west of the Fox Valley

- There is a low (currently less than 15% chance) of more impactful (1"+) slushy snow accums within the narrow zone where falling snow appears likely

- There is a slight (15% chance) that some of the precip shortly after onset could mix with light freezing rain for a short period in the 12am-3am Friday timeframe over a portion of interior northern Illinois. Due to mild temps during the daytime on Thursday and temps close to freezing *if* any brief freezing rain occurs, impacts appear unlikely (less than 15% chance)

Castro

Discussion:

A compact but strong mid-level wave will track from the southern Great Plains into the western Great Lakes during a gradual weakening phase Thursday through Friday. Copious moisture from the Gulf Coast region is expected to quickly advect NNE with persistent low-level 30-40kt flow over the next 36+ hours, ultimately bringing PWATs nearing 1" into as close as central Illinois by late Thursday night.

Meanwhile, a loosely coupled upper-jet structure over the mid- Mississippi River Valley combined with a nearly 100kt 500 hPa jet nosing into the southern half of Illinois will promote an impressive dynamic setup for widespread precipitation well into our forecast area late Thursday night through Friday morning.
Ensemble clustering continues to differ some, but did notably converge in the 12z model cycle in the handling of some important features, notably the northward placement of a strengthening low- level baroclinic zone across northern Illinois. The ECMWF EPS and CMC/CMC ensembles again favored a slightly colder solution that would bring increasing snow chances into the northwest third or so of the CWA while the GFS/GEFS favors a slightly slower and therefore warmer solution (but still support changes in precip type).

Some caveats with this event include an overall weakening system (surface low strength peaks late Thursday evening over Oklahoma)
and a decent antecedent layer of dry air aloft to the north.
Increasing low-level ENE flow on the northern periphery of the system, owing to a 1020 hPa surface high shifting across Minnesota, should offset precip amounts to some degree. But unlike a more typical Hudson Bay setup with very dry air upstream into Lower Michigan, trajectories of much drier air will focus from Wisconsin toward Iowa. Therefore, the abundant moisture and strong forcing should overwhelm the existing dry layer well into the CWA.

Given the recent trend of increasingly strong agreement in measurable precip for the CWA, the main change to the forecast database was to increase PoPs into the (80-100%) categorical range for the entire area late Thursday night into Friday morning. The anomalous PWATs around 1" point toward total liquid equivalent precip of 0.5"+ south of I-88 and amounts upward of 0.75" south of I-80 (localized 1" totals may be realized). There is much more uncertainty in the realm of precipitation types on the northern fringe of the precip shield, substantially tied to specific temperature, dew point, and wet bulb temperature trends.

Our forecast ended up closer to the ECMWF/EPS, which itself is a good compromise between the colder outlier NAM-WRF guidance and the warmer GFS/GEFS (ultimately just a hair warmer than the raw NBM). Think that this synoptic setup is more favorable for rain to wet snow where p-type ends up primarily frozen and the increased signal for freezing rain was likely an artifact of some of the colder guidance having sub-freezing temps while a warm nose aloft is briefly in place for a short portion of the overnight. For this reason, capped freezing rain mention at slight chance (15%) for a small portion of interior northern Illinois for a brief 2-3 hour window between midnight and 3am shortly after onset.

For the lion's share of the event, overall expectation is for a widespread precip event across a vast majority of the forecast area Thursday night and Friday morning. Precip. type will be rain south of a Chicago to Peru line, with increasing chances for a period of all wet snow (ratio 5:1 or less) for areas near and west of the Fox Valley and I-88 and north. In particular, north of Dixon to McHenry line could hold onto wet snow past daybreak Friday. Given the increasing signal in guidance (especially EPS and CMCens) for accumulating snow and the factors noted above, have again increased snow probs with some minor accumulations (<1") in the forecast late Thursday night into Friday morning.

While still a lower (<20%) chance exists for one inch or more of accumulating snow in the aforementioned area, the low ratios and marginal surface temps greatly increase uncertainty in any notable impactful slushy accums. If the low-level baroclinic zones further contracts and supports a longer residence time band of strong f-gen in a dynamically cooled thermal profile, then the chances for some travel impacts Friday morning will increase. We will message the low potential for slushy snow related impacts and continue to assess trends. Lingering light rain should taper off by Friday afternoon as temps warm well above freezing area wide.

Castro/Kluber

Key Message for Friday Night Onward:

- An active weather pattern with multiple periods of precipitation will persist through early next week

Discussion:

Beyond the daytime hours of Friday, guidance remains in an active WSW/SW flow pattern with multiple systems traversing the region through the weekend until deeper troughing drifts east across the Great Lakes region by mid-week. A secondary jet streak behind the system Thursday night into Friday will force another (lighter)
swath of precip into the area Friday night along the existing low- level baroclinic zone. Increased PoPs into the likely range Friday evening for most of the CWA for this potential and also nudged PoPs upward overnight across northern Illinois for the exit of the short-wave. While thermal profiles generally favor rain for most of the area, some wet snow accumulations are possible on the northern edge of the system into far northern Illinois and southern Wisconsin.

There should be a lull Saturday, followed by multiple potential rounds of light precip Saturday night through Tuesday. The most prominent of these appears to be Saturday night-Sunday when PoPs are about 40%, and then generally in the 20-30% range thereafter.
Confidence is low in the specifics, which will be refined in the coming days. With frequent potential bouts of precip. and associated cloud cover, temperatures should generally be average to slightly above normal through the period.

Kluber/Castro

AVIATION
For the 06Z TAFs...

Forecast concerns include...

Low level wind shear Thursday morning.
Gusty southwest winds Thursday.
Wind shift to northeast late Thursday evening.
Rain/ifr/lifr cigs early Friday morning.

Overall, no significant changes to the previous forecast through Thursday afternoon. Southwest winds under 10kts to start will increase into the 10-12kt range by daybreak. Low level winds will increase overnight with low level wind shear through mid morning Thursday, when gusts into the lower/mid 20kt range will develop.
These gusts will diminish in the mid afternoon as winds turn westerly by sunset. Winds will steadily, perhaps quickly turn northwest in the early evening and now appear to become northeast by late evening and then will remain northeast into early Friday morning.

There remains the potential for mvfr level clouds, possible mvfr cigs on Thursday, but the better chance appears to be in the mid afternoon into the evening but have remained scattered during this time period. Eventually, mvfr cigs are expected to become prevailing from south to north late Thursday evening, lower through low mvfr and then likely into ifr early Friday morning, possibly lifr by the end of the 30 hour ORD/MDW taf period. The lowest cigs will likely be tied to precipitation arrival, which appears to be mainly rain for ORD/MDW. There is a chance for snow across northwest IL, mainly at RFD, but this would be after the 06z end time of this forecast. cms

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.

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