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LMZ741 Expires:202310050315;;498835 Fzus53 Klot 042007 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 307 pm cdt Wed oct 4 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>743-050315- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- calumet harbor il to gary in- 307 pm cdt Wed oct 4 2023
Tonight..Southwest winds 10 to 15 kt diminishing to 5 to 10 kt. Chance of showers and slight chance of Thunderstorms in the evening, then showers likely and a chance of Thunderstorms after midnight. Waves 1 to 3 ft subsiding to around 1 ft.
Thursday..Southwest winds 5 to 10 kt. Showers likely and a chance of Thunderstorms in the morning, then slight chance of showers and Thunderstorms in the afternoon. Waves around 1 ft.
Thursday night..West winds 10 to 15 kt becoming northwest overnight. Partly cloudy in the evening, then becoming mostly clear. Waves 1 to 2 ft.
Friday..West winds 10 to 20 kt increasing to 15 to 25 kt. Chance of showers in the afternoon. Waves 1 to 2 ft building to 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 050002 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 702 PM CDT Wed Oct 4 2023

SHORT TERM
Issued at 300 PM CDT Wed Oct 4 2023

Through Thursday night...

Early this afternoon, we sit under mostly cloudy skies while a swath of showers is closing in on the far southern CWA. Rain's been attempting to fall on various parts of the CWA since early this morning. However, a great deal of low level dry air has kept any of this from reaching the ground. With that dry air still in place for the time being, any rain that does decide to inch into the southern CWA through the rest of the afternoon will likely run into a similar problem. Rain chances do build later today and tonight as a cold front marches across the region.

This front is currently just beginning to cross the Mississippi. The front doesn't look all too impressive at the moment, at least the portion of the boundary that will be passing over us. The aforementioned dry air in the low levels and subtle forcing, both along the boundary and otherwise, is keeping this portion of the front dry as it approaches the western CWA. As we approach the evening, however, we'll see an influx of low level moisture ahead of the front which may allow for some showers to develop as it begins making its way into north-central IL. Progressing into the overnight, a tightening theta-e gradient suggests an increase in forced ascent along the front and continued moisture advection at the surface will allow for as many as 1,000 J/kg of MUCAPE to build ahead of the front. Additionally, a mid level jet max is progged to park its left entrance region over the area providing additional lift through the column. 700mb and 500mb vertical velocity forecasts from various high-res models are lighting up right in the 06-09Z timeframe. All of this will result in an uptick in thunderstorm potential as the system works into Chicago metro late tonight. The rain and storm chances will continue to push off to the southeast through Thursday morning, exiting the CWA altogether likely sometime in the mid-afternoon. The trailing dry air mass wastes no time drying out our profile behind the front meaning we're not anticipating much, if anything, in the way of post-frontal precip.

Guidance is beginning to hint at a secondary frontal feature bringing additional rain chances Thursday evening and night. While we are forecast to see a bit of a mid-level moisture plume move in which could maybe get a few showers stirred up aloft, another hydrolapse in the low levels should once again do well to keep most of this precip from actually falling on the CWA. Therefore, the forecast for this period remains dry. Additionally, the cold advection behind the front will make conditions feel noticeably colder come Thursday with highs progged for the lower 70s, a mere taste of the much cooler air in store for the end of the week.

Doom

LONG TERM
Issued at 226 PM CDT Wed Oct 4 2023

Friday through Wednesday...

Key Messages:

* Noticeably colder temperatures this weekend and breezy

* Lake effect showers Friday night into Saturday into parts of northwest Indiana could be heavy at times with embedded thunderstorms

* Low potential for frost Saturday night (<20% and mainly across the I-39 corridor)

No major changes in thinking from the overnight shift on the forecast evolution in the extended, so you'll note just some minor changes to the previous long term discussion.

On Friday-Friday evening, a potent mid-upper level trough packing cold 500 mb temperatures of -25 to -30 Celsius will pivot overhead, with a secondary cold front approaching at the surface. This upper low is 2 to 3 standard deviations below normal both from a height and temperature perspective. Stout mid-level height falls as the system becomes more neutral to even negatively tilted should compensate for the much drier low level air mass to generate scattered showers in the afternoon. Steepening low to mid-level lapse rates from the strong cold advection will generate a sliver of lower level instability as highs reach the low-mid 60s. This conceptually supports low topped convective showers and even some potential for graupel in more robust cores, given the low freezing levels that will be in place. Shower activity should peak during the afternoon with daytime heating and then wane in the evening. Aside from the shower potential, expect west-northwest winds gusting up to 30 mph high fairly high confidence through Friday evening, adding to the chill as temperatures steadily cool behind the front.

An impressive lake effect rain setup for parts of northwest Indiana and southwest lower Michigan. The current forecast winds (and convergence orientation) from northwest to north-northwest should favor northeastern Lake County IN and particularly Porter County.
With the open lake buoys still in the mid to upper 60s and 850 mb temps plunging to near 0C, lake surface to 850 mb delta T will be near 20C, lake induced equilibrium levels forecast to push 25kft, and lake induced CAPE up to 1500 J/kg, if not a bit higher. These all check boxes for heavy rain in a single dominant band (if that occurs) and embedded thunderstorms, warranting the inclusion of slight chance thunder mention (and a slight chance for waterspouts on the lake). The height of the lake effect rain should be later Friday evening into Saturday morning, followed by decreased intensity during the afternoon.

The cold and breezy conditions will continue into Saturday. The upper low will make slow eastward progress as it will be blocked by upstream ridging over the Atlantic. Lows will dip into the upper 30s to mid 40s Friday night and then highs on Saturday will only be in the mid-upper 50s with a scattered stratocumulus clouds and blustery northwesterly winds. A another wave on the backside of the upper low will reinforce the mid level cloud cover Saturday and Saturday night, and reinvigorate lake effect showers near or east of Porter County into Sunday morning. Some sprinkles could materialize elsewhere. These clouds and some lingering winds still look to limit the frost concern to patchy well west of Chicago, but the air mass will still support lows in the mid to upper 30s outside of Chicago and the immediate lakefront. The GFS/CMC ensemble guidance suites support this with probabilities of temperatures below 40F in the 60-90 percent range outside of the Chicago urban footprint. (EC ensemble a bit less confident)

The upper low will be slow to depart the north/east Great Lakes given strong meridional flow, thus our region will remain in the cool northwest flow pattern for several days at least. Sunday's highs will be similar to Saturday except with winds a bit lighter and again a healthy strato-cu deck in the afternoon. Following another chilly night Sunday night, temperatures will moderate a bit on Monday, but near normal highs (and milder overnights) won't arrive until Tuesday and onward. Dry conditions are expected through at least mid next week.

KMD/Castro

AVIATION
For the 00Z TAFs...

* Scattered showers with embedded storms tonight * MVFR cigs Thursday AM with lingering showers

An earlier small cluster of showers near RFD managed to produce a few lightning strikes and continues to weaken as it lifts northeast toward Wisconsin. A separate axis of showers is currently attempting to develop near Peru, IL that is expected to fill in this evening as it lifts northeast toward the Chicago area terminals. While instability will be decreasing with time overnight, it is sufficient to maintain a VCTS mention with this update. Opted to shift the onset time to 5Z to account for models trending a bit earlier and when instability is more favorable.
Visibility could drop to IFR directly beneath any of the stronger cores, though will maintain MVFR in the going TAFs.

Gridded ceiling guidance builds cloud bases down to MVFR and near IFR toward daybreak with any lingering showers. There remains some uncertainty with how quickly precip ends Thursday morning due to variability the progression of the front. Overall the trend is for showers to end a bit earlier, so have pushed the end time of SHRA to 16Z ORD/DPA and 18Z MDW/GYY.

Wind directions will lean SSW through the evening hours and may be sporadically gusty (15-20kt). Speeds and any lingering gusts ease toward daybreak with light generally westerly winds expected through the day. A secondary cold front pushes through after sunset Thursday turning winds northwesterly.

Petr

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

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