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LMZ740 Winthrop Harbor To Wilmette Harbor Il- 949 Am Cst Thu Nov 30 2023
.small craft advisory in effect until 4 pm cst this afternoon...
Rest of today..Southwest winds 20 to 25 kt with gusts to 30 kt diminishing to 10 to 20 kt late. Partly to mostly cloudy. Waves 2 to 4 ft subsiding to 1 to 2 ft late.
Tonight..West winds around 10 kt becoming northeast by midnight and increasing to 10 to 20 kt overnight. Rain overnight. Waves around 1 ft building to 2 to 4 ft.
Friday..Northeast winds 15 to 25 kt. 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.
Friday night..Northeast winds 10 to 15 kt. Rain likely in the evening, then chance of rain overnight. Waves 3 to 6 ft subsiding to 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 301655 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1055 AM CST Thu Nov 30 2023

UPDATE
Issued at 1055 AM CST Thu Nov 30 2023

Main tweak was to adjust up high temperatures and southwesterly wind gusts today, and nudge cloud cover down until mid afternoon.
A combination of a stout departing low level jet, a bit deeper mixing, a good amount of sun, and the low level air mass, has led to temperatures already approaching the range of the previous forecast highs for the day as of this writing, including already 51F at MDW. As such, increased high temperatures to the lower to mid 50s and increased gusts to up to 30 mph per recent observations. Partly cloudy (mainly mid-high clouds) skies are expected through early afternoon, also supporting the warmer temps as noted. Finally, slowed down the PoPs in our far southern CWA locales later this afternoon, with only slight chances (~15%) by just after sunset. As such, looking at a dry daytime for the area.

No changes were made to the precipitation forecast for tonight into Friday, which will be addressed in the full forecast issuance this afternoon.

Castro

SHORT TERM
Issued at 333 AM CST Thu Nov 30 2023

Through Friday...

Key messages:

- Mild and primarily precipitation free through the daytime hours today.

- A widespread soaking cold rain is expected (for most) late tonight through Friday morning. Rainfall amounts of 0.5" up to an inch are currently forecast.

- Conditions continue to favor the rain to mix with, or even change over to a period of wet snow for period into Friday morning across interior sections of far north IL, primarily near/west of the Fox Valley.

- There is a chance (currently 20-40%) of some slushy wet snow accums up to a couple inches (mainly on non paved surfaces)
within the narrow zone generally along and north of a Dixon to McHenry line.

While a period of inclement weather is expected tonight into Friday, the daytime hours of today will be rather quiet and mild, albeit with increased cloud cover. Temperatures should have no problem warming well into the 40s this afternoon, with several locations even cracking the 50 degree mark.

The primary focus for our active weather later tonight and on Friday is the compact and strong Pacific impulse currently noted in the water vapor imagery near the Four Connors region of the Desert Southwest. This feature is expected to eject eastward across the TX Panhandle this afternoon, then east-northeastward into the lower Great Lakes Region into Friday. As it does so, the flow aloft will become strongly diffluent as a loosely coupled upper-jet structure sets up across the Lower Great Lakes region.
This in combination with a northward influx of unseasonably deep Gulf Moisture (PWATs at or above 3/4") will set the stage for a widespread precipitation event across the area late tonight into Friday morning. Forecast guidance has tended a bit farther north with the precipitation, and as a consequence it appears all of the area will see precipitation from this system. Total liquid equivalent amounts of a half to as much as an inch are expected.

Much of the precipitation tonight into Friday will be in the form of a cold rain, especially along and south of a Waukegan to Peru line. However, chances increase with northward extent towards the WI state line (and inland from Lake Michigan) that the column will dynamically cool enough into Friday morning to support a change over to a period of wet snow. Some accumulations up to a couple inches remain possible (up to 40% chance), especially for areas north of a Dixon to McHenry line. Fortunately, at this point it appears that travel impacts will be limited due to marginal surface temperatures, which would favor most of the wet accumulations on non-paved and elevated surfaces. We thus plan to continue to message this possibility in and around the Rockford area. The heavier rain and accumulating wet snow will abate by midday. Thereafter, it appears we will continue to see some drizzle our light rain at times through the afternoon as temperatures remain steady in the upper 30s north and the lower to middle 40s far south.

KJB

LONG TERM
Issued at 333 AM CST Thu Nov 30 2023

Friday night through Wednesday...

Key Messages: - Active period with multiple rain and rain/snow mix chances through next Tuesday - Seasonable temps through early week transitioning to potentially unseasonably warm conditions by late next week.

An active period is in store through at least the first half of next week. A longwave trough across the west half of the CONUS by Friday will slowly shift ENE across the Great Lakes early next week, reaching New England mid-week. Smaller troughs within the broader wave show no fewer than three disturbances affecting the CWA Friday night through next Tuesday. In each case, we will have to contend with mixed precip issues for at least north and west portions of the CWA.

In the wake of tonight/Friday's system, a weak secondary upper- level wave will cross the forecast area Friday evening and overnight as the weakening low-level low pressure drifts from near STL to near DTW Friday afternoon through Friday night. Modest low-level isentropic ascent ahead of the highly NW-tilted low with height will support a broad shield of light precip across much of the CWA. Thermal profiles support primarily all rain except for areas northwest of a Dixon to McHenry line (roughly the 850hPa low track) where snow should mix with and possibly change over from rain. Surface temps look to be marginal for any notable accumulations, but a few tenths of an inch cannot be ruled out on grassy surfaces.

For Saturday, a brief period of weak CAA and slight mid-level height rises should limit precip potential, though the area may remain shrouded in low-level stratus and patchy drizzle. The next wave, with a little colder profile, will bring another round of rain and wet snow across the area late Saturday night through Sunday. Beyond Sunday, precip chances return Tuesday with a third wave before ridging ends precip chances for the second half of next week. EPS/GEFS/CMCE guidance is showing a growing signal for prominent ridging (nearly +2 sigma 500 hPa heights) across the western CONUS early next week (assisted by a northern Greenland blocking ridge shifting arctic air into Asia) that will gradually deamplify into our area through the rest of the week. This would favor a period of unseasonably warm conditions for several days.

Kluber

AVIATION
For the 12Z TAFs...

Key Aviation Messages...
- LLWS through mid-morning - W to NE wind shift early this evening - Rapid deterioration of conditions late tonight with IFR visibility and LIFR ceilings Friday morning

VFR conditions are expected through mid afternoon before an MVFR deck develops toward sunset. SW winds under 10 knots will quickly increase with gusts to 25 knots by mid-morning as mixing ensues into a shallow inversion and 40kt LLJ. Winds will diminish to under 10 knots while veering W to NE late afternoon into early evening, with the shift east of north occurring around 02Z.

After some spotty -SHRA or -RADZ late this evening and early overnight, a moisture-laden system will quickly spread a widespread shield of RA into the Chicago metro terminals.
Conditions will quickly deteriorate prior to daybreak Friday, with LIFR ceilings and IFR visibility expected from around sunrise/13-14Z through the morning. At RFD, precip is expected to quickly become very wet SN (possibly mixed with RA at times)
prior to sunrise with IFR visibility likely through Friday morning.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
IL and IN nearshore waters until 4 PM Thursday.

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