NOTE: Some of the data on this page has not been verified and should be used with that in mind. It may and occasionally will, be wrong. The tide reports are by xtide and are NOT FOR NAVIGATION. Privacy and Cookie policy

LMZ740 Winthrop Harbor To Wilmette Harbor Il- 346 Pm Cst Thu Nov 30 2023
.small craft advisory in effect from late tonight through Friday evening...
Tonight..North winds 5 to 10 kt becoming northeast and increasing to 15 to 20 kt by daybreak. Chance of rain toward daybreak. Waves around 1 ft building to 2 to 4 ft.
Friday..Northeast winds 15 to 25 kt in the morning diminishing to 15 to 20 kt in the afternoon. Rain. Waves 3 to 5 ft building to 4 to 6 ft occasionally to 8 ft.
Friday night..Northeast winds 15 to 20 kt diminishing to 10 to 15 kt. Rain. Waves 3 to 5 ft subsiding to 2 to 4 ft.
Saturday..Northeast winds 10 to 15 kt. Cloudy. Waves 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 302345 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 545 PM CST Thu Nov 30 2023

SHORT TERM
Issued at 316 PM CST Thu Nov 30 2023

Through Friday night...

Key Messages:

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

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

- There remains a chance (currently 40%) of some slushy wet snow accums up to a inch or so (mainly on non paved surfaces) within the narrow zone generally along and north of a Dixon to McHenry line. Closer to the WI border, there remains a low (~20%) chance of amounts around 2" if the wet snow comes down at a heavy enough rate to overcome marginal temps and mild ground conditions.

- Periods of light precipitation, primarily in the form of light rain and drizzle will persist through Friday afternoon (60-70% probs), with some wet snow probably (50-60%) continuing to mix in over portions of far northern Illinois.

- Precipitation will reinvigorate (thought with continued primarily light rates) Friday evening with 80-90% probabilities near and north of the Kankakee River. A similar zone of far northern Illinois to Friday morning will likely (>60% chance)
have at least a mix with wet snow, but no additional accumulation is forecast.

Castro/KJB

Discussion:

A compact and strong Pacific impulse (and its associated surface low) currently noted in the water vapor imagery over the southern High Plains will eject 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. 12z forecast guidance remained in good agreement with the northern periphery of the system's expansive precip shield reaching into southern Wisconsin. As a result, the entire CWA remains on track to see precipitation from this system (peak PoPs of 100% pre-dawn Friday through Friday AM). 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, thanks to a strengthening low-mid level f-gen circulation in addition to the strong large scale ascent. The 12z forecast guidance did generally trend a bit warmer with temperatures and dew points during the highest precipitation rates. In addition, pavement temps will likely have a longer lag to cool off tonight following the over- performing high temps this afternoon. Given the marginal thermal set-up and mild ground conditions, it's uncertain if snow rates (where p-type does go over all snow) will be heavy enough to result in any pavement accums.

The above being said, some accumulations up to an inch remain possible (up to ~40% chance), especially for areas north of a Dixon to McHenry line. Probabilities for accumulations of 2" (or just upwards of 2") came down to only 15-20% over the far northwest CWA (roughly north and west of Rockford). All in all, at this point it appears that travel impacts will be limited due to marginal surface temperatures, which would favor most of the wet accumulations (where slushy accums occur) on non-paved and elevated surfaces. We will continue to message this possibility in and around the Rockford area.

Heavier rain rates and any accumulating wet snow will abate by mid-day. Thereafter, it appears we will continue to see waves of drizzle or light rain at times through the afternoon as broad ascent continues through a deep saturated layer from the low stratus base up to about 600 mb. Looking at a pretty murky afternoon as a result, with any persistent drizzle capable of knocking visibility down into the 1-2 mile range. In addition, as alluded to in the key messages section, sufficient saturation of the DGZ may remain vs. farther southeast to keep some wet snow mixed in over parts of the far NW CWA, including Rockford. High temperatures will range from the upper 30s over interior far northern Illinois to the lower-mid 40s for the rest of the area.

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 re-invigoration of 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 850 hPa 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 completely ruled out on grassy surfaces (confidence was too low for any formal accums in the grids).

The main change with this afternoon's forecast issuance for the Friday evening/night period was to increase PoPs up to 80-90% for areas near and north of the Kankakee River based off ensemble PoPs jumping into that range. The steady light precip will wind down after midnight Friday night. Low temps will be in the mid- upper 30s for most of the area, except locally low 30s far NW and around 40 near/along the Chicago shore.

Castro/KJB/Kluber

LONG TERM
Issued at 219 PM CST Thu Nov 30 2023

Saturday through Thursday...

Key Messages:

* Multiple chances for rain and rain/snow mix through Tuesday night

* Seasonal temperatures through mid week, but the potential for anomalously warm temperatures build late in the week

Mostly cloudy skies are expected on Saturday with temperatures in the 40s. Mid-level subsidence and cold air advection will likely keep the area drier as the previous short wave moves east. However, the 500 mb long wave trough over the western Texas panhandle is projected to move northeastward toward the Great Lakes through the day on Sunday. Overnight low temperatures Saturday into Sunday are expected to be above freezing for most of the region, but best chances for subfreezing temperatures at the surface is projected northwest of a Dixon to Woodstock line. Precipitation is likely (55%- 65%) to be a cold rain, with a wet snow for northwest Illinois Sunday morning, through Sunday evening. A secondary shortwave traversing around the long wave keeps confidence low on the exact timing of precip ending and light rain/drizzle could linger into the early part of Monday morning. Northwest flow behind the trough will provide additional cold air advection for colder, drier conditions Monday morning.

Yet another upper level trough is expected to drop down out of Saskatchewan on Tuesday and deepen as it approaches Lake Michigan providing another chance for rain or rain/snow mix. There is still a lot of uncertainty with the timing and speed of the system, but it looked like the blend was going too low with PoPs in comparison to the suite of ensemble models. So PoPs were nudged upwards to have a chance (30% to 40%) be displayed in the forecast. Lastly, the Euro has a much broader negative tilt to the upper level trough, while the GFS has a more meridional dip to it with a tighter gradient.
Winds were maintained with gusts around 20 mph, but if models trend toward the GFS it is possible that the winds are underdone.

An upper level ridge over the intermountain west will grow and strengthen as it drifts eastward over the Northern Plains.
Temperatures will be around seasonal normals for much of the week.
However as the ridge drifts east, higher pressure will grow and there is a signal for the potential for temperatures to warm to almost 10 degrees above normal for the end of the week.

DK

AVIATION
For the 00Z TAFs...

545 PM
Primary forecast concerns include

Wind shift to northeast later this evening.
Periods of rain with lifr cigs Friday.
Rain/snow mix at RFD.

Rain will spread north across the terminals in the predawn hours Friday morning and a period moderate, to perhaps briefly heavy rain is possible around/just after sunrise. The rain will taper off during the late morning, but periods of light rain or drizzle are expected Friday afternoon. Another period of light rain is expected Friday evening.

Across northwest IL and at RFD, the precipitation is likely to mix with snow with a few hour period of mainly wet snow possible Friday morning, likely just after sunrise. A mix of light rain, light snow and drizzle is expected to continue Friday afternoon at RFD and then another period of a mix of light rain and light snow is expected Friday evening and this also may have a period mainly wet snow. Confidence for snow across northwest IL is medium, but temperatures are marginal and there may not be much accumulation.

A low vfr deck, 3-4kft has developed across the area and this will continue into this evening and then slowly lower through mvfr into the early overnight hours. Once the precipitation arrives, cigs will quickly lower through ifr with prevailing lifr cigs expected for Friday and Friday evening. Visibilities may also lower into the 1-2sm range, even in areas that remain all rain though prevailing vis will likely be in the 3-5sm range.

Westerly winds will generally be under 10kt early this evening and directions will steadily turn to northwest by mid evening and shift to the northeast by late evening. Directions will likely turn more easterly by Friday morning and there is some potential for winds to turn southeast at GYY before winds slowly turn back to the northeast Friday afternoon and then more north/northeast Friday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
Winthrop Harbor to Wilmette Harbor IL...4 AM Friday to 10 PM Friday.

toggle option: (graph/table)

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

Chicago, IL,