textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Widespread light rainfall is expected this morning through early this afternoon.

- A front with potential for snow squalls will progress west to east late this afternoon into this evening, with increasing concern for at least snow showers during this time frame.

- Significantly colder temperatures (overnight lows in the single digits to low teens) expected Tonight, potentially flash freezing any surfaces that remain wet from Today's rain and snow.

SHORT TERM

Issued 415 AM CST Thu Dec 18 2025

Today and Tonight:

As depicted in the latest CAM guidance, light rain is expected to develop overhead within the next few hours (early this morning) as WAA aloft continues to intensify. Our VWP is already showing a 45-55 kt southwesterly LLJ in the lowest few thousand feet of the atmosphere. Due to the well developed surface inversion, these winds will not mix all the way down, but we may yet see some 27 knot (upwards of 30 MPH) south surface gusts at later this morning as the inversion weakens. Rain will intensify and become more persistent later this morning as an occlusion passes overhead. Winds veer west-southwesterly behind the occlusion this afternoon. Rain totals are on track for roughly one to three tenths of an inch. Temperatures peak around Noon CST today, with most areas achieving a daytime high in the low to mid 40s. Depending on the exact position of the occlusion, far southeastern WI may even reach the upper 40s as the surface warm front clips the area.

From 4 PM to 8 PM CST this afternoon / evening, we expect a potent polar cold front to track west to east across the region, with winds veering due west (and eventually northwest) behind it, with gusts likely reaching / exceeding 35 MPH. The only noteworthy change on this front (since the prior forecast iteration) is a roughly 1 hour delay in the timing. Temperatures rapidly fall below freezing behind the front, and there's at least a 60% chance of some scattered light snow showers along it. As mentioned previously, all the ingredients for snow squalls are present with this frontal passage (very strong FGEN on short-range model guidance, a transition from above to well below freezing with the front, healthy pressure falls/rises with the boundary, and several thousand feet of sharp low-level lapse rates in the cold advection sector). The only ingredient that's putting a limit on the snow squall parameter values (on the current model iteration) is low-level positive buoyancy energy (the conditional instability or conditional symmetric instability needed to give the snow showers a convective kick). That being said, the 00z and 06z NAM do feature sufficient buoyancy, the HRRR is trending stronger with that buoyancy and already resolves some weak squall-like characteristics in composite reflectivity, and personally I wouldn't be surprised to see these trends continue.

Even without a true 'snow squall' event (which again, could occur), the cold front and any scattered light snow showers would still be able to rapidly freeze any lingering moisture on pavement, and slick conditions are a concern. Overnight lows in the single digits to low teens are expected tonight, with gusty northwest winds continuing through daybreak Friday.

Sheppard

LONG TERM

Issued 400 AM CST Thu Dec 18 2025

Friday through Wednesday:

GEFS and ECMWF Ens continue to show fairly good agreement during this period. Nwly winds and cold advection will cease Fri AM as low pressure moves away across srn Ontario and Quebec. A surface ridge passage will then follow for the afternoon with temperatures remaining cold due to the polar airmass.

Sly winds and warm advection will then begin Fri nt as low pressure around 990 MB tracks from srn Manitoba into Ontario. The associated upper wave and sfc low will then move across Ontario and the Lake Superior region on Sat. The warm advection will continue into Sat AM followed by a cold front. There are 20 percent chances for mainly light snow during this time. A stronger polar cold front will then arrive Sat nt. Temperatures will warm into the 30s on Sat then dropping into the single digits for much of srn WI by 12Z Sun. Polar high pressure will then pass Sun afternoon and evening with cold temperatures continuing.

Another period of sly winds and warm advection will then ensue late Sun nt and Mon as low pressure develops in the nrn high plains. 20 percent chances for light snow are forecast for portions of srn WI during this time. This low will evolve to a weaker low across the Lake Superior region and Ontario late Mon nt-Tue with a weak and dry cold frontal passage for srn WI on Tue. High temps will warm through the 30s from Mon into Tue. A broad upper ridge over the Great Plains will then shift into the central USA by Wed with a developing warm front to the south. Light sely winds will develop with temperatures remaining relatively mild.

Gehring

AVIATION

Issued 400 AM CST Thu Dec 18 2025

TMKE and KMKX radars both confirm ongoing low level wind shear, out of the southwest at 45 to 50 knots. This shear ought to be particularly strong given that the due southerly surface winds and gusts (10 kt gusting to 20) are struggling to keep up at the moment. Broken MVFR ceilings of 2,000 to 3,000 ft have already overspread the region in advance of the rain, which is expected to begin (perhaps in a showery nature to start, becoming steadier) over the next few hours, peaking later this morning as an occluded front passes over the region. This rainfall will be accompanied by a drop to IFR or even LIFR cloud ceilings, which improve to MVFR or Fuel Alt MVFR after the occlusion passes (early to mid afternoon). The passing occlusion will also bring an end to the low level wind shear threat and allow surface winds to veer southwesterly.

A healthier cold front will then track eastward across our region from 4 PM to 8 PM CST this afternoon / evening, bringing plunging temperatures and likely some scattered snow showers and snow squalls, along with broken MVFR / Fuel Alt MVFR ceilings. The scattered snow squalls in this time window (if present) will present multiple hazards, including a brief dramatic reduction in visibility, a flash freeze of moisture on pavement (due to falling temperatures and dynamic cooling from falling snow), minor snow accumulations, and very gusty west to northwest winds. Light snow shower and flurry chances gradually taper off in a west to east manner late tonight. Gusty northwest winds linger into Friday morning before decelerating. Broken MVFR / Fuel Alt MVFR ceilings may persist through much of this period as well, gradually scattering out over time.

Sheppard

MARINE

Issued 400 AM CST Thu Dec 18 2025

Low pressure around 29.0 inches over Northern Minnesota will track into Lake Superior by this afternoon. Southerly gales are expected to continue this morning, and may briefly intensify (closer to 40 kt) later this morning through around Noon CST. A brief lull in the gales is possible this afternoon as low pressure crosses the Upper Peninsula of Michigan, but then winds become westerly and increase back to gale force behind a strong cold front this evening, then turn northwesterly tonight and remain gales through Friday morning. A Gale Warning is in effect for all open waters for these time frames. Moderate freezing spray is expected to accompany the gales late tonight into Friday morning as a bitterly cold airmass enters the region.

Winds diminish briefly Friday as low pressure exits to the west, but another low pressure system will swing through southern Canada Friday night, bringing potential for south-southwest gales once again. Winds will shift to become northwesterly behind the trailing cold front of this low Saturday afternoon, increasing to near gales once again Saturday night before diminishing again into Sunday. Winds remain light through Sunday night.

Sheppard

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646 until 6 PM Thursday.

Gale Warning...LMZ643-LMZ644-LMZ645-LMZ646...6 PM Thursday to 9 AM Friday.

Gale Warning...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563- LMZ565-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779- LMZ868-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 until 9 AM Friday.