Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 301 PM CST Sat Dec 7 2024

Through Sunday night:

Temperatures have over performed across the area this afternoon within the strong warm air advection pattern currently in place across the Great Lakes region. Accordingly, temperatures will now top out around (or a degree so warmer) the current readings, which generally range from the mid to upper 40s. High-level cloud cover continues to stream southeastward into the area late this afternoon. This will thus continue to result in partly to mostly cloudy skies for most locations into this evening.

Strong pressure falls in advance of a rather stout and fast moving clipper system tracking across extreme southern Ontario into this evening will keep our breezy southwest winds (gusting 25 to 30 mph) going for a few hours this evening. Winds will really be ripping within a couple thousand feet of the surface, however, as a low-level jet featuring westerly winds up around 50 kt continues overhead this evening. Fortunately, we will not be mixing into these strong westerly winds, so surface wind gusts will remain in check. Wind speeds will then abate overnight into Sunday morning following the quick eastward exit of the clipper system into New England.

Dry and unseasonably mild weather persists Sunday. Lighter southern surface winds are expected during the day in advance of a northern stream low pressure system shifting eastward across the northern Plains to the north of a notable southern stream impulse shifting from the Desert Southwest out across the TX Panhandle. With dry and mostly sunny continues expected in our area on Sunday, we continue to anticipate afternoon highs peaking around, or just above 50.

As we head into Sunday evening, the southern stream impulse is forecast to track eastward into the Ozarks. As it does, an increasing downstream southerly mass response will induce a pretty decent northward surge of low-level (dew points in the 40s) moisture into our area. This will result in the rapid northward development of a low-level stratus deck across much of the area after sunset Sunday evening, but not necessarily a lot of precipitation. Instead, it appears that the track of this southern stream impulse to our south Sunday night will steer the better deep mid and upper-level moisture and forcing (supportive of more substantial rain) to the southeast of much of the area. With this in mind, we continue to carry the highest rain chances in our area southeast of I-57. Farther to the northwest into much of far northern IL, I opted to maintain some slight chance pops Sunday night, mainly to account for the potential of drizzle from the low-level stratus deck overhead.
Model forecast soundings suggest these lower clouds will be around (or a bit more than) 3,000 feet deep, and thus may support some instances of drizzle northwest of the better precipitation shield shifting into the Ohio Valley late Sunday night. Eighter way, any light precipitation/drizzle threat will end into early Monday morning as a northern stream cold front, begins to shift eastward into the area.

KJB

Monday through Saturday:

Forecast thinking during the long term period has not changed from the previous thinking highlighted below.

On Monday, aggregate upper- level troughing across the western US will shift eastward anchored by a low pressure system lifting into central Ontario. Around the same time, an associated cold front should sweep eastward across the Great Lakes. While highs ahead of the front on Monday should still reach the upper 40s to lower 50s, temperatures will tumble Monday night with lows expected to fall into the mid to upper 20s. Tuesday into Wednesday, a sharp upper-level wave originating out of central Nunavut is expected to race southward and merge with the upper- level trough across the Great Lakes, effectively forcing the trough to contract and rapidly lift toward the northeastern United States. Such will set the stage for a lobe of arctic air to swing through the Great Lakes. Highs in the 30s on Tuesday will fall toward the 20s on Wednesday and Thursday (wind chills some 10 to 15 degrees lower). Finally, would have to think there should be a period or two of flurries or snow showers in the general area as the contracting upper- level trough whips overhead (perhaps on Wednesday).

Ensemble model guidance appears to be in agreement that the period of cold temperatures next week will be short lived as somewhat zonal flow becomes reestablished across the northern US. Indeed, multi- model ensemble meteograms of temperatures support highs returning to the 40s by next weekend.
Interestingly, both EPS and GEFS show a signal for wetter weather in the general region in the December 14th to 17th timeframe as well, which aligns well with the forecast for above-average precipitation by mid-month advertised by the CPC.

Borchardt/KJB

AVIATION /00Z TAFS THROUGH 06Z MONDAY/
Issued at 539 PM CST Sat Dec 7 2024

VFR conditions will prevail through the TAF cycle. Strong winds just off the deck will result in LLWS this evening, with winds aloft quickly subsiding toward midnight CST. Winds should be lighter Sunday and closer to due south, which should allow for a lake breeze to develop during the afternoon. Given the very short days, the lake breeze probably won't have time to make it to ORD or MDW prior to it dissipating at sunset, but something to keep an eye on.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CST Sunday for the IL and IN nearshore waters.