I'm starting to sound like a broken record. A Frost Advisory is in effect for at least part of Western Wisconsin for the third night in a row. Polk, Barron, and Rusk counties are now included in the Advisory, though conditions won't be much different than the past couple nights there.
The NWS can only issue Freeze and Frost products for locations that the Midwest Regional Climate Center (MRCC) declares that the ground can support vegetation. Sometimes these reports are delayed a bit by the time local entities get around to submitting reports to the MRCC.
Tonight's risk for patchy frost is mainly northwest of Eau Claire and does not include Eau Claire county, though still some patchy frost is possible. Just like last night, morning lows are expected to be in the mid 30s. Also like last night, a round of light rain is expected and the temperatures won't drop into the frost possible range until the sky clears late. Tonight's rain chances are along a weak cold front that will pass through between 9pm and midnight.
Clouds will begin to clear after that and temps will drop. Another limiting factor to frost formation especially in areas that won't drop to freezing is that there will still be a light breeze behind the front which will mix the moisture, not allowing it to settle near the surface.
Our large temperature swings from the morning 30s to afternoon 50s and 60s is due to how dry the air remains. Temperatures can change faster when there is less water vapor in the air since water resists temperature change much more than the other elements and compounds that make up our atmosphere.
The dry air continues to cause drought issues especially to our south. While the dry land could cause issues for crops this year if it gets worse, farmers have been able to plant a bit earlier thanks to the dry fields (and farmers are very good at knowing how different crops react to freezing and frost temperatures, though planted crops are insulated from colder temps by the soil).
While the area considered by the Climate Prediction Center to be in the "Abnormally Dry" category, basically in near-drought conditions, shrunk slightly in the past week, the area in drought conditions has increased.
This week's update places almost all of Pepin county and a small sliver of Dunn and Pierce counties are included in that drought category.
Rain chances tonight won't provide much help, but any rain is welcome. A line of scattered light to briefly moderate showers will move through. After that, we'll clear out and cool down as previously mentioned.
Tomorrow will be mostly sunny, though clouds will increase Saturday and Sunday as another low pressure system approaches. This system is still expected to miss us to the south, but its forecast track has trended slightly northeast in the past 24 hours.
This means that there will be a slight chance for rain on Sunday in the Chippewa Valley, but the dry air in place will fight against that chance. Temperatures look like they'll trend a bit warmer by the middle of next week, but rain chances remain few and far between.