It's Complicated

If you want to skip the set-up and get straight to the 7-day forecast, scroll down.  But don't tell me you did that.

Yup, complicated.

So complicated, in fact, that some forecasts for south-central Wisconsin are warning of the possibility of "several inches" of snow on Christmas Eve day (Channel 3), some are calling for mostly rain and just a bit of snow (Channel 27 among them), and some are being completely non-committal or taking the very unhelpful "maybe" approach to forecasting (like the National Weather Service and The Weather Channel).

NWS goes so far as to whip out the dreaded "significant uncertainty" crap...

Notice how the NWS completely skips over Wednesday in the graphic above, opting to show the easier-to-predict Tuesday and Thursday instead.  Cowards.

The one thing we all agree on is the exact thing that makes it so complicated: low pressure will approach southern Wisconsin from the west on Tuesday, and sometime Tuesday night or early Wednesday, it will combine with another low to the east, strengthen significantly, and then essentially stop in its tracks to our northeast.

Two factors create the "significant uncertainty":

1. The exact track of the first low.  If it moves directly over southern Wisconsin, as the image above and some other forecasts seem to suggest, we'd experience southerly winds on Tuesday and all precipitation would fall as rain.  If it tracks to our south, over northern Illinois, the low would wrap cold air into southern Wisconsin and Tuesday precipitation would fall as snow.

2. Where the intense "combined" low decides to slow down.  If it slows just east of upper Michigan, it drives bands of strong winds and moderate snow behind it, counter-clockwise, from the north, during the day on Wednesday.  If it moves farther to the east, we still get strong winds but less snow.

So there's one thing we can be confident about: wind.  Lots of wind from the north and northwest on Wednesday.  Add some snow into the mix, and travel will be a bitch.

Rippeology sees it thusly:

A strong southerly flow (which will actually start setting up late today) will prevent the first low from dipping too far south, which puts the low over southern Wisconsin on Tuesday.  That means rain Monday night and Tuesday.  Then I see the lows combining near Lake Huron on Tuesday night, slowing, and kicking back the wind and moderate snow on Wednesday.  Winds remain strong on Thursday, until another low approaches from the southwest on Friday to accelerate the storm to the northeast.

We'll get to that Friday low in the 7-day forecast below.

Of the 6 weather models I studied this morning, this one best represents what I expect on Wednesday:

Where I depart from this surface forecast, however, is that I think the combined low is strong enough to kick snow back further to west than what is indicated here.

This NOAA model shows heavier bands of precipitation on the back side of the low:

I tend to favor that line of thinking.  If you'd like to see the full animated loop of the above, taking us from now until Wednesday morning, click here.

I'm putting a lot of focus on Christmas Eve because of travel implications, but it's not the only area of concern in the next 7-10 days.  As we come out of the mild spell that I predicted back on December 2nd and head into a significantly more active weather pattern, there are multiple weather events that could impact southern Wisconsin between now and the new year.  One is that low pressure system I mentioned above for Friday the 26th; another storm could impact us on Monday the 29th.

Here's the 7-day Rippeology forecast for the New Glarus/Madison area:

Today: Mostly cloudy, light winds, high of 33.

Tonight: Cloudy with areas of fog developing.  South winds will squeeze a little moisture out of the upper atmosphere, resulting in some light freezing drizzle later tonight.  Low 26.

Sunday and Sunday night: Still cloudy, with drizzle (daytime) and freezing drizzle (nighttime) continuing.  A few flurries may mix in.  Winds start to increase from the south and southeast.  High Sunday of 37; low Sunday night of 30.

Monday and Monday night: Yup, more clouds. Rain (mostly light) develops late in the day as that first low approaches from the west, and continues through the night.  Breezy.  High Monday of 37; low Monday night of 32.

Tuesday and Tuesday night: Rain decreases for a time during the day as the low passes over southern Wisconsin.  Winds continue to increase from the south, then switch to the northeast at night.  Rain changes to snow late Tuesday night, with up to an inch by morning.  High Tuesday of 38; low Tuesday night of 30.

Christmas Eve: Very windy with snow.  Accumulations of 2-4 inches.  High temperature of 32 early in the day; temperatures fall to the mid 20s by late afternoon.  Snow tapers Wednesday night but strong winds continue, creating some blowing and drifting, especially on east/west roads.  Low Wednesday night of 22.

Christmas Day: Still breezy, with some peaks of sun as the combined low starts to push well into eastern Canada.  High of 29; low Christmas night of 20.

Friday: Winds switch to the south as another system approaches from the southwest.  Becoming cloudy with snow developing late.  I'll update accumulation predictions as we get closer.  High of 31.

Looking farther ahead: another snowstorm is possible early the week after Christmas, followed by colder temperatures.  New Year's Eve could be pretty chilly!

Questions?  Feel free to leave a comment.  As always, when something changes, I'll update it here on the blog.  After all, it's complicated.  And there is significant uncertainty.  :)