The (Major?) Winter Storm I'm Watching...

For the last few days, weather models have been suggesting a big storm in the middle of the country for late next weekend into early the following week (12/3-12/5).  I want to show you what that looks like in this morning's 06z GFS model run.

I'm going to show you 5 images that represent a period from next Saturday afternoon into Monday afternoon, the 5th.  Click on any map to enlarge.

Take a look at this low pressure in northern Mexico...

It moves east toward the Gulf of Mexico and starts to tap an endless stream of moisture...

Then it starts to head northwest, producing heavy rain and thunderstorms to the east/warm side of the low, and snow to the west/cold side of it.

The exact track of the low determines where the heaviest snow falls.  If this low was 75 miles further to the east, it would place Madison/New Glarus in the path of very heavy, prolonged snow.

After the low passes, it pulls cold air in from the north (remember that winds circulate counter-clockwise around a low), throwing down more snow on the back side of the storm.  It's amazing how this one low pressure system and a trailing cold front can produce precipitation all the way from Canada to the Gulf!

If this storm was happening tomorrow, I'd be predicting two FEET of snow for parts of Wisconsin.  And then, with those two highs to the west and fresh snow cover, I'd be predicting a pretty cold week.

So this will be fun to keep an eye on over the next 7-10 days.  It may not materialize, but conditions should be favorable for this kind of whopper in this time period.  Currently, storms are not tracking far enough east to put southern Wisconsin in the snow zone.  But as cold air continues to push into the middle of the country, that should slowly change.

It's a fun time of year to be a weather geek!

~Scott

Pattern Change Ahead?

If you're ready for some real Wisconsin winter, Mother Nature seems to have heard your prayers.

November has been unusually mild.  Every day of the month has been above normal -- until yesterday, when our 35-degree high broke that momentum.  By the time today and tomorrow are in the books, we'll have had 3 straight days of below normal highs.

The mild weather has been courtesy of a strong Pacific jet that is effectively blocking any Canadian cold from settling in for the long haul.  Also preventing the Canadian cold is the lack of Canadian cold.  Our neighbors to the north have also been above normal all month.

But weather models are starting to agree on a change over the next 7-10 days.  Here are 4 different model looks at the end of next weekend (11/27-11/28).

  

These "500 mb geopotential height" maps are showing the mid level of the atmosphere.  Click on one of the maps and note the numbers in the 500s.  These represent the height of a pressure surface above sea level.  The lower the number, the colder the air mass.  That's because cold air is denser than warm air.  The color blue indicates colder than normal; red indicates warmer than normal.

So, these 4 models are agreeing on a colder air mass by the time Thanksgiving weekend is done.  That means at least a temporary breakdown of the strong Pacific jet that's been keeping us mild.  The average high at the end of November is 38, so colder than normal temperatures mean precipitation will fall as snow.

The lines on those maps are also wavier than they've been in the last few weeks, and that means a stormier pattern.  So it's no surprise that models are also starting to suggest more precipitation events and stronger systems.  In fact, GFS is already showing two storm systems in our area the first week of December -- right after that cold air settles in.

Incidentally, our first shot at measurable snow could be this Friday -- the day after Thanksgiving.  Remember 4 weeks ago, when I predicted our first measurable snow would be this Saturday night?  FOUR WEEKS AGO?  Right now, this looks like 1-2".  But I'm keeping an eye on it.

Stay up to date here and on Facebook, and as always, let me know if you have any questions!