Deconstructing Decima; Assessing the Week Ahead

Like every other science, the study of weather is a continual learning experience.  When I see professional meteorologists use a term or idea that I don't know, I study it.  For the last month, I've been geeking out on Atlantic/Pacific oscillation patterns and how they impact the lower 48.  The learning part is a lot of fun for me.

I also try to learn by evaluating my own forecasts.  For instance, with this latest storm, I've learned that many Rippeology followers don't have the appetite for a 13-minute video.  If I do video again, I'll cut that down by two-thirds.  Promise.

But I just watched that video again and took inventory of what I got right and what I got wrong.  Here's that:

Got right:

• Issuance of Winter Storm Warning to cover southernmost counties on Thursday;

• Dusting of snow Friday morning;

• Snow totals of 7-11”;

• Highest snow totals (lake enhancement) in the east and southeast;

• Frozen precipitation in northern Illinois;

• Extreme cold behind the storm

Here are the preliminary snow totals from the National Weather Service out of Sullivan/Milwaukee, which shows the concentration of 8-12" (in orange) in the eastern part of the state.

 

 Got wrong:

• Timing/intensity: didn’t expect the 9-hour lull between rounds one and two, and thought heaviest snow would be Friday night;

• Leaning on Kuchera conversion ratio from liquid to snow (10:1 method averaged out much better);

• Low track – slightly farther south/east than anticipated

So that's a learning experience for me.  Thanks to all of you for being patient (and interested) as I continue to study.

That's the look-back.  Now, let's look ahead at what the coming week might bring...especially as we get closer to weekend holiday travel.

First, you'll be happy to know that today's sub-zero high temperatures are the coldest we should see for the rest of December.  As I mentioned in Wednesday's video, we're going to get a bit of a warm-up as the week wears on.  The upper 20s and low 30s (maybe mid 30s by Friday) will feel downright balmy.  Shorts weather!

There are a couple shots at precipitation in the next 7-8 days.  The first is Wednesday.  A relatively weak Alberta Clipper is forecast to move through -- mostly impacting the northern part of Wisconsin.  This could be a 2-3" snow event, if it materializes at all.  If you're traveling north Wednesday, I'll help you keep an eye on that.

The second potential system will be Saturday or Sunday.  Both GFS and ECMWF set up a boundary in our area between that milder air and some colder air pushing in from the west/northwest.

GFS has this boundary at 6am Sunday...

While the Euro sets it up 12-18 hours later...

Generally, a transitioning of air masses results in some precipitation in between.  But it's too soon to tell if this sets up in the Great Lakes region or a little farther south.  I'll watch the Saturday through Monday models throughout the week and keep you posted.

If you're getting on an airplane Thursday, Friday or Saturday, I don't see anything major in the lower 48 that would cause widespread delays or cancellations.  Of course, it's still very early.  Sunday/Monday bears watching.

If you have specific travel plans that you want to discuss, just let me know!  Otherwise, I'll update this post throughout the week as models begin to show more clarity.  In the meantime, stay warm!

~Scott

Rippeology Video Update

HUGE apologies that this inaugural video timed out at 13 minutes.  I'll work to get that down to no more than 5 minutes.

But I'm very interested in your opinion -- video good, or stick with text posts?

Thanks, as always, for your support.

Standing My Ground

I'm just going to go ahead and assume that most of you look at multiple forecasts when you're interested in the weather -- rather than just relying on Rippeology.  If you're doing that, you're seeing some forecasts "back off" yesterday's projected totals a bit, putting southern Wisconsin more in the 5-8 inch range for this weekend's storm.

Here are the National Weather Service snow maps from yesterday and today.  Note the retreat in snow totals.

Yesterday, I predicted 8-11 inches.  I'm sticking with that forecast today for 5 reasons.

First, nothing about this storm has changed in the last 24 hours.  Timing, temperature, intensity and duration are all nearly identical.  The only thing that has changed slightly is the location of the low pressure development (see the last point below), and that actually works in favor of my slightly higher totals.

Second, there are two pieces of energy in play here, and at some point, they will combine into one.  Bernie Rayno from Accuweather did an excellent job of explaining this in his video yesterday.  When and where those two pieces of energy combine makes a huge difference.  As I draw it out on paper, I have them combining somewhere near Waterloo, Iowa (my birthplace!), which bodes well for higher snow totals in southern Wisconsin.  Because...

Third, as the low strengthens and tracks through northern Illinois Sunday, it seems inevitable to me that the low will shoot heavier snow bands to its northeast -- right in the direction of southern Wisconsin.  With southeast winds and strengthening energy to the south/southwest, it just makes sense that some bands in our area will briefly produce snow rates at 1-2" per hour.  That alone can pad snow totals in a short period of time. 

Fourth, this is a 30 to 36-hour storm.  Pretend for a moment that the average snow rate is only a quarter of an inch per hour.  That's 8-9 inches.  Granted, we'll see some periods where the snow rate is .10 to .25 inches an hour...but all it takes is a couple of hours in heavier bands (as discussed above) to rack up the snow totals.

Fifth, this morning's models have the low developing farther south than prior runs, giving it the ability to tap some gulf moisture before heading in our direction.  GFS has the low moving out of the Texas panhandle and into Oklahoma by Noon Sunday.  This, too, suggests the possibility of some heavier bands.

Everybody interprets model data differently, and the dynamics of this storm are only now really starting to set up.  Truth will reveal itself at some point when the snow finally stops while we're all sleeping Sunday night.

~Scott

Caly: An Imperfect Storm (Updated)

Yes, of course the Weather Channel has given it a name.  Because Weather Channel.

Winter storm "Caly" is churning up in the west and will impact Wisconsin's weather for a prolonged period of time this weekend -- probably for about 36 hours, from 6pm Saturday to 6am Monday.

When it's all said and done, most of south-central Wisconsin will have 8-11 inches of snow to deal with.  Isolated areas in southwestern Wisconsin could be more in the 11-14 inch range.

Let's get to the pretty maps, and then we'll discuss some forecast details.

Here's how GFS sees snow totals through Monday morning.  Remember, you can click on any picture to enlarge.

And here's the wider view.  Totals in maroon are 10-12".

The National Weather Service agrees with those totals:

Because most models are in good agreement with this prolonged storm, NWS gives us a better than 50/50 chance of 8 or more inches before it all ends Monday morning.

Caly is actually two phases of one storm that will hit our area without much pause in between the two.  Here's how it sets up:

Saturday evening: light snow starts to overspread the area from west to east, and steadily increases in intensity throughout the night.  Look for about 3-5" by sunrise Sunday.  This is the flare-out snow area before the actual low approaches Wisconsin.

Sunday: the main low pressure begins a track through Iowa and northern Illinois, keeping us in moderate snow much of the day.  This adds another 4-6" by dinnertime on Sunday.  A good stiff breeze will cause blowing and drifting with this drier, fluffier snow, which is sure to piss you off as you attempt to clean off your driveway.

Here's how GFS sees low position at 6:00pm Sunday.

Sunday night: the low pulls off to the east, but light snow remains on the backside (probably an additional 1-2") -- as does the wind.  Whatever snow you cleared during the day on Sunday may relocate itself back to its original spot.  Not my fault.

Madison's average December snowfall is around 13 inches.  This storm, combined with last weekend's official 5.7", will put us over our average snowfall for December with only a third of the month in the books.  Gee, do you know any amateur forecasters that predicted above average snow this winter?  Like, on Labor Day?

So why does the headline call Caly an "imperfect storm"?  Mostly, because of its lack of organization.  This is not a classic Wisconsin winter storm that races through in 12 hours and dumps heavy snow.  Nope, it's gonna just hang around for a day and a half and annoy you like the guest that won't leave.

It's also likely that the prolonged nature of the storm is what has so far prevented the National Weather Service from issuing a Winter Storm Watch for our area.  It would be unusual to issue a 36-hour Watch.  But they'll have to issue something.

UPDATE: At 11:00am, the National Weather Service finally issued a Winter Storm Watch for south-central and southeast Wisconsin.  Area in blue is the Watch; area in purple is a very non-committal Winter Weather Advisory:

Here's the text of the Watch statement:

Note that this watch may be extended past Sunday evening, depending on backside snow rate and blowing/drifting.

(END UPDATE)

Some schools will likely be on 2-hour delay Monday morning.  Some will close.  It's too soon to pinpoint those cancellations.

Looking ahead, the fresh snow cover, along with a significant Arctic blast, will make it very cold by the middle of next week.  Wednesday and Thursday will make you curse your decision to live in Wisconsin.  Look for highs both days to hover between 0 and 10 degrees, with wind chills in the -25 neighborhood.  Check out these temps:

Looking farther ahead, models are suggesting yet another storm next weekend.  But let's get through this one first.

Over the next few days, I'll be prepping a Christmas travel forecast in diary-style, updating it frequently as we get closer to that holiday weekend.

What questions can I answer for you?

~Scott

Sunday Snow, and What Happens Next

Ten days ago, I posted a blog about the possibility of a winter storm late this weekend.  While it won't have nearly the intensity that models were suggesting back then, we are definitely getting snow tomorrow.

Here's how this morning's GFS model sees accumulations in Wisconsin.  The last 4 runs of this model have increased snow totals, but have kept the bulls eye in southwestern Wisconsin.

And here's how the National Weather Service sees it:

Judging from the storm's current intensity, I favor these 3-4" totals.  Some spots may hit 5".  My guess is that the National Weather Service will issue a Winter Weather Advisory for our area at or before the 3:30pm forecast later today.

UPDATE (2:10pm):  As expected, the National Weather Service has issued a Winter Weather Advisory for south-central and southeastern Wisconsin, in effect from 3:00am to 6:00pm Sunday.  The official advisory calls for 3-4" of snow.  Here's the statement from NWS (click to enlarge):

So, our first measurable snow will come exactly one week after the 11/26-27 prediction I made all the way back on October 24th.  Not bad.

Snow will start early in the morning, probably before sunrise, while temperatures are still in the mid to upper 20s.  That will allow for accumulation on all surfaces.  So if you're headed to Green Bay for the game, be aware that road conditions will make bad traffic even worse.

So tomorrow's snow will make it look like winter...but it'll take a couple more days to really feel like winter.  Here's the cold front that goes through on Tuesday:

That cold front will do two things: (1) pull in the season's first real rush of arctic air, and (2) prevent that other low (the one in the above map centered over Tennessee) from impacting our weather all that much -- or at least that's how it's looking at the moment.  That's the low we had all been watching the last few days, and the one Accuweather meteorologist Bernie Rayno recorded a video about yesterday.

Let's talk about that cold first.

By Wednesday morning, that arctic air spills into the west.  This map shows temperature anomaly (departure from average) in degrees Celsius for Wednesday morning.

As that cold air continues to settle in and push eastward, our temperatures here in southern Wisconsin will drop into the 20s for daytime highs, and the lower teens for nighttime lows.

Brrr.

Models see a second major cold push around December 14th (give or take):

If that materializes, we'd see our first sub-zero temperatures of the season -- perhaps well below zero.

As for additional snow beyond tomorrow, well, much depends on which models and patterns you believe.  ECMWF still seems to favor some snow on Thursday, so we'll keep an eye on that.  There is another shot at snow next weekend.  As long as that cold air stays in place, chances for snow will continue at a pace of 2-3 storms per week.

Drive safe tomorrow, and as always, let me know if you have any questions!

~Scott