Thursday, December 9, 2010

The Weekend Snow...

Before I discuss the weekend, tonight looks rather unimpressive except that a tiny amount of frozen precipitation could fall right about the time when school administrators care making decisions on weather to delay school or go on time.  No call but I wouldn't be surprised to see some school delays on the morning news.

The weekend system remains a difficult forecast.  Model shifts have been 300 to 400 miles over the past 24 to 36 hours.  There is some limited agreement from the big three (NAM/GFS/ECMWF) and time is getting close enough that forecasters must begin make public forecasts.  What appears to be the best areas for significant snow in Wisconsin, Minnesota, and Iowa have issued advisory or watches for winter weather.  That seems prudent.

The solutions which could produce a historic snow system is not completely off the table.  Some model solutions continue to show just that and even the big three shows hints.  Forecasters have identified this system rather unique and models are generally not well equipped to handle it.

We are also missing significant amounts of data.  Taking a moment to understand how the entire model process will help us better understand the data problem.  Weather models need data about current conditions worldwide and use complex math and physics to predict upcoming weather.  The data comes from sorts of places.

Data over land is easy to gather.  Besides an extensive ground network of reporting stations, weather balloons are released twice daily from various locations.  These travel up tens of thousands of feet in the atmosphere recording conditions all the way up.  Additionally commercial air planes record weather data and report it.

However much of the earth is covered in water.  For us, the last real data samples happen in Japan.  Over the Pacific Ocean, there are reporting buoys, ships, and air planes but these are not as dense as the land network.  There are no weather balloons.

This is why you sometimes hear people talk about the storm not being sampled yet.  Until the system comes ashore much less data is available to feed the computer weather models.

The current system is not expected to get sampled well until tomorrow morning.  At that time, expect models to make final corrections.  Even still it may not be 100% set in stone.  While some small shifts are expected, now and then there are surprise storms.  This system probably has one of the highest chances to do something wildly unexpected.

I remember Paul Kocin, formally the winter weather expert at The Weather Channel, telling a story about a major east coast snow storm which paralyzed the big cities up and down the seaboard over Christmas.  Nothing was forecast and everyone including the NWS thought it was going to be a quiet holiday weekend.  Something changed and he awoke and while enjoying his morning coffee he almost went into a panic when looking at current conditions.  A few hours later he was on air warning people about the system which nobody had even considered.  The moral is to never say never.

So what do I think will happen?  As long as the north track holds (and the north track seems to be the dominate group think), then we will see rain mixed with occasional sleet with a change over to all snow.  Sunday will see 2 to 3 inches of backside snow.  It will start after midnight Saturday and taper off by Monday morning.  While that seems pretty insignificant and is not more than we had with last weekends clipper, this will be anything but.

Winds will whip 15 to 25 MPG gusting to 30 to 35 MPH.  Temperatures will single digits by Monday morning.  I would expect a two hour delay for all area schools.  Conditions will be marginal for all of Sunday.

As always, things can and likely will change.  I mean we have more model runs coming right up.  You can see the CPC hazards map still has us right on the edge for heavy snow.

The two lows pressure systems might just phase and cause this storm to bomb out over Indiana.

As you can see, there is a lot of spread on the low tracks between the models.  Each mark shows another model ensemble forecasting a different locations.  Not a lot of consensus when you look. 

Stay tuned for more...

Added 11:49 PM - IndianaWeatherOnline has a great write up on this weekends forecast and the possibility of a secondary low bringing us snow.  Give it a read here.

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