Saturday, January 12, 2008

Weather update: The big one or big rain?

Well, the beginning of the new pattern we talked about earlier this week is just a few days away and it might kick off with a bang.

Most model runs show a major Gulf of Mexico low forming early next week and moving northeast by midweek.

Let's look at how the latest (18z) run of the GFS depicts this storm:

Here is the wee hours Thursday morning. This map depicts surface temps (usually, the maps I show you refer to upper air temps. As you can see, the 32 line is to our south as precip moves in. Notice the high pressure to our north east -- recently these areas of high pressure have been oriented in a west/east manner. This one is more north-south which is better for cold-air damming to set up. To wit, you can see the little wind barb (if you zoom in) over the Charlotte region is showing a north/northeast wind.

Now for Thursday after dawn. Still at or below freezing at the surface (and in upper levels) as a 2-4 inches of snow falls.

Here's early Thursday afternoon. This is the upper air map which shows us JUST above freezing -- also borderline at the surface. BUT, you can see the heavy precip that is falling. If this were all snow, it would be an additional 3-6 inches, maybe more. Or it could be a little more snow, then some freezing rain. Or it could change to all rain.

IF it were all snow ... IF ... we'd be talking about 6-10 inches. BIG IF!

By Thursday night, the storm has pulled away.

Couple of things -- before this storm moves in, it will encounter very dry air, which will lead to what is called evaporational cooling. Basically, this means that as precip falls through dry air, the air is cooled. The "wet bulb" temperature is essentially the temperature to which the atmosphere will fall if saturated. So, if you had a temperature of 40F but because the air was so dry, the dewpoint was 10F. The wet bulb would be around 30F -- that means that the temperature would drop 10F once it started precipitating. Here is a web bulb calculator.

Computer models tend to underplay two things -- CAD (cold-air damming) and evaporational cooling. So, this is one time the models may trend MORE in our favor on temps.

And that is the ingredient which will be key here -- there is strong model agreement that there will be a significant low pressure system -- the question on temps will be the close call. The last few runs of the GFS have set up a rain-snow line somewhere along I-85.

Bottom line: This is BY FAR the best looking system since 2006 or earlier. Plus, there continue to be more chances downstream in the 12-day range and beyond.

Here's a breakdown on the Thursday event by percentages:

No precipation to speak of: 10 percent
All rain: 50 percent
Marginal winter event (dusting of snow/slight ice coating) 18 percent
Minor winter event (1-2 inches of snow/minor ice coating) 12 percent
Moderate winter event (3-5 inches of snow/moderate icing) 7 percent
Major winter event (6 or inches of snow/major icing) 3 percent

So, that's a 22 out of 100 chance of accumulating snow, which is pretty darned high for 5 days out.

Of course 78 percent chance of nothing or close to nothing.

Still, half (or more) of the fun is in the tracking!

Stay tuned!


Big Mama Vikki said...

Hey Skip, I am supposed to work on Thursday and have to be in at 11:30am, so if everything goes as you predict, will it already be laying on the roads by then?

Anonymous said...

hey i hope this theory is right,i am off thursday!!!!!got a new 4 wheeler for christmas--come on snow!!

Skip Foster said...

Vikki -- wow, wish I could pin timing like that down this far in advance, but that would just be a blind guess -- I'm not even sure it's going to snow at all, much less exactly what time. Still, at SOME point in the next few days, we'll have a good idea -- stay tuned!!!!!