Storm Watch: Thursday Night - Friday

First of all:  sorry about not posting for this latest storm.  I was out of town with no Internet connection.  This was an interesting storm and at some point on Monday night I think I saw every possible type of winter precipitation possible:  snow, sleet, graupel and freezing rain.

Before we get into the possible storm Thursday night into Friday, I thought it might be good to talk about the different types of precipitation and how they form.

Snow - this is the easy one.  Temperatures in the clouds are cold enough to produce ice crystals that we recognize as snow.  As it falls from the clouds, the atmosphere is sufficiently cold all the way from cloud base to the ground (or almost the ground) for the ice crystals to remain intact.

Variants:

- "wet" snow which we sometimes see when the air between the cloud and the ground is hovering around freezing.  Some melting takes place which makes the snow "sticky".  You know this because snow with a higher water content makes better snowballs than dry snow which we get when it's very cold.  The snow flakes collide with each other and stick together making for the streaky, sometimes large flakes we see during "wet" snow.

- "graupel" which is a phenomenon that almost always happens when the precipitation type is transitioning (e.g. from snow to sleet) and is caused by snowflakes hitting droplets on the way down that are colder than 32 degrees but remains in a liquid state.  This is called supercooling.  The snowflake develops a coating of ice which continues accumulating (accreting) around the ice crystal until it resembles a small ball rather than a snowflake.  This is not the same as sleet.

Sleet - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet.  At some point in its travel toward the ground the air cools again for enough distance to refreeze the droplet of water into an ice ball which we recognize as sleet.

Freezing Rain - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet.  The temperature near the surface is at or below freezing, however (meteorologists refer to this as the cold air being "shallow").  Therefore the liquid water freezes on contact with anything on the surface that is below freezing especially cars and trees.  Over time, the ice accumulates (we say "accretes") and produces that glazed look like the one we saw this morning.


Ok - class time is over!  On to the possible Thursday storm.

As has become the norm this year, the models disagree on the track - but moreso on the intensity - of this storm.  We'll look at the NAM and the GFS for this discussion.

Oh, and a quick note: you can click on any of the images in the blog and get see the full-sized version.  This can help if you're having trouble seeing details in the smaller maps.

We'll start about 54 hours from now:  Thursday morning.

Here's the NAM forecast for that time:

You can see our storm in the mid-Mississippi Valley.  The NAM already has a strong low - especially when compared to the GFS at the same time which forecasts a weaker and more "elongated" area of precipitation:

Later that night (12 hours later) the NAM shows the storm really getting its act together.  Precipitation extends from the Northeast into the southern states - although not quite in our area yet.  You can see a storm center forming as the precipitation is forecast to be heavier in Kentucky and Tennessee at this point.

The GFS is not quite as excited about the storm although it does "close off" the low and shows its position in southwest Virginia at this forecast time.  You can tell the intensity isn't as high mostly because the precipitation field is less impressive - especially near the center.  However, unlike the NAM, the GFS has light precipitation breaking out over our area by this time. 

Moving ahead 6 hours, the NAM has a pretty nice system starting its turn up the coast.  We are in a region of moderate precipitation and this would argue for a few inches of snow overnight Thursday into Friday morning.

The GFS has a similar solution for this time frame - the storm is wrapping itself up over the Atlantic - but the overall precipitation field is somewhat lighter especially from our area northeastward into Philadelphia.

So - the timing of this storm between the two models is fairly close.  I think we can expect any snow that does fall to happen sometime after sunset on Thursday night.  The models would argue more towards midnight.

We'll need another few models runs before talking seriously about possible accumulations and snow totals.  For now, I'd say the outlook is favorable for snow on Thursday night into Friday morning and there's a 50/50 chance that the snow would be "significant" - i.e. 2 inches or more.

This isn't a big storm for us by any stretch - no Snowpocalypse this time.  But it could be a nice, quick snowfall of a few inches if things come together over the next day or two.

Stay tuned...