Tuesday afternoon, January 19, 2016

Little change in the outlook today.  Models have done a flip-flop with the NAM now a little stronger than the GFS.  But, the system drifting toward us remains a weak one, so the biggest question remains, “How weak?”

Did you notice this morning that even though we had a clear, calm night and temperatures were down to about 10 degrees, there was no frost on rooftops or lawns.  That’s a testament to how dry our air is now.  The approaching system is coming from the WNW, so it’s not going be able to pick up pick Gulf moisture.  Luckily, the system does have a good supply of mid-level moisture to produce snow.  Unluckily, a lot of that snow will evaporate before it reaches the ground.  As that snow evaporates, however, it moistens the lower level air.  Eventually, the snow works its way to the ground.  And, that’s the key to tonight’s snow…how much snow will be left to fall after it finally starts reaching the surface?

We’ll probably start falling aloft over Louisville before midnight, but it looks like we won’t start seeing it hit the ground until around 4 A.M.  Snow is likely to continue until about 10 A.M., so that gives us roughly six hours of snow to work with.  Unfortunately, the models show a gradual weakening of the system overnight, so the snow we are going to miss (Mid-4 A.M.) should be more than the snow we will get later (8-10 A.M.).

One more thing to consider…that cold air.  That really changes the so-called snow ratios.  Under normal conditions (temperatures near 32 at surface and slightly warmer air aloft) the usual snow ratio is 10 to 1.  (One inch of water content will produce about a 10″ snowfall.)  Very wet snows can have ratios as low as 7 or 8 to 1 while very dry, fluffy snows can reach ratios as high as 20 to 1 (or higher).  This snow will have a high ratio.  So, a little moisture can go a long way.

So, for the next 24 hours, here’s what I expect…clouds roll in this evening with virga (that’s the snow aloft) showing up overhead (as seen on radar) before Midnight.  Snow starts around 4 A.M. and diminishes by 10-11 A.M.  Total snow accumulation: 2″- 3″ for the Louisville area.  Snow accumulations will gradually increase as you go south and west of Louisville.  The western third of the state should see 4″ – 6″ of snow.

Road conditions should be very messy (snow and ice) for morning rush hour but should return to near normal for the evening rush hour.

What about that late-week storm?
Good question, if I must say so myself.  All signs are still pointing toward a major winter weather event around the lower Ohio Valley Thursday night and Friday.  Models keep making adjustments to this storm as it begins to move over the northwestern states.  It’s strong enough to create havoc from coast to coast over the next 3-4 days.  But exactly what it’s going to bring us is still a question.  But, the most recent model runs still put us in line for some major snow.  The GFS has now upped it’s forecast to 8.2″.  But as close as 100 miles south of us, the forecast is for mostly rain.  And, trying to pinpoint something like that 2 to 3 days in advance is very difficult.  It has been my observation over the years that the GFS tends to be too far south initially with these systems and gradually brings them farther north with time.  As small a shift as 100 miles in the storm track over the next 48-72 hours could change a major snow into a cold winter rain.  It’s just too early to tell.  Stay tuned.