Saturday, Jan. 19, 2019

Model snow solutions are far apart.

Weather forecast models…you rely on then, you love them and sometimes you hate them.  Today, I’m very confused by them.

A little background:  Models are an attempt to explain/predict the future weather conditions.  The atmosphere is extremely complicated and certainly not fully understood.  Nevertheless, we’ve taken various laws of physics and developed equations to explain the atmosphere’s behavior as best we can.  Then, we have problems with data collection (it’s far from perfect) and fitting the data to a three-dimensional grid.  So then we put the imperfect data into the imperfect physical model, adjust it to the forecast grid, and let the computer do its magic.  In spite of the weaknesses of the “initialization” the results are extremely good – most of the time.

We have have a lot of different models and model schemes.  They all have the same goal – a correct portrayal of the upcoming weather.  The two big workhorses of the National Weather Service (for forecasts of a few days) are the GFS (a global model) and the NAM (North American regional).  We have two very short term models that model 18-21 hours ahead.  These are the RAP and HRRR.  The GFS and NAM run every six hours while the higher resolution models run every hour

Different Physics Packages!

Each model has a different physics package at its core.  Sometimes the results match almost perfectly.  Other times, they don’t.  It’s up to the human forecasters to try to figure out what the best solution is.  Model differences are wide apart today.

Let’s get down to the nitty-gritty.  What we’re most interested in today is snow!  When and how much.  Last evening’s (7 P.M.) computer runs showed a wide divergence from earlier model runs.  All models increased their snow forecasts for tonight – some by several inches. (This much change is quite unusual so close to the weather system’s arrival.)  Since then, they’ve divided into two camps – the GFS/RAP are at the lower end of the spectrum with 1″ – 3″ inch forecasts while the NAM and HRRR are forecasting 4″-8″ overnight.  Quite a difference!

Since last night the GFS and (today) the RAP have been trending down while the NAM and HRRR have been trending upward.  Go figure!

So, what’s going to happen?

Good question.  I don’t know for sure, but I think I’ve got a pretty good idea.  Yesterday’s blog pointed out some problems I had with the NWS forecast.  The rain looks like it’ll end at the lower end of expectations.  The cold air system and the rain system have still not gotten together and it looks like the phasing of the two systems won’t happen until after midnght.  And, the falling temperatures this evening will take several hours – so no concern about the much-feared “flash freeze.”  Temperatures will fall.  Snow will fall.  Roads will become slick.  Just like always.

So, the GFS inspired forecast from yesterday has worked very well.  There are just more things I like about today’s GFS than the other models, so I’m sticking with it.  The GFS has upped its snow forecast from yesterday and seems quite reasonable to me. So…

Snow forecast:

1″ -2″ metro Louisville.  Up to 3″ north of the Ohio River.

Rain should change to snow around 9-10 P.M. and it will be quick – most of the snow should be over by 1-2 A.M.

Note:  This is my “best guess.”  The National Weather Service is at 3″ – 5″ and some of the model forecasts discussed above go even higher. This could turn out to be a snow-lovers dream.  I’m just not buying it, yet.

Think snow!