Noon SAT 2/19/11
I don’t say this lightly… this is probably the most difficult weather forecast we have made in a long time.
A storm system now over Texas will head northeastward toward the Appalachians by tomorrow. On the northwest side of it, colder air will wrap into the storm bringing a changeover from rain to snow on the northern extent of the precipitation shield.
Even less than 24 hours from this storm’s impact on the viewing area, many questions remain.
To illustrate, I want to give you some figures for this storm from a couple models. Usually within a couple days of a weather event you have great continuity between models. However, this storm has not exhibited this continuity. If anything, there is just as much spread in the model data as there was yesterday. Hard to believe, but it’s true.
For example… take Louisville first. NAM gives downtown a dusting and the Snyder 2-3″. Meanwhile the GFS would argue for a few flurries and no accumulation county-wide.
Another example… E’Town. The NAM gives E’Town a whopping 6 to 8″ of snow, while the GFS has only a dusting at best.
To repeat… a 6 to 8″ difference from two models within a 24-hour window of the storm arrival!
There are a couple reasons for the differences. First the NAM has a stronger storm with colder air and a precipitation shield that extends farther northward. The GFS on the other hand has warmer low layers, a dry layer that inhibits precipitation from reaching the ground, and thus little precipitation.
Both models have had fair run-to-run continuity, meaning each update of the individual model is similar to its predecessor.
But something has to give!
There are some other models out there worth noting. The ECMWF from overnight (the new run is not in yet), which has performed okay during this storm… is pretty much spot-on with our forecast from yesterday of up to 1″ in the metro, with 2-4″ south of the Parkways.
Another of our exclusive models is just now coming in, and has the precipitation shield farther south — but it is only updated to midnight tonight, which makes it difficult to make any assumptions yet.
Here’s the bottom line, until we see some clear-cut agreement among the models, I am inclined to make no changes to our forecast… particularly considering the agreement we have from the usually reliable ECMWF model. However, I have some concern that the GFS may in fact be onto a weaker storm… and if other models come in and agree with this assessment, we may have to revise storm snow totals downward.
Of course, if the NAM model is correct, adjustments upward would be made in the forecast.
One solid fact remains: Southern Indiana will really not be impacted by this weather event.
More updates as I get them here and on Facebook with a final updated forecast tonight at 6 on WLKY.