Miguel Cervantes
Jedi Master
The NAM 3km takes in to account more factors than the 12km and 32km do, such as air temp at altitude. Here is the argument I just posed on the "book of faces" as one other member he phrased it.
The critical question that needs to be answered is the altitude of the moisture and can the air support the moisture.
Temps during the estimated timing of this event will be in a favorable range at the surface up to perhaps 850 mb, but the way a low pressure system distributes moisture is by taking in moisture from the surface and dispersing it at altitude creating rain, or in this case snow. At 500mb the air temperature will be in the -27 to -31 celsius range. For this argument we will use -30 celsius which translates to -22 degrees Fahrenheit over the region where the 12k NAM is projecting snow.
Moisture evaporates at that temperature and doesn't stand a chance. If the moisture intrudes the area at 700mb height the temps are projected to be -12 celsius or 10 degrees fahrenheit. This is a marginal range to support moisture transfer to the surface. If the moisture comes over us at 850mb the temps will be around -8 celsius or 17 degrees fahrenheit.
The other thing to consider is this is a progressive tropopause that will be deepening with every hour that passes so temps at altitude won't be static and will be getting colder as the day goes on. Further minimizing the chances for moisture transfer to the surface.
IF the moisture is in place before the tropopause deepens over our area then chances are good, but looking at the EURO temp maps at differing altitudes the chances aren't good.
I believe this is what the NAM 3km is recognizing by showing rain at lower latitudes in Georgia where it is possible for the atmosphere to support it. We are at a point now where model agreement between the NAM 3km and the EURO should start coming together over the next 24 hours and will have a much clearer picture of what our potential is.
