This is a common issue when a major day or holiday approaches. The desire to provide specifics far in advance. 10-day forecasts for trick-or-treating weather on halloween. Guarantees of a white Christmas, etc. Unfortunately, the existence of a holiday does not magically clear up the meteorological crystal ball or alter the fundamental chaotic nature of the atmosphere.
I saw one forecast this weekend for a brutally cold Christmas day with a high of 18ºF. Presumably this was based on forecasts like the one below from yesterday morning's GFS which did indeed have some exceptionally cold air over northern Utah with 700-mb temperatures below -20ºC.
However, last night's GFS paints a totally different picture with a ridge over the west and 700-mb temperatures of -4ºC over northern Utah. For the ski areas, that's a swing in temperature of about 17ºC (30ºF)!
Cold air outbreaks on a continental scale can often be anticipated may days in advance, but the specifics for one particular area are more difficult to nail down. Northern Utah is difficult because we tend to be on the western edge of the cold air. The shift in the model forecasts above reflects the uncertainty in the forecast more than a model trend. It doesn't mean we're going to miss the cold air (or be warm). It simply means that we have a range of possibilities due to the chaotic nature of the flows that generate these cold-air outbreaks.
We don't produce a 700-mb temperature thumbnail plot from the GEFS, but below is one for the "1000-500-mb thickness" (dashed lines, some in red), which is proportional to temperature in the lower atmosphere. These are valid for 11 PM Christmas Eve, when Santa will be delivering presents to your home. Some of these runs would be quite cold (e.g., lower left panel and 2nd panel from right in center row), but others would be more seasonable or perhaps even warm (e.g., first and second panels from left in center row).
|Source: Penn State E-wall|