Monday, December 18, 2017

Predictability Limits Don't Change Because of the Holidays

Christmas is still a full week away, but that isn't preventing people from making specific forecasts for the big day. 

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
Thus, endorsing a specific, individual forecast makes little sense at this stage.  On the other hand, it is important for meteorologists to discuss the range of possibilities for big holidays when travel volumes are likely to be high. In that regard, mentioning that there could be a cold surge into the area seems appropriate, if it is also noted that it is still unclear if that surge will push into the Great Basin or remain east of the divide and over the northern Plains. 


  1. Planning a fatbike ride at North Fork Park this afternoon. Im curious if the prevailing SSW wind today will result in lake effect snow on the Lee side of Ben Lomond and Willard. Sorry to hijack this post, but I searched your archives and could not find much on lake effect in Southwest winds. I read the "pound for pound" post with great interest, but what more do you need besides SWerly flow for snow up there?

  2. Just ordered your book - RTFM, right? :)

    1. ha ha. Not much lake effect up there on the Ben Lomond Massif...