Wednesday, August 31, 2011

Relief in Sight, with Complications

Sick of the heat?  So am I, but relief is in sight, although there are some complications.

It has been almost 2 weeks since we had a below average minimum temperature, and event longer since we had a below average maximum temperature at the Salt Lake Airport.

Source: NWS
Further, this is the longest continuous stretch of days with a maximum temperature above 90F this year.  We all need a break.

And, as luck would have it, it is on our doorstep.  This morning a surface trough is centered near the Utah-Nevada-Idaho triple point, with cool, Pacific air somewhat upstream over southwest Idaho and Oregon.  700-mb temperatures over Salt Lake City this morning are >14C, but drop to <0C over northwest Washington.  Aahhhh.

Today we'll be in the warm southwesterly flow ahead of the trough, but cooler air is coming.  The question is how cool.  There are large differences in the forecasts produced by the models over the next couple of days.

Let's begin with the 0600 UTC NAM.  It takes its time bringing the cold air in, but when it does, it brings it in in earnest, dropping the 700-mb temperature to 3C by Friday morning.

NAM 700-mb wind and temperature forecast from 0600 UTC 31 Aug –
1200 UTC 2 Sep
The 0600 UTC GFS has a different idea.  It brings the cold air to our doorstep, but the cold air stalls, and warmer air moves in quickly by Friday morning.  In fact, the 700 mb temperature on Friday morning is 11C, 8C higher than the NAM!  That is a HUGE difference, especially if you are planning on camping out at Albion Basin.

GFS 700-mb wind and temperature forecast from 0600 UTC 31 Aug –
1200 UTC 2 Sep
Model sensitivity at such short forecast lead times rarely gets so large.   The two models essentially handle the large-scale pattern differently, with the NAM producing a compacting, amplifying 500-mb trough and the GFS a wimpier 500-mb trough that lifts into Wyoming .

NAM 500-mb height, absolute vorticity (color fill), and vertical velocity
forecast valid 0000 UTC 2 September.
GFS 500-mb height, absolute vorticity (color fill), and vertical velocity
forecast valid 0000 UTC 2 September.
I typically lean toward the GFS in situations like this, primarily because it is a global model and usually conditions over the upstream Pacific Ocean, which is not fully covered by the NAM domain, are often critical for forecasting the structure and amplitude of upper-level troughs over the western United States.  Perhaps one of our readers can dig into the cause of this huge model spread and provide a more detailed analysis.  


  1. Looking back at the 06z runs of the GFS and the NAM, it seems that the NAM brings a slightly more compact 500mb low into British Columbia. This then appears produce a more pronounced shortwave ridge at that level over the WA/OR, leading to a sharper downstream trough over us.

    Looking at the 12z data, these two models are moving towards agreement, with a nudge to the GFS. If this bears out, it might be owing to what you said regarding sampling over the Pacific.

  2. ... on a less objective note, go NAM!

  3. In some cases, I have noticed that such a large difference between the models at one particular height level (e.g. 700 mb) can suggest a sharp discontinuity in the vertical profile, which actually might only vary slightly in height from one model to another. In this case, though, the SLC sounding profile shows that the disagreement is even more pronounced near the surface (9-10C difference between models)with the NAM forecasting a much cooler post-frontal air mass extending up to near 700 mb, and the GFS showing little if any cold frontal passage by Friday morning.