The Usefulness of Spaghetti…Charts

September 26, 2012

The past few days we’ve seen an unusally high level of uncertainty in the long range forecast.  Models are having major difficulties deciding whether strong upper level ridging will develop over us Sunday/Monday and whether a deep and cold trough will drop in over us beyond that next week.  This is the type of forecast nightmare we don’t want to see in the cold season.  This exact same issue would have us worrying about a major arctic blast vs. sunshine and 50 degrees.   There would be lots of unhappiness as models swing wildly from cold to warm over us with chances for snow too. 

However, it’s early October next week so it’s mainly a matter of whether we’ll be sunny and 85 or partly cloudy/showery and 60 for highs.  Not such a big deal this time of year.

One tool that has been very helpful is the “spaghetti chart”.   It displays one parameter from models out in time on a chart.  But instead of just one model, you see the many different versions (ensemble members).  When all those runs are similar, you have a high level of confidence. When they are all over the chart, like that pasta, reliability is low.  This 2nd situation continues this evening. 

Let’s take the 850mb temperature (5,000′ temp) over Portland (in celsius).  This is the 12z ECMWF model run:

Several obvious issues…look how quickly the solutions diverge Sunday and beyond.  By Tuesday, the 2nd of October, the range is anywhere from -1 to +24!  The “operational” (blue) or main run of this model is +10 at that time and headed downhill quickly.  Then look how the operational run with it’s deep cold trough bottoms out as the coldest of all 51 ensemble members next Thursday, the 4th.  That tells me it’s extremely unlikely that run will verify.  This is where these charts come in useful.  It’s generally safer to stick closer to the red line, that’s the average of all the runs.  That is why I didn’t go crazy warm/hot on the 7 Day forecast Monday and Tuesday.  Also didn’t go with such chilly temps next Wednesday.  This ensemble forecasting is where we are headed the next 10 years when we think of forecast improvement.

How about the fresh 00z GFS:

Some more clues.  About half the members are showing a flatter ridge and quicker cooling late Monday-Wednesday.  Even the operational model says 80 might be a little high both Sunday and Monday.  The trough must drop down out of Canada more quickly on those members.  Then, like the ECMWF, the operational GFS is the coldest or near-coldest run next Thursday and beyond.  Luckily, just about all of that is beyond our 7 Day forecast duties.

Chief Meteorologist Mark Nelsen