NCEP "Spaghetti" map

[Andy Wehrle]

Originally posted by Mike Johnson
I have to qualify the following by noting it is very long range,
but maybe there is hope for the rather slow start to the season!

Models are showing a split flow pattern setting up after this
weekend, NCEP, for example:

http://www.cdc.noaa.gov/map/images/ens/spa...ag_f192_nh.html

Things become really interesting after May 6th. -- although it is a long way out:

http://www.cdc.noaa.gov/map/images/ens/spa...ag_f264_nh.html

Oh well, it's something to think about!

Mike

As a newbie forecaster...I must ask, how to make sense of all this "spaghetti"? :?

 

[William Scheftic]

Well the image on the left in the links above show the mean 500mb geopotential height pattern that is the average of all members of the NCEP GFS ensemble package. Basically the NCEP runs several perturbations (slight changes to Initial Conditions [I.C.]) of the GFS, which produces forecasts from each run. The image on the right shows all the member's forecast for 5780 m (red) and 5580 m (light blue). If the members are agreeing on a solution then the lines will be close together, representing a continuity of solutions between the members, therefore more dependability in the operation GFS forecast. However, if the ensembles are all over the place then the forecast would be difficult to ascertain.



Here at 264 hours (11 days) the control run of the GFS is showing a split flow upper air pattern with a strong closed low between the split. While it is certainly difficult at 264 hours to truely ascertain the possibility of the operational GFS playing out, I think there is some good signs that point towards a split flow pattern. None of the ensemble members place a trough in the 5580 m contour over the CO/WY area. also none present a very amplified ridge pattern along the same location in the 5760 m pattern. Now the green lines which represent the climatological 500mb pattern at this time of year, certainly indicate the same basic split flow that this GFS's ensemble run have produced in the 11 day range. When the mean ensemble run does not deviate much from climatology then there is not much predictability in the run. The fact is, that out to 264 hours you have just as good of a chance of the 500mb climatological pattern playing out as the operational GFS. Typically the ensemble members are much more useful in the 5-7 day range as a forecasting tool.