A deep low pressure system is predicted to develop in the Gulf of Alaska that will affect the US and Canadian west coast around 99020612. This cyclone apparently is associated with very high predictability: The NCEP ensemble indicated its development first at 11.5 days   lead time, and the ensemble mean mean sea level pressure forecast has not changed much since the initial time of 99012700 (10.5 days lead time), with a closed low predicted first with 984, then with 968 mb.  The probabilistic quantitative precipitation forecasts gave a high probability of heavy precipitation on the west coast with 10 days and shorter (9 days, 8 days, etc) lead time. Note that the NCEP ensemble spread, normalized by the average forecast spread over the previous 30-day period (see on ensemble mean charts), is well below average not only over the predicted low pressure system but over an extended area that covers much of the US and parts of the Atlantic, suggesting  an especially high degree of predictability. For example, the estimated uncertainty associated with the center of the low at 8.5 days lead time is on the order of 2-3 mb (see light blue colors indicating actual ensemble spread in this figure) - that is the range typical of systems with large uncertainty at initial time    (see, for example, the spread associated with  the low pressure wave approaching the west coast in the same forecast at initial time).  This may be an extremely predictable system; however, verification statistics indicate that the ensemble correctly identifies 10-15% of all forecast cases as most predictable - and those cases at 12 days lead time verify as well as the "least predictable" 10-15% of cases at day 1. This is the power of the ensemble: it lets you know in advance how much we can trust the forecasts. On the 27th of January, one could have made a 10.5 days lead time forecast with the same (or higher) confidence as often 1-day forecasts are associated with.
 
 
 
 

A deep low pressure system is predicted to develop in the Gulf of Alaska that will affect the US and Canadian west coast around 99020612. This cyclone apparently is associated with very high predictability: The NCEP ensemble indicated its development first at 11.5 days   lead time, and the mean sea level pressure forecast has not changed much since the initial time of 99012700 (10.5 days lead time), with a closed low predicted first with 984, then with 968 mb.  Note that the NCEP ensemble spread, normalized by the average forecast spread over the previous 30-day period, is well below average not only over the predicted low pressure system but over an extended area that covers much of the US and parts of the Atlantic, suggesting  an especially high degree of predictability. Every day since 990128, the ECMWF ensemble mean forecast is almost identical over the predicted low pressure system with the NCEP ensemble mean: see forecasts with 9.5 (10 for ECMWF), and 8.5 days lead times.This is clearly a forecast feature that is well predictable at 10.5 days or shorter time scales. For example, the  99012900 NCEP, and the 99012812 ECMWF ensemble means (8.5 days lead time) differ not  more than 4 mb near the center of the low pressure system at 204 nhours lead time! Compare this with a difference of 6 mb or more at zero hour lead time (difference between 12-hour ecmwrf ensemble mean and NCEP control analysis) over  a short scale wave approaching the US west coast.  The large difference there may be partly due to the fact that the NCEP analyses have been enhanced by dropsonde observations from two reconnaissance flights taken around 99012800 and 99012900, while they were not used in the ECMWF analysis.