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 (9.5 days, 8.5
days, etc). 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), with 10.5
days or shorter lead time 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.