A deep low pressure system was predicted to develop in the Gulf of Alaska,
affecting the US and Canadian west coast around 99020612. This cyclone
was apparently associated with a very high degree of predictability: The
NCEP ensemble indicated its development first at 11.5
days
lead time, and the ensemble mean mean sea level pressure forecast did not
change much after the initial time of 99012700 (10.5
days lead time), with the deepest predicted closed isobar of the low
somewhere between 972 and 964 mb at 9.5
days, 8.5 days, and
shorter lead times. Note the predominantly blue and even light blue colors
on all of these forecast maps, indicating that the ncep ensemble spread,
nomalized here by the average ensemble spread over the preceding 30-day
period, was much lower than usual at these forecast ranges over a large
area including and surrounding the low pressure system, suggesting
an especially high degree of predictability. Despite the extreme nature
of this event, all these forecasts, as expected, verified very well against
the verifying analysis that
had a central pressure of 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 times.
Note also that the ecmwf ensemble mean forecast at 10
days and shorter lead times (9
days, 8
days, 7
days, etc) were very similar to the ncep ensemble mean. It is
also interesting to note that, for example, in the ensemble forecasts started
on 99012900 (NCEP) and 99012812 (ECMWF) there is more difference between
the two ensemble means at very short
lead time (12 and 24 hours respectively, see area of low pressure wave
approaching the west coast of the US, marked by green and yellow colors
as highly uncertain, with a maximum difference in the ensemble means of
7 mb) than at a much
longer
lead time (204 and 216 hours, respectively, see the less
than 4 mb difference near the center of the predicted large-scale low pressure
system, marked as highly predictable by the light blue colors). The high
degree of predictability of the low pressure system is also highlighted
by the fact that 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
the actual, not the normalized ensemble spread in this figure) -
the same range of uncertainty that unpredictable systems are associated
with at
initial time - like the 2-3 mb spread
over the low pressure wave referenced above, approaching the west coast
in the same forecast at initial time . Though the large-scale low pressure
system in the Gulf of Alaska may be a weather event with exceptionally
high predictability, verification statistics indicate that the ensemble
can correctly identify 10-15% of all forecast cases as highly predictable.
In these cases the 12 days lead time forecasts are as reliable as the
"least predictable" 10-15% of cases at day 1. This is the power of
the ensemble: it lets us know in advance how much one can trust the forecasts.
On the 27th of January 1999, one could have made a 10.5 days lead time
forecast with the same (or higher) degree of confidence as sometimes 1-day
forecasts are associated with.