Over the northeastern Pacific, much of January and February of 2002 was dominated by anomalously low/high pressure areas north/south of 55 N. A 24-hr forecast initiated on the 17th shows a flow pattern typical of January-February 2002. At the beginning of March, however, this pattern changed by the formation of block, see, e. g., a 24-hr forecast valid on 4 March 2002. Note the reversal of the conditions valid on 17 February in terms of an establishement of lhigh/low pressure areas over the northern/southern latitudes of the northeast Pacific; and the replacement of a low pressure area and southwesterly flow by a high pressure area and northwesterly flow over the western half of the north American continent. All these features were relatively well predicted by a the NCEP ensemble mean forecast at 12-15 days lead time. Note that the forecast was associated with relatively high confidence and probability values in the area of the forecast block and in the northern part of the northwesterly flow. The 15-day lead time forecast initiated on 17 February verified relatively well other parts of the Northern Hemisphere, too. Interestingly, the 11-8 day lead time forecasts did not capture the blocking development well and were associated with lower confidence/probability values. From day 7 lead time the blocking again was predicted with generally increasing skill (not shown).
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