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Data for these analyses was obtained from the US National Centers for Environmental Prediction, Climate Prediction Center.

Aids to interpreting plots:

Definitions

Climatology	The long-term average of a given variable, often the 30-year period 1961-1990.
Anomaly	The difference between the observed value of a variable and its climatological value.
Pentad	A 5-day average of a variable, starting from 1-5 Jan, 6-10 Jan, ..., 27-31 Dec.  On plots using pentad data, the date for the "middle" of the pentad is displayed: 3 Jan, 8 Jan, ... , 29 Dec.
hPa	The abbreviation for "hectopascal", a unit of atmospheric pressure.  For comaprison, the atmospheric pressure near the earth's surface is roughly 1000 hPa (its less at higher elevations).  The 250 hPa level in the atmosphere is roughly 10 km above the earth's surface.  One hPa equals one "millibar" or "mb", these pressure units are also displayed on some plots.