Today’s GFS-derived (MOS) temperature departure forecast for Friday illustrates the battle going on between the PNA (Pacific-North American pattern) and the NAO (North Atlantic Oscillation). The PNA is in a negative phase, which means we have a southward dip in the jet stream over the west and a ridge of high pressure trying to form downstream over the Southeast.. Meanwhile, the NAO is likewise negative, with a blocking high over the northern Atlantic attempting to force a southward dip in the jet stream over the eastern U.S., in opposition to the PNA’s Southeastern high. The net result is illustrated nicely in the image below, which is the forecast departures from normal for Friday, February 22, 2013.
The GFS’ MOS temperature departure from normal (scale at the bottom, in degrees F) for Friday, Feb 22, 2013 is shown here. Areas shaded in yellow and orange are expected to be warmer than normal, while the opposite holds true for blues, purples, and white. The primary weather features from the negative phase of the PNA are denoted by the red arrows (jet stream) and weather features (“H” and “L” for High and Low), with a southward dip in the jet stream over the western U.S. while a ridge of high pressure becomes established downstream over the Southeast. But the negative phase of the NAO – whose impacts are denoted by the blue “H” and arrows – feature a southward building high over the Northeast and a cold north-northeasterly flow over the Mid-Atlantic. Negative phases of the NAO are actually warmer than normal for New England since the trajectory is more onshore – vs the typical trajectory from Canada. You can see the PNA pattern “attempting” to warm up the eastern U.S., while the NAO does its best to keep the East Coast cold. The result is usually a mixed mess for Maryland.