Discussion

The long wave trough in eastern North America along 75-80 W as indicated by the 5-wave 500 hPa height pattern lines up very well with the actual 500 hPa height pattern indicating that the current flow pattern in this region matches the long wave pattern. In other words, the flow pattern is dominated by the long wave pattern with minimal short wave activity moving through the long wave trough.

In western North America the 5-wave 500 hPa heights indicate a pronounced long wave ridge with two closed contours along 118W. The actual 500 hPa heights show a ridge a bit farther to the east along 110 W. Since the long wave ridge will tend to remain quasi-stationary for several days, it is reasonable to expect the actual 500 hPa ridge to retrograde during that time to ultimately match the long wave ridge position along 118 W.

Over the Pacific there is a long wave trough with two closed contours along 155 W as seen in the 5-wave 500 hPa heights. The cut-off in the actual 500 hPa heights is located a bit farther to the east at 150 W. Since the cut-off in the long wave trough will tend to remain quasi-stationary for several days, it is reasonable to expect the actual 500 hPa cut-off to retrograde during that time to ultimately match the cut-off in the long wave pattern along 155 W. The argument is the same as for the long wave ridge in western North America. Thus, in considering the factors that will influence the day-5 forecast for TLH, it is important to consider a probable retrogression of the long wave pattern over the eastern Pacific and western North America by about 5-8 degrees of longitude.

Over the Pacific there is a notable lack of line up between the broad long wave trough with its associated cut-off at 155 W in the Gulf of Alaska, as seen in the 5-wave 500 hPa height pattern, and the actual 500 hPa height pattern. This incongruence is due to the presence of two very strong short wave troughs that have already been mentioned in Section 1a above. One is the short wave trough approaching the Washington/Oregon coast (130 W), and the other is the short wave trough with one closed contour (GFS) located to the south of the western tip of the Aleutian Island chain (175 W). In the next several days these two short waves can be expected to move rapidly eastward, riding along a track defined by the quasi-stationary long wave trough/ridge pattern over the eastern Pacific and western North America. This would carry these short waves eastward, and then “up and over” the western North American ridge, where they would then “drop into” the next long wave trough over eastern North America. They can be expected to intensify as they move northeastward toward the long wave ridge line and then weaken as they move southeastward toward the long wave trough line. This whole scenario is further complicated by the likely retrogression of the long wave pattern, as discussed above. If this retrogression occurs the two short waves would be expected to “drop into” the next long wave trough over the U. S. but at a more western longitude. To use forecaster language, they would be expected to “dig” farther to the west and south.