Two types of NCEP EPS guidance from the two previous days' runs valid at forecast time are shown below. First is the ensemble mean 168-hour 500-hPa height forecast for the time of interest, with spread normalized by the mean spread at each grid point over the past 30 days. We see an ensemble mean trough in the eastern and central U.S. with relatively low spread (high certainty) relative to the last 30 days, as indicated by the blue colors. In western North America, we see split flow, with ridging in northwestern North America, troughing in the southwestern U.S., and higher spread (lower certainty) indicated by the warmer colors.
There is also larger uncertainty over the western Atlantic at about 40°N, 60°W, the maximum of which is bisected by the ensemble mean 5640-m height line. We will investigate these areas further with the use of spaghetti diagrams of the 5640-m and 5400-m height contours.
Two spaghetti diagrams from the 12 UTC run seven days prior to the forecast time of interest are shown below. From the spread of the contours in the western Atlantic, we see that this area of uncertainty results from some members being faster and more strongly amplified with a northern jet stream trough. Locations for this trough range from 65°W off the Atlantic coast to about 50°W southeast of Newfoundland. The slower solutions are deeper. Meanwhile in the western U.S., considerable uncertainty exists with the location and amplitude of a southern stream disturbance moving eastward from the Pacific, located anywhere from about 140°W to 120°W. Some ensemble members also indicate a third trough of varying location and intensity, from southwest AZ (cut off from the circulation at 115°W) to the mid-Mississippi valley (about 90°W).
