Discussion

The correct answers are (b) and (d), as explained below.

By 12 UTC 25 October 2003 (Fig. 4 for GFS; Fig. 8 for UKMET) the anticipated retrogression in the long wave pattern over the Pacific and western North America has occurred in both models. The vorticity center off the coast of Washington/Oregon in the initialization of 00 UTC 23 October 2003 has moved “over” the long wave ridge and into the central U. S. near Colorado in both models. The vorticity center that was initially located to the south of the western tip of the Aleutian Island chain (175 W) has moved to southeastern Alaska (60 N, 140 W) in the GFS but noticeably farther eastward (60 N, 135 W) in the UKMET. The vorticity center associated with Patricia, which by this time has weakened into a depression (See Table 1), is farther to the northeast and more clearly defined in the UKMET than in the GFS.

By 00 UTC 26 October 2003 (Fig. 5 for GFS; Fig. 9 for UKMET) the UKMET begins to “dig” the Colorado vorticity center (previous paragraph) into New Mexico, while the GFS moves it southeastward into a NE/SW trough through North Texas. Thus, the central U.S. trough is beginning to “hang back” in the UKMET. The vorticity center with depression Patricia continues to be much better defined in the UKMET as it moves northeastward, and for the first time there is the hint that this vorticity center might be “picked up” by the New Mexico trough and vorticity center in the UKMET. The vorticity center that had been at 60 N and 140 W (GFS) or 135 W (UKMET) continues to move more rapidly in the UKMET, as it is now moving southeastward toward the central U.S. long wave trough, while it is located in the long wave ridge line in the GFS.

Twelve hours later, at 12 UTC 26 October 2003, it is clear that the two global models, the GFS (Fig. 6) and the UKMET (Fig. 10), have developed quite different solutions for the trough over the central U.S. The GFS shows a trough with one major vorticity center in southwestern Missouri. The Canadian vorticity center that had been located in the long wave ridge line (previous paragraph) has weakened and remains in Canada well to the north of the major vorticity center in Missouri. The UKMET, on the other hand, shows a more complex trough system in the central U.S. The New Mexico vorticity center (previous paragraph) has moved into western Texas, maintaining a positively tilted trough over the central U.S. The Canadian vorticity center that had been moving southeastward toward the central U.S. long wave trough (previous paragraph) has continued to do so, and is now in the Dakotas. It is likely that this trough will “dig” southward and maintain the positively tilted trough over the U.S. Both models now show the vorticity with depression Patricia moving eastward (GFS) or northeastward (UKMET). The UKMET solution holds open the possibility that this vorticity center, and its associated moisture, will be able to move northeastward into the Gulf of Mexico region ahead of the positively tilted central U.S. trough.