EXTENDED-RANGEH FORECASTING BY WEATHER TYPES 
coast of North America. The depth of the trough as a 
rule increases with the westward shift. With the zonal 
flow types, the farther southward the westerlies are 
shifted over a broad sector, the stronger the zonal 
flow character of the type. In the extremes of this last 
category the southward shift occurs over a sector 90° 
(or more) wide and is accompanied by the formation of 
extensive polar anticyclones to the north of the prin- 
cipal storm path. This storm path is unusually far 
south. 
In the meridional category, the Bn-a type has a 
definite upper-level trough at about 90°W and a crest 
over the Great Basin (eastern Washington, Idaho, east- 
ern Oregon, Utah, Nevada, and northern Arizona). 
Because only minor polar outbreaks occur in this type 
and because of the presence of a large dynamic anti- 
cyclone over the Great Basin which deflects Pacific 
storms northward, the United States and southern 
Canada experience relatively warm dry weather. As one 
proceeds to the Bn-b type, it is found that whereas 
conditions to the west of the Rockies are nearly iden- 
tical with those for Bn-a, in the Hast a polar outbreak 
occurs as indicated on the diagram. This tends to 
make the eastern trough aloft deeper than in the case 
of Bn-a and produces below-normal temperatures east 
of the Rocky Mountains. In the case of Bn-c type, the 
meridional character of the flow becomes quite marked 
with a well-developed trough aloft at about 100°W. 
Frequently a strong dynamic cyclone remains trapped 
for several days in the southern Great Basin. The polar 
outbreak, instead of being confined to the area east 
of the Rocky Mountains as in the case of Bn-b, flows 
westward through mountain passes as well as south- 
ward along the eastern front of the Rockies and pro- 
duces much-below-normal temperatures throughout 
most of western United States. Ahead of the mean 
upper-level trough, surface temperatures are consider- 
ably above normal, and precipitation is likely to be 
excessive near the mean trough. It is interesting to 
note that the Great Basin anticyclone, present in the 
two preceding types, is now weaker and appears to be 
merged to the west with the Pacific anticyclone, which 
has shifted northward. A further westward shift of 
the mean upper-level trough position is exhibited in 
type A. Here the Basin anticyclone has disappeared 
from the scene as a semipermanent feature of the 
circulation at the surface. The eastern lobe of the 
Pacific anticyclone has moved unusually far north and 
the polar outbreak associated with this type consists 
of a mass outflow of cold air moving southward along 
the Pacific Coast, then southeastward into the Great 
Basin. In contrast to polar outbreaks over the land 
areas of North America, this outbreak is characterized 
by cyclonically curved isobars at the surface. Surface 
temperatures are below normal along the Pacific Coast 
and in the Great Basin but are above normal in the 
East. Because of the interaction of tropical and polar 
air masses in an area north of normal in the East, 
precipitation is excessive in the Mississippi and Ohio 
Valleys. This type and the closely related Bn-c type 
837 
were responsible for the abnormally cold weather in the 
West and warm weather in the Kast during the winter 
of 1948-49 and also during January, 1937. Type D 
represents the most extreme of the meridional flow 
types. In this case the principal trough lies entirely 
off the West Coast with an unusually intense, north- 
ward-displaced Pacific anticyclone, which is merged 
with the cold anticyclone over Alaska. The principal 
outbreak of cold air is so far west that a second minor 
outbreak occurs downstream in eastern North America. 
There exists a somewhat infrequent, yet important 
additional meridional flow type not shown in Fig. 1. 
This is the C type which resembles the Bn-a type 
except that there is a low-latitude trough aloft just 
off the southern California coastline. In this way the 
westerlies are split into two branches, the northern- 
most branch moving along a crest at about 125°W 
and the southern branch moving along a trough at this 
same longitude. The two streams converge downstream. 
For this reason there are essentially two storm tracks, 
a northern and a southern track. Very heavy precipita- 
tion occurs along the southern track in southern Cali- 
fornia, Arizona, and New Mexico. 
The first of the zonal flow types, B, is characterized 
by a northerly storm track with a belt of well-devel- 
oped subtropical anticyclones to the south. As a result, 
relatively dry and warm weather prevails over most 
of the United States and the southern Prairie Prov- 
inces of Canada. In the second zonal flow type, Bs, 
the upper-level westerlies are farther south in the East 
but in the West they are as far north as in type B, thus 
tending to accentuate the western crest. Although the 
Great Basin anticyclone is actually more persistent m 
the case of type Bs, it is considered to be more zonal 
than B because the westerlies are farther south on the 
average over a broad sector and are definitely stronger 
as reflected by the greater rapidity of movement of 
migratory systems along the principal storm path. 
Types Hu, Hm, and Hu are each in turn characterized 
by a more extensively developed polar anticyclone 
and a more southerly shift in the westerlies aloft and 
consequently in the storm tracks. The great masses of 
cold polar air associated with these types seldom move 
southward in any form of organized polar outbreak 
such as occurs with the meridional flow types but 
merely linger north of the mean frontal zone accentu- 
ating the air-mass contrast across the front and leading 
to excessive precipitation along the mean frontal zone 
itself. In the extreme Wu type the cold air occupies 
most of the United States and entirely blocks the 
eastward progress of Pacific storms beyond the Rocky 
Mountains. 
GENERAL CIRCULATION 
CONSIDERATIONS 
A recent study [13] of broad-scale and long-period 
weather processes occurring in connection with certain 
abnormal meridional flow patterns has supplied in- 
formation providing more justification for the weather- 
type treatment thus far discussed. Among other things, 
