LARGE-SCALE SYNOPTIC PROCESSES UNDERLYING PRECIPITATION 39 
associated with frequent overrunning (ascending) 
moist air masses from the Gulf. Thus, precipita- 
tion amounts about 25% of normal in December 
changed to over 200% of normal in January. 
Of course, this heavy precipitation must also 
be associated with synoptic-scale systems (cy- 
clones) interactive with the mean planetary 
trough, as well as the meso-scale convective ac- 
tivity released in those portions of the tropical 
air rendered unstable by ascent imposed by the 
larger systems. There was a dramatic change in 
LATITUDE °N 
=e 
—! 
LATITUDE °N 
LATITUDE 
LATITUDE °N 
LATITUDE 
LAST HALF OF JAN 1959 
14 16 18 20 22 
-8 -6 -4 — 024 6 8 
MPS 
10 
Fig. 7—Fifteen-day mean zonal wind speed 
profiles corresponding to charts shown in Fig. 6 
the tracks of the cyclones during January incident 
to the development of the new trough, as shown 
in Figure 8. Note that the precipitation-produc- 
ing type of storm (the Colorado or Southwest 
disturbance) became prevalent during the latter 
half of January. 
The strongest development occurred from Jan- 
uary 20 to 22, when a cyclone first appearing in 
New Mexico on the 20th deepened from 994 mb 
to 968 mb by the 22nd. This storm is of great 
interest not only as a manifestation of the great 
change in the general circulation, but also in its 
role as a factory for many types of precipitation 
ranging from snow and sleet to drizzle and thun- 
derstorm. The period of this storm also coincides 
with the January thaw documented by Wahl 
[1952] and the associated precipitation singularly 
noted by Bowen [1956]. 
Precipitation associated with the January 21- 
22, 1959 cyclone over the Midwest—The great 
cyclone of January 21-22 over the Midwest pro- 
duced floods in the Ohio Valley. The surface maps 
for 12h 00m GMT (1200Z) on both days are 
shown in Figures 9 and 10. While the electron- 
ically computed vertical motions are computed 
for the 600 mb level (from a baroclinic model 
employing 850 and 500 mb data), these may be 
considered to represent a mean vertical motion 
in mid-troposphere (Figs. 11 and 12). Details of 
this model and the vertical motion computations 
may be found in Thompson [1957]. 
A glance at the charts of vertical motion and 
areas where precipitation is and is not occurring 
shows a reasonably good general correspondence 
between ascent and precipitation, and descent 
and lack of precipitation. The large area domi- 
nated by more vigorous ascent over the Great 
Plains and the Northeast on the 21st is related to 
the precipitation occurring ahead of the warm 
front and behind the cold front. An area of no 
rain in the warm sector appears in an area of 
lesser, but still upward, motion. On the 22nd, the 
instability snow showers over the southern Great 
Lakes and adjacent areas are indicated to be part 
of a larger-scale system of upward motion (in- 
duced by a lagging upper air trough) and not ran- 
dom flurries. Then again, the frontal showers in 
the south are associated with induced ascent. 
If these charts are scrutinized more carefully, 
however, it becomes clear that the correspondence 
is only general and that there are large discrep- 
ancies. For example, on the 21st directly in the 
area of greatest upward motion there is a pre- 
