254 MONTHLY WEATHER REVIEW JULY 1'367
THE WEATHER AND CIRCULATION OF JULY 1957''
Drought in the East
H. F. HAWKINS, IR.
" ExtendedForecast Section, U. S. Weather Bureau, Washington, D. C.
1. MONTHLY CIRCULATION AND ITS CHANGE
FROM JUNE TO JULY
The most impressive single aspect of the mean circula-
tim for July 1957 was its complete reversal in phase from
the previous month. From Japan eastward through
Europe there occurred a shift in the westerly wave train
equivalent to a phase change of about 180'. Although
the definite establishment of this new regime coincided
with the transformation of hurricane Audrey into a major
perturbation in the westerlies, the need for shortening of
the wave length as the westerlies weakened and shifted
northward had been quite evident during latter June.
The preceding article in this series [l] examined in con-
siderable detail the June circulation (fig. 1) and described
the initiation of the new circulation regime which occurred
near the end of that month. Figure 2, the mean 700-mb.
pattern for July, shows a typically summer circulation
with a trough off either coast, a ridge over the Great
Plains, and an upper level anticyclone near the center of
the United States. Comparison with June (fig. 1) reveals
the striking change between the two months; i. e., the
westerly troughs of June were replaced by ridges in July,
and the ridges of June were replaced by troughs. In addi-
tion, there was; in effect, an increase in the mean wave
number from five to six. I n the average, normal seasonal
trends may be accompanied by a similar increase, but the
changes are seldom accompanied by so nearly a complete
opposition in phase.
Since local weather can, in general, be associated with
the abnormalities i n , the 700-mb. mean patterns, it .is
profitable to compare the departures from normal in June,
figure 1, with those of July, figure 2. The oyerall contrast
is quite impressive, even in many areas far removed from
the main belt of westerlies. When we used the data at
50' N. latitude as roughly representative of the westerly
belt, the linear correlation coefficient between the 700-mb.
height anomalies of June and those of July at 36 points
10' longitude apart was found to be --.61. Thus it be-
comes obvious that not only did the phase of the circula-
tion change, but so also did the anomalous features or flows.
Another way of illustrating this aspect is presentled in
figure 3 which shows the change in 700-mb; height am-
malies (seasonal trend thus excluded) from June to July.
In almost every case rises show where troughs and below
normal heights (June) were transformed to ridges of above
normal height (July), and vice versa. Monthly oscilla-
tions-or reversals in circulation characteristics-have
been noted quite frequently in previous articles of thk
series [2 , 3, 4, 5, 61. However, July is usually more apt to
reflect persistence of the June regime than a sharp re-
versal [7].
Aside from these rather unusual changes in local circu-
lation characteristics, the monthly.pattern also reflects a
continuation of blocking activity. It is tempting to relate
the general prevalence of blocking in the last several years
to the rise and continued high level of sunspot activity,
but conclusive statistical proof of the association has not
yet been forthcoming. During July (fig. 2) the most con-
spicuous area of such 'action was over the Bering Sea,
where heights were 340 feet above normal and sea level
pressures (Chart XI, inset) some 7 mb. higher than normal.
Also affected were the Greenland area.and north-central
Siberia. I n fact, above normal heights and sea level
pressures extended from Hudson Bay eastward (at 65' N.)
all the way through. northern Asia to Alaska-an almost
1~1c:urts I.-Mexn 700-mb. contours (solid) and height departures
from monthly normal (dotted) (both in tens of feet) for June 1957.
Dominant feature in the United States was the mean trough
I See Charts I-XVII following p. 268 for analyzedclimatologicaldata for the month. (heavy vertical line) through central part of country.
JULY 1957 MONTHLY WEATHER REVIEW 255
FIGURE 2.-Mean 700-mb. contours (solid) and height departures from monthly normal (dotted) (both in tens of feet) for July 1957, with
troughs indicated by heavy vertical lines. Note complete change in phase from June (f i g . 1).
complete circumpolar ring. The only other conspicuous
high Iatituiie feature was the strong, persistent, closed
polar Low around 75' N. latitude and 160' E. longitude.
The Icelandic Low (or Lows), although still present, was
considerably weakened by the blocking.
An aspect of no little importance was the stability of
the major westerly wave train. This was most note-
worthy in the United States where successive &day mean
maps reveal only minor shifts in the long wave pattern
throughout the month. Variations in depth of the coastal
troughs, minor fluctuations in their position, and changes
in the upper level anticyclone over central United States
constituted the major 5-day circulation features. More
sizable alterations were noted in Canada where the direct
effects of blocking surges were stronger and more apparent.
The 200-nib. circulation features, figure 4, were essen-
tially similar to the 700-mb. pattern except for the dis-
appearance of the Greenland anticyclone a t higher levels.
As usual, the summer maritime troughs were better defined
aloft, but the westerly waves retained the same relation
and identity. Both Arctic and temperate mean jets were
present with a clear westerly maximum just about girdling
the globe between 40' and 50' N. The mean jet over the
United States also demonstrated the phase change from
June [l] noted in other circulation features.
256 MONTHLY WEATHER REVIEW
FIGURE 3.-Changes in monthly mean 700-mb. height departures
from normal-June to July 1957. The lines of equal anomalous
height change are drawn at 100-foot intervals with the centers
labeled in tens of feet. Rises in central United States were ac-
companied by falls along each coast.
2. WEATHER OF THE MONTH
Mean temperatures for July, Chart I-B, were 1' to 4 O
F. above normal from Texas to North Dakota in central
United States where, under the upper-level High and the
generally anticyclonic circulation, there was sufficient in-
solation and warm air advection. To both the east and
west the coastal troughs were effective in producing cooler
regimes. Stronger than normal maritime influences were
operative in the Northwest, while the Southwest was cooled
in part by the cloudiness attending shower activity in the
western moist tongue. Coastal California was warm, al-
though well inland it was cool-a condition commonly
attributed to a weakening of the sea-breeze regime.
Temperatures oyer eastern United States were interest-
ing in that near to below normal temperatures did occur
as usual to the rear of the coastal trough, but a coastal
strip from the Outer Banks of North Carolina to New
Hampshire remained above normal. Considering the
depth of the offshore trough, this warmth appears rather
difficult to rationalize, especially since the thicknesses from
1000 to 700 mb. (not shown) were definitely below (mean
virtual temperatures cooler than) normal over the entire
area. However, temperatures in interior regions of t,he
East averaged only 1' to 2' F. below normal a t the surface,
and even these departures seemed intimately associated
with precipitation (Chart 11) and the cloudiness associated
with it (Chart VI). Thus, for the most part, air 0111~-
slightly cooler than normal a t lower levels was advccted
over the coastal strip and, in the descent from the Appa-
lachians, was heated enough to account for the coastal
FIGURE 4.-Mean 200-mb. contours (solid, in hundreds of feet) and
isotachs (dotted, in meters per second) for July 1957. Solid
arrows indicate the position of the 200-mb. mean jet stream,
which was displaced northward and curved sharply anticycloni-
cally around strong mean High in central United States.
warmth. Further warming was produced by abundant
sunshine (Chart VII) in an area where little rainfall was
recorded.
Precipitation (Charts I1 and 111) was reasonably plenti-
ful in many parts of the country. On the northwest coast
it was caused by forced ascent of cool moist maritime air
and frontal activity. In a wide band from New Mexico
and eastern Arizona northward through the western Plains
and eastward through the Great Lakes to New England,
showers in the western moist tongue spread northward
and then eastward. Some of the moisturo also eddied
anticyclonically over the central Mississippi Valley where
as much as twice the normal amount of precipitation was
recorded. I n t h e Southeast heavy rains were associated
with quasi-stationary frontal activity and the instability
induced by the attending cyclonic circulation. Less than
X inch of rain was recorded over a wide belt of the Far
West lying between the rains on the southern edge of the
westerlies in the extreme Northwest and the continental
summer showers in the Southwest and central Rockies.
It was also a period of drying-out in Texas and Oklahoma,
where little flow of Gulf moisture and no perturbations
of consequence were noted. The only other dry area of
significance occurred in the East behind the mean trough
off tho East Coast.
In t,he preceding discussion of circulation changes the
o!):.ioas rr~~~c~rs:21 in flow characteristics from June to July
\vas Ilighligh~ed to some extent. It would seem appropri-
ate therefore also to indicate the accompanying changes
in temperature and precipitation. The monthly changes
in temperature by class are shown in figure 5A. (For
JULY 1957 M O N T H L Y W E A T H E R 257
FIGURE 5.-Number of classes the anomaly of temperature (A) and
precipitation (B) changed from June to July 1957, with increased
values considered positive and decreased values negative. Areas
with increases of two or more classes are cross-hatched; areas
with decreases of two or more classes are dotted.
definition of classes, see reference [7].) Temperatures fell
one to three classes in the Far West (except coastal Cali-
fornia) due to increased trough activity in the north and
cloudiness in the south. Temperatures rose two to three
classes from Texas to Minnesota as heights rose and the
ridge of July replaced the trough of June. Along the
east coast cooling was generally in order as the trough
deepened offshore.
Precipitation changes, also by class, are indicated in
figure 5B. Drying occurred under the upper-level High
in central United States and also over most of the East
to the rear of the trough, except the Southeast which
remained wet, and the eastern drought area which re-
mained dry. The continental showers of summer pro-
duced wetter conditions from New Mexico north-north-
eastward as well as in eastern Nevada and 1da.ho. Most
of California received no rain a t all as the westerlies shifted
northward and typically arid summer weather a as estab-
lished. However, in northern California westcr13 trough
activity caused an increase in coastal precipitation,
TABLE 1.-Total precipitation (inches) and its departure from normal
in eastern drought area.'
April 8 to August 18 period
1957 11 Record minimum for previous 60 years Station
1 Total I Dep. /I Total 1 Dep.
Boston, Mass .._._._____________________
Nantucket, Mass .._________._____._.___
Providence, R. I _...____________._._____
Bridgeport, Conn ________.._._._.____".
New York. N. Y .._______..._._________
Philarielphia, Pa ... ._.___._._._.______.
Harrisburg, Pa .._______....._._._______
Baltimore. Md. ._.____._...._....______
Washington, D. C .______...._._________
Norfolk. Va
~
______.___...._.____----.
-7.05
-7 86 -5.13
-8 23 -9 51
-6.23
-6.14
-9 31
-8 60 -8 72
-10.03
-
Year
1950 1949
1914
1911
1939
1923
1939
1930
1930
1911
1944
-
I Fron Weekly Weatherand Crop Bulletin, National Summary, VOl. XLIV, NO. 33,
U. S. Weather Bureau, Washington, D. C., Aug. 19,1957.
3. EASTERN DROUGHT
Certainly one of the major items of topical interest was
the intense drought which gripped parts of the East.
From North Carolina to southern New England the coastal
areas received only 25 to 50 percent of normal July
precipitation. The lack of rainfall could be directly asso-
ciat,ed with the offshore trough and the dry northerly and
northwesterly components of flow which prevailed a t most
levels. The upper-level continental anticyclone appeared
to be too weak to carry the moisture on the rear side up
and across central United States far enough to be effective
in the East.
In June,, when the circulation was reversed, the central
United St'ates trough was apparent'ly too far west for
significant precipitation in the East [I]. A similar condi-
tion prevailed during April and May. The transit'ion
from one regime to the other was sharp and rapid with
little opportunit'y for effective relief. As a result, much
of the East, which had enjoyed a satisfactory ground
moisture condition in early spring, was suffering a very
severe drought. I n duration, this could not compare to
many quite recent occurrences in the central and southern
Plains, but in severity the eastern drought was quite note-
worthy. Numerous July minimum precipitation records
were set, but of greater significance is table 1 which com-
pares the indicated 19-week precipitation totals with
record minima for the previous 50 years at 11 selected
eastern drought stat,ions. Thus one sees t h a t a t Provi-
dence, R. I. ; Bridgeport, Conn. ; Philadelphia, Pa. ; Harris-
burg, Pa.; Baltimore, Md.; and Washington, D. C., this
has been the driest growing season in 50 years.
The riddle of basic causation as related to a phenomenon
such as the eastern drought still lies beyond the scope of
present meteorological theory. Although certain aspects
of atmospheric behavior are better understood and better
explained than ever before, the role of instantaneous inter-
actions within the mean ensemble is just beginning to be
appreciated and analytically investigated. As of now, one
can only point out that the very st,able pattern of July-
258 MONTHLY WEATHER REVIEW JULY 1951
strong trough off the east coast and small upper-level
anticyclone over central United States-was sufficient to
maintain extreme drought along the coastal strip. The
cyclonic activity associated with the trough was too
far east, and the High was too weak and/or too far west
to transport adequate moisture into the East. How
much the preceding dryness (April-June) contributes to
such a regime may be evaluated in the not far distant
future. But, given such knowledge, the essential problem
is still only once removed. There rema,ins basically a
problem in understanding which will never be satisfied
with the old saying: All signs fail in time of drought,.
REFERENCES
1. W. H. Klein, The Weather and Circulation of June
1957-Including an Analysis of Hurricane Audrey
in Relation to the Mean Circulation, Monthly
Weather Review, vol. 85, No. 6, June 1957, pp. 208-
220.
2. C. &I. Woffinden, The Weather and Circulation of
February 1957Another February with a Pro-
nounced Index Cycle and Temperature Reversal over
the United States, Monthly Weather Rewiew, vol. 85,
No. 2, Feb. 1957. pp. 53-61.
3. C. R. Dunn, The Weather and Circulation of Novern-
ber 1956-Another October to November Circulation
Reversal, Monthly Weather Review, vol. 84, No. 11,
Nov. 1956, pp. 391400.
4. W. H. Klein, The Weather and Circulation of May
1956Another April-May Reversal, Monthly
Weather Review, vol. 84, No. 5, May 1956, pp. 190-
197.
5. J. F. Andrews, The Weather and Circulation of July
1955-A Prolonged Heat Wave Effected by a Sharp
Reversal in Circulation, Monthly Weather Review,
6. W. H. Klein, The Weather and Circulation of May
1954-A Circulation Reversal Effected by a Retro-
gressive Anticyclone During an Index Cycle,
Monthly Weather Review, vol. 82, No. 5, May 1954,
7. J . Namias, The Annual Course of Month-to-Month
Persistence in Climatic Anomalies, Bulletin of the
American. Mdeorological Society, vol. 33, No. 7,
Sept. 1952, pp. 279-285.
V O ~. 83, NO. 7, July 1955, pp. 147-153.
pp. 123-130.
JULY 1957 M. W. R. LXXXV-99
Chart I. A. Average Temperature (OF.) at Surface, July 1957.
B. Departure of Average Temperature from Normal (OF.), July 1957.
A. Based on reports from over 900 Weather Bureau and cooperative stations. The monthly average is half the sum of the monthly
average maximum and monthly average minimum, which are the average of the daily maxima and daily minima, respectively.
B. Departures from normal are based on the 30-yr. normals (1921-50) for Weather Bureau stations and on means of
25 years or more (mostly 1931-55) for cooperative stations.
c; U
Lxxxv-100 JULY 1957 M. W. R.
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JULY 1957 M. W. R. LXXXV-101
Normal monthly precipitation amounts are computed from the records for 1921-50 for Weather Bureau stations and from
records of 25 years or more (mostly 1931-55) for cooperative stations.
Lxxxv-102 JULY 1957 ,M. W. R.
Chart VI. A. Percentage of Sky Cover Between Sunrise and Sunset, July 1957.
B. Percentage of Normal Sky Cover Between Sunrise and Sunset, July 1957.
A. In addition to cloudiness, sky cover includes obscuration of the sky by fog, smoke, snow, etc. Chart based on
visual observations made hourly at Weather Bureau stations and averaged over the month. B. Computations
of normal amount of sky cover are made for stations having at least 10 years of record.
JULY 1957 M. W. R. LXXXV-103
Chart VII. A. Percentage of Possible Sunshine, Julv 1957. -
B. Percentage of Normal Sunshine, July 1957.
LXXXV-104 JULY 1957 M. W. R.
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