MOXTHLT WEATHER REVIEW 163
THE RAINS OVER ARIZONA, AUGUST 26-29, 1951
J. A. CARR
WBAN Analysis Center, U. S. Weather Bureau, Washington, D. C.
INTRODUCTION
On August 26, 1951, showers and thunderstorms
occurred over southern Arizona bringing to a temporary
end a prolonged period of above normal temperatures.
During the succeeding three days the area of precipitation
expanded as the thunderstorms gave way to a more steady
type of rainfall. Idow maximum and minimum tempera-
tures were noted during thc rain period with some stat'ions
reporting record, or near record, low maximum tempera-
tures.
This article will present the results of a bricf investi-
gation into the causes of this unusual weather.
MAJOR CIRCULATION INFLUENCE
The rain over Arizona presents an interesting example
of the connection between the weather a t one point and
the weather at another; in this instance, the rain in
Arizona and the heat in Texas.
Throughout August very dry weather prevailed over
Texas as the land lay in the grip of one of the most per-
sistent periods of heat in its history. Temperatures, over
the State, ranged from the nineties to the hundreds
throughout the entire. month. Under clear skies the air
was heated to high levels and helped maintain a warm,
circular High, aloft, over west Texas and the west Gulf
region. Toward August 25, 1951, this High increased in
size to the extent t.hat it covered the entirc Gulf and the
ocean areas adjacent to the west coast, of Mexico [I].
At the surface the air moved over the warm waters of
theGulf of Mexico absorbing moisture as it went. Over
the middle and northern half of tho Gulf the air moved
inland over Mexico curving anticyclonically a s i t ap-
proached the hills and mountains of the Sierra Madre
Range (in northern Mexico, just west and parallel to
106' W. Long.). In time the anticyclonic curvature
brought the air northeastward, on a downslope path,
across New Mexico and Texas.
However, west of the Sierra Madre Range a different
situation developed. Judging by the flow, some air from
the lower Gulf of Mexico and air over the tropical waters
off Central America joined forces to the west of the
Pacific coast of Mexico. These winds, under the same
waters, picking up some of the moisture associated with
the hurricane southwest of Lower California, acquiring
additional moisture as i t channeled northward, through
the Gulf of Lower California, and finally, arriving at
Yuma where the first topographical feature intervened.
THE ROLE OF TOPOGRAPHY
Once on land the air continued on the anticyclonic
path, as directed by the pressure pattern, but was deflect-
ed somewhat to the left of the free air path by the in-
fluence of the terrain.
The land surface is lowest in the region of Yuma from
where it rises to the east and north. To the east and
northeast of Phoenix the land rises abruptly to form the
Colorado Plateau. The effect of the land formation was
such that the air moved nort,hrastward over rising ground
without too much interference until the western fringes
influence as the air to the east of Mexico, moved poleward FIGURE 1.-700-mb.chartfor 1500 GMT. Auaust 29.1951. Contours (solid lines) at
curving anticyclonically with tirne (fig. 1). Thus, we find 100-footintervalsarelabeled in hundreds of geopotential feet. Isotherms'(dashed lines)
arc at intervalsof5C.Barbs on wind shafts arefor speeds in knots (pennant=50
.-
air becoming more moist during a long fetch over warm knots, full barb=lO h o t s , half barbe5 knots).
FIGURE 2.--24-hour precipitation cnding 1230 GMT, A4ugust 29, 1951 (Shaded=Trace or
<1 inch, encircled white=l inch or more).
of the Colorado Plateau were reached. Wit'h this ob-
struction in its path, the air, still trying to move north-
eastward, was deflected more to the north of its path in
the Phoenix-Prescott region. This idea is supported by
the reported southeast surface winds, during the rain
period, a t these stations.
SURFACE EFFECTS
Rainfall began on the 26th, as moist winds moved in-
land along the southwest border. The combination of
hot ground, strong solar heating, and a low level of con-
densation in the rising air set the stage for the numerous
thunderst,orms which were reported by various stations.
By late evening of the same day the strong convectivc
stage was passed as was indicated by the absence of re-
ports of thunderstorms and the frequent observation of
showers and steady rain.
The rains spread across Arizona in a ciiscernable pattern
so arranged that the long axis of the heaviest precipita-
tion pattern was orientated northeastward from the
southwest border of the State, and then north across
Flagstaff (fig. 2). Such an orientation suggests the
topographical influences previously discussed. The heavi-
est falls were recorded along the axis and in, or near, the
center of the line (near Flagstaff). Tucson was to one
side of the main flow as revealed by the &day total of
only 0.62 inch, which fell in 1 day. A second axis of flow
existed on the 27-28th in the Yuma to Needles (Calif.)
region. A tabulation of rainfall amounts and related data
is presented in table I.
When the month came to a close, Phoenix had received
a total of 5.33 inches, marking the month as the wettest
August in the 56-year history of the station. The rains
of the 26th to 29th yielded the greatest tot'al evcr recorded
TABLE 1."24-hour fotals of r a i n f a l l f o r selected stations in Arizona,
Aug. 26-99, 1951 (with comparative data) *
I I I
Flagstaff" ..... ~..~~
T .O 1 2.82 5,014 Prescott (W'BAS)---
2.li 3.80 .50 .84 2.43 .03 .96 1,083 Phoenix.- ~. . . . . . . . . .
.96 7.77 1.58 2.12 2.12 2.00 2.15 5,000 Payson ..............
1.92 3.00 1.62 2.28 .10 T 2.83 6,993
1.5: 1.06 2.58 2.11
lucson -...-- ........ 2,558
4.01 1.13 .2 5 .68 .15 .05 .57 203 Yuma-.-. . . . . . .. .~..
1.10 1.30 ,IO .94 .I 9 .07 1.34 4,880 Winslow-
~ .~~~ ......
1.50 .G2 ...~~~~ I .6 2 T 2.17
Feet Inch Inch Inch Inch Inch Inch
r .
~~~~
___ ___ _____~__~___
Bngust 1951. *Source-U. S. Weather Bureau, Station Meteorological Summary (for stations shown),
during the month of August. Tucson had it's heaviest
total for August since 1946. At Payson a @-year record
for gr:rcat>est t,ot,al raminfall for any mont'h was broken by
t'he August t,otsLl of 10.38 inches. Flagstaff report'ed the
rain on the 28th was the greatest of record for any day
in August.
The rain plus a thick layer of clouds produced int'er-
esting effects upon the temperature readings. Maximum
readings dropped markedly under the influence of evapora-
tion of falling rain, evaporation of rain from the ground
surface, and cooling of the air by passage of cold rain
through the lower layers of the afmosphere. These
factors, added to a ?,-day period of 9 t'o 10 tenths cloud
cover over a large portion of the St'at'e, heIped maintain
relat'ively low temperatures.
At Yuma, for instance, t'he month was consistently
above normal wit'h maximum temperatures above 100"
F. for t'he first 25 days of the month. From a high of
110' on thc 25th to a high of only 80" (27th) was a sharp
change for t'he residents, especially when it is recalled
that the humidity on t'he latter dat'e was quite high.
Phoenix reported a maximum of 73' on the 27th aft'er a
period of 22 consecutive days of 100 or above. In fact
t'he maxima of 73" (27th) and 76' (29th) were the lowest
ever observed at Phoenix in any August. A t'abulation
of t,he maximum and minimum tempcrat'ures is presented
in t'able 2.
TARLE 2.--Maxirn,um and minimum temperatures for selected stations
in Arizona, August 25-31, I951 *
I Maximum temperature (" F) Minimum temperature (" F)
i l
80 63 58 57 fi9 73 37 44 46 51 49 44 38
86 (i3 70 63 77 82 58 57 56 55 50 47 48
100 73 83 76 92 93 76 72 68 88 67 65 69
8G 72 69 6G 77 80 51 56 54 55 52 47 46
88 73 86 88 91 94 7fi 70 66 71 71 68 63
90 73 66 74 83 84 57 00 59 55 55 49 52
99 80 87 93 95 100 72 73 72 71 70 71 72
Bugust 1'351.
'Source-IJ. S. Weather Bureau, Station Meteorological Summary (for stations shown),
AVGCST 1981 MONTI-ILP WEATHER REVIEW 165
FEATURES OF THE SURFACE MAPS
A survey of the surface weather maps for August 26
to 30 yielded little of value as an aid to the explanation of
the weather. Figures 3 , 4, and 5 illustrate the relatively
unchanging features of the surface pressure over the south-
western States. Two interesting features did come to
light, however. The first, and not too surprising, effect
was the disappearance of thc thermal Low over Arizona,
especially on the 27th. The second feature was a west-
ward migration of a High cellwhich on August 26 had
been centered over New Mexico. As the rain area spread,
the High moved toward castern Arizona where it pcr-
sisted and grew in size till near the end of the period.
This High was related to the western end of the upper
level High which on the 27th extended from Florida to
New Mexico, roughly parallel to 36" N. Lat.
To some extent the western periphery of the High was
"fictional." As the temperatures dropped within the
storm area, they soon reached values lower than the
readings a t stations outside the rain area. With such a
situation two mountain stations, one in the rain and one
outside the rain area, a t the same altitude and observing
approximately the same station pressure, but with a
difference of 20" to 30" between their surface temperatures
would report entirely different sea level pressure readings
after reducing their station pressures. This, of course,
would result from one station calculating a higher sea
level pressure by using a relatively low value for the mean
temperature of the air column from the station level to
sea level.
The apparent sprea>ding of the western side of the High
wiped out the form of the heat Low. When the rains
ended and solar heating once more took over, the edge
of the High retreated from the southwest portion of the
FIGURE 3.-Surface weather chart for 1830 QMT, August 27, 1951. Shading indicates
areas of active precipitation.
State while the center itself moved north toward the
Utah border.
LOWER TROPOSPHERE MOISTURE AND TEMPERATURE
DISTRIBUTION
Graphs showing the vert>ical distribution of moisture
a t Phoenix were prepared, with figure 6 showing the period
of increasing moisture. The curve for August 25 shows
the moisture content on a dry day during which the
temperature rose to 104" E'. under a cloudless sky. The
winds aloft were southwest as was to be expected of the
return flow of the upper level High. The significant
point on this curve is the layer of moist air centering
around 700 mb. The curve for the 26th shows a large
FIGURE 4,"Surface weather chart Cor 1830 GMT, August 28, 1951.
FIGURE b.--Surface weather chart for 1830 GMT, August 29,1951.
166 M O N T H L Y W E A T H E R AUGUST 1951
Mixing Ratio (gm/Kg ) Mixing Ratio (gm/Kg )
FIGURE 6,"Moisture soundings (grams of water vapor/kilogram of dry air) at Phoenix,
line), 1951.
Ariz., 1500 QMT, August 25 (solid line), August 26 (dashed line), Augtut 27 (dotted
FIGURE 8,"Moisturc soundings (grams of water vapor/kilogram of dry air) at Phoenix,
Ariz., 1500 (*&IT, August 28 (solid line), August 29 (dashed line), August 30 (dotted
line), 1951.
TEMPERATURE "C TEMPERATURE "C
FIGURE 7,"Temperature soundings (on a pseudo-adiabatic diagram) at Phoenix, Ariz., FIGURE 9.-Temperature soundings (on a pseudo-adiabatic diagram) at Phoenix, Ariz..
0300 QMT, August 25 (solid line), August 27 (dashed line), 1951. 03W QMT, August 29 (solid line), August 30 (dotted line), 1951.
AUGUST 1951 M O N T H L Y W E A T H E 167
24-hour increase from the surface to approximately 800
mb. with some decrease from 730 to 550 mb. Over the
next 24 hours, the curve (27th) shows increases from the
surface to 550 mb., with the greatest increase centering
around 700 mb. and with a second maximum from 600
to550 mb. Comparing tho curves for August 25 and
27, the 48-hour change shows a region of maximum mois-
ture increase from 900 to 770 mb. with a second maxi-
mum from the surface to 900 mb. In other words, the
greatest increase of moisture was in the lower levels.
This fits in with the other evidence previously presented.
As shown in figure 7, the Phoenix temperature soundings
for the same 2 days (25th and 27th) indicate that the
air on August 25 was conditionally unstable from the
surface to 680 mb. where it became stable with respect to
the moist adiabatic lapse rate. It is interesting to note
the development of an inversion at the surface at 0300
GMT, on the 25th. I n 48 hours tho change of air mass
was reflected by the moist adiabatic conditions from the
surface to near 550 mb. At the same time strong cooling
had taken place as indicated by the separation of the two
curves from 800 mb. down to the surface.
The period of decreasing moisture a t Phoenix is shown
by the curves of figure 8. The curves of August 28 and
27 (fig. 6) show close agreement as conditions in the free
air were not too different during the interval. In the
following 24 hours, the curve of the 29th shows the
moisturc content of the air had decreased slightly from the
surface to 750 mb. without any particularly significant
change above 750 mb. However, the curve of August 30
indicates a strong drop off of moisture had taken place at
all levels from the surface to 550 mb. The smallest change
was found from the surface to 900 mb., which is to be
expected in light of the 3-day soaking rain and some
flooding over the land. The curve shows the greatest
influx of drier air took place a t about 800 mb.
The curve of August 30 seems to have been approaching
the same shape and values as the pre-rain curve of the
25th, although the moisture values were still generally
higher.
The temperature soundings for August 29 and 30 (fig. 9)
show a 24-hour change to warming between 920 and 720
mb. and a cooling above to 400 mb. Apparently on Au-
gust 30, the heat from the sun was being spent, mostly, in
evaporating water from the earths surface. This is
indicated by the slow return of the moisture curve to the
velues preceding the rain.
While the rains continued in Arizona the upper cireula-
tion remained essentially unchanged until the 29th. On
this date a deepening upper level Low (fig. 1) over Wash-
ington and Oregon changed the circulation along the Pacific
Coast. The air that now arrived over Arizona, like the
air it displaced, had moved long distances over water.
However, the water, this time, was quite a bit colder and
consequently the cold air from the more northern latitudes
possessed a more stable lapse rate. The stable air with
lower moisture content moved in over the Southwest and
brought clearing weather that finally enabled the tempera-
ture to return to the more normal 100 readings.
REFERENCE
1. V. J. Oliver, The Weather and Circulation of August
1951, Monthly Weather Review, vol. 79, No. 8,
August 1951, pp. 160-163.
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