March 1965 Julian W. Posey 1 89
THEWEATHER ANDCIRCULATION OF DECEMBER 1964
Record-Breaking Floods in the Northwest
JULIAN W. POSEY
Extended Forecast Division, National Meteorological Center, U.S. Weather Bureau, Washington, D.C.
1. HIGHLIGHTS
The floods of northern California and Oregon in the
latter half of December were the most important weather-
related phenomena during the month in the United States.
Very he:~vy rtlin that fell from the 19th to the 23d caused
unprecedented flood waters in scores of river communities
from northern Ctdifornia coustal drainage areas through
western Oregon. Ewlier storms had set up antecedent
soil and surface conditions that contributed to the flood
situation, m d h t e r heavy rnins delayed the return of the
rivers t o normnl. Mtlny high water records established
during the Cttristn1;Ls week of 1955 were exceeded, in some
instances by as much ns 10 ft. or more. Thousands of
persons were driven from their homes by the flood waters.
Numerous commrt rtities were isolated as highways and
bridges were closed or washed out. Some communities,
such as IUnmatth Glen, Calif., were totally destroyed.
At least 40 persons lost their lives as a direct or indirect
result of the floods [I ].
Scattered and locdly severe flooding also occurred in
Idaho, Washington, ;~d Newda. The Sacranlento River
in California rose to signific:mt levels, but flood control
projects of recent years kept this river in the bankful
stage at Smratnento. Scattered flooding also occurred
tts far south 11s the SrLn Jonquin Basin in California.
Another nmjor weuther feature of December was a
blizzard t h t crossed the Northern PlrLins during the 15th
and 1Gth wtd cnrlsed heavy losses in cattle and sheep.
This storm also contributed to the flood situation of the
west coast, for i t precipitated hetLvy snows o~7er the
western 1nollnt;Lins. .Later tnuch .of the snow in the
Coastal m d ClLsctLde Ranges melted during the heavy
rains.
2. MEAN CIRCULATION
The nlew 700-nlb. circulation for December (fig. 1)
represents t i decreme of the mid-latitude westerlies over
the Western Hemisphere from the previous month.
The average wind speed I L t these latitudes was S.9 m./sec.
compa,red to 9.S m./sec. in November. This decrease
was first manifested in :I slow-down of the progression
described by Dickson [2] in his discussion of the November
circulation, nnd then after the middle of December retro-
gression occurred. Strong blocking ridges became en-
trenched over both the Atlttntic and Pacific Oceans
(figs. I tmd 2). Blocking was also present during the
month over centrd Asia but was not nearly as pronounced
as over the two ocems. The Pacific block had been present
during November but became much more intense in
December. Both the negtttive 700-1nb. height anomaly
to the south and the positive anomaly over the Aleutians
increased considerably in magnitude. The Atlantic block
was not present early in the month while the progression
mentioned earlier continued; however, soon after the
middle of December blocking became quite evident in the
eastern Atlantic and moved steadily westward as i t
intensified during the lnst h l f of the month. The short
w2tvelengtlls of the Western Hemisphere circulation during
December we typical of blocking patterns. Over North
America the main trough was in the central part of the
continent, but :L second trough was located just off the
west coast .
Figure 3 illustrtites how markedly the westerlies split
around the two ocemic blocks. Over North America in
between tbe strong blocks, where upper-level westerlies
are norndly split in December, the separation of the two
branches was less thurl usud. The flow over nearly all of
North America w~ts quite zonal with much less amplitude
than I I O ~J ~, which was in sharp contrast with the flow
over the tLdjacent ocems.
3. TEMPERATURE
Frequent incursions of warm moist air from the Gulf of
Mexico to the east of the main trough (fig. 1) resulted in
above normal temperature in most of the eastern United
States (fig. 4). At the same time, stronger than normnl
westerlies brought mild Pacific air into the West from
Oregon and Idtho southwttrd which resulted in above or
near normal temperature for California, the high Plateau,
t'he southern Rockies, und much of Texas.
The extremely low average temperature in the Northern
Plains reflected the flux of cold continental air into the
ttreas east of the meat1 ridge dong the Rocky Mountains.
Many areas in hfontalla, including Billings, Great Fdls,
and Havre, reported the coldest December of record.
The source region for this continental air, Alaska and the
Yukon Territory, wns unusually cold during the month.
Temperatures a t Fairbanks, Alaska averaged 19' F. below
normal (fig. 4) nnd a t Dawson in the Yukon 24' %. below
normal [3]. The very strong and persistent upper-level
ridge over Bering Strait contributed to the cold ofAlasktx
and the Yukon by favoring surface anticyclones which
in turn favored clear skies stud strong radiational cooling
during the long nights.
4. PRECIPITATION
FLOODS IN THE NORTHWEST
The extremely hetLvy minfall over much of the Fur
West during December was associated with the strong
190 MONTHLYWEATHERREVIEW Vol. 93, No. 3
FIGURE 2.--Mcan 700-mb. height departures from normal for
December 1964, at intervnls of 50 f t . with centers labeled in tens
of feet and the zero isopleth heavy. A strong blocking pattern
prevailed over the Atlantic and Pacific.
FIGURE I .--Mean 700-mb. contours (tens of ft.) drawn at intervals
of 100 f t . for December 1964.
southwesterly flow and trough off the coast (figs. 1 and 2).
More than 300 percent of normal precipitation (fig. 5)
fell over large areas that usually have considerable
amounts of precipitation during December. Most of this
very heavy rain fell during a &day period and cnused
disastrous floods.
The large-scale synoptic situation over the eastern
Pacific that brought the flood conditions was analogous to
the regime that led to the record-breaking floods of De-
cember 1955 [4] in northern California and Oregon and to
several earlier major flood situations in the sa,me region.
Most of these past flood situations in northern Cali-
fornia and nearby States have been characterized by a
strong southwesterly flow aloft that brings very warm and
moist air from the vicinity of Hawaii up over the western
mountains. Reference to figure 6 will show that during
the period of heaviest rain this December (19-23) the
main jet flow approached from the southwest just north
of Hawaii in a similar manner. The only difference was
one of intensity. Not only was the jet stream very strong
when averaged for the 5-dsy period, but also the wind
March 1965 lullan
I I.
W. Posey 191
FIGURE 3."hlcnn isotachs at 700 mb. (ill. m./sec.) for December
1964. Solid :moms indicate the observed itxcs of maximum winds
and cl:Lshcd lines intlic:ttc the normnl LLXCS.
FIGURE 4.-llep~trturc of the nvcr:Lgc surface temperature from
normal (" F.) for December 1964 (from [l]). Record-breaking
low tcmpcrutures mere observed in the Northern Plains.
speeds were very persistent from day to day. At 40' N .,
140' mi. the 300-mb. wind speed averaged 134 kt. during
the 5 days, and at no time was the speed less than 110 k t .
u t this point according to the objective wind analysis of the
National Meteorological Center.
Hetivy rains that occur in association with strong jet
maximi are well documented [5]. Added to the dynamic
lifting associated with the jet stream mas the much
stronger lift supplied by the winds approaching the
western mountains in an almost right-angle trajectory.
The isohyets in figure 6 give the broad outline of the rain-
fall during the flood period. Some of the heavier amounts
are shown in table 1. The crest of the floods was greatly
increased by the rapid melting of snow in the mountains.
At Government Camp, Oreg., snow that was 45 in. deep
;?S'$& ~.. PERCENIAGE OF NORMAL PREClPlTAIlON
FIGURE Fj.-Percentage of normal prccipihtion for December 1964
(from [I]). Very heavy prccipilation fell over much of thc
West.
FIGUHE 6.-Mean isotachs (kt., thin solid lincs) over the c:tstcrn
Pacific and isohyets (in., tl:ishccl lincs) of precipitation in the
Northwest for December 19-23, 1964. Ifenvy solid arrows are
jet axes.
on the morning of the 21st T V ~S only 6 in. deep two days
later.
PRECIPITATION ELSEWHERE
Southwesterly flow to the eftst of the mean trough
through central United Sttites resulted in above n o r n d
precipitation in most sections enst of the Mississippi
River (fig. 5), thus further alleviating the drought condi-
tions of the previous summer nnd fall, following the im-
provement that occurred in November [2]. I n Ohio,
Dayton reported above normal precipitation for the first
time in S months and Cleveland also recorded above
normal precipitation following the driest November since
1924. Southern New Engltmd, southeastern New York,
and pnrts of Pennsylvania received precipitation amounts
in excess of 4 in. However, other areas in the Northeast,
TABLE 1.-Precipitation amounts for selected stations in northern California for December 19-24, 1964
Station Precipitation Station Precipitation (in.) (in.)
" __~- -
Brush Creek _._............
Blue Canyon ____._........
Shasta Dam _____.._.......
l.Ioopa ___................__ 22.5
18.4
Camptonvik .....___......
18.5
Whispering Pines ........_. '24.4 Carlmville __.....__._..___ 20.3
12. 0
21.1 Willits Howard Forest-.-..
1'2.9
1)eer Creek _...._________. 21.9
192 MONTHLY WEATHER REVIEW Vol. 93, No. 3
FIGURE 7.-(A) iOO-mb. contours (solid) and height departure from
normal (dotted) both in tens of feet, for Ilecember 1-5, 1064.
(B) Total precipitation (in.). (C) Surface temperature dcparture
from normal (" F.). (B) and (C) for week ending Deccmbcr 6,
1964 (from [I]).
while receiving some drought relief, still did not have
normal rainfall for the month, and the long-time moisture
deficiency became greater. Burlington, Vt. recorded 1.63
in. which WRS 0.50 in. below normal, and Williamsport,
Pa. measured 2.66 in. (0.3s in. below normal). For
Williamsport the deficiency for 1964 was 7.24 in. and
nearly 17 in. for the past two years.
The shortage in annual moisture remained large in
widespread areas of central and eastern United States.
FIGCRE 8.-(A) 700-mb. contours (solid) and height tlepnrturcs from
normal (dotted), both in tens of feet, for December S-12, 1964.
(B) Total precipitation (i n .). (C) Surface temperature departure
from normal (" F.). (B) and (C) for week ending Dcccmbcr 13,
1964 (from [l]).
hlIadison, Wis., with only 0.34 in. during December,
reported below nornlal precipitation during S months of
1964. Rochester, N.Y., ended the driest year since 18i7
with a monthly deficiency of 0.34 in. E l Paso, Tex.,
reported 0.52 in. for December, which was slightly above
normal, but with seven unusually dry months during the
year El Paso had only 5.35 in. during all of 1964. Clley-
enne, Wyo. reported only 40 percent of normtd precipita-
tion during the year, wllich was the driest since 1900.
March 1965 Julian W. Posey 193
FIGURE 9.-(A) 700-mb. contours (solid) a d height departures from
normnl (clotted), both in tens of feet, for I~ecembcr 15-19, 1964.
(B) Total precipitation (in.). (C) Surface temperature departure
from normal (" F.). (B) and (C) for week ending December 20,
1064 (from [l]).
Fort Myers, Fla., although reporting 32.83 in. for 1964,
received the least annual rainfall since records began in
1891.
5. WEEKLY VARIATIONS
A mean upper-level trough (fig. 7A) advancing slowly
through the eastern part of the country during the first
week of December caused heavy rn>itl in the lower Missis-
sippi Valley and excessive rain in southern G.eorgia and
northern Florida (fig. 7B). The central ,p u t of the
Nation ancl the Southwest which were dominated by the
FIGUI~E 10.-(A) 700-mb. contours (solid) a 1 1 d height departures
from normd (clotted), both in tens of feet, for December 22-26,
1964. (B) Total precipitation (in.). (C) Surf:tcc temperature
departure from normal (" F.) (B) ancl (C) for meek ending
December 27, 1964 (from [l]).
mean ridge had generally light or no precipitntion. Along
the northwester~l coast and the northern Rockies westerly
winds carrying moist Pacific air'resulted in fairly heavy
rain for that section during the week. Most of the Na-
tion east of the Rocky h4ountains experienced a very cold
week (fig. 7 C ), as the position and large amplitude of the
mean ridge allowed cold continental air to penetrate to
the Gulf of Mexico. Only a small area east of the Mid-
west trough had warni Gulf or Atlantic air present long
enough to give above normal temperatures for the week.
194 MONTHLY WEATHER REVIEW Vol. 93, No. 3
West of the Rocky Mountains the westerly flow brought
mild Pacific air and above normal temperatures.
As the progression of November 121 continued into the
second week of December, the deep Low in eastern
Canada and associated trough were replaced by a strong
ridge (fig. SA). However, the southernmost portion of
the mew trough over Mexico and Terns did not progress
but remained almost stationary. Then ns the next major
trough that moved eastward in the mid-latitude westerlies
merged with this low-latitude trough very heavy rain
sttwted along the Texas coast and spread up the Missis-
sippi and Ohio Valleys (fig. SB). Although the ridge over
the Rockies was much weaker than normd in the second
week it did result in light or no precipitation over the
Northern Plains and Rocky Mountains. Westerly winds
continued to give appreciable rnin or snow to the North-
west. I n response to above normal 700-mb. height and
anomdous southerly flow over the East, temperatures in
most of the eastern half of the United States were above
normtd (fig. SC). The western half of the country had
generally below normal temperature associated with
below normd 700-mb. heights.
Strong amplification of the broad-scale circulation oc-
curred in the third week of December over North America
and the adjacent oceans (fig. 9A). A very strong mean
ridge developed over the eastern Pacific and northwud
across Alaska and over the Arctic Basin. Two cent,ers
of positive 700-mb. height anomaly were associated with
the ridge, one over the Aleutians (910 ft.) and the other
over the Arctic (890 ft.). The resulting downstream
trough produced a blizzard across the Northern Plains
that gnve from 1 to 5 in. of snow from the Rocky Moun-
t i n s t o New England. Sleet and freezing rain fell along
the southern boundary of the snow belt. This storm
reached its greatest intensity just east of the Rocky
hllountnins (wind speeds reached 56 1n.p.h. at Rapid
City, S. Dak. and 57 m.p.h. at Cheyenne, Wyo.), then
weakened as i t moved eastward over the Great Lakes
into southeastern Canada. Before reaching the blizzard
stage this storm moved through the Northwest from the
Pacific and gave heavy precipittltion along the west coast
(fig. 9B). Because of the distance from the Gulf of
Mexico and the zonal character of the flow in southern
United States during the passage of this storm, the total
precipitation was relatively small over central ancl eastern
United States. The heavier areas of precipitation in the
Southeast occurred with trailing cold fronts. Following
the blizzard a very large and extremely cold air mass
spread southward from Alaska, in response to the ridge
amplification, and covered all of the United States before
the end of the week (fig. 9C). The lowest minimum
temperature observed in December during the 31 years
of record a t t h e Billings, Mont., Airport was reported
this week.
High-latitude retrogression during the fourth week
caused the ridge connection from the southeastern Pacific
to the upper-level High near the Aleutians to be broken.
The resulting flus of vorticity ru.ound the large High
centered over the western Aleutians in the fourth week
of December carved out a deep trough in the central
Pacific west of Hawaii (fig. 10A). The very long fetch
of warm, moist, southwesterly winds to the east of this
trough resulted in the flood-producing rainfall (fig. 10B)
discussed in section 4. This current was very deep with
almo;t parallel flow from the surfuce t o t h e 300-mb.
level (figs. 6 and IOA). Although the heaviest 7-clay
period of rain occurred during the week ending on the
27th, the heaviest &day rainfall fell from the 19th to 23d.
A redistribution of vorticity downstream following the
breaking of the ridge connect,ion in the eastern Pacific
led to 21 sharp trough development over the eastern United
States. One of the storms which cilused the floods of
the Northwest swung southenstward through this trough
a s i t deepened and produced the heavy rain shown in
the Southeast (fig. IOB). The strong southwesterly
current in the eastern Pacific also resulted in rapid and
intense warming that spread over most of the West and
the South Central States.
Temperatures were 15’ to 18’ 17. above normal for the
week over large a,reas of the high Plateau and the southern
Rockies (fig. lOC). In the meantime, retrogression of
the Atlantic block coupled with the sharp trough over the
East gave a very strong sontherly anomalous flow (fig.
10h) that warmed the East rapidly also. Temperatures
ranged from 6’ to 15’ F. above normal over much of the
country east of the Mississippi River, and many daily
maximum temperature records were set in the East.
The strong ridge over the Aleutitms and Alaska combined
with northwesterly flow in west8ern Canada continued
the very cold regime in the Northern Plains. Some of
this cold air moved across the mountains into the State
of Washington which was north of the main storm track,
ancl most of Washington had below normal temperatures.
Retrogression continued during the last few clays of
December with the positive anonmly over the Atlantic
centered neur Newfounclland by the end of the month.
This brought continued warmth to the East, b u t t h e
west coast had begun to cool agnin under the influence
of a deep trough near the coast. Heavy precipitation
continued in the East and Far West the last few days of
December.
REFERENCES
1. U.S. Weather Bureau, Weekly Weather and Crop Bulletin,
Nationul Summary, vol. LI, Nos. 40-52, December 7, 14, 21,
25, 1964 and vol. LII, No. 2, January 11, 1965.
2. I t . 13.. Dickson, “The Weather and Circulation of November
1064-Co1~trasting Weather R.cgimes Related to Long Wave
Progression,” Monthly Weather Review, vol. 93, No. 2, Feb.
3. C:landa Department of Tr:lnsport, Rlcteorological Branch,
Canudiun Weather Review, vol. 2, No. 12, Dcc. 1964.
4. J . F. Andrcws, “Thc Wenthor and Circulation of 1)ecembcr
1955,” Monthly Weather Review, vol. 83, No. 12, Dee. 1955,
5. 13. T-T. Johnson and S. M. Daniels, “Rninfall in Relation to thc
Jct Stream,” Quarterly Journal of the Royal Meteorological
Society, vol. SO, No. 344, Apr. 1954, pp. 212-217.
1965, pp. 123-126.
pp. 327-336.
U.S. GOVERNMENT PRINTING OFFICE:1965