The rainy season in western Oregon runs from October through
May, while eastern Oregon experiences its rainy, or snowy, season from late
autumn through early spring. Strong storm systems develop in the upper level
flow over the Pacific during the rainy season, bringing rain to the lower elevations
and snow to the higher elevations. Occasionally, a sub-tropical feed of moisture
often referred to as the Pineapple Connection, will be tapped by the stronger
storms. The Pineapple Connection is just a term used to describe a continous
stream of upper level moisture originating from the tropics, often near Hawaii.
This stream of moisture is warm and as a result the air can hold more moisture.
The subtropical moisture will enhance the precipitation process in the storms,
producing more precipitation than would normally be expected. Flooding can occur
if the several storms move across the same area in succession, with heavy snow
falling in the higher terrain. There is at least one subtropical connection
that brings heavy rain to some part of the Pacific Coast nearly every year.
The key to how much precipitation falls during a storm is closely related to
how strong and persistent the subtropical connection is.
Another process that brings heavy rain to Oregon occurs during
the spring and summer months. Thunderstorms can produce heavy precipitation
is a very short amount of time, sometimes on the order of minutes. Again, a
strong feed of upper level moisture can enhance the rainfall. However, the source
of upper level moisture can vary during the summer. Often the feed of moisture
originates over the southwestern United States, often as an extension of the
monsoon moisture from Baja California. Monsoon moisture comes from the tropics
of Mexico as a result of clockwise circulation around a large high pressure
area over the southern Rocky Mountains. In addition, weaker Pineapple connections
occur, but are often short-lived.
November 24 to 26, 1999
[This narrative written by George Taylor, Oregon
Climate Service]
An intense rain storm caused by a flow of warm, moist air from the southwest,
struck Oregon on Wednesday and Thursday, November 24 to 25. This marked the
first truly intense winter storm of the season, following a relatively dry
and mild fall in most of Oregon. Rainfall totals were very high. Tillamook
recorded 3.98 inches in 24 hours (and now has over 24 inches for the month),
Newport had 3.07 inches, and Florence received 3.63 inches. Willamette Valley
totals included over 4 inches in 2 days at Corvallis, 2.4 inches in two days
in Salem, and 3 inches in 3 days in Eugene. Clay Creech of the Hatfield Marine
Science Center in Newport sent a chart showing hourly
precipitation (and cumulative 2-day total) for Nov. 24-25 at the Center,
from the National Weather Service gage located there. Note how the highest
rainfall intensities occurred Thursday evening (during supper?). Note also
the 2-day total -- more than 10 inches!
In the Coast Range, rainfall was even heavier. John Kwait
of Siuslaw National Forest (USFS) sent data from several Remote Automated
Weather Stations (RAWS) stations: Cedar (southern Tillamook County), Cannibal
Mountain (northwest Lane County) and Dunes (along the coast just south of
Florence). Considering that Oregon's all-time one-day record is 11.65 inches,
Cedar's total on November 25th is rather significant! Here are the daily
totals for the 25th and 26th:
Range of Thanksgiving Day 1999 Rainstorm Totals
(inches)
Location
Nov 25 Rainfall
Nov 26 Rainfall
Cedar (near Tillamook)
11.00
5.54
Cannibal Mountain
3.92
4.33
Dunes (near Florence)
1.68
1.83
Rains were of sufficient duration and intensity to produce
local flooding in many parts of Oregon and Washington. The daily river gage
plots for the Siletz River on the central coast and the Trask River on the
north coast showed how quickly and how high the river levels rose. River
levels fell in a few days in most places, but remained somewhat high.
November 18 to 20, 1996
[This narrative written by George Taylor, Oregon
Climate Service]
Record-breaking precipitation throughout much of Oregon caused local flooding, landslides, and power outages over
much of the state during November 18-20. The rain resulted from a broad upper-air weather system of moist
subtropical air which originated over the tropical Pacific. The air mass reached central California over the previous
weekend, producing rainfall daily amounts as much as 8 inches. Gradually the system moved northward, reaching
southwestern Oregon on the 17th and spreading to the remainder of the state the following day. High rainfall amounts
were reported throughout the state.
The infrared satellite
image for November 18 shows the areas of warm and cold air over the
Northwest. To the north of the frontal boundary, the air mass was quite
cool; snow was reported from Portland northward into Washington. South of
the front, very mild air and heavy rains prevailed. Winds were also very
strong, exceeding 70 mph at some coastal locations in southern Oregon.
December 20 to 24, 1964
The December 1964 rainstorm was undoubtedly the most severe rainstorm to ever
occur over central Oregon, and among the most severe over western Oregon since
the late 1870s. Several observing stations across central Oregon recorded
two-thirds of their normal annual rainfall in just 5 days. Scores of stations
set new records for both 24-hour totals and December monthly rainfall totals.
Widespread severe flooding occurred, with at least 30 major highway bridges
receiving such damage as to make them unuseable! The new John Day multi-million
dollar bridge was destroyed as were scores of bridges on county and secondary
roads. Hundreds of miles of roads and highways were washed out or badly damaged.
Thousands of people had to be evacuated due to ensuing floods. The Willamette
River at downtown Portland had a stage of 29.8 feet. This was a record high
for the winter season, and was within inches of the peak stage during the
Columbia River spring flood of 1948. Hundreds of homes and other buildings
were destroyed and an even greater number were badly damaged. Following is
a table of December 1964 rainfall totals and the normal rainfall for December.
December 1964 Rainstorm Totals (inches)
Location
December 1964
Normal December Rainfall
Comments
Albany
12.55
6.75
none
Arlington
6.87
1.33
Record December rainfall
Ashland
11.28
6.74
Record December rainfall
Bend
8.74
1.73
Record December rainfall
Crater Lake
38.47
11.69
Record December rainfall
Detroit
30.86
12.83
Record December rainfall
Eugene
20.99
6.61
Record December rainfall
Falls City
27.05
13.83
none
Government Camp
28.62
12.78
none
Grants Pass
16.06
5.52
Record December rainfall
Heppner
4.40
1.38
Record December rainfall
Illahe
41.43
16.76
Record December rainfall
Lakeview
8.96
1.88
Record December rainfall
Medford
12.72
3.38
Record December rainfall
Newport
20.94
11.02
none
Pendleton
3.23
1.49
none
Portland, downtown
11.45
7.42
none
Portland, airport
9.97
6.38
none
Reedsport
22.01
11.94
none
The Dalles
9.05
2.36
Record December rainfall
Valsetz
40.25
22.27
Record December rainfall
July 13, 1956, near Mitchell, Oregon
A small, but very intense thunderstorm occurred just north and east of Mitchell, located in north central Oregon in
Wheeler county. The region is drained by Bridge Creek, which flows through the center of Mitchell and is usually less
than twelve inches deep during July. Within minutes, Bridge Creek became a raging torrent of water that swept
buildings away easily. The raging water completely destroyed or heavily damaged twenty buildings in the town center,
and washed out several bridges and sections of highway. The observing station was carried away in the flood so no
record of actual rainfall is available. Immediately after the flash flood, a United States Geological Survey person
visited the region. After careful surveying, he estimated that in fifty minutes, about 3.5 inches of rain fell, and at the
storm's center, nearly 4.0 inches fell.
November 22 to 24, 1953
Rainfall was very heavy over western Oregon, with the heaviest rain occurring
over the southern coastal area. At the peak of the storm, most observing stations
along the south Oregon coast reported 24-hour rainfall totals of 4 to 7 inches
and 72-hour totals of 6 to 10 inches. Widespread heavy flooding occurred in
all southwest Oregon streams and in most tributaries and the mainstem Willamette
River. Here are some rainfall totals:
Some November 1953 Rainstorm Totals (inches)
Location
1 day Rainfall
Storm Total
Gold Beach
6.56
9.80
Port Orford
4.84
7.25
Powers
5.00
7.76
Reedsport
4.45
7.34
Sitkum
5.72
9.97
Valsetz
5.84
10.22
January 16 to 19, 1953
The heaviest rain fell over the south coastal areas, where 24-hour rainfall
totals were generally 4 to 8 inches, and storm totals of 15 to 20 inches were
common. Here are some of the more spectacular rainfall totals:
Some January 1953 Rainstorm Totals (inches)
Location
1 day Rainfall
Storm Total
Bandon
4.00
8.72
Brookings
6.19
14.14
Cape Blanco
6.31
15.77
Denmark
8.07
16.24
Gold Beach
6.46
16.41
Illahe
7.17
17.37
Port Orford
5.80
14.03
Reedsport
4.11
8.52
Valsetz
5.40
15.05
Williams
4.26
9.77
October 26 to 29, 1950
This strong storm extended over the entire state, but reached its greatest
intensity over southwestern Oregon and the higher elevations of the Cascades.
In western Oregon, storm totals generally ranged between 10 and 12 inches
in the extreme south, gradually decreasing to near 4 inches at the Columbia
River. Even in the drier part of central Oregon, heavy rainfall occurred.
About one-fourth of observing stations in eastern Oregon reported 24-hour
rainfall totals that were greater than their normal October rainfall! Here
are some of the rainfall totals from around the state:
Some October 1950 Rainstorm Totals (inches)
Location
1 day Rainfall
Storm Total
Bend
2.72
4.98
Brookings
4.68
11.50
Cascade Locks
2.31
4.89
Cottage Grove
4.15
7.73
Crater Lake
5.17
9.30
Chemult
2.00
4.10
Glendale
4.49
11.81
Grants Pass
5.27
10.76
Illahe
5.42
14.80
Odell Lake
5.01
9.06
Persist
4.78
10.04
Sexton Summit
4.22
9.52
December 26 to 29, 1945
This rainstorm began over western Oregon late on the 26th and was over by
late on the 28th. Most rainfall fell on the 27th and 28th. During the peak
of the storm, western Oregon 24-hour rainfall totals of 3 to 5 inches were
not uncommon. Illahe, located in southwestern Oregon in eastern Curry county,
reported a 24-hour rainfall total of 8.25 inches of rain, and received 12.91
inches during the entire storm. Heavy precipitation fell across central Oregon
as well. Several observing stations in north-central Oregon reported 24-hour
rainfall totals of 3 to 5 inches, and storm totals of 4 to 6 inches.
December 26 to 30, 1937
While the entire state received some precipitation from the storm, it was
only in the northwestern Oregon that rainfall amounts were heavy. Two-thirds
of the observing stations in northwestern Oregon reported 24-hour rainfall
totals of 2 inches or more and at least a third of those stations had 24-hour
rainfall totals of 3 inches or more. Here are some of the rainfall totals
from around northwestern Oregon:
Some December 1937 Rainstorm Totals (inches)
Location
1 day Rainfall
Storm Total
Falls City
5.50
15.35
Portland Headworks
4.20
10.17
McNamers
5.15
17.13
Newport
3.83
8.90
Portland
5.01
9.34
Valsetz
7.86
24.00
Zigzag
3.91
11.50
Corvallis
5.37
16.24
November 19 to 21, 1921
Unusually heavy rainfall occurred over most of the state, but the heaviest
was over the northwest and north-central Oregon. Here are some of the rainfall
totals:
Some November 1921 Rainstorm Totals (inches)
Location
1 day Rainfall
Storm Total
Cascadia
4.69
8.59
Estacada
4.50
8.56
Government Camp
4.96
11.19
Portland Headworks
5.13
11.22
Mount Angel
5.10
9.00
Portland
4.13
6.71
Welches
6.55
11.93
Hood River
4.05
8.64
The Dalles
4.20
8.90
Moro
2.25
3.20
Wasco
2.23
3.69
November 18 to 24, 1909
This was actually two storms following each other in rapid succession with
no significant break in the weather between the first and second. During the
seven days period, rainfall totals were 10 to 20 inches along the coast, 4
to 6 inches in the western inland valleys, 7 to 14 inches along the western
Cascades slopes, and only 1 to 3 inches in eastern Oregon. Along the coast
and in the upper western Cascades, 24-hour totals of 4.50 to 5.50 inches were
common. At least 80% of the stations west of the Cascades received one inch
or more and 24-hour totals of at least 2 inches, with several stations receiving
3.00 inches or more.
June 14, 1903, near Heppner and Ione [historical photo]
This was surely the most deadly natural disaster in Oregon's recorded history. A strong thunderstorm,
accompanied by extremely heavy rain and hail, moved near Heppner, Oregon. The storm covered a very
small area, probably no more than 50 square miles. Heavy rain fell in a very short time, creating severe
flash flooding along Willow Creek, normally a peaceful stream flowing through the town center. The entire
town was swept away in just a few short minutes, drowning nearly 247 people. Eyewitnesses say
thunderstorm rains arrived as a 40-foot wall of water and the ensuing flood raged through town for over
an hour. In all, one-third of the towns' structures were wiped out. The massive runoff of water was a
result of heavy rain falling on the barren rocky hills, then flowing into the Willow Creek watershed. Only
fifteen minutes separated the first rainwater in Willow Creek at Heppner and the flood crest! There are
no rainfall records available for this storm because the weather observing station was completely
destroyed, drowning the observer and his entire family.
A similar fate would have been in store for the citizens of Ione, just 20 miles downstream. However,
telephoned warnings prompted an immediate evacuation and residents escaped to high ground. At least
150 homes were destroyed at Ione and bodies were washed more than 40 miles downstream to the
Columbia River.
November 12 to 17, 1896
Heavy precipitation fell over the full length of the Oregon coast and inland
over the northern three-fourths of the state west of the Cascades. Between
the 12th and 17th of November 1896, 15 to 20 inches fell in the coastal areas,
and 5 to 10 inches fell further inland. Maximum 24-hour totals of five to
seven inches were observed at many coastal observing stations. Most of the
Willamette valley locations reported 24-hour totals were of at least two inches.
January 28 to February 3, 1890
Very heavy rainfall fell over most of western Oregon, particularly along the
coast and in the Willamette valley. The seven-day totals ranged from 15 to
20 inches along the coast with 26.52 inches at Glenora in Tillamook County.
Rainfall in the Willamette valley was generally between 10 and 15 inches,
while 5 to 10 inches fell in the southwestern interior valleys. Precipitation
east of the Cascades was light.
December 12 to 13, 1882
In Portland, one 24-hour period had 7.66 inches of rain and the two-day total
was 10.75 inches. It is not known how widespread precipitation approaching
this intensity may have been due to the very few existing observing stations
at that time. From the limited information available, however, it appears
that the heavy rainfall was only in northwest Oregon.
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