National Weather Service Forecast Office
National Oceanic and
JOSE GEOGRAPHY TOPOGRAPHY
is located in the Santa ClaraValley which extends to the
southeast from San FranciscoBay. The valley is oriented
in a northwest-southeast direction with mountain ranges on both sides.
To the west and south lie the Santa CruzMountains. The most prominent
peaks in this range are south of the city and rise as high as 3791'
on Loma Prieta. East of the valley lies the
including the famous Mt.Hamilton
and Lick Observatory. The highest peak in this part of the range is
at 4570'. Southern and eastern reaches of the city extend into the foothills
of these ranges.
Most of the city
lies in the bay flats. Near Alviso the elevation
is about 20 feet above sea level, the airport lies at 60 to 75 feet,
and downtown about 90 feet. Outer edges of the city rising into the
hills are higher than 300 feet. These areas are the upper sections of
Alum Rock, Evergreen, AlmadenValley and CambrianPark. There are several hills
within the city. A group of hills just south of the Fairgrounds rise
to over 400 feet. Some of the hills subdivide areas of the valley into
smaller valley appendages. Most notably in the southern part of the
city, the SantaTeresaHills demarcate the AlmadenValley and rise to over 1000
Jose=s latitude and location on the west coast of
North America place the city in a Mediterranean
type climate. This classification is mainly identified by sharply contrasting
wet and dry seasons. The wet season runs from November through March.
82% of the yearly precipitation total falls within this period. Rainfall
is sparse from May through October. Rain during the summer months of
June, July and August normally totals only 0.20". Wet seasons are
cool, but mild. Dry season weather is very consistent, with warm sunny
experiences a marine dominated climate largely influenced by the cold
California Current. During the dry season, a
large and semipermanent high pressure area
over the eastern Pacific establishes itself. This produces steady northwest
winds parallel to the coast. These winds propel other physical forces
which transport the surface water offshore. The surface water is replaced
by the colder upwelling water. Strong upwelling during the summer and
weak upwelling during the winter keep sea surface temperatures normally
in the 50 to 60 degree range all year. Air coming in contact with the
ocean surface is cooled and often condenses to form stratus clouds.
The air near the surface cooled by the ocean waters
contrasts sharply with the warm summer air aloft. This creates
the stable Amarine inversion.@
The cool layer underneath the warm air is called the marine layer.
Dry season weather
is a combination of nocturnal low cloudiness, sunny days, and an afternoon
sea breeze. The interior of California
is heated by the hot summer sun, creating a thermally induced low pressure
area. This creates a strong temperature (and pressure) contrast (gradient)
from the coast inland. Cool air from the ocean moves inland with a strength
proportional to the strength of the pressure gradient. This pressure
gradient has been termed Aonshore
San Jose lies in a zone between true coastal influence and true inland
influence. These influences are governed by the strength of the onshore
flow. Onshore flow is the typical pattern during the dry season. Under
neutral or offshore flow, the weather in San Jose
has little marine influence.
Summer weather includes
nighttime low clouds to some extent for roughly two-thirds of the nights.
In the presence of a strong and deep marine layer, clouds will develop
during the evening and not clear until late morning. With a weaker marine
layer, areal coverage of the nighttime clouds
is often incomplete and random, or absent. Summer stratus clouds are
almost never low enough to produce fog with visibilities that hamper
temperatures, assuming a weak to moderate onshore flow pattern, normally
range from 80 to 85 degrees. Lows are consistently between 55 and 60.
Afternoon high temperatures are tempered by the sea breeze off the bay.
Offshore flow occurs
when high pressure at the surface builds over the Great Basin
province, with lower pressure off the coast. During offshore flow, all
marine influence disappears, and temperatures can soar to 100 degrees
or more during the dry season. These events occur during any season,
but are most common in Fall, and secondarily
in Spring. From Spring through Fall, dry warm air inland migrates westward.
Additionally, this warm air is further heated by adiabatic compression
as it descends the DiabloRange into the Santa
During winter, the air inland is not so dry and warm, so the warming
effect is minimal.
Dry season winds
are nearly calm every day during the time period of an hour or two after
sunset to around . Then,
almost like clockwork, a northwest sea breeze of 10 to 15 knots will
develop in the afternoon. Occasionally during the summer the pressure
gradient is oriented from south to north, producing a southerly flow.
The onshore flow then reaches San Jose
through the Aback
the southern part of the valley. Southeast sea breezes then develop,
mainly in southern sections of the city. During wet season storms, strong
southeast winds blow ahead of the cold front.
the dry season is rare, but does happen as a result of late or early
season storm fronts or southerly surges of subtropical moisture. These
surges can occur when the late summer monsoonal flow over Arizona
backs up into coastal California,
or with the residue of a dying eastern Pacific tropical storm. Coastal
drizzle, which can accumulate to measurable levels along the coast with
a sufficiently deep marine layer, hardly ever amounts to more than a
trace in San Jose.
mid-winter are generally temperate with highs between 55 and 60 degrees
during the day, and around 40 at night. Overnight low temperatures can
vary widely during a calm, clear winter night. The bay flats are milder
because of the modifying influence of the bay waters and the urban heat
island effect near downtown. Wintertime lows in this area average between
40 and 45 degrees. Away from downtown or the bay waters are the slightly
higher valleys of Almaden or the Evergreen
area. Here, the modifying bay and urban influence is smaller, and lows
average between 35 and 40 degrees.
Radiation fog from
the Central Valleyadvects into the Bay Area through the CarquinezStrait
during wintertime offshore flow. It can also spill into the Fremont
area over SunolPass from the LivermoreValley. Rarely, if ever,
does this fog advect all the way to San
Jose. Rather, radiation fog is more likely to
form on its own, especially in sheltered pockets away from the bay such
as the AlmadenValley,
Evergreen, and Coyote Creek areas. Dense fog can develop overnight when
the antecedent conditions include a moist lower layer (i.e. following
a recent rain event), clear skies, and calm winds. Dense fog is not
very common at San JoseInternationalAirport, but it can have
a great impact upon air transportation because of reduced visibilities.
Winter storms normally
occur due to occluded fronts moving into the
region from the west northwest. The Pacific High moves southward as
the westerlies of the Polar jet stream migrate
southward into California
latitudes. Occasionally, cold fronts from the Gulf of Alaska
or warm rain events induced by an active subtropical jet,
can produce significant rainfall. Thunderstorms are rare, but can occur
in cold and unstable air masses following a cold front. Normally, the
thunderstorms only produce brief heavy rainfall. Infrequently, they
produce small hail or exhibit severe behavior such as a funnel cloud
or very rare tornado.
early spring and fall is infrequent. Most storms during this period
produce light showers. However, polar and subtropical air masses can
collide, producing very heavy rain events. External weather altering
events, such as El Nino, can produce a significant increase to normal
rainfall and extend the duration of the wet season. Two of the three
wettest seasons of recorded history in San Jose
were during strong El Nino events (1982-83 and 1997-98). The other season
could have been an El Nino event as well, but it occurred in 1889-90,
long before sufficient understanding of El Nino developed.
Jose lies in the rain shadow of the Santa
Air flowing over the mountains sinks upon reaching the valley, decreasing
the necessary lifting mechanism for cloud formation and rainfall. Distribution
of average annual rainfall over the San Jose
area is largely dictated by topography, gradually decreasing from south
to north. San Jose averages
about one fourth of the rainfall received in the mountains. Contours
of equal rainfall nearly parallel elevation contours. The 20"
contour and foothill sections ring all but the northern part of the
city in a tilted U shape. The 14" contour runs east-west along
the northern part of the city, indicating rainfalls of less than 14"
north of the airport. The annual rainfall within city limits ranges
from about 13.5" near the bay, to about 24" in the southern
extremities near Los Gatos.
Snow is extremely
rare in San Jose, with only
a handful of well documented instances in over 100 years. Amounts are
usually a trace and have never exceeded one inch.
JOSE STATION HISTORY
for San Jose began in January
1874 when rainfall data were kept by the Southern Pacific Railway Co.
At this time the railroads coming into California
wanted weather records. In 1898, readings began at the Southern Pacific
depot on Bassett Street.
From May 1892 to September 1905, the records were kept by prominent
local citizens A.C. Simonton, W.H. Hunt, E.P. Llewelyn,
W.G. Taylor, and A. Bettens.
In September of
1905 a full time Weather Bureau station was established in the old Daugherty
building on South Second Street
(near San Fernando Street).
The earthquake and
fire of April 18, 1906
destroyed the building and some of the weather records. On July
1, 1906 the U.S. Weather Bureau took over the station and
re-established it in the FederalBuilding (now the San Jose
Museum of Art) located at the corner of San Fernando
and Market Streets. The station was equipped with more recording instruments
and became a full service station.
The depression years
of the 1930s demanded cuts in some of its city services, and in 1933,
the weather station was on the chopping block. However, the San Jose
City Council, through the efforts of City Manager C.B. Goodwin, arranged
to continue the work and assumed responsibility for the record-keeping.
On August 5, 1933,
the station was moved to City Hall on North
First Street across from St. James Park,
and Andrew B. Bennett became the record keeper.
In 1939 the instruments
were installed on the roof of City Hall after a year of comparative
readings because the CityHallPark
site was Abecoming
In November of 1940 Mr. Bennett resigned his position. Mae L. Bennett
(relation unknown) then continued the record until May of 1943.
In June of 1943
Ernest O. Billwiller of the San Jose Public
Works became Director of the Municipal Weather Station, with assistance
from fellow Public Works employees Ernest Walker and Leo Raiche.
The station remained
on the roof of City Hall until 1957 when it was moved to the new
City Hall at 801 North First Street.
passed away in May 1964 and the services of he and his assistants were
terminated the following September.
of Civil Defense Russell Lunsford petitioned for locating the weather
station at the Civil Defense Department because of the value of weather
data to weather-related emergencies. With the support of the U.S. Weather
Bureau and State Climatologist Robert Elford,
the San Jose climate station
was moved to the Civil Defense Department, its present site at 171
W. Mission Street.
With the passing
of Mr. Lunsford in June of 1965, the new Assistant Director of Civil
Defense, Charles Rehling, assumed the responsibility
of the weather station.
From 1971 to March
of 1992, Staff Technician Carol Sisemore served
over 20 years as the cooperative observer. She served under the direction
of Charles Rehling (1965-1977), and Emergency
Services Coordinators Robert H. Black (1977 to 1991), and Dr. Frances
Winslow (1991 to present).
In March of 1992,
Ms. Sisemore finished her work and handed
the observing responsibility to OES Executive Secretary Robin Joseph,
who keeps the record at the present time.
provided by Jan Null, retired NWS forecaster, in putting together and
making sense of an enormous amount of weather data.
Special thanks to
Robin Joseph, the San Jose
cooperative observer at the Office of Emergency Services, for lending
out data sets and answering numerous questions.
A great appreciation
to all observers who have maintained the integrity of weather records
in San Jose for over 124
B., 1939: The San Jose
Weather Station: A Description of the Station, Scope of Work, Services
Rendered and Notes of General Interest. Unpublished.
Black, Robert, 1983:
San Jose Municipal
Weather Station: A Full-Service Cooperative Sub-Station of the National
Weather Service. City of San Jose Memorandum.Unpublished.
1991: Weather Station Chronology.City of San
1962: Weather of the San FranciscoBay Region.
University of California Press.
1996: Selected Cities Guide: An Introduction to Microclimates in
the San FranciscoBay Area and
Null, Jan, 1995:
Climate of San Francisco.
NOAA Technical Memorandum NWS WR-126.
address unknown) Railway Co
Second Street and San