This time of year is characterized by subsidence drying and warming under the
northwesterly flow of the North Pacific high pressure cell. The dominant upper
level flow is southwesterly, which is primarily dry. Major changes in clouds
and precipitation patterns are generally due to a change in the synoptic pattern
which results in:
1) A northwestward surge, or a southeastward retreat of middle and upper level
monsoonal (subtropical) moisture.
2) Deep moisture associated with a weakening tropical depression or storm being
advected north and east.
Northern Mountains
In the early portion of the summer, there is usually enough low level moisture
to produce afternoon cumulus, and occasionally a shower or thunderstorm. As
the season progresses into July, however, the lower levels slowly dry out and
fire danger increases, with forests in this area often experiencing afternoon
relative humidities near 20%. Low dew points in the valleys below allow for
strong daytime heating and the generation of correspondingly strong afternoon
mountain-valley circulations which drive the surface winds. The dominance of
the North Pacific high provides ridge top winds from the west to north. Toward
the middle of summer, strong mid-level high pressure over the southwest United
States (‘Four Corners') veers this upper flow to the south or even southeast.
This can advect subtropical moisture over the region, resulting in clouds, widely
scattered high-based thunderstorms, and the inherent lightning-caused forest
fires.
Western Coastal Mountains/Clear Lake
This area is most strongly influenced by the North Pacific high pressure cell
for ridgetop winds that are sometimes gusty from west to north. This flow becomes
downslope into the Clear Lake area, and with a wide basin-type topography which
is exposed to much solar heating, produces afternoon max temps near lake level
that are nearly as hot as those in the adjacent Central Valley. In some of the
east-west oriented canyons, this onshore (westerly) wind can overwhelm the east
to southeast upslope winds by the late afternoon as these slopes lose their
sunshine. This can result in gusty dry downslope winds that may penetrate all
the way into the western valley. Marine stratus can occasionally affect this
area if the onshore pressure gradient is exceptionally strong, with the southern
and western valleys seeing this the most often. Relative humidities in the valleys
and the hills of these areas seeing this marine air will of course be higher
than those to the north and east. Precipitation and even afternoon clouds are
practically non-existent over these mountains, and tend to come in episodes
of days that generally coincide with when the Northern Mountains also are seeing
thunderstorms. The greatest incidence of this occurs early and again late in
the summer as the ‘Four Corners' high develops north and veers 700mb winds
to an easterly direction. Moisture can then be advected southward along the
ranges by the northwest flow of the Pacific High.
Southern Cascades and Northern Sierra Nevada
This is the most variable area of the CWA in the summer. Variations depend
upon the relative influence of two high pressure cells. If the North Pacific
High is very strong, then the area will see mainly clear skies and light surface
and ridgetop winds with low relative humidities. Several days of light winds
in this regime will generally allow evaporation and evapotranspiration to accumulate
enough moisture in the boundary layer to produce afternoon clouds and occasionally,
an isolated thunderstorm. If the ‘Four Corners' high strengthens, with
more southerly mid-level flow developing, moisture advected northward produces
more afternoon clouds, with more afternoon thunderstorms over the crest. By
August, both the North Pacific high and the ‘Four Corners' high tend to
shift north and a relative low pressure cell develops off the coast of Baja
California. The resulting strengthened southeasterly mid-level flow transports
more subtropical moisture north and east over the area. This allows afternoon
moist convection to grow deeper, as mid-level air entrained into the cumulonimbus
has a higher moisture content. Because the boundary layer remains relatively
dry with these mid-level moisture surges, the first day of cumulonimbus development
produces high-based thunderstorms with little rain and numerous cloud to ground
lightning strikes, usually causing fire starts in the National Forests. The
easterly component of the mid-level flow in the subsequent day's thunderstorm
environments can at times be mixed down to manifest itself as a gust front or
outflow boundary that propagates downslope, providing a focusing mechanism for
new cells, or enhancement for existing cells. These can produce locally heavier
storms with larger volumes of rain and hail. A few times each summer, thunderstorm
cells are able to propagate or redevelop as far down as the eastern portions
of the Central Valley floor in the evening, as the foothills cool and relative
humidities begin to recover. The large majority of western slope and foothill
thunderstorms are high-based. 40 to 50 thousand foot MSL echo tops have been
observed with the strongest thunderstorms, but when developing over elevations
of 6 to 10 thousand feet, and with bases 4 thousand feet or more above ground
level, these largest thunderstorms end up only being 30 to 40,000 feet deep.
Central Valley and Delta
Very warm afternoon temperatures and low relative humidities in the valley
are a result of:
a) Adiabatic warming due to subsidence from the North Pacific high pressure
cell.
b) The nocturnal inversion formed due to downslope drainage flow from adjacent
mountain ranges.
c) Inhibition of convection in morning hours due to combined effects of a)
and b). This inability of atmosphere to transport heat aloft results in a net
positive heat flux into boundary layer.
The boundary layer is generally mixed to at least 1km (> 3000 feet) over
the valley by afternoon on a typical summer day. This produces surface winds
that are generally up-valley, or, southerly in the center of the Sacramento
Valley, and northerly in the San Joaquin Valley. In the Sierra foothills are
seen light upslope (southwest to northwest) surface winds. Shortly after noon
in the Sacramento River Delta at the outlet of the Straits of Carquinez, an
onshore sea-breeze circulation develops. As the afternoon progresses, and the
onshore thermal gradient increases, the sea-breeze front attempts to advance,
continually being mixed out at its leading edge - usually somewhere between
Suisun and Davis - by the convective currents of the hot and dry valley air.
If the marine layer is sufficiently deep to sustain this erosion, the sea-breeze
front can advect to and through the Sacramento and Stockton metro areas. This
produces not only a marked temperature decrease, but also a wind shift and enhancement
known as the Delta Breeze. This is seen generally as southwest to southeast
winds in the Sacramento Valley and northwest to north winds in the San Joaquin
Valley. In the Delta itself, southwest to northwest winds can gust to over 30
mph, and can persist throughout the night and into the next morning. Generally,
a depth of at least 1500 feet is necessary to produce a cooling Delta Breeze
in the Sacramento and Stockton metro areas. Areas near the Straits, however
can observe an evening sea breeze and cooling with a marine layer as shallow
as a couple hundred feet. In these strong high pressure scenarios however, daytime
max temperatures in these areas will climb much higher. With the depth of the
marine layer being the major limiting factor on the cooling effects of the Delta
Breeze further inland, changes which indicate a lifting of the subsidence inversion
are key to accurate diagnosis and forecasting - namely 700-500mb height falls
and cooling. This can be very difficult to forecast, as the changes are sometimes
subtle, but generally the maximum marine layer deepening happens if the shortwave
producing these changes times its arrival to coincide with the diurnal pressure
minimum in the evening. This is of course too late to affect the current day's
max temp, but will likely have larger affects on the next days min and max than
if it had arrived at another time, as marine air subsequently advects into the
valley most of the night. The 24 hour maximum temperature cooling in these ideal
cases can be as much as 12 or 15 degrees F. In many instances, the models will
indicate what appears to be a sufficient surface pressure gradient (e.g., 4mb
from SFO to SAC) to advect marine air inland, when all that results is an increase
in wind speed with negligible cooling in the Sacramento metro area. In these
cases, the marine layer deepened some, but the sea breeze front was mixed out
somewhere just southwest of Sacramento, with only momentum keeping warm southwest
winds blowing through the city.
Precipitation in the Central Valley is very rare in the summer months and is
due to either to the remnants of a tropical system, or a thunderstorm that is
able to propagate into the valley off the foothills after sunset.
COOL SEASON
The strongest feature in the entire Northern Hemisphere during the winter is
the jet stream in the Northwest Pacific. The tremendous baroclinicity created
by arctic and polar lows from the Asian continent interacting with the warm
southern currents of the Western Pacific produces a large jet maximum. Wind
velocities at jet level typically exceed 100 kt over a large area of the Northwest
Pacific the entire winter. At the surface, a semi-permanent low pressure area
forms, or more correctly, a preferred area for cyclogenesis is found in the
vicinity of the Gulf of Alaska. Periods of moist southwesterly flow east of
this semi-permanent feature provides the climatologically wetter winters of
the west coast of North America north of 40N. In a typical year, less north-south
baroclinicity in the Eastern Pacific weakens the jet to 60 kt or less, and the
upper flow becomes diffluent, sometimes with northern and southern branches
of the jet forming. Beneath this pattern, surface low pressure systems occlude
well offshore and are typically filling as they reach California in the southern
branch of the diffluent jet with deep cyclogenesis in the Eastern Pacific relatively
uncommon. This split and southern branch can at times be strengthened considerably
by the development of a blocking ridge or high in the 120 to 140W longitude
range north of 45N. As one might imagine, the North Pacific surface high weakens
in the winter, and is displaced south and slightly east.
In an El Nino winter, the Eastern Pacific baroclinicity increases, allowing
the strong Western Pacific jet energy to exist further east. This decreases
the amplitude of the long wave pattern in the Eastern Pacific, making it more
zonal, so that there is an increased likelihood of cold polar waves propagating
into California. In a La Nina winter, the Eastern Pacific longwave amplitude
tends to increase, with less splitting, and fewer southern stream precipitation
systems affecting California. When and if the high amplitude longwave trof pattern
shifts east, precipitation tends to persist for extended periods, bringing warm
sector precipitation to large areas of the CWA. This shift can occur as the
ocean-atmosphere system is transitioning out of a La Nina pattern and into an
El Nino pattern.
Northern Mountains
Winter precipitation-producing weather systems rarely miss this area, unless
the Eastern Pacific split is very strong and the southern stream carries the
low into Central and Southern California. Since the majority of the precipitation
that occurs over these mountains in the winter is the warm sector variety, the
form of precipitation can vary greatly over the area. With low level flow in
the absence of forcing generally being north to northwest and downslope, inversions
form readily, and dry, dense air often becomes entrenched in the valleys. In
the presence of moist southerly pre-frontal flow over these valleys, wet bulb
cooling can lower the rain-snow line 2000 feet or more below the large-scale
wet bulb zero elevation despite warm advection. The heaviest precipitation areas
tend to be the ridgtops and south-facing mountains in northern Shasta County,
with the western ridges generally receiving more than the eastern ridges. Under
moist southerly low level flow, this area is due north of the converging effects
of the Northern Sacramento Valley. This acts to focus the heaviest precipitation
downstream from low level trajectories directed from about 150 -210 . This area
is usually closest to the coldest air associated with a winter system, and so
the most significant post-frontal lowering of snow levels occurs in these mountains,
with 1000-2000 foot levels not uncommon. A relatively infrequent but potentially
dangerous situation develops occasionally after the front has passed. A unidirectional
southwesterly wind profile interacting with the terrain produces training convective
cells which tend to set up in a line which runs from roughly from southwest
Shasta County northeast into the McCloud Arm of Lake Shasta.
Western Coastal Mountains/Clear Lake
While snow in the western coastal mountains is infrequent, it does usually
occur each winter. Precipitation is light across much of the region, as it is
generally in the precipitation shadow of the west slopes of the coastal range.
East slope totals are greatest as one looks further north where some of the
moisture-converging effects of the northern portion of the valley can be realized.
Also, extremely deep low pressure systems off the Pacific northwest coast will
produce strong isallobaric southeast winds, this in turn enhancing the orographic
precipitation on these east slopes.
Southern Cascades and Northern Sierra Nevada
Beginning in the north, aside from Lassen Peak, which at 10,437 feet towers
over all nearby mountains and has distinctly harsher weather, the ridges here
range from 6000 to 8000 feet in elevation and are oriented northwest to southeast.
This makes weather systems in which the moist warm sector flow in the lowest
10,000 feet has trajectories in the 220 to 250 range the most productive orographic
precipitation producers. Indeed, this is the wettest portion of the Sierra Nevada,
with some areas averaging over 80 inches of precipitation each year. This area
runs roughly from the Cascades near Mt. Lassen southeast to include the mountains
of the Feather and Yuba river drainages of the extreme Northern Sierra. As one
looks further south in the Northern Sierra into the I-80/US50 corridor and the
American and Cosumnes basins, the ridge lines becomes oriented more north-to-south.
Looking further south in the range into the Stanislaus drainage, the Sierra
ridge again becomes oriented northwest to southeast, but by the time a frontal
system progresses this far south, the condensate supply has usually been significantly
reduced. Large precipitation events here tend to be more infrequent and are
usually associated with either a stalled front or an unusually strong southern
branch of the polar jet. Post frontal training of convective cells, similar
to those that set up in Shasta County can occur in the foothills of the Sierra
as well, with major southwest-northeast oriented canyons like that of the American
River providing the focusing. In the typical evolution of a California precipitation
event, the veering of low level flow from south to west generally coincides
with the shearing of the deepest moisture east and north out of the state. However,
even post-frontally, there is typically plenty of low level moisture to move
upslope, with the increase in convective instability near and behind the front
at times more than compensating for the less-than-favorable flow orientation.
These situations may be difficult to discern from satellite imagery, as persistent
rain and snow showers can be produced from clouds with tops barely exceeding
15,000 feet MSL. This is because the bases of these clouds are often on or near
the ground level, with local IFR conditions quite common. In addition to the
north-south variations in precipitation, there also exist gradients with elevation,
in which precipitation rates increase with elevation up to around 6000 feet,
and then remain fairly constant, or drop off slightly with increasing elevation
to the crest. This of course varies from storm to storm, with cold (drier) storms
seeing the maximum lower and warm (moist) storms, higher. As winds generally
continue to increase with height independent of the moisture profile, winter
storms that are cold and dry typically have mainly blowing snow problems, while
warmer, more moist storms produce the blizzard conditions of snow and blowing
snow near the passes.
Central Valley and Delta
Because differential heating of the continent and ocean is much weaker, the
onshore component of the northwesterly flow from the North Pacific high is non-distinct,
especially inland, and can be easily overwhelmed by an offshore flow and/or
synoptic-scale forcing. Thus, Central Valley and Delta temperatures and winds
are virtually isolated from the effects of the ocean in the cool season. In
the absence of storm systems, the low level wind field is generally dominated
by downslope drainage flow off the colder mountains, strengthening the nocturnal
inversion. Under the influence of a strong inversion and with little mixing,
shallow radiation fog forms readily overnight. The fog depth generally ranges
from 200 feet in the lower portion of the valley, to only 10 or 20 feet in the
Northern Sacramento Valley areas. An up-valley wind will usually develop and
mix the fog out by late morning, but can persist through much of the daytime
hours in the portions of the season (Dec-Feb) where daylight is very short and
diffuse and upslope flow is never realized. Clearing becomes more difficult
each day, as the fog layer itself reflects solar energy that might otherwise
be heating the ground.
The influence of a given Pacific shortwave on the populated areas of the Central
Valley varies greatly and is a function of the magnitude of the high pressure
that tends to exist over the Western U.S. coast relative to the pressure falls
being produced by the shortwave. In general, however, the evolution of phenomena
are as follows, with weak systems stopping early in the list and stronger systems
producing extremes of each item on the list:
a) South winds increase and back to southeast, with the magnitude of the wind
change somewhat related to future rainfall rates. The exception to this is in
the extreme Northern Sacramento Valley, where the winds stay southerly and intensify.
These pre-frontal winds can become extremely strong with a rapidly deepening
Pacific system. In December, 1995, sustained winds of 52kt and gusts to near
70kt were measured in the Northern Sacramento Valley with a 957mb low off the
Oregon Coast.
b) Light rain begins in the Northern Sacramento Valley. In the weaker storms,
dry dense air near the surface can be left undisturbed by the south-southeast
flow in areas like the northern and western portions of the valley, including
the I-5 corridor north of Redding. This can produce local wet bulb cooling and
snow down to 100-200 foot elevations.
c) Light rain begins in the Southern Sacramento Valley.
d) Moderate to heavy banded precipitation forms ahead of the upper cold front
over the ridges of the coastal range and move east over the valley.
e) Winds speeds increase to their maximum. Light rain begins in the Northern
San Joaquin Valley as rainfall rates level off in the Northern Sacramento Valley.
f) With the passage of the upper cold front, winds shift from south-southeast
to west-southwest.
g) Rapid clearing may occur, or, if the frontal passage occurs in the late
morning, convective clouds, scattered showers, and possibly thunderstorms may
form.
Valley thunderstorms are actually possible at any time during the cool season,
as this is the time of year when low level moisture is most plentiful. The likelihood
is further enhanced by the fact that, as cold advection occurs aloft behind
the upper front, terrain blocking often diverts and modifies the surface front
to the point that there is little, if any, cooling and drying occurring in the
lowest kilometer over the valley, except directly adjacent to the Delta and
in the extreme Northern Sacramento Valley. This increases the convective instability
in the post frontal environment, leading to showers and thunderstorms. Afternoon
valley thunderstorms become more numerous later in the cool season in the months
of March and April as there is more solar heating at this time, producing greater
instabilities before boundary layer moisture has been significantly reduced
by many weeks of heating. While funnel clouds are quite common in these environments,
severe thunderstorms and true tornadoes in the valley are relatively rare. With
thunderstorms essentially being cool season, large amounts of small hail are
typically produced by these storms, producing agricultural damage without being
severe. While strong persistant rotating updrafts can form in valley thunderstorms
given the proper shear, the resulting supercell is often not given the space
to fully develop features that lead to the production of significant tornadoes,
such as a rear flank downdraft interacting with the updraft. This is mainly
due to the limiting width of the valley, being only about 40 miles wide at its
greatest girth. Thus, an F0-2 tornado forms and disspates within a time frame
of about 15 minutes. Damaging vortices are more commonly produced by either:
a) a landspout, in which the subcloud thunderstorm updraft, with some pre-existing
(weak) rotation along a convergence zone, is vigorously stretched to induce
increased rotation, or
b) a gustnado, in which a thunderstorm downburst locally enhances horizontal
vorticity which is then tilted into the vertical.
While the supercell tornado provides the greatest opportunity for detection
by radar, it is by far the least common. Landspouts and gustnadoes on the other
hand occur more freqeuntly, but provide few radar clues.
Following the passage of a shortwave, cold dry air typically settles in over
Oregon and then the Great Basin. This will greatly enhance the northerly pressure
gradient down the Sacramento Valley, producing gusty winds that can be damaging.
These winds tend to be strongest in the early morning as the pressures north
and east of California diurnally peak near this time, with heating in the afternoon
having the tendency to form more up-valley gradients. The effects of the north
winds on the valley temperatures and humidities depend on the strength of the
passing system. A strong system will have a strong front and a lot of cold air,
with the level of maximum cold advection occurring in the lowest couple of kilometers.
This produces north winds which actually are advecting cold air into the valley
and may even introduce the chance for a freeze. On the other hand, a weaker
system will have its cold advection max at a higher elevation. This is the more
common situation in the spring and fall. This cold advection aloft causes air
to descend two or three kilometers before reaching the Sacramento Valley floor
and actually will produce adiabatic warming and rapid drying rather than cooling.
When this occurs early in the cool season before precipitation has begun, fire
danger can increase dramatically.
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