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WFO Sacramento County Warning Area Meteorology

WFO Sacramento County Warning Area Meteorology

(Updated Published Version - July 8, 2003)

Scott J. Cunningham


WARM SEASON

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.


(Updated Jan 2002)



Northern California Topography


 



Northern California Topography

Northern California Topography



Annual Average Precipitation


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