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In the preceding days, "omega" type blocking in the North Pacific had developed a "breakthrough" branch of the jet stream at around 36N. The jet was associated with relatively high 500 mb heights in northern California which reflected the subtropical nature of the air masses being advected into the state. |
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North American Analysis, 500 mb By the 11th, the breakthrough portion of the flow was over
western North America. Mild air spread over most of the continent
in association with this flow. |
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IR Satellite on ETA 300 mb Analysis, West Coast View The first major cloud band entering California was associated with what many operational forecasters term "overrunning" or "warm frontal" rain. In reality, many times actual frontal boundaries are difficult to discern for this type of pattern. In reality, warm advection (horizontal transport of warmer air into a given region) in of itself can be associated with large-scale lifting even if no front is present or if no "major" (short wave trough) disturbance aloft is evident. This was the case in the early morning hours of December 11, when very strong warm advection was associated with considerable upward motion, generation of clouds and generation of precipitation in north-central and northern California. |
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IR Satellite on MRF Surface Analysis, West Coast View
The surface pressure pattern favored the development of southwesterly
flow at the surface. Such flow is at right angles to the coastal
mountains and maximizes orographic rainfall mechanisms. |
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Plot of 700 mb Absolute Vorticity/1000-500 mb Thickness Forecasters may have been initially thrown off by the lack of strongly defined features (termed "vorticity maxes") to account for this strong vertical motion field. This is because the warm advection field is often ignored and put into a separate, secondary category when operational meteorologists try to diagnose vertical motion fields. (On the other hand, there is a relatively simple way to combine the vorticity advection terms and temperature advection terms on one chart. By overlaying the thickness field with the vorticity field midway in the layer, upward motion is "forced" when the thickness contours extend from "high to low" vorticity and vice versa, termed, "positive or cyclonic vorticity advection by the thermal wind (CIVA)" or " isothermal cyclonic vorticity advection." This chart shows the implied forcing for vertical motion at the 700 mb level (roughly 10000 feet). Note the area of CIVA over north-central California.) |
