The West Coast Low Latitude Storm Type ("Pineapple Connection")


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12 UTC 1/21/97 Sfc Isobars/1000-500 mb Thick

The locations A and B are in regions downstream of probable positions of surface warm fronts. Forecasters often refer to the vertical motion that occurs far ahead of surface warm fronts incorrectly as "overrunning." If this were the case, soundings in locations experiencing such "overrruning" would show apotentially warm, moist air overlying a colder (sometimes drier) air mass near the ground.

This model may work to DESCRIBE certain situations, particularly regions immediately north of surface warm frontal boundaries on the continent, but certainly does not account for the precipitation observed with low latitude storm types in California. A feature of such storm types is a large area of warm advection on the eastern side of the surface cyclones. In the case illustrated by the figure here, both A and B are in regions of warm advection.

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20 UTC 1/21/97 IR GOES-9 Imagery

Warm advection itself is associated with layer lifting of the moist boundary layer. This explains why warm advection often is associated with heavy precipitation in California even in the absence of strong surface boundaries (warm fronts).

In this satellite image, note the cloudiness over California. Moderate rain was ongoing at the time of the image in the San Francisco Bay Region in the area of warm advection shown on the previous surface chart. It was tempting for forecasters, applying the inaccurate view of "overrunning," to assume that warm moist air was "being lifted" over colder air northeast of the warm frontal boundary off the coast. This was not the case.

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Sounding A in Warm Advection Area In California

The 12UTC OAK sounding shows that despite the temptation to label the cloudiness and precipitation in California as "overrunning", that the roots of the precipitation were NOT aloft, as they would be in the classical "warm front" model. Note the nearly saturated air to 800 mb surmounted by dry air aloft. Note also the wind vear with height, a directional shear that is consistent with warm advection through the layer affected.

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Sounding B Near A Surface Warm Front In Michigan


The 12 UTC APX sounding from Michigan shows a more conventional thermodynamic structure associated with what forecasters assume occurs in the classic warm front model. In this case, the upper air site is near a strong surface warm front. Note the relatively warm, nearly saturated aloft, surmounting the colder air in the boundary layer. The upper air site, in this case, may be close enough to the surface warm front such that the conventional model of the result of "overrunning" may actually work here.

The point here is that the term "overrunning" is improperly used. Forecasters using the term are really trying to characterize one of the forcing mechanisms for synoptic scale lift (warm advection) that is as much a "dynamic" forcing mechanism as is cyclonic (positive) vorticity advection and is distinct from the mesoscale processes of frontal lifting. (See Chuck Doswell's short essay on overrunning to see his discussion on this misuse.)