Heads Up California – Atmospheric River Hitting Now

March 19, 2018

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New threat of mudslides, avalanche to burned area in Southern Cal.

Local folk: If asked to evacuate, don’t hesitate.

Daniel Swain’s blog WeatherWest:

The strongest storm of the year (and perhaps longer) for southern California is rapidly developing over the Eastern Pacific west of California. This system already has a visually spectacular presentation on satellite imagery, and is exhibiting almost textbook structure for an atmospheric river of the “Pineapple Express” variety (so named for the subtropical origins of the associated moisture transport axis near Hawaii). This slow-moving storm will take its time getting here, but will also linger after making landfall on Wednesday. As a result, a long-duration heavy precipitation event is expected from the Central Coast and southern Sierra Nevada (in the north) to the coastal plain in SoCal (in the south). The focus of very heavy precipitation appears to be Santa Barbara and possibly Ventura County (plus or minus 50-100 miles of coastline), but everyone in that above-mentioned region is going to get soaked.

This does *not* appear to be one of those atmospheric rivers (AR) with “all bark and no bite.” Why not? Well, an AR moisture plume is not itself sufficient for heavy precipitation–there still has to be an additional “lifting mechanism” that acts to “squeeze” all of that water out of the atmosphere. Sometimes, a very moist AR can come along that only produces significant precipitation in the mountains (as winds hitting the topography and forcing air to rise upslope is the only thing providing the requisite lift). When a very moist AR “attaches” itself to a strong surface low, though, the associated dynamical lift provided by the associated cold front can produce intense precipitation just about everywhere, which adds to the topographical enhancement.

The present storm appears to fall into the latter case: a surface low is expected to spin up west of San Francisco, with frontal lifting combining with favorable jet dynamics aloft to produce what could be a very impressive storm system across the southern half of California Wednesday into Thursday. Models are currently keying on a “frontal wave” late Wednesday as the surface low deepens, which could cause the atmospheric river to temporarily lift back northward before ultimately shifting back southward. The net effect, especially in Santa Barbara County, could be that an initial period of heavy rain will slowly taper off before rapidly intensifying a second time early Thursday when the cold front approaches. It is this second wave of rainfall that may be of most concern, as rainfall rates could easily exceed a half inch per hour. There are also some increasing signs of convective instability, especially on Thursday–and while this may not be enough for thunderstorm activity, it could easily be enough for intense convective bursts of rain with rainfall rates as high as an inch per hour!

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The big question: to what extent, for how long, and precisely where will this AR stall out on Wednesday-Thursday? If it moves relatively quickly, flood concerns will be considerably reduced; if it shifts northward, the Thomas Fire burn scar will be at less risk; but if a stall occurs near Santa Barbara–as currently appears possible–then watch out. Right now, it’s not possible to distinguish between these scenarios. But all of them entail quite a bit of water for a broad swath of SoCal. It is worth noting that the majority of major historical flood events in California have been the result of a slow-moving or stalled atmospheric river; such stalling is often as a result of a frontal wave, similar to the one described above. This is a storm to watch very closely on the Central Coast southward to LA County.

Just how unusual is a storm of this magnitude? When measured in terms of the “vertically integrated water vapor transport” (IVT, i.e. the amount of water in motion in the entire column of air above your head), the inbound atmospheric river may be among the strongest (or perhaps *the* strongest) on record for the months of March or April in Southern California. With water vapor fluxes of this magnitude, it is possible that the models are actually underestimating the precipitation potential with this event (and they’re spitting out some huge totals for much of SoCal already).

 

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3 Responses to “Heads Up California – Atmospheric River Hitting Now”

  1. dumboldguy Says:

    Ho-Hum! It looks like no one on Crock wants to get too excited over this. More rain and flooding/mudslides in CA?—-Ho Hum again, it’s NIMBY, so good luck folks. thoughts and prayers, etc.—good luck with FEMA.

    When are enough people ever going to realize that more of this is coming to a backyard near ALL of us? Oh, forgot—-President Pussy Grabber and the Porn Star are more important news (and that’s a good name for a rock group).


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