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Out West; An appearance by the 'pineapple express'

Poised just off the west coast, will California finally receive badly needed rain and snow from this massive plume of Pacific moisture?
Poised just off the west coast, will California finally receive badly needed rain and snow from this massive plume of Pacific moisture?NOAA

It has the potential to bring massive amounts of rain and snow to a desperately dry and parched western U.S.

Finally, after two months of continuous and persistent western high pressure, changes taking place over the eastern Pacific offer some hope of relief to record drought conditions in California.

But don't hold your breath.

While satellite images of a deep and well-developed plume of subtropical moisture off the west coast are impressive, it's the kind of thing that could fall under that category of “close, but no cigar.”

Upper air models are congruent with a potentially wet pattern, and some precipitation is either already occurring or predicted for the west coast. But any hopes for really substantial precipitation, or an even remotely possible deluge with this pattern are up in the air due to a few important factors.

High pressure is still somewhat in command, although weaker and displaced from its former blocking position, still has influence upon the overall scenario. Latest models suggest also that this new pattern, while hopeful, might be relatively short lived, with stronger high pressure once again returning and re-establishing itself along and off the west coast. This could in effect shut off the 3000 mile-long fetch of moisture now extending from south of Hawaii all the way to northern California.

There are many complex and interacting factors involved here, and several differing scenarios which could play out over the next several days. West coast forecasting is made more challenging by absence of a well-established network or grid of surface and upper air observations and data across the vast expanse of Pacific Ocean, from which California and western weather originates.

The change is in place. Abundant moisture is available. Elements of lift, and timing suggest that precipitation may be somewhat limited, especially south of central California. The dynamics are such that while the potential for significant compensation for many months of drought are in place, there still needs to be more atmospheric instability, and some colder air aloft to maximize the current moisture potential.

Current rainfall estimates are for less than half an inch in Los Angeles this week, with amounts increasing to the north. Cool, mostly cloudy conditions can be expected all along the west coast through the week with perhaps an inch or more of cumulative rainfall mostly across northern California, Oregon and Washington.

Snow levels with this type of pattern tend to be relatively high due to warmer sub-tropical air. For California, 5 to 7 thousand feet would be a reasonable expectation, lower in the Pacific Northwest. The California Sierra snow pack is very diminished, currently at 20% of normal. It will take weeks if not months of persistent precipitation to replenish it.

But for the time being, a change has finally arrived. While maybe not monumental, there are possibilities and potential. Nothing can be ruled out.