Bad Weather Coming

West coast of North America, look out:

Originally Posted by USGS
From: UC Environmental Protection Services Issues [mailto:UCEPS-
Subject: Winter Storm Warning starting Sunday

Currently, the strong El Nino is reaching its peak in the Eastern
Pacific, and now finally appears to be exerting an influence on our
weather. The strong jet has been apparent for quite some time out over
the open water, but the persistent block had prevented it from reaching
the coast. Now that the block has dissolved completely, a 200+ kt jet
is barreling towards us. Multiple large and powerful storm systems are
expected to slam into CA from the west and northwest over the coming
two weeks, all riding this extremely powerful jet stream directly into
the state. The jet will itself provide tremendous dynamic lift, in
addition to directing numerous disturbances right at the state and
supplying them with an ample oceanic moisture source. The jet will be
at quite a low latitude over much of the Pacific, so these storms will
be quite cold, at least initially. Very heavy rainfall and strong to
potentially very strong winds will impact the lower elevations
beginning late Sunday and continuing through at least the following
Sunday. This will be the case for the entire state, from (and south of)
the Mexican border all the way up to Oregon. Above 3000-4000 feet,
precipitation will be all snow, and since temperatures will be
unusually cold for a precipitation event of this magnitude, a truly
prodigious amount of snowfall is likely to occur in the mountains,
possibly measured in the tens of feet in the Sierra after it's all said
and done. But there's a big and rather threatening caveat to that
(discussed below).
Individual storm events are going to be hard to time
for at least few more days, since this jet is just about as powerful as
they come (on this planet, anyway). Between this Sunday and the
following Sunday, I expect categorical statewide rainfall totals in
excess of 3-4 inches. That is likely to be a huge underestimate for
most areas. Much of NorCal is likely to see 5-10 inches in the
lowlands, with 10-20 inches in orographically-favored areas. Most of
SoCal will see 3-6 inches at lower elevations, with perhaps triple that
amount in favored areas.

This is where things get even more interesting, though. The models are
virtually unanimous in "reloading" the powerful jet stream and forming
an additional persistent kink 2000-3000 miles to our southwest after
next Sunday. This is a truly ominous pattern, because it implies the
potential for a strong Pineapple-type connection to develop. Indeed,
the 12z GFS now shows copious warm rains falling between days 12 and 16
across the entire state. Normally, such as scenario out beyond day
seven would be dubious at best. Since the models are in such truly
remarkable agreement, however, and because of the extremely high
potential impact of such an event, it's worth mentioning now. Since
there will be a massive volume of freshly-fallen snow (even at
relatively low elevations between 3000-5000 feet), even a moderately
warm storm event would cause very serious flooding.
This situation will
have to be monitored closely. Even if the tropical connection does not
develop, expected rains in the coming 7-10 days will likely be
sufficient to cause flooding in and of themselves (even in spite of dry
antecedent conditions).

In addition to very heavy precipitation, powerful winds may result from
very steep pressure gradients associated with the large and deep low
pressure centers expect ed to begin approaching the coast by early next
Though it's not clear at the moment just how powerful these winds
may be, there is certainly the potential for a widespread damaging wind
event at some point, and the high Sierra peaks are likely to see gusts
in the 100-200 mph range (since the 200kt jet at 200-300 mb will
essentially run directly into the mountains at some point). The details
of this will have to be hashed out as the event(s) draw closer.

In short, the next 2-3 weeks (at least) are likely to be more active
across California than any other 2-3 week period in recent memory. The
potential exists for a dangerous flood scenario to arise at some point
during this interval, especially with the possibility of a heavy
rain-on-snow event during late week 2. In some parts of Southern
California, a whole season's worth of rain could fall over the course
of 5-10 days. This is likely to be a rather memorable event. Stay

Samuel Y. Johnson
Western Coastal and Marine Geology
U.S. Geological Survey
Pacific Science Center

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Dinah from Kabalor

Author. Discardian. GM. Current project: creating an inclusive indie fantasy ttrpg

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