HUNKER DOWN

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 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
week.


Thoughit'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 tuned.*


*******************************************************************
Charles N. Alpers, Ph.D.
U.S. Geological Survey, California Water Science Center Placer Hall 6000
J Street Sacramento, CA  95819-6129

tel.       916-278-3134
fax.      916-278-3013
e-mail   cnalpers@usgs.gov

<http://profile.usgs.gov/cnalpers>