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Area forecast discussion 
National Weather Service San Diego California
401 am PST Monday Dec 29 2014

Synopsis...
a storm system moving all the way from Canada to Southern
California will bring rain and snow beginning Tuesday afternoon
and continuing through Wednesday...with snow levels down to 2000
feet above sea level. It will also be very cold...with highs only
in the upper 40s and lower 50s Wednesday and Thursday at lower
elevations...with below freezing weather above 2000 feet in the
mountains. Isolated thunderstorms also possible near the coast
late Tuesday night. Fair weather will return by Thursday
night...with a gradual warming trend back to near normal
temperatures by the weekend.

&&

Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
counties...

The cold Canadian sourced storm system is still on track for
Tuesday through new years evening across socal. There are some slight
variations in the various models and we can now examine the high
resolution short term modeling as well to see how the smaller
scale features will vary throughout the region. Overall though
impacts still look significant due to very cold temperatures in
locations that do not normally see these conditions...as well as
accumulating snow as low as 2000 feet.

The most significant variation this morning is that the moisture
parameters do not look quite as significant for the San Bernardino
Mountains and the High Desert for significant snowfall...primarily
due to predominant northerly flow through that region...keeping
the low level moisture tap mainly turned off. This factor is seen
in the model soundings for locations in the High Desert where you
can see substantial near saturation aloft but significant dry air
entrainment in the low levels...therefore the snow will have to
overcome that dry layer before reaching the ground. Precipitation
will be showery and accumulations of snow will be a little more
hit and miss than we have previously forecast. Still...some
accumulations are likely in the High Desert...1-3 inches...with
3-7 inches for the higher san Bern mountains. Further south...the
Riverside and San Diego mountains will be more under the gun for
heavier accumulations of snow due to more favorable transport of
low level moisture...perhaps 6-10 inches there in the higher
peaks. In addition...the higher peaks of the coastal valleys and
Santa Ana Mountains could see a few inches of snow too...therefore
we have added those areas above 2000 feet in the Winter Storm
Watch.

A slight risk for thunderstorms still is on the menu as well for
the coastal waters and adjacent portions of San Diego and Orange
counties. The interaction with the relatively warmer waters will
bring decent surface based instability with elevated instability
just inland. Cells seem most likely to form in the relatively
warmer airmass late Tuesday night. Inland precipitation will be
convective but likely too cold for thunderstorms...but convective
snow showers could be briefly heavy.

Another factor with this storm is a concern for public safety due
to the fact that temperatures will be abnormally cold in the
metropolitan areas...combined with brisk winds and wet weather may
cause a significant impact for those without adequate shelter.

Dry air entrainment from northwest to southeast will take place
Wednesday night into Thursday morning shutting down the
precipitation machine. We have ended the precipitation a little earlier
than previously forecast. Cool weather looks to continue through
Friday...and then east Pacific ridging will bring a gradual warming
trend through the upcoming weekend.

&&

Aviation...
291019z...VFR with scattered clouds near 2000 feet mean sea level through 00z Tuesday. After
00z...areas of broken ceilings 2000 feet mean sea level with higher layers developing
through 4000 feet mean sea level Monday night and spreading inland to the coastal
slopes by 12z Tuesday...clear above.

&&

Marine...
219 am...no marine concerns through Monday. Winds will increase
Tuesday ahead of a cold winter storm that will bring scattered
showers and a chance for thunderstorms Tuesday night. Heavy ice pellet
showers and isolated waterspouts are possible with thunderstorms.

Onshore flow will increase to near 15-20 knots Tuesday afternoon/evening...
turning from the north overnight into Wednesday morning. A Small Craft
Advisory may be needed for this period...but confidence not high
enough at this time...and winds are marginal.



&&

Skywarn...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.



&&

Sgx watches/warnings/advisories...
California...Winter Storm Watch from Tuesday afternoon through Wednesday
afternoon for the apple and Lucerne valleys...Riverside
County mountains...San Bernardino County mountains...San
Diego County mountains.

Winter Storm Watch from Tuesday afternoon through Wednesday
afternoon above 2000 feet for the San Bernardino and
Riverside County valleys-the inland Empire...San Diego
County valleys...Santa Ana Mountains and foothills.

Pz...none.

&&

$$

Public...Brotherton
aviation/marine...jad

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