Frost occurs on still, clear nights when heat from the ground radiates to
the sky until it drops below freezing. As water vapor turns to liquid and
then to ice, some heat is released, so temperatures on these frosty nights
hover at 29 - 30F.
This nips the newest foliage on subtropicals such as
Citrus, but a string of holiday lights (not the little twinkle lights)
generates enough heat to prevent damage.
A ridge of high pressure then settles over us; conditions are stagnant and
air is 'trapped'. Temperatures drop steadily, ground and river fogs
coalesce into one big foggy mass. Satellite images show a big bank of fog
nestled in the soup bowl of the Sacramento Valley: an hour's drive east or
west gets you up and out of it.
This gloom can settle in for days, lifting by
mid-day (but not clearing) and settling back down at night. In one record
event we went 15 days without seeing the sun, and the day's high
temperatures barely topped the mid 40's. That can make folks pretty
cranky, and it's usually right around the holidays. "White Christmas": has
a different meaning here.
Weather folks on TV give us a day or so warning, but they tend not to differentiate between frosts and freezes. Don't panic on a frosty night, but take action when a regional freeze threatens. Here's a hint: still & cold = frosty, windy & very cold = freezing. Don't pick all your lemons on a frosty night, but you may lose your crop during a prolonged freeze.
Freezes occur between mid-December and early January, when days are shortest and preceding days have already been foggy and cold. The 1998 freeze happened December 26, so folks returned from holiday travel to find dead plants. Going away? Tender plants should come inside, and marginal ones should be tucked into south or east facing nooks near the house for added warmth.
The greatest risk to plants in a freeze is desiccation (damage to leaves and stems caused by drying), so soak well (especially container plants). Succulent leaves rupture as the water inside freezes, so your jade plant should be brought indoors. Our goal with woody plants is to keep them alive-don't worry about the foliage of Citrus or Bougainvilleas at this point.
Protection measures will each give a couple of degrees of protection. Securely drape with plastic to trap heat and prevent drying; spray the plant with an anti-transpirant; leave the aforementioned holiday lights (or a shop light with a 40-watt bulb) on all night.
Winter storms mostly originate in the Gulf of Alaska and head towards the
Pacific Coast. When the "storm door" is open these hit every few days.
Whether they hit us (sometimes) or Oregon and Washington (usually) depends
on how far south the jet stream has shifted.
These storms are cold and
fast-moving, dumping 1/2" to 1" of rain in less than a day. The snow level
drops to a couple of thousand feet, and there may even be little snow
flurries on the valley floor. Although water may puddle for a few hours,
our soils can readily absorb this amount of water.
Some storms come off the warmer part of the Pacific, and warm air can hold more moisture. These storms are bigger, wetter, and slower, lasting a couple of days. The snow level is usually above 5000'. When these hit the coastal mountain range the clouds elevate and cool rapidly, leading to some very high rainfall totals on this side of the Valley. Last December [2002] folks behind Winters recorded 6 - 7" of rain during one storm in early December! These "Pineapple Express" storms really pack a punch when they collide with cold air from the Gulf of Alaska as the sudden cooling squeezes out the moisture.
Clay soils can't absorb water when it comes down this fast, so runoff and standing water are inevitable. Plants are surprisingly tolerant of having their roots stand in water for a day or so in the winter, because the fungi which attack roots are mostly inactive in cold weather.
More
than a day without oxygen in the root zone can be harmful, and root damage
isn't noticeable until warmer weather. Digging a quick channel (a fun
project for kids!) to drain the water to the curb or lawn can save
valuable plants.
These are the storms which break branches and topple trees. Fall or early
winter would be good times for a visit from a certified arborist to
correct poor branch angles or reduce the weight on large-limbed
evergreens.
Check the staking of young trees, as these storms whip the
branches and tops. Trees that fall suddenly usually had defective roots to
begin with. A young tree whose trunk is an inch or more in diameter may be
impossible to straighten, and should probably be replaced-but check with a
professional first.
Climatologists and oceanographers know that ocean temperatures follow a pattern along the Pacific coast: some years they are warmer than average (El Niño), and some years they are cooler (La Niña). This gradual movement of warm water back and forth across the Pacific is called the Southern Oscillation. El Niño years tend to be wet here; La Niña years tend to be drier and colder overall (but with more Pineapple Express storms hitting Northern California). In between are years like this [2003] (La Nada?).
So expect many days this winter with morning fog, a week or so of Valley Fog, a dozen or so storms at 1 - 2 week intervals, 17 - 20" of rain, and about a dozen frosts between Thanksgiving and Valentine's. But watch that Weather Channel, especially around the holidays!
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