Quick update - rain delay

It's finally raining in San Francisco after much delay.  The system that impacted the Pac NW with heavy rain, ice, and snow yesterday finally slid south and brought light rain to the Bay Area, which will continue on and off overnight.  Rainfall totals will remain below a quarter inch in SF though, which is less than previously expected.  A considerably stronger system will move in Friday afternoon and night, with up to an inch of rain possible along with gusty winds, so evening plans tomorrow should include an umbrella.  Much of Saturday and Sunday will likely be dry with only passing showers, so weather impacts for the 49ers game should be minor.  A third system will move in Sunday night, with moderate rains possible.

After that, unfortunately, it appears the persistent ridge of high pressure that was over the West Coast for most of the past few months will rebuild.  This will send heavy rains northward, leaving us with dry and warmer conditions likely through the end of January.  So, it does appear that this transition to a wetter pattern across California will be quick.

Rain still on the way

The major change in the large-scale weather pattern is fortunately still on track (and actually is already underway).  Today, a cold upper-level trough moved into the Pacific Northwest, bringing rain and even snow down to low-elevation cities like Seattle and Portland.  This system also cooled our temperatures dramatically as onshore winds increased.  This system has also dislodged a stubborn ridge of high pressure that has been present much of this winter along the West Coast.  This will allow a parade of storm systems to impact northern California starting mid-week.

Monday, Tuesday, and much of Wednesday will still be dry in the Bay Area.  Despite mostly sunny skies though, the air mass aloft is much cooler, and cannot support the temperatures in the 60s to near 70 we have been enjoying lately.  The first in a series of cold fronts will impact our area late Wednesday, with rain developing in the North Bay and spreading south into San Francisco after dark.  Then, a couple more cold fronts will move in, with the strongest one expected late Friday into early Saturday.  This strongest front will bring rain even to southern California.

What is uncertain though is where the heaviest rain will be and how much will fall.  The models have been fairly consistent in producing the bulk of the heavy rain north of San Francisco, from Sonoma County northward into Oregon.  The image shown is model estimated cumulative rainfall from now until just after midnight on Saturday.  The model depicts a tight precipitation gradient from north to south across the Bay Area, with 2 to 5 inches across the North Bay, 1 to 2 inches around San Francisco, and perhaps only an inch or less across the South Bay and southward.  This creates a very difficult forecast - 5 inches of rain has considerably different ramifications than 1 inch, so nailing down exactly where the rainfall bulls-eye will be key.

The weather pattern looks to remain active into early next week before quieting down again.  A few inches of rain (and heavy snow in the mountains) will be much welcome.  But we must also keep in mind that such rainfall amounts are not unheard of for this time of year, and will not erase the huge deficits from November, December, and now the first half of January.

Finally, winter. Eventually.

It's been a long time in coming, but signs are pointing to a major change in the weather pattern across the northern Hemisphere.  So far this winter, the coldest air has been largely confined to the Arctic, with an absence of strong storm systems to pull that cold air southward.  Along the West Coast, a strong ridge of high pressure has been the dominant feature.  The first image is a computer model forecast for tomorrow (Jan 12), showing the position of upper-level features (about 5 to 6 km above the surface).  The colors indicate vorticity, or spin.  And in general, wind follows the black lines.  The ridge is noted by what looks like a "hill" in the black lines stretching from California northward almost to the North Pole.  This has steered incoming Pacific storm systems northward, usually into British Columbia, and leaving us with dry, mostly sunny weather under northerly winds.

Finally, this high pressure ridge is forecast to break down and shift northward starting this weekend.  The second image shows the forecast for next Wednesday (Jan 18).  The big high pressure ridge that was along the West Coast strengthened and moved northwestward (or retrogressed, because its position moved opposite the usual west to east motion).  It's forecast to be a huge high pressure system over eastern Russia.  While this happens, strong westerly flow from Asia undercuts the high pressure system, stretching from Japan eastward and aimed at the Pacific Northwest.  And, embedded in that westerly flow are several storm systems, noted by the vorticity (colored) regions.  The first one will move into Washington and Oregon on Wednesday, but will drag a cold front across northern California, and finally bring us some rain.  Then, several more systems will follow.  It's still way too early to tell how much rain we will get.  Much of the heaviest rain may be focused north of here, but we can at least bet on some decent rainfall totals, and accumulating snow in the Sierra above 4000 to 5000 feet.  It's also uncertain how long this new pattern will last.  We could be in for an active few weeks, or the atmosphere could reset back to a pattern similar to what we've had most of this winter.

Lake Effect Snow

Since our weather here in the Bay Area remains benign, let's look at some interesting weather occurring further east.  After a mild December, very cold air is finally making its way into the Midwest and East.  For many locations, strong winds with temperatures in the teens and 20s are expected, but only light snow.  The major exception will be locations downwind of the Great Lakes.

This time of year, the water temps of the Great Lakes are usually in the 30s and 40s.  When an Arctic air mass drops south over the lakes, with air temps in the teens and 20s, a large vertical temperature gradient is created, from the relatively warmer lake waters to the very cold air above.  This results in instability.  As the cold air moves over the lakes, it is heated and moistened from below and rises.  As this lake-modified air rises, it cools and releases its moisture in the form of clouds and precipitation, impacting areas downwind of the lake.  It's a relatively simple atmospheric process but the devil is in the details.  Forecasting lake effect snow with precision is difficult.  Lake snow tends to set up in narrow bands of heavy snow - in one location, whiteout conditions may be occurring, while a few miles away, the sun is shining.  Wind direction is the major determiner in what locations receive the most snow.  Usually for the Midwest and East, cold air comes from the west, northwest, or north.  Thus, the heaviest lake effect snows are usually found to the south, southeast, or east of the Great Lakes.  Cities like Marquette and Grand Rapids in Michigan, Cleveland, and Buffalo are all on the east or southeast sides of the Great Lakes, and typically receive much more snow than cities on the west sides of the lakes, such as Detroit, Chicago, and Milwaukee.

As I said earlier, a cold air mass is currently moving southeast over the lakes, triggering heavy lake snows along the Lake Superior and Michigan shores, and will develop along the eastern Great Lakes shortly.  The radar image shows the narrow bands of heavy snow (darker green).  This event is expected to produce about a foot of snow some spots, along with 40-50 mph wind gusts.  In the worst historical lake snow events, some locations receive several feet of snow - this tends to occur most along Lake Superior and downwind of Lake Ontario.

Still no major rains coming to California anytime soon.  If this dry pattern continues through January, we will risk a serious drought situation heading into the spring and summer.