Winter Solstice and Current Weather

The winter solstice occurs at 9:30 PST this evening.  At that time, the sun will be directly over the Tropic of Capricorn, or 23.5 degrees S, in the Southern Hemisphere, the southernmost location on Earth at which the sun is directly overhead.  This represents the longest day of daylight for Southern Hemisphere locations and the shortest daylight period here in the Northern Hemisphere.  Starting tomorrow, the daylight period will become incrementally larger for us until the Summer Solstice next June.

It certainly hasn't felt much like winter here in the Bay Area - we have received very little rain this month and are quickly building up a precipitation deficit.  A persistent upper-level ridge of high pressure located over the eastern Pacific has been responsible for the dry but occasionally cool and windy weather.  This pattern will likely last into early January.
Right now, a strong offshore pressure gradient is developing across California, something we have seen repeatedly this month.  Just of the past couple hours, these offshore winds have produced major temperature and humidity differences across the Bay Area.  In the North Bay, temps rose several degrees and humidities dropped - after the sun went down.  This was due to northeasterly flow descending the Sierra and then the Coastal Range.  As air descends, it is compressed, causing it to warm and dry.  The graph attached shows the temperature, dew point, and relative humidity at the Santa Rosa airport for the past three days.  Notice the final few hours - temperature jump and dew point (and relative humidity) decrease.  The temperature jumped from 42 to 58 between 6 and 7 p.m., a pretty remarkable increase with the sun already down.  Similar conditions were experienced in Napa and some locations in the Central Valley.  Oakland, meanwhile, is already down to 40 degrees with 86% relative humidity.

Happy Winter!

Where's the rain?

It's certainly been dry around the Bay Area for the past couple weeks.  As of this evening, San Francisco has only received about 57% of its average rainfall for the rainy season to date (since July 1).  The strong upper-level ridge that was offshore last week, and resulted in the strong offshore winds across California, has moved inland and has steering incoming Pacific storms well north into Canada.  Along with the dry weather, we've also had poorer air quality across northern California.  First, winds have been light, resulting in stagnation.  Second, the upper ridge overhead acts to trap pollutants near the surface.  And third, with the cold nights, many residents turn to their fireplaces for warmth - but this burning is a huge source of particulate matter and soot.  So on days like these, please try to avoid using your fireplaces if possible.

So where has the rain been?  Lately, and much of this year for that matter, many cities in the Ohio River Valley surrounding areas have been setting rainfall records.  A snowy winter last year was followed by a very wet spring, frequent summer thunderstorms, and now a very wet autumn.  Cleveland, Cincinnati, Louisville, and Philadelphia have all now set new records for yearly precipitation - breaking some records that had been standing for over a century.  The map of year-to-date precipitation shows parts of Kentucky, Ohio, Pennsylvania, and New York with over 20 inches of rain above normal, or 150 to 200% of normal.


In contrast, the southern Plains and Gulf Coast are running large precipitation deficits, which in tandem with strong winds and hot temperatures this summer, produced the major wildfires across Texas.

Back to the future, our weather pattern should slowly change over the weekend and more so next week as an upper-level trough finally approaches the West Coast.  We should see a few chances for showers around San Francisco, but no major storm system are in the foreseeable future.

Windy weather

As I'm sure everyone has noticed, it's been quite windy at times over the past week in the Bay Area, along with southern California and much of the Southwest.  We had a classic set-up for a high wind event, similar to the event I described in the previous post, but much stronger.  Looking at the 500 mb pattern from last Thursday, we see a strong ridge over the eastern Pacific and a strong trough over the western U.S.  Imbedded in the trough is a strong shortwave over southern California, indicated by the tight gradient in heights.  This 500 mb pattern resulted in a strong offshore gradient at the surface - very strong winds blowing from the Great Basin westward across California.  Winds at higher elevations were easily above 50 mph with gusts well over 100 mph at the Sierra crest.  The winds are also enhanced locally through canyons, notably in the Los Angeles area, where most of the wind damage and power outages occurred.

As from the previous post, offshore winds also warm and dry as the air descends the Sierra down to sea-level at the California coast.  Temps reached the 70s on several days in the Bay Area.  In fact, outside of South Florida and South Texas, San Francisco was the warmest location in the country for a time on Friday - how often does that happen?!  The winds also scoured out the low-level moisture associated with the marine layer; you may have noticed how clear the skies were.  I noticed how clear Mount Diablo looked, which often appears hazy.  Finally, we can thank the dry offshore flow for the lack of dew on our car windows in the morning.

Looking ahead, aside from Monday, winds will be much lighter most of this week as the upper-level ridge over the Pacific slowly moves inland.  This will bring continued sunny skies with seasonably warm temps, but it will feel more humid again near the ocean and bays as the offshore flow weakens.  No rain in sight for the next week - certainly a dry start to December.