Elevated Fire Danger

Tuesday night into Wednesday we have an elevated risk of fires for parts of the Bay Area.  Fires have not been a major concern across California this year due to last winter and spring being very wet, with generally cooler than average temperatures this summer.  But in autumn, while much of the country is cooling down, the wildfire risk can reach its peak in California.  In many years, the wet season does not begin until November, meaning the ground and vegetation have been dry for several months.  Also in autumn the marine influence weakens, with less fog and lower relative humidities.  The fire danger is highest when winds turn offshore (usually they are onshore, hence, the persistent overnight/morning fog).

Tuesday night and Wednesday, the weather pattern will lead to offshore flow, due to a strong surface high over Idaho and the Great Basin with lower pressure over coastal California.  This is indicated on the model forecast for early Wednesday morning.  The yellow lines are isobars, or lines of equal pressure.  When isobars are packed closely together, such as over northern California, winds are stronger.  In this case, winds will be quite gusty from the northeast, especially in the hills of the North and East Bay.  Air originating from the Great Basin is already quite dry, and becomes even more dry as the air descends the Sierra Nevada.  By the time that air arrives here, relative humidities are very low - perhaps down to just 10%.  The combination of these gusty, dry winds combined with sunny, warm weather will result in the enhanced fire danger.  Fortunately, the rains we received earlier this month have resulted in some greening of the vegetation, which will somewhat mitigate the fire threat.  Also, temperatures will not be excessively warm.  This weather pattern also leads to Santa Ana winds in southern California.  Santa Ana winds are also warm and extremely dry, and are accelerated through the valleys connecting the California deserts with the coast.

Otherwise, the longer range models are keeping the storm track well to our north into next week, which will keep us dry with slightly above normal temperatures.

Midwestern Autumn

Autumn may have some of the nicest weather of the year here in California, but I still find that I miss autumn in the Midwest.  The combination of mild days (50s and 60s) with low humidity and crisp nights - and the plethora of deciduous trees changing colors and losing their leaves - is less prevalent in coastal California (at least the leaves part).  The image below is a true-color visible satellite image (basically what the human eye would see from space) a couple days ago.  Skies were clear across the Great Lakes region, allowing us to see the vegetation below.  Notice the pockets of orange and red across the region - northern Illinois, Michigan's Upper and northern Lower Peninsulas, much of Ontario, etc.  Those are the dense deciduous forests that have changed color.  Further south, the vegetation is still mostly green.  Also of interest is the Great Lakes themselves.  The hazy swirls in southern Lake Michigan are sediment/sand stirred up from the bottom of the lake.  At the beginning of the month, a powerful low-pressure system moved across the Midwest; as the system moved east, strong northerly winds behind it persisted for several days, blowing down the length of Lake Michigan.  Those winds produced large waves (10-20 feet), which stirred up the lake bottom.

We have some great fall weather for the rest of this week in the Bay Area - high pressure building in will result in sunny skies and offshore flow, which will push the marine stratus westward away from the coast.  Highs should easily be in the 70s in San Francisco and 80s further inland.

Fantastic weather ahead

The storm system that has been plaguing the West Coast for the last few days has finally moved east, after bringing unseasonably heavy rains and mountain snows to much of California.  Now, a much different weather pattern is developing.  The system responsible for the wet weather was an upper-level trough (lower pressure) - this is being replaced by an upper-level ridge (higher pressure).  These system are apparent in this 500 mb model forecast map for tonight.  The upper-level trough is over the four-corners region while an upper-level ridge is along the immediate West Coast.  I'll get into more detail about these types of systems in a future post, but for now, suffice it to say that upper-level ridges are usually associated with "nicer" weather.  This ridge is forecast to strengthen over California right into the end of next week.  This will shunt weather systems over the Pacific northward into the northwestern U.S. and Canada.  And in the Bay Area, our air flow will become more offshore instead of onshore - this will largely prevent coastal stratus/fog from forming, resulting in nice weather even near the ocean.  Highs this weekend will be in the 60s to near 70º, and gradually warming into the 70s (coast/bay) and 80s (inland) by Wednesday and Thursday. 

Storms are over

The strong cold front has moved well inland now, after bringing several hours of moderate rains across much of northern California last night and southern California today.  After the front moved through the Bay Area early this morning, colder air moved in aloft.  At the same time, we had sun this morning, which quickly warmed the surface/ground.  This resulted in a big decrease in temperature with height - warmer air beneath much colder air - or instability.  This instability helped produce scattered brief, but heavy, rain showers and even a few lightning strikes across the area.  Now that the sun has gone down, we've lost the surface heating, and the pocket of cold air aloft is moving east, so the instability has decreased.  Thus, we will remain mostly dry tonight.  Officially, SFO has received 1.10" of rain this month, which already eclipses the average rainfall for October.  We won't see any more rain for a while though.  The computer models are showing sunnier and warmer weather building in this weekend and next week with highs in the 70s.

One storm down

The cold front is now moving into the Bay Area. Ahead of it, we had several hours of light to moderate rain, with the heaviest amounts north of the Golden Gate. Santa Rosa has received about 0.30" with San Francisco receiving 0.15" to 0.20". Not a big event by any means, but it was the first widespread rain since May. The front will weaken and move east overnight. The radar image also supports this, with very little rain to our west. Tuesday looks at least partly sunny.

The next storm will be stronger, with periods of heavy rain and gusty winds.  This storm will start to impact the Bay Area after midnight Tuesday night, with most of the rain falling Wednesday morning.

Next week's storms

In the last post, I talked about the active weather pattern predicted by computer models for the first week of October, but there was considerable uncertainty in that forecast. Well, as the event(s) draw closer, we have a better idea of what will happen.

Storm #1: The first storm is already impacting the West Coast, with a weak cold front moving into Washington, Oregon, and northern California. This is evident on the visible satellite image below. This will bring some light rain to those areas, but the precipitation will stay well north of San Francisco. This front is, however, helping to disturb and breakup the marine layer. That's why we're not seeing much fog around the Bay today. Also of note is the big swirl west of Mexico. Those are the remains of Hurricane Hilary. A few days ago, Hilary was a very strong hurricane, but has since moved north over cooler waters and weakened (hurricanes get their energy from warm ocean waters, usually mid-80'sºF or warmer).

Storm #2: The second storm is still well offshore but is stronger and will move further south than the first storm. The computer models are still a little uncertain on timing, but it looks like light rain will spread over our area Monday afternoon, starting in the North Bay. We won't get a lot of rain out of this system (maybe a quarter-inch), but this will be the first widespread rain event of the season.

Storm #3: We'll have a break on Tuesday with partly sunny skies. The final and strongest system will move in Tuesday night into Wednesday. This system will bring several hours of rain, which will be moderate to heavy at times - rainfall totals may be over one inch. Let's keep in mind that the average rainfall at SFO for the entire month of October is 1.04 inches. This storm also may bring a period of gusty winds as the cold front itself moves through. And, colder air associated with this front will result in snow in the high Sierras - mostly above 7000 feet.