Advection Fog

Fog is a cloud that is actually on the ground!  There are many different ways that fog forms.  What’s behind advection fog, though, is pretty interesting.  Advection fog is most common along the West Coast of The United States during the spring and first few months of summer.  In fact, most of the fog that affects San Francisco, and the nearby coastal areas, is due to the advection fog process.  Namely the fog that affects the West Coast is known as sea fog,  Advection fog may last for hours, or even days.

The way advection fog, or more specifically sea fog, is winds blow over warm water that is relatively far out to sea.  There, the air picks up warmth and moisture.  The winds then take that air and drag it over cooler water nearer to the coast.  This process condenses the water vapor out of the air and develops the cloud.  The winds then take the cloud, which is near the surface of the water, and drag it over the near-shore areas of the land.

Advection fog can also occur in interactions between warmer water and cool landmass areas.

In fact, advection fog can occur during other seasons and in other parts of the country.  On a smaller scale, advection sea fog occurs in the Gulf South during the wintertime months.  Here, winds out of the southeast drag air over the warm waters of the central Gulf of Mexico and eventually over much cooler near-shore waters developing fog over places like New Orleans and Southeastern Louisiana.  In fact, sea fog will be socking in Boothville in far southeast Louisiana prompting fog advisories through Friday morning.

Cool Air Dips South, El Nino Rains in Cali?

Cool air is finally surging toward the South. On this visible satellite image, captured at about 3pm EST today, you can see the dense overcast clouds across the Central Plains. Temperatures are dropping to the 30s, 40s, and 50s in many locations. In Lake Charles, Louisiana, the forecast high is expected to drop from the upper 80s to the mid 70s by Friday.

In California, rain will finally taper off this evening into Thursday. At the bottom of this article, you’ll find the one day record rainfalls posted by the National Weather Service offices for Los Angeles and San Francisco.

Los Angeles TV forecasters have the latest on the storms in Southern California, while the Bay Area forecast can be seen on San Francisco TV.

Above average rainfall across the southern United States is usually forecast during an El Nino year.

According to the NOAA scientists, “El Niño’s impacts depend on a variety of factors, such as intensity and extent of ocean warming, and the time of year. Contrary to popular belief, not all effects are negative. On the positive side, El Niño can help to suppress Atlantic hurricane activity. In the United States, it typically brings beneficial winter precipitation to the arid Southwest, less wintry weather across the North, and a reduced risk of Florida wildfires.

El Niño’s negative impacts have included damaging winter storms in California and increased storminess across the southern United States. Some past El Niños also have produced severe flooding and mudslides in Central and South America, and drought in Indonesia.” ( Source: NOAA )

According to NOAA, the 2009-2010 El Nino is expected to strengthen during this upcoming winter period.

…RECORD 24 HOUR RAINFALL SET OR TIED AROUND THE SAN FRANCISCO AND MONTEREY BAY AREA TUESDAY OCT 13…

STATION NEW RECORD OLD RECORD

KENTFIELD 6.14 4.20 SET IN 1957

OAKLAND MUSEUM 3.86 0.37 SET IN 1988

RICHMOND 3.38 2.47 SET IN 1962

SAN FRANCISCO AIRPORT 2.64 2.62 SET IN 1962

SAN FRANCISCO DOWNTOWN 2.49 1.80 SET IN 1962

SANTA ROSA 2.74 (TIED) 2.74 SET IN 1962

KING CITY 1.65 0.30 SET IN 2007

MONTEREY CLIMATE STATION 2.66 1.14 SET IN 1962

SALINAS 1.05 0.39 SET IN 1992

SANTA CRUZ 3.16 2.49 SET IN 1957

…RECORD RAINFALL REPORTS FOR TUESDAY OCT 13…

STATION NAME NEW RECORD OLD RECORD

PASO ROBLES 1.80 INCHES 1.41 INCHES IN 1968

SANTA BARBARA APT 1.67 INCHES 0.62 INCHES IN 1957

WFO OXNARD 0.51 INCHES 0.34 INCHES IN 1962

BURBANK APT 0.59 INCHES 0.33 INCHES IN 1957

WOODLAND HILLS 0.66 INCHES 0.27 INCHES IN 2007 (PIERCE COLLEGE)

SANDBERG 0.57 INCHES 0.09 INCHES IN 1957

TV Forecasting in the Bay Area

If you go to my t-shirt shop, you’ll see a couple of shirts I made to represent San Francisco weather. My niece loves “The Horror! The Fog!”, because if you’re on the inland side of the coastal ranges, and you start to see the fog roll in, it can feel like a winter snow storm hitting on a summer day. San Francisco experiences heavy rains, thunderstorms, even snow on nearby mountaintops. During the summertime, forecasting high temperatures can be extremely difficult. You can forecast a high of 80 degrees, and the marine layer will move in, and your forecast high will be blown by 15 degrees! My friend Billy Poon, meteorologist at KPIX in San Francisco, helped me with this blog.

NWS Office San Francisco/Monterey Bay (I use this to check out the weather changes in the last 24 hours, surface pressure very important for forecasting shifts in winds. Wind shifts are not only important for marine forecasts in the Pacific and San Francisco Bay, but for fog forecasting as well.)

Weather Underground Bay Area Weather Sites (This map from Weather Underground shows all the personal weather stations available, much more than the local National Weather Service Office provides. Each station has a record of weather information.)

Unisys Weather Upper Air Models (Place I go to start on my 7-day forecast.)

GFS MOS Guidance (GFS MOS forecast guidance for temperatures around the Bay.)

NAM MOS Guidance (NAM MOS forecast guidance for temperatures.)

Raw FOUS Data (The FOUS… Billy uses this to check out the Relative Humidity at 3 levels, for Low, Mid & Highs clouds & rainfall over 60 hours.)

NWS San Francisco/Monterey Bay Weather Tables (This is a clickable map that will show 7-day model based weather forecast.)

Air Quality (Check the air quality for the day—big concern in the summertime.)

Airport Delays (Check for airport delays, especially San Francisco!)

Rainfall Forecast (Rainfall forecast broken down over 3 days, very fine detail. Billy says it’s a great tool during rainy season.)

California Department of Transportation – Road Conditions (Knowing the actual road conditions, can help you determine if you blew the rain/snow elevation on the roads leading up to Lake Tahoe!)

USGS Earthquake Map (And, of course in California, you will need this earthquake map from the USGS.)

Thanks Billy!

-Dawn Brown, Billy Poon, KPIX Meteorologist