Here Comes The Fog Again

After an unusually cold and dry winter so far across the Gulf South, the pattern has taken a more typical turn.  High pressure is building back in, winds out of the southeast are setting up, and warmer temperatures are ushering back in, but also so is dense overnight fog.  As the pattern stagnates over the next several days, reduced morning visibilities can be expected across portions  of the Gulf states.  In fact Dense Fog Advisories have been issued for the coast of Southeast Louisiana for visibilities below 1/4 mile at times.  Cities that can expect slow morning commutes are places like New Orleans, Baton Rouge, and Biloxi.

Dense Fog Advisory February 15, 2011, Image: NOAA

What’s the deal with this Gulf Coast fog during the Fall, Winter, and Spring?  Well, its something called sea fog and it doesn’t just happen in Louisiana and Mississippi.  In fact, it also occurs in other places in the United States, like San Francisco, and even in other places around the world, like Hong Kong!

Sea Fog Formation, Image: Hong Kong Observatory

Take a look at the image above.  This is what causes sea fog to form.  Winds from a warm and humid source region blow over an area of colder water.  This helps to condense the moisture out of the warm, moist winds as they pass.  This process forms a low cloud.  Those same winds then push the newly-formed cloud over a nearby landmass and voila: fog.

In the case of the Gulf South, these winds are southeasterly and pick up warmth and moisture from the Central Gulf of Mexico, where the deep waters still have temperatures well into the 70s.  These rather tropical winds then run northwestward over shallower near-shore waters, that are much more subject to cooling by the cold wintertime land temperatures.  These coastal waters have temperatures only in the 50s.  This helps condense the moisture out of the warm, relatively muggy southeasterly winds.  Hence, a bank of low clouds forms.  The southeasterly winds then push that batch of clouds onshore and there you have it: sea fog and rough morning commutes across the Central Gulf states this week.

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.

Advection Fog From Above

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.

US Watches & Warnings, Image: NOAA

Ida Strengthens… Cat 1 Hurricane Again

Hurricane Ida, Infrared Satellite, Image: NOAA click to enlarge

Update from the The National Hurricane Center:

1115 PM EST SAT NOV 7 2009 …IDA REACHES HURRICANE INTENSITY… DATA FROM NOAA BUOY 42056…LOCATED IN THE NORTHWESTERN CARIBBEAN SEA ABOUT 120 MILES…195 KM…EAST-SOUTHEAST OF COZUMEL MEXICO… RECENTLY REPORTED A 1-MINUTE SUSTAINED WIND OF 74 MPH…119 KM/HR. THIS INDICATES THAT IDA HAS ONCE AGAIN REACHED HURRICANE STATUS… WITH MAXIMUM SUSTAINED WINDS OF AROUND 75 MPH…120 KM/HR. AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT IS CURRENTLY APPROACHING IDA AND WILL PROVIDE ADDITIONAL INFORMATION ON THE INTENSITY AND STRUCTURE OF THE HURRICANE IN THE NEXT FEW HOURS.

Ida Track Forecast, Image: NOAA click to enlarge

The track forecast has not changed much over the last 24 hours, with the exception that Ida may retain tropical characteristics until it is near landfall in the panhandle of Florida. Portions of the northern Gulf Coast continue to be under coastal flood watches. Heavy rain will also be a problem as a cold front slams into the approaching tropical air mass. Why does the National Hurricane Center think Ida will weaken in the center of the Gulf of Mexico?

Wind Shear Analysis, Image: University of Wisconsin click to enlarge

Once again, we need to revisit wind shear. Taking a look at this current wind shear analysis from the University of Wisconsin, the yellow lines in the Gulf of Mexico show us how much wind shear the storm will run into once Ida cross the Yucatan Straits. This map is difficult to read. What you need to know is the current wind shear over the Gulf exceeds 30 knots or about 35 miles per hour. This can be a hostile environment for tropical systems. National Hurricane Center forecasters expect Ida to remain a hurricane for the next two days, let’s see if that occurs.

-Dawn Brown