Advection fog is caused by a strong temperature gradient
between the open Gulf waters and the inland water areas adjacent to the
coast. This temperature gradient extends from Appalachicola, Florida to
the Mississippi River. The temperature gradient is attributed to
cold outflow from the rivers and bays along the Gulf Coast. There
is a potential for a fog bank to form off the immediate coast when the
temperature of Pensacola Bay is at least 5 degrees F colder than the Gulf.
The fog bank is produced by the return flow of continental polar air. The
lower layers of air are modified by a 24-48 hour trajectory over the Gulf.
The formation of a fog bank can be detected by an increase
in the radar ground return pattern east of KNPA during the afternoon and
in the visual GOES image. Keesler AFB (KBIX), Tyndall AFB (KPAM), and Hurlburt
Field (KHRT) can also observe coastal fog banks.
There must be a sustained onshore surface wind from 110
degrees to 250 degrees at 6-10 knots for a fog bank to move over immediate
coastal areas. When the onshore winds are less than 6 knots or calm to
light northeasterly during the early evening, the fog bank will remain
offshore until early morning.
An advection fog cycle usually begins 3-4 days after a
frontal passage and does not dissipate until the next frontal system moves
through. Early in an advection fog cycle, the fog bank over land
will dissipate by mid-morning due to normal heating. In a true advection
fog situation, when the surface winds are sustained at 6-10 knots, the
fog bank will remain adjacent to the immediate coast over water. When the
land areas begin to cools, the fog bank will move back over land areas.
Later in the advection fog cycle, dissipation occurs later in the day.
By the third day 300' ceilings and 1 mile visibility occurs around 1100
local. During the early afternoon, brief periods of VFR conditions
are observed when the stratus layer based at 600-900' becomes scattered.
Zero-zero (0-0) conditions with occasional periods of 200-1/2 from 1800
until 2400 local is a good forecast. 0-0 conditions from 0000 to
0900 local usually prevail. Visibility will improve briefly to 1-3
miles in prefrontal rainshowers and thunderstorms.