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Fog in late Spring, Summer and Fall
During late spring, summer and early fall, fog of
any type does not create a problem except for brief
periods near sunrise when the visibility is reduced
to less than 4 miles. The following are scenarios
that have been researched to identify the onset and
dissipation of summertime fog regimes.
1) When high pressure sets up over east central AL/GA
with northeast to easterly flow from 500 mb to the surface,
fog will form after 06Z/01 local and dissipate about 12Z/07
local. Visibility can be reduced to 4 nm.
2) When the Bermuda High ridges at the surface from the
western Atlantic over FL/AL/MS, and the 850 mb high is
located over central AL, and the 500 mb high is located
over southern AL/FL border, (east of NAS Whiting Field),
expect fog in the early morning and haze late morning into
the late evening. Fog and haze will persist for 24 to 36
hours or longer as long as the SE wind component persists
at the lower levels. Approximate sky conditions: 020
Scattered to Broken and 250 Broken. Visibility will
decrease to 4-6 nm in haze, and 3-5 nm in fog. Fog will
begin to develop by 07Z/02 local.
3) When a surface and and 850 mb ridge/high moves or
develops to the NW of NPA, from LA through AL, and the
surface and upper level winds are northwesterly, a drying
condition will exist and fog and haze will not form. Sky
condition: Scattered Cirrus. When the high pressure system
moves eastward and a southeasterly flow develops at the
surface as well as the 850 mb, expect fog to develop after
06Z/01 local and haze to develop after 11Z/06 local reducing
visibility to 4-6 nm in haze, 3-5 nm in fog.
4) When a surface high develops or moves over central GA
and the 850 mb high develops or moves over the east coast
of FL (JAX, ORL area) or southern GA, expect the onset of
fog to develop after 06Z/01 local and continue for two to
four hours. Expect light SSE-SSW surface winds, <8 knots,
sky condition: 015-020 Scattered, Scattered Cirrus,
visibility: 4-6 nm in fog.
5) When the 850, 700, 500 and 300 mb upper level high and/or
ridge is stacked or nearly stacked over central TX, expect
visibility to lower to 5-6 nm in fog. Predominant winds are
a SE flow at all levels. If this condition persists for 36
hours or more (NPA on the strong side of the high), expect
haze to develop by the 2nd day and reduce visibility to 4-5
nm and fog to develop after 03Z/22 local reducing visibility
to 2-3 nm in fog. Expect daytime surface winds from 120-150,
at 10-12 knots, decreasing to less than 8 knots after sunset.
Expect haze to dissipate as the high moves eastward to the
NW of NPA as per para 3.
6) Occasionally a quasi-stationary front will extend north of
NPA across the N FL/ Southern AL/GA border. The weak surface
high will usually set up over the eastern Gulf of Mexico,
Central Florida, or the Western Atlantic. With the frontal
boundary just to the north, fog and haze formation is rare
due to the northwest-northeasterly component at 850 to 300
mbs. If the frontal system is located more to the north
across Central AL/GA and the surface high pressure center
is located over the northeast Gulf of Mexico, Central FL,
expect fog to form after 06Z/00 local decreasing the visibility
to 4-6 nm then dissipate after 10Z/05 local. If the front moves
to the south of NPA and there is a surface high located over
NC/VA look for fog to develop north of the field and inland,
but not at the station.
d. True frontal fogs are a rarity, but slow moving and
quasi-stationary frontal systems in combination with advection
fog produce widespread fog and low stratus that can persist
for days.
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