a. Non-frontal stratus presents a problem for local VFR flying during the late fall, winter, and early spring. Forecasting stratus formation along the coastal areas requires accurate forecasting of wind direction and speed. Non-frontal stratus is produced under three different synoptic situations. It may be advected into the area during
periods of prolonged easterly flow, produced by mechanical lifting during
periods of southeasterly to southwesterly flow, or advected into the area
during situations that produce light ground fog north and east of KNPA.
1) The first synoptic situation
that produces stratus begins when a high pressure area is centered over
the mid-eastern seaboard with a long northerly wind trajectory over the
western Atlantic. As this flow turns back to the west across southern
Georgia and northern Florida, East Coast stations will first detect stratus
layers with bases between 800-1,500'. The stratus will be steadily
advected westward at a rate equal to the gradient wind. It usually arrives
in the local area within 18 hours after it reaches the east coast.
Ceilings in the local area range from 800-1,000' from midnight to noon,
local time, and 1,000-1,500' from noon to midnight, local time. KPAM,
KHRT, and Pensacola Regional Airport (KPNS) will normally get the stratus
before KNPA, and a check of these stations will provide a 30 minute to
1 hour advance notification of when the stratus will reach KNPA.
2) The second synoptic
situation that produces stratus in the local area is a moderate (12-15
knot) southeast to southwesterly low-level wind. When moist air moves over coastal land areas, stratus is produced due to mechanical lifting. This occurs when the return flow around a high becomes stationary and centered over the southeastern seaboard. After the second day of the return flow, KNPA has clear skies until the wind reaches the critical
velocity of 12-15 knots. When the critical wind velocoty is reached, stratus begins to develop from Panama City, Florida to New Orleans, Louisiana. All local coastal
stations rapidly become (within 15-30 minutes) broken to overcast at 800-1,000
feet. The Lifted Condensation Level (LCL) for the local area is a good indicator of the base of the stratus. Water areas off the
immediate coast remain clear in this synoptic situation.
Under continued southeasterly flow the stratus is gradually
advected inland. With heating, the stratus will lift to a maximum
of 1,500-2,000' and become broken to scattered from 1200-1500 local.
As cooling begins, the stratus returns to broken variable overcast with
the bases near the local LCL (Lifted Condensation Level).
Occasionally, stratus does not form when the critical
wind is reached. In this situation, low-level divergence has overcome the
3) In the third synoptic
situation, ground fog forms over inland areas and the wind velocity
is such that only light ground fog is reported. Stratus forms around
sunrise and rapidly covers the local flying area. Normally, KNPA
is the last station to be affected during this situation. This situation typically occurs during the spring and fall, when a quasi-stationary high produces a northerly flow during
In this situation, the wind is usually influenced by the sea breeze. At night the wind becomes calm and only a light ground fog forms. Near sunrise the land breeze reinforces the gradient wind and produces a resultant velocity of 12-15 knots. This produces surface frictional lifting which causes the stratus produced to be advected over
KNPA. KPNS generally detects the stratus about 30 minutes to 1 hour
before it reaches KNPA. Bases are found at the LCL (Lifted Condensation Level). Tops are usually not above 1,000-1,500 feet.
Visibility beneath all stratus layers remains good--generally
4-6 miles when bases are 500' and above, and 1/2-1 mile when bases are
less than 500 feet.
b. Stratus is often associated with frontal systems that
become stationary over the central and southern Gulf. Stratus becomes
widespread in the return flow as relatively cold air moves across the warmer
As the frontal system begins to move inland along the
coastal areas, ceilings are generally 2,000-3,000' at the onset lowering
to 800-1,000' after 12-18 hours. When the frontal system is within
100 nm of the coast, the ceiling lowers to 400-800 feet. These
conditions will continue until the frontal system passes to the north of
KNPA. Typical warm-frontal multi-layered cloudiness accompanies the
frontal system. The amount and intensities of rain with the frontal
system depend on the frontal slope. Clearing aloft occurs with frontal
passage, but low cloudiness persists due to moderate southerly flow after
Advection fog is likely if the wind drops below 15 knots
and remains between 6-12 knots. Satellite pictures are a valuable
aid to determine cloud amounts and associated weather accompanying the
Post-frontal stratus will be a brief problem during
the early morning hours after a sharp cold front has passed, and clear
conditions prevail after passage.
c. Stratus formation may occur on the morning following an afternoon cold-frontal passage. The frontal cold passage typically has produced abundant rainfall west through northeast of KNPA. Stratus forms as a result of surface frictional lifting of moist air by a moderate (12-15 knots) northwesterly flow. This is similar to the stratus formed with the land breeze reinforcing the gradient wind, except that this occurs during periods when the land breeze and sea breeze do not have an influence on the prevailing wind.
Shortly after sunrise, with a moderate northwesterly flow,
stratus forms west through northeast of KNPA and gradually becomes broken
variable overcast with bases of 800-1,200' over KNPA. By 1000 local,
the stratus becomes scattered with bases 1,500-2,000 feet.