In the local Pensacola area, the primary wind direction
produced by the surface gradient is northerly from September through March,
and shifts to southerly from April through August. Wind speed is
usually less than 20 knots. In some cases sustained winds will require
small craft warnings for 24-36 hours. Gusty winds are fairly common
and can be expected with frontal passages, thunderstorms, southerly gradient
winds intensified by the sea breeze, and occasional tropical storms or
hurricanes. The terrain is such that there is no effective up-slope motion
Sea and Land Breeze regimes.
From May through September, the primary influence on surface
wind is the land and sea breeze effect. Sea breezes are more pronounced in summer, but will develop in any season when the land warms to 7-10 degrees F above local bay and gulf temperatures.
During an average summer day, the following usually occurs:
Between midnight and 0800-0900 local, the land is cooler then water and a land breeze sets up as winds drain from the NNW-NE 3-5 knots. When the land warms to above the bay temperatures (generally 0930-1030 local) the wind shifts to the SE, 120-160 degrees, at 5-8 knots. By early afternoon (1300 local) the true sea breeze veers and sets in from the SSW
(220) at 8-12 knots and increases to 15-20 knots at max land heating (1400-1600 local). The SW flow continues as the winds gradually decrease as evening cooling occurs. As the land slowly cools to below the surrounding water temperature, the winds initially become light and variable, then calm by 2200 local. By midnight, the air has cooled enough for the land breeze to set up again.
In August, the sea breeze maintains a SE component throughout
the day due to the position of the Bermuda High (its northern-most position
over the central North Atlantic). During August and early September, the
SSW sea breeze does not normally occur.
Thunderstorm Maximum Gusts.
The maximum expected gusts associated with thunderstorms
can be derived from maximum thunderstorm cloud tops (in thousands of feet)
and speed of cell movement obtained from weather radar. To use the following
rules, the thunderstorm must pass over the forecast point, and moderate
to heavy rain must occur to attain the forecast gust speed. The forecast
wind direction of the maximum gust is generally the mean wind direction
from 10,000-14,000 feet.
The following empirically-based rules can be used to obtain
expected maximum gusts.
1) Convective Overwater Thunderstorms: 1/4
times the maximum top plus the speed of the cell movement.
2) Convective Overland Thunderstorms:
1/2 max top plus the speed of cell movement;
3) Frontal/Squall Line Overwater Thunderstorms:
3/4 max top plus the speed of cell movement;
4) Frontal/Squall Line Overland Thunderstorms:
1/2 max top plus the speed of cell movement.
During the fall, spring, and early summer seasons, a high
pressure system centered in the vicinity of Pensacola will create a weak
northerly gradient, which influences the thermal gradient winds in the local area. The resultant gradient wind will usually double in velocity near sunrise, and continue for two hours with occasional gusts to 18 knots. This is especially true when
the air temperature and local water temperatures are equal.
A very good tool for surface wind throughout the Southeast
is the Boundary Layer Wind chart.
Rules of thumb: utilize 75% of the wind velocity over
land, 90% over water. Wind direction will back ten degrees over land
and five degrees over water.
1) During the fall, winter,
and spring, a radiation inversion will de-couple the gradient wind from
the surface, resulting in a very weak surface wind during periods of strong
gradients. The gradient wind will return to the surface when the
radiation inversion has been broken.
2) Using the 1,000' and
2,000' actual winds aloft, the average surface wind speed will be 80% of
the average speed of these winds with the maximum surface gusts equaling
the highest of these two levels.