In the Pensacola region, air mass and frontal thunderstorms are most frequent during the summer months, but can occur at any time of the year. The chances of a thunderstorm in Pensacola between 1200-1800 local are better than 30% on any summer day. However, the worst flying conditions are found when thunderstorms form squall lines associated with fast-moving cold fronts. This most commonly occurs during late fall, winter, and early spring.
The high-frequency period of thunderstorms (1200-1800
local) is evident in all seasons. Pensacola is fifth in the United States in number of summer thunderstorm occurrences. Individual thunderstorm cells are rarely larger than 10 miles in diameter and their life cycle varies from about 20 minutes to 1 1/2 hours.
When an El Nino is present, there is a marked lack of afternoon thunderstorm activity normal to the area and drought conditions are more likely to exist in the southeast U.S. High heights ridging from the Pacific vice the Atlantic are prevalent through the mid and upper levels of the atmosphere. This produces upper-level winds from the northwest over the area, which provides subsidence and suppress of convection over the area. Local afternoon thunderstorm activity will occur only when the Pacific ridge breaks down, which allows the Atlantic ridge to prevail in the mid and upper levels.
Rules of Thumb. When the Panhandle is under the influence of the Bermuda High (located along the eastern seaboard) the following rules of thumb may apply:
1) In June, July, and August, thunderstorms on avergae should be expected on or in the vicinity of the station 1 to 3 hours after the southwest sea breeze begins. The sea breeze front usually extends 3 to 8 miles inland for this area. Thunderstorms over I-10 are common in July and August. Thunderstorms should be expected on or near the station at or 1 hour after max heating (normally between 1100-1300L).
2) In August, the sea breeze is predominately from the southeast with a weaker gradient force. Thunderstorms should develop overhead more frequently.
Types of Thunderstorms
1) Air mass (overland)
Thunderstorms are normally not evident before May. They form in the
unstable mT air, are generally less severe, and as a rule extend to lower
heights than frontal thunderstorms. Convergent air flow is absent during the winter, thus convective thunderstorms are rare. The most severe air mass thunderstorms may have surface wind gusts in excess of 50 knots, but generally only produce maximum wind gusts of 35 to 40 knots.
Most of the time, KNPA is not directly affected by air mass thunderstorms, although they frequently occur within 25 nm of the field. The most prolific area for development is northwest of a line from Innerarity Point through Pleasant Grove to the Ensley subdivisions. The prevailing direction of movement is northeastward at 5-10 knots. Thus the majority of thunderstorm activity occurs northwest of the station and moves away from the station.
a) Generally, convective activity begins about 1000 local with cumulus
humulus appearing west through north of KNPA. Thunderstorm activity begins about 1200-1300 local. Thunderstorms continue until just before sunset with the majority of
the activity ending by 1900 local. A short wave trough or a broad trough at 500 mb over the local area will increase the amount and intensity of convective activity. This will produce lines rather than areas of convective activity. Longer periods of convective activity are observed during this synoptic situation. Occasional funnel clouds have been reported in the local forecast area under these conditions.
Occasionally, a thunderstorm will form over KNPA and cause
IFR field conditions for the normal life cycle of the cell. Isolated
severe activity can also develop over KNPA and move to the east, causing
moderate wind damage in the immediate vicinity.
b) The movement of air mass thunderstorm cells in
the near vicinity can be reliably forecast by using the direction of the 700 mb wind
and the speed of the 850 mb wind. With troughs at 500 mb in the local
area, the convective activity tends to move due eastward (rather than the usual
northeastward). This increases the chance of NASP being affected by thunderstorm
The best method to forecast the most likely location of thunderstorm activity
is a single degree thermal analysis of the 500 mb chart. Using the 1200Z, 500 mb ANAL, analyze for each degree over the southern U.S., including the Gulf. If a cold pocket is located over KNPA and sufficient moisture exists, thunderstorms will develop on
or near the field. Movement will depend on mid to high-level wind
flow. With a subsidence inversion evident, thunderstorm development
will be delayed until mid to late afternoon and will be isolated.
The absence of sufficient moisture does not mean a "no
thunderstorm" forecast. The sea breeze can supply enough low-level
moisture to produce isolated activity with little moisture indicated on
mid or high-level charts. A "no thunderstorm" forecast is justified
if both a warm pocket and no moisture are indicated on the 1200Z 500 mb
2) Convective nocturnal
thunderstorm activity over water occurs over the Gulf from May through
September. The most prolific area is found 25-50 nm south of the
coast during July and August. Activity begins to develop after 2200 local with cumulus
humulus gradually developing into thunderstorm activity by 0400 local.
Activity continues until 0900 local, when dissipation begins. The
majority of thunderstorm activity dissipates by 1100 local with generally
clear skies over the Gulf by 1200 local. The normal movement of nocturnal
thunderstorm activity is a slow drift to the north.
Early in the season, dissipation usually begins before the activity reaches land. Only light rain and mid-level cloudiness reach the coast. Later in the season, the water adjacent
to the coast becomes warmer and the activity reaches the immediate coastline before dissipation (up to 5 miles inland). In this case, KNPA experiences brief periods of ceilings below 1,000' and visibility below 5 miles. Surface wind gusts are minimal with a maximum of less than 30 knots.
When 500 mb or 700 mb troughs move over the local area,
thunderstorm activity increases, prevails longer, and on occasion produces
funnel clouds and waterspouts along the immediate coast.
3) Perdido Bay Nocturnal
Thunderstorms are a local phenomenon. From May through October, nocturnal
thunderstorm activity occurs over Perdido Bay, west of KNPA. This
activity begins about 0300 local and reaches its maximum intensity shortly
after sunrise. Movement, if any, takes the storm over land with dissipation
beginning as soon as landfall is reached.