The most adverse weather conditions experienced in the
Pensacola region are due to active cold fronts and returning warm fronts
from the Gulf.
The movement of winter frontal systems through the Pensacola
area depends on the trajectory of the air mass behind the front, the strength
of the Bermuda High, and the 500 mb flow associated with the frontal system.
Cold fronts may occur between October and April, and peak in December and
There are two scenarios that are associated with the movement
of active cold fronts.
In the first scenario, the Bermuda High is in its normal
position, and another high moves down the Mississippi Valley to north Texas,
then eastward. The cold front will move through the local area and well
into the Gulf. Cold frontal systems (especially during fall, early winter,
and spring) pass through the local area. Cloudy conditions will persist
for 36-48 hours after the front has passed. A sharp 500 mb trough will
accompany the cold frontal systems. Clearing in the low cloud levels with
follow the passage of the 850 mb trough. Mid and high cloudiness will remain
until the the direction of the 500 mb flow has changed from a southwesterly
to westerly to northwesterly flow.
The second scenario is that cold fronts may be associated
with wave development on quasi-stationary frontal systems. A wave typically
develops in the southwestern Gulf and moves northeast. The wave center
usually moves inland around Lafayette, Louisiana, and the cold front trails
southwest into the western Gulf. As the low center tracks northeast, the
cold front moves in relation to the low. Cold frontal weather is experienced
during passage, with rapid clearing after passage.
1) Cold frontal systems
tend to become quasi-stationary through central Mississippi, southeastern
Louisiana, and southeastern Texas when the air mass behind the system has
a trajectory down the Mississippi Valley to central Kansas, and then eastward.
This is especially true when the Bermuda Ridge has pushed westward.
Another indication that a frontal system is going to become stationary
west of the local area is digging of the jet stream westward along the
West Coast producing a cut-off low or a retrograding trough along the California
The sequence of events that produces a prolonged quasi-stationary
frontal system in the local area is a continental polar/arctic air mass
that moves down the Mississippi Valley to northern Arkansas and then east-southeast
into western Georgia and becomes stationary. This moves the cold
frontal system south of the coast and into the Gulf where the front then
becomes stationary. Simultaneously, a westward extension of the Bermuda
High moves into the north central Gulf.
2) The atmospheric conditions
are similar to that of a cold frontal passage with prolonged postfrontal
cloudiness. The difference is that with a quasi-stationary front,
short waves move eastward through the axis of the long wave trough and
continue eastward producing open waves along the front. This situation
should be closely monitored to ensure detection of low pressure development
and that proper warnings and forecasts are disseminated in a timely manner.
About 24 hours after the front becomes stationary, weak
short waves at the 700/500 mb level begin to move along the frontal system.
This produces stable waves on the surface that ripple along the frontal
The open waves that ripple along the surface frontal system
produce an increase in rain showers at and northward of the frontal wave.
Radar is helpful in detecting the associated shower activity.
Severe weather is often associated with the development
of an unstable wave or closed low on the frontal system. A strong short
wave trough with associated positive vorticity advection moving off the
Texas coast just south of Galveston will trigger a closed surface wave
development on the quasi-stationary front.
Depending on the upper-air pattern, this wave may continue
to develop as it moves east and northeast, producing thunderstorm activity
that has a high potential of being severe as it passes through KNPA's area
Warm frontal systems will usually pass through the local
area when the low moves into the east Arkansas/west Mississippi area.
Weather associated with the frontal passage usually is multi-layered cloudiness
with widely scattered light rain. Warm-sector cloudiness and visibility
will depend on the wind velocity in the warm sector. Warm fronts
influencing the local area originate from two synoptic situations:
1) In the first situation,
a wave develops on a quasi-stationary frontal system in the southwestern
Gulf and moves northeast. The wave center moves inland, usually around
Lafayette, Louisiana with the warm front trailing southeast into the central
Gulf. As the low center tracks to the northeast, the warm front moves
in relation to the low. Typical warm frontal weather is experienced
as the front moves through the local area. As explained in frontal
stratus situations, the cloudiness and visibility experienced in the warm
sector will depend on wind velocity in the warm sector after the warm front
moves north of the station. Embedded thunderstorm activity is a frequent
occurrence with the passage of the warm front.
2) In the second situation,
a cold front moves through the local area and into the central Gulf trailing
westward to southern Texas. The local area experiences clear weather for
48-72 hours as the high pushing the cold front moves to the East Coast.
With continued progression of the high eastward, the front in south Texas
begins to move northeast as a warm front and usually becomes connected
with a new low developing on the cold front moving into central Texas.
As this low moves east, the frontal system moves in relation to the low.
The local area may be influenced by Tropical Easterlies
during August and September due to The location of KNPA at 30 degrees N.
The forecaster should be alert for northward extensions of tropical troughs
that influence the local area with activity moving from east to west. This
northward extension occurs when the Bermuda Ridge is displced from the
northern Gulf to overland areas along the Gulf Coast. This easterly low
irregularities will increase shower activity along the Gulf Coast.
Easterly low irregularities can be detected in the low
level wind chart for the Florida Peninsula and Panhandle. A low-level
continuous streamline analysis that covers the Atlantic and Caribbean can
give the forecaster adequate warning of tropical development in NAVTRAMETOCFAC's
area of concern. Easterly waves are also readily identified in the