This is one of a series of pamphlets dealing with the weather aspects of Naval and Amphibious Warfare. The data on which these studies are based are taken from official documents and reports submitted to the Navy Department. The material has been collated and presented in a semi-technical form with particular attention given to the operational aspects of weather.
During the preparation of this study, it was found that weather data submitted by the various commands were occasionally at variance. An effort has been made to reconcile these differences in order to provide an accurate account of the sequence of weather conditions as forecast for the Force Commander, to describe the actual weather conditions observed during the operation, and to give a practical explanation of these weather conditions.
It is hoped that these studies will afford a clear view of the use of weather information during the planning, strategical, and tactical phases of the operation, and that they will form a basis for a better understanding of the application of weather information in future operations.
The primary objective of these analyses is to assist those officers who are charged with the responsibility for the planning and execution of similar operations.
[Signature: Aubrey Wray Fitch]
Vice Admiral, U.S.N.,
Deputy Chief of Naval Operations, (Air).
The Invasion of Southern France
The significance of weather during the Invasion of Southern France was noted by Commander, Western Naval Task force, as follows:
1. A storm plan should be a standard part of any plan for amphibious operations.
2. Landing craft personnel should be thoroughly indoctrinated in small craft handling under storm conditions.
3. A rear-echelon aerological unit should be organized and equipped in order that weather information may be supplied all forces in the event of a casualty to the flagship.
A major landing operation in support of the Invasion of Normandy started on 15 August 1944 on the southern coast of France between Cape Cavalaire and Cape Roux. Altogether, over 2,250 ships and craft took part in the landing operations and supporting operations. Between 15 August and 25 September, 325,000 troops, 68,500 vehicles, 490,400 tons of dry cargo, and 325,750 barrels of wet cargo were landed in the objective area.
The general directive defining the purpose and scope of this operation was received early in 1944 from the Supreme Allied Command. Most phases of the planning were, therefore, allowed sufficient time for a critical investigation of all of the major aspects of the projected operation. In regard to the weather planning, a detailed study was made of:
1. Weather expected.
2. Strategic and tactical weather considerations.
Weather along the Mediterranean coast of France reflects those major climatic controls which extend their influence over the area. Most important of these controls is the Mediterranean Sea which produces mild year-round temperatures and generally favorable living conditions.
The other major climatic control of the area is the position and intensity of the Atlantic high pressure area. In summer, the center of this high pressure area has retreated to the south and weakened to some extent, with a result that the Mediterranean area is not greatly influenced by the wind,
weather, and frontal systems associated with it. In winter, however, the area of high pressure moves to the north of the Atlantic Ocean and becomes somewhat more intense. Resulting winds in France, including the Mediterranean coastal area, are then prevailing westerly and northwesterly. With this arrangement, cold fronts and polar outbreaks frequently reach into the Mediterranean bringing heavy precipitation and strong winds. These polar outbreaks are known in the Mediterranean countries as "mistrals". The following charts illustrate the yearly trends of the major weather elements in the locale of this operation.
OCCURENCE OF STRONG MISTRALS
|ANNUAL MARCH OF PRECIPITATION
Since the Mediterranean Sea is entirely surrounded by land areas, the development of sea and swell results from deep migratory depressions which intrude into the area, or, more frequently, from low pressure centers which generate in the Gulf of Genoa. Since the development of unfavorable sea and swell conditions thus depend upon manifestations of a purely local nature, they are subject to analysis and prognostication on the basis of wind predictions for the Mediterranean area. There is little or no counterpart to conditions such as exist on the shores of large continents where heavy surf and swell generated by a wind system far in the distance is observed at times when the local weather is good.
Strategic and Tactical Weather Considerations
The study of weather conditions along the southern coast of France produced several direct operational applications. As recorded by the Naval Commander, Western Task Force, these are:
Relating to the Establishment of D-Day:
"Early in the planning period, this command had set forth the view that D-Day should not be later than 1 September. This was based on the considerations of beach maintenance and weather. It was believed that the Western Task Force should plan for a period of at least 30 days beach maintenance before ports could be opened sufficiently to cross the beaches. The following storms along the coast of Southern France, locally known as 'mistrals', could be expected frequently after 1 October, making beach maintenance no longer feasible.
Relating to the Commitment of Forces:
"Airborne forces had been offered. Because of their dependence on weather conditions, their value was viewed with considerable doubt."
"The simple feature, timing; of (1) the operation, (2) neutralization of the islands, (3) troop landings, (4) gunfire and air support, was early considered by this Command far more important to the success of an amphibious invasion than any additional fair weather forces such as paratroops or airborne forces."
Relating to Preliminary Operations:
It was expected that weather in the entire area would permit extensive pre-invasion aerial bombardment, assembly of transports, and all other operations relating to that phase.
Relating to the Approach:
It was further expected that generally good weather would not hamper the formation and movement of convoys composed of several types of small craft as well as larger units. At the same time, it was realized that assembly and movement of convoys would not be concealed from the enemy by clouds or low visibility.
In providing for the contingency of a violent storm in the area at a critical time, Storm Plans were drawn up in advance of the operation.
"Each plan called for issuance of a preliminary warning by radio or visual methods when the forecast indicated that certain conditions were likely to arise, and of a final warning when more severe conditions were predicted. Steps to be taken upon receipt of these two classes of storm warnings were carefully elaborated. General instructions regarding storms were included, and the old problem was made more vivid by citing instances of actual storms encountered at Anzio, Salerno, and Sicily and the resulting damage."
Since most storms in Southern France are accompanied by offshore winds in the area of projected operations, it was felt that while unloading operations would not be seriously affected, convoys at sea would be completely exposed to the full force of any well developed mistral.
Relating to Possible Postponement:
"Because of the possibility of unexpected changes in weather along the southern coast of France, a postponement plan was prepared. Because of the overwhelming force involved in the Invasion of Southern France and the favorable naval and military situation, it was agreed that postponement would be executed only in event of adverse weather.
"It was decided that the postponement as ordered, should take place prior to 1200 August 14, D minus 1 for two reasons:
(1) At 1200 D minus 1, the slow assault convoys of LCTs [Landing craft, tank], SS-1, SS-1A, and SS-1B would have advanced one hour in the approach lanes. After that time, it was felt that turning convoys of this nature about with poor navigational equipment LCTs carry would be a greater risk than pushing them on in the face of the weather.
(2) (Discussion not applied to weather)."
Weather and the Operation
The Aerological Unit attached to Naval Commander, Western Task Force, was divided into two sections on 3 July 1944. One section went aboard the Force Flagship, and the other was established ashore at Calvi to provide weather information and forecasts during periods when the forces afloat would observe radio silence. Routine weather forecasts were supplied by the section ashore to subordinate Task Force and Task Group Commanders until 15 August, when the flagship section took over this duty.
Embarkation of troops started on 9 August at various ports. Preliminary assembly of convoys and pre-invasion bombing attacks continued. Weather during the day was generally favorable for all scheduled operations.
On 10 August, the weather was less favorable for air activity from bases on Corsica, but surface operations were unhampered, and
most air operations continued as scheduled. Over Southern France, the weather was clear with moderate winds and slight seas. Several convoys were underway for the assault area via various staging points.
11, 12, 13, and 14 August were noted for the excellent weather and sea conditions prevailing over the Western Mediterranean. All operations continued as scheduled. Assault craft and support craft underway for the assault area were clearly observed by enemy reconnaissance planes, weather offering little or no cover.
On D-Day, 15 August, low clouds and fog patches were present over Southern France. Lower clouds thinned and broke by mid-morning. The weather elements most important to a landing operation - sea and swell conditions - were perfect. Winds were light and variable. The seas were calm. All forces seized D-Day objectives. A total of over 60,000 men and 6,500 vehicles were landed during the period.
The Build Up
In general, the weather remained mostly favorable from D-Day until the Navy's task of lending full support had been accomplished. On 17 August, airfields on Corsica were rendered unserviceable by heavy shower activity. Unloading operations at the beachheads were hampered on August by the presence of a fresh easterly wind with a resulting 3 to 4 foot surf. The winds gradually subsided on the 22nd, and by early morning of the 23rd it was calm again, with no sea or swell.
Weather continued excellent until 2 September, when the area was suddenly struck by the first real "mistral" of the season. A cold front entered the Gulf of Lions [Lyons] at about 1800, and moved eastward at 25 knots. In the assault area ahead of the front, winds increased to fresh northeasterly. With the passage of the front, skies cleared, and the wind shifted to the northwest. Five hours after the frontal passage, the wind suddenly increased to 50 knots, with gusts to 60 knots. Offshore seas rapidly developed to a height of 10 feet. Near the beaches, the short distance of wind travel over the water precluded the development of more than a moderate sea.
During the next day, considerable improvement was noted in the assault area, where winds gradually subsided to 20 knots. In the convoy lanes, however, seas were heavy, with a swell of 16 feet observed in the Ligurian Sea - Corsica - Sardinia area. Visibility was abnormal, exceeding 100 miles.
By 4 September, weather in the assault area was excellent. Clear skies, excellent visibility, and gentle winds again aided unloading and support operations.
On 6 September, the weather took another turn for the worse. Continuous rain, low ceilings, poor visibility, fresh to strong northeast winds, and a surf of 5 to 7 feet presented operating conditions far from optimum. In general, these conditions prevailed for the next three days.
The remainder of the period to 25 September was clearly marked by a shortening of the periods of good weather and an increase in the number of unfavorable periods.
The decision to conduct the operation before 1 September was firmly underlined as sound.
Weather conditions throughout the first phase of the Invasion of Southern France were essentially the same as expected. Late summer and early fall weather in the area was expected to favor, and did favor, all operations. The most notable contribution to the operation made by a critical examination of weather conditions in the area was that which prompted the decision to conduct the operation before 1 September.
Tactical weather information disseminated to all forces was also reliable and accurate, within the limitations imposed by the absence of certain weather reports. The final weather forecast for the assault was issued on 12 August proved to be substantially correct.
In his final report on the Invasion of Southern France, the Naval Commander, Western Task Force made the following statement in regard to the aerological aspects of the operation:
"The need for a storm plan in setting up the scheme for an amphibious invasion is clear. For the invasion of Southern France, where one of the largest landing craft forces ever assembled was to be sent into an area known for its severe autumnal mistrals, a storm plan was an absolute necessity. The provisions of the individual plans put forth by the Attack Force Commanders were based on long experience in this theater and are considered sound. The value of these plans was demonstrated during the storm of 2-3 September.
"Training of landing craft personnel should include thorough indoctrination in the art of handling landing craft in storms.
"A storm plan should be a standard part of any plan for amphibious operations.
"The recommendations contained in the report of the Sicilian invasion were affirmed as valid. Additionally, it is recommended:
"That necessary measures be taken to insure complete, accurate surface and upper air reports from friendly and neutral countries in the vicinity of operations.
"That a rear-echelon unit be regularly organized to provide information to task force commanders during periods of radio silence in the attacking forces and to make all forecasts in the event of a casualty to the force commander's flagship of such severity as to prevent the functioning of the flagship unit after H-Hour.
"That radio-sonde equipment be installed ashore as near as practicable to the vicinity of the assault beaches. This could well be a part of the
equipment of the rear-echelon unit recommended in 5.
"That parachute-type droppable automatic weather stations be stocked in Aerological Supply Pools."