. Precipitation Heavy precipitation interferes with the movement ashore and the push inland. Strike capability is greatly diminished.

. Lightning Poses a grave danger during boat operations.

. Low visibility Hampers small boat operation.

. Wind direction/speed Can modify breaker type in the surf zone and affect flight operations. May also reduce visibility in the surf zone.

. Modified surf index (MSI) Most critical parameter in a waterborne assault.

Cloud cover (bases and tops).

Prevailing and sector risibilities,

Surface and upper level winds.

Density altitude (DA) and pressure altitude (PA).

Air/sea temperature, icing, freezing level.

Contrail formation.

Bingo fields.

EO weapon/sensor performance.

Any other significant weather.

In the discussion of currents we will first discuss, offshore currents, followed by rip currents, and shore currents.

OFFSHORE CURRENTS. These currents are found outside the surf zone (both tidal and nontidal). . Tidal currents are predominant near entrances to bays and sounds, channels, between islands, or between islands and the mainland.

. Tidal currents usually reverse direction on a periodic basis (every 6 to 12 hours) and can reach speeds up to several knots.

RIP CURRENTS. These currents result from waves piling water up against the coast. They flow along the coast until they are deflected seaward by bottom irregularities or until they meet another current. . Flow can reach speeds as high as 12 kts, but usually attain speeds of 2 to 4 kts. Prevents most landing craft from making any headway ashore.

. The head (leading edge) of the current is often discolored by silt in suspension.

. If the beach is irregular, they will flow along the beach for a short distance and then flow out to sea.

. They are easily identified by aircraft, as they create a turbid flume offshore.

SHORE CURRENTS. The following discussion deals with the formation and characteristics of shore currents.

. Generated by waves breaking at an angle to the beach.

. Littoral or longshore currents flow parallel to the beach inside the breakers.

. Speeds increase with increasing breaker height, with increasing angle of the breaker with the beach, and with steeper slopes.

. In areas where longshore/littoral currents are common, sandbars are usually present.

. Longshore/littoral currents must be considered in selecting a beach or landing site. A littoral current can cause a landing craft to broach.

Further discussion of beach topography, beach composition, beach surveys, breakers, and offshore sealswells may be found in the technical manual, CWOSM, Part 1, TM 04-92.

For further discussion of AMW operations refer to the technical manual, CWOSM, Part 1, TM 04-92 and Joint Surf Manual, COMNAVSURFPAC/ COMNAVSURFLANT Instruction 3840.1. The last topic of discussion in this chapter will be the briefing of METOC services available from OA divisions.

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