MODIFIED  SURF  INDEX calculate  the  modified  surf  index  as  part  of  the  surf CALCULATION forecast. LEARNING   OBJECTIVES:   Describe   the procedures used to calculate the Modified Surf Index  (MSI).  Identify  features  of  the  SURF program available in TESS/MOSS. The modified surf index (MSI) is the most critical parameter  in  a  waterborne  assault.  Both  the  SUROB observer and weather personnel aboard the task force command   ship   normally   calculate   the   MSI   from information contained in a SUROB. Additionally, surf forecasters at a center, facility, or detachment routinely The  modified  surf  index  is  a  calculated,  single dimensionless  number  used  as  an  objective  decision aid.  It  is  an  assessment  of  the  combined  effects  of breakers,  littoral  current,  and  wind  conditions  on landing  craft.  If  the  MSI  exceeds  the  MSI  limit  for  a particular craft, the landing is not feasible with that type of  craft  without  increasing  the  casualty  rate.  The modified   surf   index   is   calculated   on   a   locally reproduced  Modified  Surf  Index  Calculation Worksheet, shown as table 4-1. Tables 4-2 through 4-6 are used in computing table 4-1. Table 4-1.—Modified Surf Index Calculation Worksheet SUROB / SURFCAST NO. 5: VALID TIME: 2200Z - RED BEACH SURF REPORT ITEM MODIFICATION ENTRY SIG BREAKER HT (ALFA) No modification, enter significant breaker height. +4.5 4.5 FT BREAKER  PERIOD Enter  value  from  breaker  period  modification  table +1.6 (CHARLIE) (table  4-2). 7 SEC BREAKER  TYPE  (DELTA) NO ENTRY FOR 100% PLUNGING SURF. -2.8 70%  SPILLING 30%  PLUNGING Enter only a value for the percent spilling surf or the 0%  SURGING percent  surging  surf  from  the  breaker  type  modification (see note below) tables (table 4-3). BREAKER  ANGLE  (ECHO) Enter   modification   value   from   the   breaker   angle Enter only greater 15 DEG modification table (table 4-4) as value (A)= 2.4. value, (A) or (B) +2.7 LITTORAL   CURRENT (FOXTROT) 0.9 KT Enter  modification  value  from  the  littoral  current modification table (table 4-5) as value (B)= 2.7 RELATIVE WIND (HOTEL) 15 Enter the value from the wind modification table (table kt at 35 DEG ONSHORE 4-6). SECONDARY  SIG  BREAKER  No  modification,  enter  secondary  significant  breaker HT   (HOTEL-ALFA) height. OFT +0.5 0 (add all entries) - - - - MODIFIED  SURF  INDEX=  6.5 NOTE: Surging breakers should occur on beaches with steep gradients and should not normally occur with spilling breakers. 4 - 1 1