Why do they base their fcsts on 1800Z surface data?

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
805 PM EDT SAT 07 AUG 2004

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS
OF SOUTH AMERICA...AND ATLANTIC OCEAN TO THE AFRICAN COAST
FROM THE EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED
ON SATELLITE IMAGERY...WEATHER OBSERVATIONS...RADAR...AND
METEOROLOGICAL ANALYSIS.

BASED ON 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
2315 UTC.

..TROPICAL WAVES...
POORLY-DEFINED TROPICAL WAVE LOCATED JUST W OF THE CAPE VERDE
ISLANDS ALONG 27W S OF 17N MOVING W 10-15 KT. POSITION IS BASED
ON LONG-TERM SATELLITE LOOPS. THIS IS A RELATIVELY WEAK WAVE
WITH LITTLE SURFACE REFLECTION NOTED IN SURFACE DATA OR THE GFS
MODEL... THOUGH THE LATTER SHOWS WEAK TURNING AT 700 MB IN THE
AREA. ISOLATED MODERATE WITHIN 60 NM OF AXIS BETWEEN 8N-13N.

TROPICAL WAVE LOCATED ABOUT 1100 NM E OF BARBADOS ALONG 42W/43W
S OF 16N MOVING W 20-25 KT. WAVE IS RAPIDLY MOVING WESTWARD
WITH A STRONG 700 MB JET TO THE N. SATELLITE IMAGES INDICATE
THAT THE WAVE HAS A BROAD CIRCULATION ELONGATED W-E ALONG 9N AND
AN ANTICYCLONIC CIRRUS PATTERN. DRY AIR/AFRICAN DUST IS LOCATED
NEAR AND TO THE N BETWEEN 35W-50W... SOMEWHAT LIMITING TSTM
FORMATION. HOWEVER WIDELY SCATTERED TSTMS DOT THE WAVE FROM
10N-13.5N BETWEEN 45W-51W ENHANCED WITH SOME DIVERGENCE ALOFT.
THE GFS INDICATES THAT THE OUTER FRINGES OF THE WAVE WILL AFFECT
THE WINDWARD ISLANDS OVERNIGHT MON WITH THE MAIN WAVE AXIS
CROSSING 60W AROUND MIDDAY MON.

TROPICAL WAVE IS NEAR THE LESSER ANTILLES ALONG 61W/62W S OF 20N
MOVING W 15-20 KT. TSTMS BLEW UP NEAR THE WAVE AXIS THIS
AFTERNOON BUT HAVE DIMINISHED IN THE WINDWARD ISLANDS. THE WAVE
IS NOT WELL-DEFINED BUT CAN BE SEEN IN HOVMOELLER PLOTS.
ISOLATED SHOWERS S OF 12.5N E OF 65W.

W CARIBBEAN TROPICAL WAVE...THE REMNANTS OF T.D. TWO...IS ALONG
82W S OF 21N MOVING W 10-15 KT. WAVE HAS WEAKENED WITH A LESS
PRONOUNCED WIND SHIFT THAN YESTERDAY AND AN ELONGATED APPEARANCE
ON SATELLITE. INCREASING CHANCES FOR RAIN ARE LIKELY FOR THE
YUCATAN AND BELIZE LATE SUN INTO MON. WIDELY SCATTERED MODERATE
FROM 15N-20N BETWEEN W JAMAICA AND 82W.
http://www.srh.noaa.gov/data/NHC/TWDAT.0408072355