Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1012 PM AST Friday Feb 12 2016
Update...latest sounding from San Juan at 13/00z showed slightly
drier conditions and a slightly weaker cap below 10 kft. The top
of the moist layer also decreased from around 9 kft to 6.6 kft.
Nevertheless small showers continue to develop in the Atlantic
waters and continue to pass over the local Caribbean waters from
about 080 degrees. This presents a slightly onshore flow for
northeast Puerto Rico and will maintain the chance for showers
over the coast there. In Saint Thomas and Saint John showers have
also been forming around the islands and drifting onshore during
the period. Expect this trend to continue into Saturday.
Discussion...previous discussion holds with increasing rain
chances beginning late Saturday night and continuing into Monday.
Models still showing better moisture the following weekend.
Although a strong high will develop in the north central Atlantic
and continue to build through the end of the week generating
increasing winds Monday into Tuesday...winds will diminish slowly
Tuesday through Thursday as the high moves east. Currently GFS
shows a much more dramatic increase in the wind speed at 850 mb
than at the surface.
Aviation...VFR conds will prevail across all taf sites through the
forecast prd. Scattered-broken passing clouds fl025...fl050...mostly clear above. Isolated
rain showers en Route by the way PR and northern leewards. L/level winds from east 5-10 kts below
fl100...then veering and becoming from west above fl250. Surface wind mainly light/vrb
but mainly from east until 13/14z...then increase to 10 to 15 kts...with local
sea breeze variations along the coastal areas. No other sig
operational weather impacts at this time.
Marine...seas at outer buoy 41043 reached 8.5 feet again just
before 12/22z and wind wave direction was registering as NNW-N.
This will keep seas from quieting as rapidly as originally
anticipated. Hence have extended the rip current risk to Saturday
afternoon. Small craft in the outer Atlantic waters should hold
until about that time. 7-foot seas return on Monday.
Previous discussion... /issued 318 PM AST Friday Feb 12 2016/
Synopsis...a deep polar low will move into the western Atlantic
eroding the middle to upper ridge during the weekend. A surface high
pressure over the eastern Atlantic will hold through the early
part of next week. A surface perturbation and a surge of moisture
will move across the region early Saturday morning. Fair weather
conditions will prevail through tonight.
Discussion...partly to mostly cloudy skies prevailed across most
of the islands...except for the southwest end of Mainland Puerto
Rico...where sunny skies prevailed during the morning and into
the afternoon hours. The best shower activity detected by the
Doppler radar was downwind from St Thomas/St John and affecting
the islands of Culebra. During the rest of the day expect a few
passing showers over the eastern half of Puerto Rico and the U.S.
Virgin Islands from time to time. In addition...the west and
interior portion of the island can expect shower activity with
isolated periods of moderate rainfall late this afternoon.
A surface trough accompanied by a surge of low level moisture will
enhance shower activity overnight and early Saturday morning.
Therefore...a wetter and more unstable pattern is forecast from
Saturday to early next week as the upper ridge collapses and
short wave energy provides favorable conditions for showers and
thunderstorms. As the ridge flattens...moisture will surge across
the northeast Caribbean islands. At this time...the wettest day
appears to be Sunday...and Monday when an easterly perturbation
will cross the islands.
Aviation...mainly VFR conditions will continue to prevail across
all taf sites through the forecast period. Some cloudiness are
expected over western interior and west sections of Puerto Rico
through at least 13/22z. Low level winds will continue mainly east
at 10 to 15 kts.
Marine...a northerly swell will continue to move across the
region producing seas at 4 to 6 feet across most of the local
waters and seas up to 7 feet over the Atlantic offshore waters. A
east to southeast wind flow around 15 knots are expected to
continue tonight into Saturday. A Small Craft Advisory remains in
effect for the offshore Atlantic waters. Small craft should
exercise caution over the near shore Atlantic.
Preliminary point temps/pops...
sju 73 86 73 86 / 30 30 30 30
stt 74 84 73 84 / 50 50 40 40
PR...high rip current risk through Saturday afternoon for northeast-
San Juan and vicinity-southeast.
High rip current risk until 5 am AST Saturday for Culebra-north
Am...Small Craft Advisory until 2 PM AST Saturday for Atlantic waters of
Puerto Rico and USVI from 10nm to 19.5n.