Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Juan PR
942 am AST Sat Dec 27 2014

Update...morning tjsj sounding indicated a very dry and stable
air mass over the islands. Precipitable water under one inch and
dew point depression at 850mb of 19 will maintain sunny skies with
little or no precipitation across the islands today. However...
moisture will surge on Sunday as a cold front approaches from the
north. The front is forecast to move over the northeast Caribbean
islands on Sunday...increasing the cloudiness and the chance of
showers across the Puerto Rico and the U.S. Virgin Islands. We
lower the pop across the forecast area this morning as dry and
stable air mass remains in place.


Aviation...VFR conds are expected across most of the taf sites
through late tonight. Then...cold front reaches the flying area
from the north on Sunday...with increasing cloudiness and shower
activity as well as the possibility of MVFR/IFR ceilings in rain showers.
East-northeast winds 10-20 knots will prevail below fl200.


Marine...building seas of 7-9 feet are forecast later this morning
as large northeast swells invades the local Atlantic waters and
Caribbean passages. Therefore...a high surf advisory and a Small
Craft Advisory are in effect through at least Monday night. Beach
goers should avoid the northern facing beaches of the Atlantic
waters due to high risk of rip currents.


Previous discussion... /issued 547 am AST Sat Dec 27 2014/

Synopsis...the local weather regime will continue to be dominated
by a middle to upper level ridge across the Caribbean basin into the
tropical western Atlantic. A cold front will reach the area on
Sunday. Migratory surface highs north of the local islands will
continue to promote fresh to moderate trade winds through midweek...
becoming strong late next week.

Discussion...mostly clear to partly cloudy skies prevailed across
the local islands overnight...with a few sprinkles observed across
the north coastal areas of Puerto Rico. Easterly winds were light
with coastal temperatures in the middle 70s.

A generally fair and mainly stable weather pattern will continue to
prevail today under below normal precipitable water and subsidence
aloft. A wetter pattern is then expected on Sunday as remnants of
an old frontal boundary move closer to the area...with best moisture
remaining over the Atlantic waters. Scattered to numerous showers
and cloudiness can be expected as soon as early Sunday morning
across the northern USVI and the north coastal areas of Puerto
Rico...spreading across St Croix...the northern slopes of Puerto
Rico as well as over the Cordillera Sunday morning into the early
afternoon hours. Lingering moisture associated with this boundary
is expected to prevail through at least early next week... therefore
the continued trend of passing showers across windward areas is

A sharp decrease in precipitable water is expected Wednesday with
values remaining below normal through Friday. However...showers
embedded in the trades cant be ruled out each day. By Friday...
windy conditions are possible as a broad surface high tightens the
local pressure gradient.

Aviation...VFR conditions are expected across all taf sites through
late tonight. Then...a cold front will reach the flying area from the
north on Sunday...increasing cloudiness and rain showers activity with
possible MVFR/IFR ceilings. Northeasterly winds at 10 to 15 knots.

Marine...large northeast swells will continue to invade the Atlantic
coastal waters today and peak late tonight into early Sunday before
subsiding Monday and Tuesday. Based on latest buoys observations...
still expect breaking waves between 14 and 16 feet. Therefore
coastal flood criteria still marginal. However...high surf and small
craft advisories are in effect through Monday.


Preliminary point temps/pops...
sju 83 74 82 72 / 0 70 70 50
stt 85 74 85 74 / 0 70 70 40


Sju watches/warnings/advisories...
PR...high surf advisory until 6 PM AST Monday for Culebra-north
Central-Northeast-northwest-San Juan and vicinity-southeast-
western interior.

Vi...high surf advisory until 6 PM AST Monday for St Croix-St.
Thomas/St. John/adjacent islands.

Coastal waters...
Small Craft Advisory until midnight AST Monday night for Anegada
Passage southward to 17n-Atlantic waters of Puerto Rico and
USVI from 10nm to 19.5n-Caribbean waters of Puerto Rico
from 10 nm to 17n-coastal waters of northern Puerto Rico
out 10 nm-coastal waters of northern USVI and Culebra out
10 nm-coastal waters of northwestern Puerto Rico out 10 nm-
coastal waters of southern USVI Vieques and eastern Puerto
Rico out 10 nm-Mona Passage southward to 17n.




National Weather Service Glossary of Abbreviations