Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1103 am AST sun Dec 28 2014
Update...a weakening frontal boundary have moved over the
forecast area this morning. As a result...cloudiness and shower
activity have increased across the northeast Caribbean islands.
Variably to mostly cloudy skies with occasional showers of light
to moderate intensity have moved inland across portions of the
USVI and PR. The most frequent showers have been affecting the
northern half of Puerto Rico where a few tenths of an inch have
been observed so far this morning. Occasional light to moderate
showers will be pushed by the northeast winds over the local
islands the rest of today. No changes were made to the forecast
package this morning.
Aviation...brief MVFR ceilings possible with the rain showers...but overall
VFR conds will prevail through the forecast period. NE winds of
10-20 knots are expected below fl100 through Monday morning.
Marine...although NE swell has peaked across the regional
waters...breaking wave height of 10 feet or greater are likely
across the Atlantic beaches of PR and USVI. As a result...a high
surf advisory continues in effect for the north coastlines of PR
and USVI through 10 PM AST.
Previous discussion... /issued 420 am AST sun Dec 28 2014/
Discussion...cold front remnant north of PR/USVI approaching
islands at about 12 knots by extrapolation of satellite images and
GFS has a good handle...will reach North Shores around sunrise.
Soundings from this model have the moisture limited to under 650
mb. That appears accurate based on implied cloud tops from infrared
images. But think GFS moisture levels are probably a little low
above 800 mb. Moisture to this depth implies quite a few showers
this morning after 600 am for north half of PR and into St.
Thomas/Vieques/Culebra. Radar currently shows them spaced out
pretty well and moving quickly. Showers will be widespread but not
last very long or be heavy. Uncertainty over how far south this
narrow band of moisture will sag by afternoon leads to low
confidence in probability of showers then. Sunshine on land should
be enough to cause some convection but the moisture is pretty
shallow anyway so do not expect it to be widespread. Frontolysis
is turning this front into a bunch of minor easterly waves to
pass over PR/USVI over the next several days. The best one
probably arrives tonight. Good convergence at 1000-850 mb as
noted on GFS at 06z tonight depicted between eastern PR and USVI.
This is a climatologically favored zone for nighttime showers and
have included likely showers overnight there.
Aviation...a cold front will reach the flying area...creating
MVFR/brief IFR ceilings between after 28/12z mainly at tjsj/tjbq/tncm...
followed by scattered rain showers across most the whole area late in the morning
and into the afternoon. Inclement weather is expected during the
rest of the forecast period. Northeasterly surface winds at 10 to 20 kts
with higher gusts are expected today.
Marine...long period northeast swells peak today but will take
several days to diminish. Winds to increase somewhat today which
will make the wave groups more complex between short period wind
wave and swell.
Preliminary point temps/pops...
sju 82 70 82 71 / 80 60 50 50
stt 84 74 87 76 / 90 60 50 50
PR...high surf advisory until 10 PM AST this evening for Culebra-
north Central-Northeast-northwest-San Juan and vicinity-
Vi...high surf advisory until 10 PM AST this evening for St Croix-St.
Thomas/St. John/adjacent islands.
Small Craft Advisory until 6 PM AST Tuesday 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 USVI and Culebra out 10 nm-
coastal waters of southern USVI Vieques and eastern Puerto
Rico out 10 nm-Mona Passage southward to 17n.
Small Craft Advisory until noon AST Tuesday for coastal waters
of northern Puerto Rico out 10 nm.
Small Craft Advisory until midnight AST Monday night for coastal
waters of northwestern Puerto Rico out 10 nm.