Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 240045
Area Forecast Discussion
National Weather Service San Juan PR
845 PM AST Mon Jan 23 2017
Skies became mostly clear across the region during the late
afternoon and early evening hours. No precipitation was detected
or reported during the afternoon and evening across the islands
and coastal waters as dry and stable conditions prevailed under a
dominant southerly low level wind flow. Recent TJSJ 24/00z upper
air sounding as well as model guidance suggest no significant
change in the overall dry and stable weather pattern at least
through Tuesday. The low level winds are however expected to
become more southwesterly by Tuesday in response to the surface
high shifting farther east of the region and a low pressure
system and associated cold front moving rapidly across the west
Atlantic. This cold front or it remnants is so far forecast to
reaching the area by Wednesday. Thereafter winds are to become
more northerly as surface high will build across the west
Atlantic, bringing cooler and stable conditions to the region once
again. No significant changes made to the inherited forecast
package at this time.
.AVIATION UPDATE...VFR conditions should prevail across all
terminals through 24/24z. Will not mention VCSH as any precip
activity should be very isolated and will not have any impact on
operations. Winds should be light southerly or variable overnight
becoming mostly S-SW at 5-10 kts...except mostly SE at TNCM and
.MARINE UPDATE...Recent data from the surrounding buoys suggest
a north northeast swell continued to affect the local Atlantic
waters and passages with Seas between 4 to 6 feet and winds up to
20 knots. For this reason small craft operators should continue
to exercise caution across Atlantic waters and local passages.
.PREV DISCUSSION... /issued 202 PM AST Mon Jan 23 2017/
SYNOPSIS...Southerly flow is bringing back some moisture, but
flow will turn westerly with the approach of a weak cold front
and push the moisture east of the local area. Mostly dry weather
with few showers will ensue through at least Friday.
At upper levels...A ridge will moving through the area will shield
us from the impacts of a mid latitude trough passing to the north.
A sharp trough will dig south across the Dominican Republic after
Tuesday causing flow to turn southwest. The trough will weaken
before it passes through the area late Sunday. High pressure
returns early next week.
At mid levels...High pressure will continue over the Caribbean Sea
from the west and slowly move north during the week. It will pass
just north f the area over the weekend to reside to the northeast
over the west central Atlantic Ocean. Mid level moisture will
approach the islands from the south, brush by Saint Croix on
Tuesday, but the main axis of the moisture will form over the
Leeward Islands with limited effect for the local area.
At lower levels...Southerly flow will increase overnight, but a
weak cold front will pass through Tuesday night and turn flow
lightly back to the north. High pressure will build just north of
the area and continue into the central Atlantic ocean bringing
relatively dry air through Friday. As the air passes over the
ocean it will gradually pick up moisture over the weekend to bring
back isolated shallow showers.
DISCUSSION...No showers were seen on our noisy back up radar as
mostly dry air returns from the south. Mid level moisture is
approaching the area, but the approach of a front will turn it
east before it has a chance to move over Puerto Rico and the
northern U.S. Virgin Islands. Saint Croix will have a slightly
better chance of showers on Tuesday. Then a weak cold front will
pass through and even low levels will dry somewhat. This will
cool the north coast somewhat, but will also end all but the
slimmest chances of rain for the next few days. A weak band of
moisture returns from the southeast on Saturday, but this will
bring only a slight chance of showers. Modest moisture from the
east in the lowest layers--generally below 700 mb--will continue
those showers which will fall mainly on the windward slopes nights
and early mornings and in the interior during the day. Little
change from this is expected through early next week.
AVIATION...Mainly VFR conditions expected to prevail across the
local flying area for the next 24 hours. However, the combination of
the limited low level moisture with sea breeze convergence will
induce the development of -SHRA across the NW quadrant of PR
possible affecting TJBQ after 23/18Z. Latest TJSJ sounding indicated
a southerly wind flow up to 10 knots from the surface to 9k feet,
becoming light and variable between 9-24K feet and then from the
northwest and stronger aloft.
MARINE...Seas will increase somewhat as swell move in from the
north northeast. The northern buoy (41043) never quite reached 7
feet as winds have remained gentle. Then, after a brief swell
episode on Wednesday from the northwest, seas will diminish. Seas
may reach 7 feet briefly in the northern outer local Atlantic
waters, but this is not expected to be a prevailing condition.
.PRELIMINARY POINT TEMPS/POPS...
SJU 73 90 75 84 / 0 20 0 10
STT 74 82 75 83 / 10 20 0 10
PR...High Rip Current Risk until 6 AM AST Tuesday for Culebra-
Mayaguez and Vicinity-North Central-Northeast-Northwest-San
Juan and Vicinity-Southeast-Western Interior.