Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 120038
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
838 PM AST Mon Dec 11 2017

.UPDATE...Partly cloudy skies tonight and early tomorrow with a
slight chance of a passing shower through tomorrow morning.
Prevailing easterly winds of 5 - 10 kts tonight and tomorrow. By
mid week conditions will slowly change, with prevailing southerly
winds ushering in moist tropical air which should aid in shower
development.

&&

.AVIATION...VFR conditions will prevail for the reminder of the
TAF period. Easterly sfc winds light to variable and increasing
to 5 to 10 kts by 12/10Z. Increasing moisture from the east
southeast could cause isolated showers across terminal sites
during the overnight and early morning hours but these showers
will be short-lived.


&&

.MARINE...Seas will continue to subside across the region waters.
In general, mariners can expect seas between 3 and 5 feet across
the Atlantic Waters and Anegada Passage, and between 1 and 4 feet
across the Caribbean Waters. A northerly swell is expected to
arrive by the end of the work-week.


&&

.PREV DISCUSSION... /issued 352 PM AST Mon Dec 11 2017/

SYNOPSIS...
The local weather conditions will be dominated by an upper level
ridge. Shower activity if any, will be wind driven due to patches
of low level moisture across the regional waters. Then, a
southerly wind flow is expected to result in afternoon showers
along and to the north of the Cordillera on Tuesday.

SHORT TERM...Tonight through Thursday...
The upper level ridge over Hispaniola will slowly weaken during
the next 3 days but will not cross PR and the U.S. Virgin Islands
definitively until Wednesday. Moisture at mid levels remains weak
through the period even though precipitable water climbs from a
minimum of 1.1 inches at 12/12Z to 1.54 inches at 14/00Z. But the
overall deciding factor in who gets rain appears to be the lower
level winds which will shift with an approaching trof, to pass the
Leeward Islands Tuesday morning and Puerto Rico shortly
thereafter after a brief passage of drier air. This will cause low
level winds to veer from easterly tonight to almost southerly
early Wednesday. Winds will be light however on Wednesday and some
sea breezes should be able to break through on the north side to
moderate what would ordinarily be a rather warm day for the north
coast of Puerto Rico. Nevertheless increasing moisture and light
winds bode well for showers, albeit shallow, over the interior of
Puerto Rico Wednesday. The cold front, now over eastern Cuba,
becomes tenuous in its advance toward the area and does not
advance beyond the highlands of the Dominican Republic which it
reaches by Wednesday night. Showers on Wednesday and Thursday will
occur mainly due to weak pre-frontal trough that tries to form
over and east of the area. For the U.S. Virgin islands, the
southerly winds on Wednesday will bring up a little moisture and
slightly warmer temperatures, but will not have as strong effect
as for Puerto Rico.

LONG TERM...Thursday through Tuesday...
An upper level ridge is expected to build across the northeast
Thursday through the upcoming weekend. This feature is expected to
maintain a relatively stable weather pattern across the local
islands during the forecast period. A dissipating frontal
boundary/shear line is expected to remain across the Atlantic
waters to the northwest and north of the local islands Thursday
and Friday. Strong surface high pressure across the northeastern
Atlantic will promote an east to east-northeast winds across the
local islands Thursday and until at least Saturday. Another
surface high pressure is expected to move just north of the local
area late in the upcoming weekend, tightening the pressure
gradient and causing breezy conditions early during the upcoming
week. Small patches of moisture embedded in the trades will bring
brief periods of showers across the windward side of the islands
especially during the nights and early in the mornings. However,
the upper level ridge over the region will inhibit the development
of widespread or significant precipitation during the forecast
period.

AVIATION...
Isold SHRA ovr Atlantic waters movg SW into PR with brief MVFR
due to CIGS. Areas of mtn obscurations in Cordillera Central til
12/02Z. A patch of moisture will bring sct SHRA to TJSJ/TIST/TISX
btwn 12/04-10Z where brief will be MVFR psbl. For the Leeward
islands CIGS mainly abv FL030 but moisture patch btwn 11/20-12/01
to bring CIGS arnd TKPK to 018-024. SHRA aft 11/06Z should remain
south of TNCM. Apchg trof to shift winds to SE TNCM/TKPK aft
12/12Z. Maximum winds northwesterly 30-40 kt btwn FL350-470 til
11/12z.

MARINE...
Seas will continue to subside across the region waters. In
general, mariners can expect seas between 3 and 5 feet across the
Atlantic Waters and Anegada Passage, and between 1 and 4 feet
across the Caribbean Waters. A northerly swell is expected to
arrive by the end of the work-week.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  74  86  76  86 /  20  30  10  30
STT  74  85  75  84 /  20  10  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...99
LONG TERM....99


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