Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 141506
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1106 AM AST Thu Dec 14 2017

.UPDATE...Fair weather conditions prevailed during the morning
hours across the islands. Mostly sunny to partly cloudy skies were
observed. For this afternoon, diurnally induced showers are
expected to develop over portions of western Puerto Rico. Minor
changes were made to the short term grids but overall no change
from previous discussion.


&&


.AVIATION...Mainly VFR conditions to prevail across all terminals.
Except possible MVFR in SHRA between 14/16z-22z at TJBQ/TJMZ. The
14/12z TJSJ upper air sounding indicated ESE winds at 4-16 kt BLO
FL050...ENE winds at 10-19 kt from FL050-FL150 and westerlies abv
FL250 with a max 61 kt at FL420.


&&

.MARINE...Buoys across the coastal waters of the islands were
indicating seas up to 3 feet and easterly winds below 10 kts. A
long period northwest swell will reach the local waters tonight
and will result in a high risk of rip currents along the Atlantic
coast of Puerto Rico and Culebra. Small crafts should exercise
caution tonight as seas up to 6 feet are expected across the
Atlantic waters.


&&

.PREV DISCUSSION... /issued 430 AM AST Thu Dec 14 2017/

SYNOPSIS...A surface front will continue to stall west of the
area while it dissipates. As the surface front dissipates, the
surface high across the central Atlantic will spread across the
area tightening the local pressure gradient. At upper levels,
ridge will continue to build and hold through much of the upcoming
week. A fair weather pattern with a few passing showers at times
is expected to prevail across the local islands during the next
few days.

SHORT TERM...Today through Saturday...

Surface high pressure across the east central Atlantic will begin to
spread westward, causing the easterly winds that we expect today and
Friday, becoming ENE on Saturday before a SFC high pressure moves
north of the local islands late Saturday, starting to tighten the
pressure gradient and causing breezy conditions late in the weekend
with an ENE wind flow by Sunday.

For today, isolated showers expected across the local waters, the
USVI, and eastern PR, leaving minimal accumulations if any. The
available moisture will be below normal today but the local effects
may help in the development of scattered showers across the
northwestern quadrant of PR this afternoon. Even drier air is
expected overnight tonight into Friday and even less shower activity
is expected on Friday as the precipitable water values could be as
low as one inch or below at times, which is near 2 standard
deviations lower than normal. By late Friday the winds are expected
to start shifting to a ENE direction. A patch of moisture expected
to approach the islands on Saturday but the lack of upper level
support will likely result only in scattered showers across the area
on Saturday. An upper ridge is expected to be over the local islands
on Saturday with a trough to our east, which is not particularly
conducive for prolonged shower activity, but scattered passing
showers are expected across the local forecast area.

LONG TERM...Sunday through Friday...

A mid to upper level ridge will hold across the forecast area
through at least midweek next week. The ridge pattern aloft is then
expected to erode Thu-Fri as an upper trough establishes across
the tropical Atlantic and into the eastern Caribbean. At lower
levels, high pressure north of the area will prevail much of the
forecast period resulting in a moderate east northeast wind flow.
With the ridge pattern from surface through the upper levels,
expect a fair and mainly stable weather pattern across the local
islands much of the upcoming week with a few passing showers
possible. Seasonable temperatures are also expected under the
east northeast wind flow.

AVIATION...Prevailing VFR conditions expected for the next 24 hours
across the local terminals. Isold/Sct SHRA through the day may cause
VCSH. However, SHRA is expected across NW-PR after 14/16Z, which may
affect TJBQ and the vicinity of TJMZ, possibly causing brief MVFR
conds at TJBQ. Winds will be generally from the east today at around
10KT with sea breeze variations.

MARINE...Seas will be 3 to 5 feet during the day, increasing
tonight as a northerly swell moves in. The local seas will
increase up to 6 feet through Saturday then 7 feet or higher on
Sunday and into early next week. There is a moderate risk of rip
currents throughout the day becoming high tonight.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  87  76  86  76 /  30  30  20  20
STT  86  75  86  76 /  30  30  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from 6 PM AST this evening through Friday
     afternoon for Culebra-North Central-Northeast-Northwest-San
     Juan and Vicinity-Southeast-Western Interior.

VI...None.
AM...None.
&&

$$

DS


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