Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230045
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
845 PM AST Sun Jan 22 2017

.UPDATE...Partly cloudy skies are prevailing across the local
region this evening. Terminal doppler radar is showing little no
rainfall echos over Puerto Rico and the U.S. Virgin Islands. The
rest of tonight and overnight...fair weather conditions will
continue across the local islands as generally dry conditions
dominate the area. Low level winds will veer to the southeast on
Monday... resulting in warmer temperatures across the
islands...especially the northern half of Puerto Rico where max
temperatures could reach 85 degrees early in the afternoon. Minor
changes were introduced to the forecast package this
evening...mainly to remove the PoPs over Puerto Rico during the
overnight hours.

&&

.AVIATION...VFR conditions expected across all terminals through
23/24z. Winds should light and variable overnight becoming mostly
ESE at 5-10 kts with sea breeze variations after 14z.

&&

.MARINE...A moderate northerly swell will continue to impact the
local Atlantic waters and Caribbean Passage over the next few days.
As a result...a high risk of rip currents continue across most of
the Atlantic beaches of Puerto Rico through at least Monday
afternoon. Seas will build to 4-6 feet by midweek as a northwest
swell reaches the regional waters.

&&

.PREV DISCUSSION... /issued 215 PM AST Sun Jan 22 2017/

SYNOPSIS...Generally dry air aloft will allow only a slight
chance of showers. The best chance will be on Tuesday. Winds will
shift to a southerly direction Monday and Tuesday but return to
light northerly with some land/sea breeze influences by Wednesday.
This will increase temperature--especially on the north coast of
Puerto Rico.

At upper levels...Northwest flow will increase tonight before a
ridge moves across the area Monday. A trough will develop over the
Dominican Republic beginning on Wednesday and hold southwest flow
over the forecast area through the rest of the week.

At mid levels...High pressure ridges in from the west early this
week and is mostly unaffected by a weak trough passage--mainly
north of the area--Tuesday night. High pressure then shifts to
northeast of the area over the weekend but remains in control of
the local area. Moisture will be lacking in the mid levels except
for a brief shot across the southeast corner of the forecast area
on Tuesday.

At lower levels...High pressure retreats to the east over the
central Atlantic ocean while strong low pressure moves into the
western Atlantic east of Virginia on Monday. High pressure then
builds over the Bahama Islands on Wednesday as a weak front moves
through the area while dissipating. The ridge across the sub-
tropical Atlantic will reorganize during the latter part of the
week and remain through early next week.

DISCUSSION...A thin skiff of cirrus was seen over the area as
north northeast flow gently nudged some cumulus formation over
Puerto Rico north of and more solidly over the Cordillera Central
and the interior of Puerto Rico. No showers were seen except over
the outer Atlantic waters. A few streamers developed off the U.S.
Virgin Islands.

The GFS is backing off the moist conditions expected on Tuesday
and has not yet verified even the mediocre POPs that it had for
the central interior of Puerto Rico for today. Southerly flow will
commence beginning tonight due to the high in the central Atlantic
and the low advancing into the western Atlantic. And, for now,
this looks like a return of the somewhat drier air at lower
levels for Monday. The southerly flow over the island will bring
down-slope effects to the lower elevations north of the
Cordillera Central and temperatures Monday will be 3 to 4 degrees
warmer than today which saw light northerly winds in place.

Since the GFS is now taking the mid level moisture from the south
and southwest just southeast of Puerto Rico it now looks like
temperatures could warm even further on Tuesday with even 90
degrees possible just inland from the northeast Atlantic coast.

This also means that chances are less that significant shower
activity will form.

Mid levels then dry considerably, along with the precipitable
water so most of the rest of the week, beginning Wednesday will be
dry and sunny with only very few showers.

AVIATION...Mainly VFR conditions expected to prevail across all
terminals. Clouds and -SHRA have developed along the Cordillera
Central and southern slopes of PR. This could result mainly in BKN
cigs and -RA/VCSH periods at JPS/JMZ through the rest of the
afternoon hours with mtn tops obscured. Winds at TNCM/TKPK are
already E-ESE...NE at the USVI terminals and sea breeze variations
across the PR termimals around 10 kts or less. Light and variable
winds expected overnight at the sfc.

MARINE...Seas will come up slightly tomorrow and winds become
southerly. This will increase the risk of rip currents--but mainly
on the north coasts. Seas are still expected to remain below 7
feet for the next 7 to 10 days.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  71  85  73  88 /   0  10   0  20
STT  71  80  74  82 /  20  20  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Monday afternoon for Mayaguez and
     Vicinity-North Central-Northeast-Northwest-San Juan and
     Vicinity-Southeast-Western Interior.

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

$$

08/11


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