Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 210024
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
824 PM AST Mon Jan 20 2020

.UPDATE...

GOES-16 detected cloudiness and moisture associated with a
dissipating old frontal boundary. Winds will continue from the
southeast across the region. Under this wind flow, the current
moist air mass will be replaced by a drier air mass later tonight
into tomorrow, limiting shower activity across the islands and
increasing afternoon maximum temperatures during the afternoon
hours. However, the typical wind-driven rains and diurnal induced
afternoon showers will continue to affect the local islands each
day.

&&

.AVIATION...

Clouds and -SHRA/SHRA will continue across the local flying area.
Some of these SHRA/-SHRA will affect JSJ/IST/JPS/NCM/KPK overnight
from time to time. SHRA activity will diminish aft 21/20z. The
wind will blow from the ESE-SE at 10 kt or less increasing at
10-15 kt after 21/13z.

&&

.MARINE...

Mariners can expect hazardous seas through at least Tuesday
evening. Therefore, small craft advisories are in effect for the
local waters. The combination of a northerly swell and wind-
driven seas will maintain seas up to 10 feet overnight. Breaking
wave heights between 10 and 13 feet can be expected with this
swell action. Refer to local marine products for additional
information.

&&

.PREV DISCUSSION... /issued 219 PM AST Mon Jan 20 2020/

SYNOPSIS...The cold front that pushed into the area Saturday
night and Sunday, bringing pervasive showers, is slowly backing
away and drier air is invading from the southeast. A stronger cold
front will move into the area from the northwest late Thursday
and Friday with more shower activity. Although drying is expected
behind the front over the weekend moisture will return by mid week
next week.

SHORT TERM...Tonight through Wednesday...
The cold front that pushed into the area Saturday night and
Sunday, bringing pervasive showers, is slowly backing away and
drier air is invading from the southeast. Only light showers are
currently indicated over Puerto Rico or the U.S. Virgin Islands at
12:30 PM AST. Low level flow, as indicated by shower motions is
clearly east southeast, but there has been sufficient warming over
Puerto Rico to produce slightly onshore winds on the north coast
of Puerto Ricoand temperatures under a partially hidden sun have
been slow to rise but are still in the low to mid 80s in the lower
elevations. Only scattered showers are expected tonight and
Tuesday in the drier air and these shwoers will focus mostly on
the windward southeast-facing plains and slopes of Puerto Rico,
except that currently the GFS is forecasting an area of moisture
to develop just east of the area and move east until it gets over
the U.S. Virgin Islands. Then that moisture is to move northwest
with little effect in Puerto Rico. Therefore, POPs in the USVI are
40 and 50 percent, but generally less in Puerto Rico. With
southeast flow continuing, the air over Puerto Rico will be slow
to gain moisture as the front approaching from the northwest will
not yeild any moisture until a pre-frontal band moves into the
area Wednesday afternoon. At upper levels a passing ridge will
flatten out over the area and flow will become westerly. A
powerful jet will continue to develop northwest of the area and
migrate over the local outer Atlantic waters with speeds of 90-100
knots. With drier air in place at mid-levels and generally
increasing mid-level temperatures thunderstorms or heavy showers
are unlikely.

LONG TERM...Thursday through Sunday.../issued 531 AM AST Mon Jan
20 2020/

By Thursday, the surface ridge across the Atlantic is forecast to
shift farther northeast into the central Atlantic, as a cold front
will move into the western and southwest Atlantic and approach
the region. An associated amplifying mid to upper- level polar
trough will also cross the west Atlantic and weaken the ridge now
in place overhead. This will result in a weakening of the trade
wind inversion and promote instability aloft through at least
early Friday. The shift in the surface to the mid- level high
pressure ridge pattern should also result in better low level
moisture pooling and convergence as the frontal boundary is
expected to arrive and sink southwards across the region by the
latter part of the work week. Model guidance also continued to
suggest a vigorous mid to upper level jet max crossing the region
Thursday through Friday. The combination of the instability aloft
and good low level moisture convergence accompanying the frontal
boundary will therefore favor the development of scattered to
numerous to showers with periods of locally heavy rains across the
islands and coastal waters at least through early Friday.

By late Friday and into the weekend, model guidance still suggest
lingering low level moisture but gradually improving and stable
conditions aloft. Therefore expect less frequent early morning
shower activity across the islands and coastal waters. This will
be followed by isolated to scattered showers of short duration
during the afternoon hours. Improving conditions are forecast
by Saturday and through Monday, as the surface high pressure will
once again build and spread across the west Atlantic, resulting
in a more northeasterly wind flow and a stable near normal weather
pattern.

AVIATION...VFR are conds are expected drg prd. SHRA/VCSH will
continue through 20/23Z for TJBQ and TJMZ. VCSH are possible across
TJPS AND TJSJ terminals dgr prd with conditions improving by 20/20Z.
ESE winds of 10 to 15 kts with higher gusts.

MARINE...Seas will continue to diminish. Seas at the outer buoy
were around 12 feet at 20/16Z and had fallen below 10 feet at the
inner buoy. Nevertheless it will take some time for seas to become
quiet enough to take down small craft advisories. Also seas are
sufficient to extend the high surf advisory another 12 hours. The
Coastal flood advisory has been allowed to expire.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  73  88  73  87 /  10  20  40  40
STT  74  81  73  82 /  30  40  40  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Tuesday night for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity.

     High Surf Advisory until 6 AM AST Tuesday for Culebra-Mayaguez
     and Vicinity-North Central-Northeast-Northwest-San Juan and
     Vicinity-Southeast.

     High Rip Current Risk through Tuesday afternoon for Mayaguez and
     Vicinity-Vieques.

     High Rip Current Risk until 6 AM AST Tuesday for Southwest.

VI...High Rip Current Risk through late Tuesday night for
     St.Thomas...St. John...and Adjacent Islands.

     High Rip Current Risk through Tuesday afternoon for St Croix.

AM...Small Craft Advisory until 8 PM AST Tuesday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-Caribbean Waters of Puerto Rico from 10
     NM to 17N-Coastal Waters of Northern Puerto Rico out 10 NM-
     Coastal Waters of Northern USVI and Culebra out 10 NM-
     Coastal Waters of Southern USVI Vieques and Eastern Puerto
     Rico out 10 NM.

     Small Craft Advisory until 2 PM AST Tuesday for Mona Passage
     Southward to 17N.

     Small Craft Advisory until 8 AM AST Tuesday for Coastal Waters
     OF Northwestern Puerto Rico out 10 NM.

&&

$$

SHORT TERM...CAM
LONG TERM....MB
PUBLIC DESK...RVT


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