Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 182213 AAA

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
613 PM AST Sat Jan 18 2020

.UPDATE...Marine conditions will deteriorate rapidly and to a
greater extent than expected. Please see the details in the
marine section below.

Scattered showers are moving into the area as the first
bands of the cold front poised near 20 north enter the area. The
GFS continues to show the front moving into the area tonight and
into Puerto Rico Sunday morning. Satellite imagery however shows
no cool tops in the clouds and so anticipate that showers will be
generally light to moderate with flooding not anticipated.
Moisture at 700 mb begins to return to Puerto Rico and the U.S.
Virgin Islands late Sunday afternoon through Monday morning. This
is when better shower and better probabilities of rainfall should
be expected--focusing mainly on the U.S. Virgin Islands and the
eastern half of Puerto Rico. It is during this time also that
winds transition to easterly. On Monday winds will be coming more
from the southeast. This will likely bring slightly cooler
temperatures to the south coast, and two to four degrees warmer
temperatures to the north coast both Monday and Tuesday. The GFS
model also shows 1000-850 mb temperatures warming during the


.AVIATION...TAF sites are likely to remain VFR during the next 24
hours, however some mountain obscurations should be expected. An
area of moisture is currently moving across TKPK and has brought
broken CIGS arnd FL045. A band of moisture associated with the
cold front is moving into the FA and is expected to move into
PR/USVI btwn 19/06-12Z. Stronger than normal nighttime winds: NE
10-20 kt with gusts to 26 kt near SHRA. Aft 19/14Z sfc winds ENE
10-25 kt with gusts to 30 kt psbl--strongest near the north coast.
Max winds WNW 65-75 kt btwn FL400-500.


.MARINE...Swell from a storm moving through the western Atlantic
have begun moving past buoy 41043. Seas up to 13 feet have been
seen but are expected to increase up to 17 feet by 8 AM AST
Sunday. Northeast winds are also adding to the confused seas. This
will result in considerably higher breaking waves than previously
expected. Breaking waves up to 19 feet are expected along the
north central coast of Puerto Rico. This will increase the amount
of coastal erosion and some salt water intrusion from the
Atlantic. The models have been consistently 3 to 5 feet lower than
the actual buoy reports and although this may be in part due to
the early arrival of the swell from the north, it is still going
to bring in much worse conditions. Hazardous seas should begin
diminishing by Sunday afternoon, but the last small craft
advisories will likely not go down before Tuesday.


.PREV DISCUSSION... /issued 200 PM AST Sat Jan 18 2020/

SYNOPSIS...An approaching frontal boundary will increase low-
level moisture and instability over the region tonight and early
next week. However, with the increasing winds expected, shower
activity will be of fast moving nature and therefore no
significant accumulations are forecast. The arrival of a long
period northerly swell by late tonight, together with wind-driven
seas will continue to generate hazardous coastal and marine
conditions over most of the local waters. Another frontal boundary
will arrive by mid-week, thus producing shower activity across
most of the region during the rest of the forecast period.

SHORT TERM...Today through Monday...
Dry conditions aloft will prevail through Monday. However, due to
low-level moisture associated with an approaching frontal
boundary, isolated to scattered showers could be expected from
time to time over eastern portions of PR and USVI with the highest
activity over the local waters, mainly during the evening hours.
However, rainfall estimates are not expected to be significant as
shower activity will be of fast moving nature. Starting late
Sunday night, a change in wind speeds and direction more to the
east will result in breezy conditions and a return of late evening
and early morning trade wind shower activity to portions of the
local waters and coastal sections of the islands. By Monday
afternoon, lingering moisture will combine with local and diurnal
effects to produce mostly isolated to scattered showers over parts
of the northwestern quadrant of the islands.

LONG TERM...Tuesday through Saturday...From Prev Discussion...
The long- term period looks unsettled as a result of several weak
frontal boundaries, which will move across the U.S. Virgin
Islands and Puerto Rico. The frontal boundaries will increase low-
level moisture and instability; this will lead to the development
of frequent passing showers.

Tuesday through Wednesday, a surface ridge is progged to slide
northeast ahead of an intensifying mid to upper-level trough over
the western Atlantic. The shift in the surface to the mid-level
ridge will result in low-level convergence and weak upper-level
divergence across the region. Also, guidance shows a robust low-
level jet with winds ranging from 60 to 90 kts persisting across
the region through the end of the week. The above conditions,
combined with weak troughiness aloft, increasing instability and
moisture will result in unsettled weather conditions. Therefore,
expected continuous showers to move across Puerto Rico and the
U.S. Virgin Islands during the middle of the week.

Thursday, an area of high pressure is expected to move off the
eastern seaboard, this will cause the surface ridge to abate over
the region as the frontal boundary strengthens over the Atlantic
waters. A cyclonic col is forecast to develop over the region,
and this will cause winds to slow and allow moisture to pull over
the area from the north/southeast.

Friday, the GFS shows the front boundary across Puerto Rico during
the afternoon. The frontal boundary and northerly winds will aid in
the development of showers across most of the region on Friday.

Saturday, the GFS shows the boundary draped across the U.S. Virgin
Islands and Puerto Rico through the afternoon, before moving the
boundary south of the area late Saturday night into Sunday morning.

Fair weather will persist throughout much of the forecast period.
However, a few quick passing showers are expected across the
local flying area at times. Clouds and showers will increase from
the north by 19/02z. The wind will blow from the ENE at 10-20 kts
with gust up to 30 kts.

Marine conditions will deteriorate throughout the day. A
northerly swell combined with wind-driven seas will increase seas
up to 15 feet late tonight and continuing through at least Monday.
Breaking wave heights between 10 and 17 feet can be expected with
the arrival of this swell. Therefore, small craft and high surf
advisories and a are in effect for most of the local waters.
Also, coastal flood advisories will go in effect late tonight for
the northern coasts of the islands. Refer to local marine products
for additional information.


SJU  72  84  73  85 /  60  60  80  50
STT  73  85  76  84 /  40  50  70  70


PR...High Rip Current Risk through late Monday night for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity-

     High Surf Advisory until 6 PM AST Monday for Culebra-Mayaguez
     and Vicinity-North Central-Northeast-Northwest-San Juan and

     Coastal Flood Advisory until 6 PM AST Sunday for Culebra-
     Mayaguez and Vicinity-North Central-Northeast-Northwest-San
     Juan and Vicinity-Southeast.

     High Rip Current Risk through late Monday night for Central
     Interior-Mayaguez and Vicinity-Southwest-Vieques.

     High Rip Current Risk from Sunday morning through late Sunday
     night for Ponce and Vicinity-Western Interior.

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

     High Surf Advisory until 6 PM AST Monday for St.Thomas...St.
     John...and Adjacent Islands.

     Coastal Flood Advisory until 6 PM AST Sunday for St.Thomas...St.
     John...and Adjacent Islands.

AM...Small Craft Advisory until 8 PM AST Monday for Coastal Waters of
     Southern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 AM AST Tuesday for 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
     Northwestern Puerto Rico out 10 NM-Coastal Waters of
     Northern Puerto Rico out 10 NM-Coastal Waters of Northern
     USVI and Culebra out 10 NM-Mona Passage Southward to 17N.

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

     Small Craft Advisory from 2 AM Sunday to 8 PM AST Monday for
     Coastal Waters OF Southwestern Puerto Rico out 10 NM.



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