National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXCA62 TJSJ 021525
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1125 AM AST Tue Mar 2 2021

.UPDATE...Drier than normal air is dominating the local area, and
the relatively dry air will continue through the rest of today.
The 12Z sounding indicated that there is a fairly stable
atmosphere and the precipitable water is 1.05 inches, which is
below than the 25th percentile for this time of year. The ENE
winds will continue today at around 15 mph with occasional gusts,
but these are expected to turn easterly and slightly lighter on
Wednesday, then becoming ESE and even lighter by Thursday and
Friday. There is a quick patch of moisture moving in early
Wednesday, which could once again bring brief showers, but
accumulations are expected to be minimal.

&&

.AVIATION...VFR conds expected for the next 24 hours. Brief -SHRA
may be possible in the vicinity of the local terminals, but no
significant hazards are expected. The local winds will be from the
E to ENE at 15 to 20 knots with occasional gusts through 02/22Z.
The winds will gradually decrease overnight as they become more
easterly.


&&

.MARINE...Small Craft Advisory continues in effect for the local
waters until 2 PM. The latest observations and model guidance
indicates that we will be able to let the advisory expire early
this afternoon. The winds across the local waters will be between
10 and 20 knots for the rest of today and tonight. There is also
a high risk of rip currents for many of the local beaches.


&&

.PREV DISCUSSION... /issued 352 AM AST Tue Mar 2 2021/

SYNOPSIS...Overall fair weather conditions are expected to
prevail through the end of the workweek. However, areas of low-
level clouds and light showers will move at times across the
islands. An induced surface trough is forecast to increase
moisture content and shower development between Friday into early
in the weekend. Trade winds are forecast decrease and turn more
from the southeast during the second part of the week.

SHORT TERM...Today through Thursday...
As the surface high pressure to the north of the islands continues
to weaken, the pressure gradient is gradually slacking off,
resulting in a decrease in sustained wind speed over the area. At
the mid levels, high pressure also dominates the western
Caribbean, which is resulting in a trade wind cap around 850 mb.
This will maintain moisture trapped in the lower levels of the
atmosphere. According to Total Precipitable Water from GOES-16, a
relatively dry air mass, with values around 0.9 inches, is
currently located over Puerto Rico and the U.S. Virgin Islands. In
fact, the forecast soundings and the latest model guidance are
not suggesting any significant increase in moisture over the
islands, meaning that rainfall should be very limited through the
next few days. Even with the low moisture content expected, trade
wind showers may still form and affect the eastern half of Puerto
Rico and the U.S. Virgin Islands, with additional showers
developing in the afternoon over western Puerto Rico, but none of
these showers are expected to be significant.

As the surface high pressure migrates toward the eastern Atlantic
and a cold front exits the eastern coast of the United States,
winds will begin to shift more from the east-southeast on
Wednesday and from the south-southeast on Thursday. In fact, the
southeasterly wind flow is expected to extend into the mid-layers
of the atmosphere. As a result, warmer temperatures are expected
by the end of the workweek, with possible high temperatures
reaching the upper 80s, especially across the coastal areas.

LONG TERM...Friday through Wednesday...
The mid-level ridge over the region is forecast to weaken by
Friday, but it is expected to build and strengthen once again
over the central Atlantic. Meanwhile, an amplifying mid to upper
level trough over the western Atlantic will extend into the
northeastern Caribbean. A surface front is expected to remain well
north of the region, while an induced pre-frontal trough is
expected to develop just west of the local area, promoting a weak
and moist southerly wind flow on Friday and Saturday. Although
global models are not as aggressive as last night solutions,
precipitable water (PWAT) content is forecast to remain just above
climatology around 1.40 inches. Therefore, expect diurnally
induced afternoon showers to develop over the interior and
northern sections of PR, due to the light steering winds, slow
moving showers could lead to minor flooding.

On Sunday, trade winds are forecast to return to around 15-20 kts
and overall fair weather conditions are expected with afternoon
showers developing over western PR. For early next week, another
induced surface trough is forecast to develop over the local area
and increase shower development across the islands. PWAT content
is expected to continue above climatology at 1.40-1.50 inches
through midweek, favoring afternoon shower development over
western PR.

AVIATION...Mainly VFR conditions expected to prevail through the
forecast period. Surface Winds will be out of the ENE at 10 to 15
knots with stronger gusts. VCSH may occur over the Leeward and
U.S. Virgin Islands, as well as TJSJ early in the period, which
could cause brief periods of low ceilings.

MARINE...Choppy wind driven seas up to 8 feet will continue
through at least today across the regional waters. Small Craft
Advisories will expire later this afternoon. Seas will gradually
improve during the next few days across most of the local waters,
however, seas up to 6 feet are expected to continue mainly across
the Atlantic waters and passages. East to northeast winds up to 20
kts will prevail through Wednesday, thereafter, winds turn more
from the east to southeast at 10-15 kts. A 3-5 ft northerly swell
will move briefly across the Atlantic waters and passages between
Thursday afternoon and Friday.

There is high risk of rip currents today for the northern and
eastern beaches of PR, Culebra, St. Croix and northwestern beaches
of St. Thomas.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  82  72  85  72 /  30  40  40  30
STT  84  73  83  74 /  20  40  40  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 PM AST this evening for Culebra-
     Northeast.

     High Rip Current Risk through late tonight for North Central-
     Northwest-San Juan and Vicinity.

VI...High Rip Current Risk until 6 PM AST this evening for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 2 PM AST this afternoon for Caribbean
     Waters of Puerto Rico from 10 NM to 17N-Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-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-Mona Passage Southward to 17N.

&&

$$

SHORT TERM...JA
LONG TERM....CS
PUBLIC DESK...FRG