Area Forecast Discussion
Issued by NWS San Juan, PR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

645
FXCA62 TJSJ 210200
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1000 PM AST Wed Mar 20 2019

.UPDATE...
A patch of low level moisture brought clouds and a few passing
showers across the islands through this evening. However, shower
activity is expected to slowly diminish overnight, as a drier air
mass filter from the east over the islands. However, under an
east to southeast wind flow, trade wind showers will move across
the east, south and north sections at times, followed by
afternoon showers over the interior, north-central and western
sections on Thursday.

&&

.AVIATION...
VFR conditions will persist through much of the forecast period.
Trade wind showers will move across the Leeward, USVI and E-PR
terminals at times. Afternoon convection is forecast to develop
across the interior and NW-PR between 21/17-23z. This may result
in brief MVFR conditions at TJBQ/JMZ. Calm to light and variable
winds are expected til 21/13z, when ESE winds of 10-15 kts will
return.

&&

.MARINE...
Tranquil marine conditions will persist through much of the work-
week. No changes were made to the inherited forecast.

&&

.PREV DISCUSSION... /issued 425 PM AST Wed Mar 20 2019/

SYNOPSIS...Ridge aloft will continue to maintain a fairly stable
weather pattern across the forecast area with limited shower
activity expected through the end of the work week. Surface high
pressure across the northeast Atlantic and a frontal boundary over
the western Atlantic will continue to maintain a moderate east to
southeast wind flow during the next several days.

SHORT TERM...Today through Friday...

Very dry air above 700 mb as a result of a ridge aloft will
continue to suppress the development of deep convection and
therefore, the chances of significant rainfall activity to
materialize during the short term period.

Currently, a surge in low-level is moving through the area and has
resulted some limited shower activity across portions of interior
and northwest PR as well as near the vicinity of Vieques and
coastal areas of northeast PR during the afternoon hours.
However, given the hostile environmental conditions in place
aloft, most of the these showers were light to moderate in
intensity. As a result, rainfall accumulations generally remained
under a quarter of an inch.

Later tonight into the day on Thursday, an area of drier air will
be moving over the region with precipitable water values expected
to drop below 1 inch. Therefore, less cloud cover and mostly dry
conditions can be expected across most of the region. A few
showers, however, cannot be ruled out during the afternoon hours
across the northwest quadrant of PR. These showers should be brief
and only result in very light rainfall accumulations.

Another surge in low-level moisture is expected by tomorrow night
into the day on Friday with precipitable water values rising
to between the 1.2 to 1.4 range. This would increase the low-
level cloudiness and shower activity once again. Some scattered
showers are possible across portions of eastern PR, the USVI, and
surrounding waters during the overnight and morning hours followed
by the development of locally and diurnally induced activity
across the northwest quadrant of PR during the afternoon hours.

LONG TERM...Saturday through Thursday...

 /issued 411 AM AST Wed Mar 20 2019/

For the weekend, drier than normal air will return briefly on
Saturday, but another patch of moisture could move in late in the
night on Saturday into Sunday but not enough to cause any
significant amounts of rain. Even though some showers are
expected to develop across the western sections of PR in the
afternoon hours, these are not expected to be too significant. A
frontal boundary is still forecast to stay well north northwest of
the local islands, but the latest guidance has some moisture
pooling just west of the local islands on Sunday into Monday,
which could bring more significant moisture than previously
expected, but at this time the more significant rain is expected
to be just west of PR across the local waters. Long range models
are showing a surface high pressure move through the western
Atlantic into the central Atlantic by Monday, then a surface low
to move across the eastern seaboard of the US on tuesday and
through midweek, causing southeasterly winds locally and possibly
dragging some moisture from the south and over the local islands,
but the mid and upper levels for those days are still not looking
favorable for prolonged and significant vertical development of
showers. The model solutions have some rainfall accumulations but
less than you would expect given the amount of moisture that it
puts over the local area with the GFS model having close to 2
inches of precipitable water. Confidence is low this far out in
the forecast, the models have changed slightly over the past few
days, but have been consistent in bringing good moisture to the
local area by midweek next week.

AVIATION...Advective or locally induced showers will bring SCT
occasional BKN between FL040-FL060 and VCSH to TJSJ, TJBQ, TJMZ,
and TJPS through at least 20/23Z. This may result in brief MVFR
conditions. ESE winds of 10-15 kts with occasional higher gusts up
to 20 knots will continue, becoming light and variable after
20/22Z.

MARINE...Fairly tranquil marine conditions with seas generally
below 5 feet and east to southeast winds up to 15 knots are expected
to continue during the next few days. A moderate risk of rip
currents is expected for portions of the north-facing beaches of
PR and Culebra on Thursday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  74  88  74  88 /  20  20  30  30
STT  74  85  74  85 /  20  30  30  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

UPDATE...CAM
AVIATION...CAM



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.