Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170104
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
904 PM AST Sat Nov 16 2019

.UPDATE...

Showers diminished over the land areas just before
sunset today. The heaviest rainfall was observed to the south of
Las Ollas. TJUA Dual Pol storm total showed rainfall amounts
ranging from 1.5 to 2.2 inches. Also, to the north of Lajas TJUA
Dual Pol had rainfall totals ranging from 1.0 to 1.3 inches.
Elsewhere, rainfall amounts where less than a tenth of an inch.
Currently TJUA doppler radar shows isolated to scattered showers
moving southward across the northern areas of San Juan, and
adjacent islands. A weak tropical wave moving through the
Caribbean is causing this activity. The weak tropical wave will
continue to produce isolate to scattered showers through the
overnight as it continues to move to the west. Tomorrow scattered
to isolated showers are possible over portions of eastern Puerto
Rico and the U.S. Virgin Islands. Light to moderate showers are
forecast to develop across the interior and western areas of
Puerto Rico during the afternoon. This activity will develop as a
result of sea breeze convergence coupled with diurnal heating, and
local effects. Tomorrow afternoon guidance shows light low-level
steering flow across the region. This light flow will cause
afternoon convection to move slowly, and could cause urban and
small stream flooding as well as ponding of water on roadways.

&&

.AVIATION...

VFR conds are forecast to continue through 17/18Z. ISO/SCT
showers are possible across E PR and USVI terminals now through
17/12Z. VCSH/SHRA are forecast to develop across terminal sites
TJBQ and TJMZ by 17/18Z. VCSH/SHRA should diminish across all
terminal sites by 18/02Z. Sfc winds light and variable during the
overnight hours, then by 17/12Z winds out of the east at 5 to 10
kts.

&&

.MARINE...

Sea up to 5 feet across the nearshore and outer waters. Winds from
the east at 10 to 15 kts. A moderate risk of rip currents for the
northern beaches of Puerto Rico. Another northerly swell is
expected to result in choppy seas by Monday night into Tuesday.

&&

.PREV DISCUSSION... /issued 146 PM AST Sat Nov 16 2019/

SYNOPSIS...Ridge aloft and a drier than normal airmass will maintain
stable weather conditions across the area through the rest of the
weekend, with limited shower activity expected. Moisture is
expected to increase by early next week, resulting in an increase
in shower activity, mainly across interior and western Puerto
Rico during the afternoon hours. The approach of an upper-level
trough by the end of the week will result in unsettled weather
conditions.

SHORT TERM...Rest of today through Monday...

A weak low-level trough, located just east of forecast area, will
continue to maintain an east northeast to northeast flow through
Sunday morning. Latest analysis is indicating a drier than normal
airmass enveloping the region currently, with precipitable water
values ranging between 1.2 and 1.4 inches. Therefore, shower
activity is expected to be very limited today with some activity
possible across southwest and south-central Puerto Rico later this
afternoon due to local effects. However, any showers that develop
are expected to be short-lived with no significant rainfall
accumulations expected. Later tonight, some passing showers will be
possible across portions of northern and eastern Puerto Rico and the
USVI.

On Sunday, the low-level steering flow is expected to diminish to
less than 5 knots as a frontal system moves across the western
Atlantic and the aformentioned low-level trough moves south of the
region. This low-level trough will also shift the low-level flow to
east-southeast. Moisture content is expected to remain below normal
with precipitable water values ranging between 1.2 and 1.4
inches. Therefore, even though showers are expected mostly across
interior portions of Puerto Rico during the afternoon hours, the
drier than normal airmass will limit the duration of the activity.

An increase in shower activity is expected on Monday as low-level
moisture increases with precipitable water values rising to around
1.8 inches. With the low-level flow expected to remain very light
from the east-southeast, showers are expected across interior and
northwestern portions of Puerto Rico during the afternoon hours and
also be slow-movers. Therefore, the potential will exist for more
significant rainfall accumulations to materialize, resulting in
some urban and small stream flooding. Across the USVI, only some
isolated to scattered shower activity is expected.

LONG TERM...Tuesday through Saturday...

PREV DISCUSSION... /issued 518 AM AST Sat Nov 16 2019/

The weak low pressure at lower levels will have split into two
parts during the weekend. By Tuesday the more southerly part will
have transited the Caribbean Sea south of us, while the northern
portion will continue moving north and away from the area. This
will leave a weak ridge on Wednesday that rapidly dissolves as
another weak trough over the Lesser Antilles drifts through the
eastern Caribbean, and finally, the local area by Saturday night.
This brings bands of moisture, but also pockets of drier air
through the area. Total precipitable water, however, will peak on
Friday at close to two inches and then fall off.

Mid level dryness will persist through Thursday and return on
Saturday night. The break occurs when a passing trough to our
north at 500 mb weakens the pressure field over the area for a day
or two. Instability also increases then, as 500 mb temperatures
cool and approach minus 7 degrees Celsius.

At upper levels, Tuesday will find a ridge extending northeast
over the local area. It will slide east over the tropical Atlantic
as a sharp trough makes its passage late Friday night.

The combination of the cooler mid levels accompanied by the best
moisture of the week and the approach and passage of the upper
level trough will make Friday and early Saturday the most
unsettled of the period, with scattered to numerous showers.
These showers will likely lead to urban and small stream flooding
in localized areas, but will for the most part will not be
particularly impressive in coverage or amount. Otherwise the
general patter of scattered showers in our typical diurnal pattern
will continue.

AVIATION...Mainly VFR conditions are expected to prevail across
all terminals. However, brief SHRA may cause tempo MVFR at
TJPS/TJMZ thru 22z. Passing trade wind showers could move at times
across the USVI/Eastern PR terminals through the forecast period.
Low level winds will continue from the ENE at around 10 kts.

MARINE...Northerly swell continues to diminish across the regional
waters with latest buoy observations across the Atlantic waters
reporting seas between 5 and 6 feet. Seas are expected to diminish
even further on Sunday and Monday, with less than 5 feet expected
across all the regional waters. Another northerly swell is
expected to result in choppy seas by Monday night into Tuesday.
For beachgoers, a high risk of rip currents continues for the
northern beaches of Puerto Rico and Culebra.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  76  87  76  87 /  40  40  40  40
STT  77  88  76  87 /  40  30  20  40

&&

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

$$

SHORT TERM...TW
LONG TERM....RAM


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