Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 200844
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
444 AM AST Mon May 20 2019

.SYNOPSIS...
An unstable weather pattern is forecast to persist today through
the end of the week. The unsettled weather is due to a mid to
upper-level trough north of Puerto Rico and low-level moisture
that will pool over the region. The factors above will increase
the likelihood for the development of deep convection, and
isolated thunderstorms across northern, interior and western areas
of Puerto Rico during the afternoon. Wednesday current guidance
shows a week sub- tropical jet with winds of 50 to 80 kts near the
area through Monday of next week. The sub-tropical jet combining
with a mid to upper-level trough that develops during the middle
of the week will increase the divergence aloft across Puerto Rico
and the U.S. Virgin Islands. The divergence aloft will aid in the
development of deep convection and thunderstorms across the
western, interior and northern areas of Puerto Rico during the
afternoon. The thunderstorms and deep convection could cause urban
and small stream flooding with ponding of water on roadways.

&&

.SHORT TERM...

The proximity of a mid to upper level trough will continue to weaken
the ridge aloft. This trough is forecast to move close to the
northeastern Caribbean through the next few days providing upper
level support for the development of showers and thunderstorms
across the islands of Puerto Rico and the U.S. Virgin Islands. A
trough of low pressure located off to the north-northwest of the
islands will induce a moist southeast wind flow across the region
through the short term period. Model guidance are suggesting
precipitable water values between 1.75-2.10 inches, which are near
or above normal values for this time of the year. In addition, a sub-
tropical jet will reach it maximum around Wednesday increasing upper
level divergence. Under this weather pattern, the development of
showers and thunderstorms is likely, especially along and north of
the Cordillera Central each afternoon.

Soils are already saturated across portions of the interior,
northwest and northeast Puerto Rico. Therefore, as the potential for
showers and thunderstorms increase, the risk for flooding will
increase. Under a southeast wind flow, the potential for urban and
small stream flooding is high across the San Juan Metro Area.

High temperatures will reach the low 90s along coastal areas each
afternoon.

&&

.LONG TERM...Thursday through Monday...

A mid to upper-level trough in the Western Atlantic will increase
upper-level instability across Puerto Rico and the local waters
through the end of the workweek. As a result, an increase in
upper-level divergence aloft for portions of northern, central,
and western areas of Puerto Rico. Analysis of the 500 mb
temperature showers values ranging from 8 to 9.5 degrees celsius.
In addition, precipitable water values will range from 1.7 to 2.00
inches through most of the long-term period. Also, the sub-
tropical jet will provide added instability and increase
divergence aloft with winds ranging from 50 to 80 kts across the
region through the long-term period.

The combination of significant low-level moisture combining with
mid to upper-level instability with local and diurnal effects will
increase the likelihood for the development of deep convection.
The deep convection will be in the form of thunderstorms over the
interior, eastern, western areas of Puerto Rico during the
afternoon. Also, showers and isolated thunderstorms are possible
across the U.S. Virgin Islands and other areas of the region
during the afternoon.

This weekend another mid to upper-level trough is forecast to
develop in move north of Puerto Rico. However, the mid to upper-
level trough will provide upper-level instability and aid in the
development of deep convection across the Puerto Rico and the U.S.
Virgin Islands.

Current model guidance shows steep lapse rates, veering winds,
and deep tropical moisture across most of the area. For the
reasons above an increase in deep convection and thunderstorms are
forecast during the afternoon across the Caribbean waters, U.S.
Virgin Island, and eastern sections of Puerto Rico Saturday
through early next week. Showers and isolated thunderstorms that
develop could cause urban and small stream flooding with ponding
of water on roadways.

&&

.AVIATION...

VFR conds expected through the morning hours. But, VCSH
will move from time to time across the terminals of E-PR/USVI/Lesser
Islands. SHRA/TSRA will develop along and north of the Central
Mountain Range of Puerto Rico between 20/16-23. This activity will
create MVFR or even IFR conds at TJBQ and possibly at TJMZ/TJSJ.
Winds will continue calm to light and VRB, becoming from the ESE at
10-15 KT with sea breeze variations and occasional gusts after
20/13Z.

&&

.MARINE...
Tranquil marine conditions will continue over the next several
days. Seas will range from 3 to 5 feet across all local and region
waters. Winds will be out of the east to southeast at 10 to 15
kts. A moderate risk of rip currents across portions of the north
facing beaches of Puerto Rico today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  77  89  78 /  50  30  30  40
STT  86  77  86  77 /  40  20  20  30

&&

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

$$

SHORT TERM...CAM
LONG TERM....TW


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