National Weather Service San Juan PR
424 AM AST Sun Nov 24 2019
.SYNOPSIS...
Mostly quiet weather is expected today with a few showers possible
during the afternoon. Southerly flow on Monday will pull moisture
from the Caribbean across the region, and increase rain chances.
During the middle of the workweek scattered to isolated showers
are possible during the afternoons due to local and diurnal
effects.
&&
.SHORT TERM...Today through Tuesday...
A ridge aloft will continue to promote dry air at mid and upper
levels today. This will inhibit the vertical development of showers
across the islands. A surface high pressure moving across the
Atlantic Ocean will promote an easterly wind flow and cool air
advection. As a result, pleasant temperatures, stable and fair
weather conditions will continue in Puerto Rico and the U.S. Virgin
Islands. Afternoon convection, if any, will be shallow and limited
to portions of the western PR. But, passing showers should not be
ruled out over the windward sections of E-PR/USVI.
Model guidance has backed off, and now the increase in moisture
should begin later on Monday afternoon/evening. Therefore, a similar
weather pattern is forecast for the first part of Monday. The
increase in moisture associated with a trade-wind perturbation will
enhance the potential for shower development late Monday
afternoon/evening into Tuesday. The lack of upper-level support will
continue to inhibit vertical development, consequently, the
intensity and areal coverage of this activity.
&&
.LONG TERM...Wednesday through Monday...
Long-term guidances continues to show a relatively quiet period with
a few passing showers over the U.S. Virgin Islands and eastern
Puerto Rico during the morning. Followed by isolated to scattered
showers during the afternoons across northern, interior, and western
areas of Puerto Rico. The shower activity will be driven by local
and diurnal effects. Thursday, morning surface and 700 mb winds will
be mainly from east to northeast. This will lead to shallow moisture
from the Atlantic moving into the region from the north. The GFS and
EURO models continues to depict marginal instability and relatively
limit moisture which is needed for thunderstorm development.
Therefore, thunderstorms where not put in the long-term forecast.
Wednesday, guidances continues to show a weak trough passing through
the area, then deepening across the central Atlantic on Thursday.
Moisture from the Caribbean is forecast to lift north across Puerto
Rico; the added moisture will aid in the development of isolated to
scattered showers during the afternoon. Late Wednesday night the GFS
and EURO shows a drier air mass filtering into the region from the
central and eastern Atlantic waters. The GFS has precipitable water
values peaking at 1.72 Wednesday afternoon, then falling through the
remainder of the day. Similar to yesterday`s guidance the GFS and
EURO keeps 500 mb temperatures warm and shows limited divergence
aloft. Therefore thunderstorm development is not anticipated at
this time.
Thursday through Friday, the GFS and Euro has a slight drying trend
with a few patches of moisture embedded in the easterly trades
pushing over the area from time to time. During the afternoons
shower activity will be driven by local and diurnal effects; with
relatively light rainfall accumulations. The next weather maker for
the area will be a front that guidance shows moving into the western
Atlantic on Thursday. Current guidance shows several jet streaks
with winds ranging from 115 to 140 kts will round the base of the
trough Friday through Saturday. The jet streaks will help deepen the
trough with a frontal passage across Puerto Rico possibly late
Saturday night or early Sunday morning. If the frontal passages
comes to fruition this could lead to the development of
thunderstorms as a result of increased instability and upper-level
forcing. At this time the current thinking is scattered to isolate
showers will develop during the afternoons mainly across northern,
interior, western areas of the islands.
Monday, southeast winds will draw moisture from the southern
Caribbean waters across the U.S. Virgin Islands and Puerto Rico.
This will result in an increase in showers across the region on
Monday.
&&
.AVIATION...
VFR conds will continue across the local flying area.
Passing -SHRA/SHRA should not be ruled out at times, mainly across
JSJ/IST/ISX/NCM/KPK thru the fcst period, and across JBQ/JMZ during
the afternoon. But, fair and stable weather conditions will prevail.
Calm to light/vrb winds, returning from E-ESE at around 10kt with
sea breeze variations.
&&
.MARINE...
Seas will continue to improve across the region as the northerly
swell continues to dissipate over the area. Today seas up to 5
feet with easterly winds of 10 to 15 kts. Small craft operators
are urged to exercise caution. There is a high risk of rip
currents for northern beaches of Puerto Rico today.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 87 75 89 75 / 10 20 30 40
STT 86 78 87 78 / 10 30 30 40