National Weather Service Today's Weather Impact Levels

751
FXCA62 TJSJ 210917
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
517 AM AST Sat Dec 21 2024

.SYNOPSIS...

An approaching front from the western Atlantic and a low pressure
over the southern Caribbean will promote increasing moisture,
light to moderate southeasterly winds, and warmer than normal
temperatures through the weekend. A building high pressure behind
the front will promote moderate to locally fresh east to
northeast winds early next week. Choppy seas and life-threatening
rip currents continue today.


&&

.SHORT TERM...Today through Monday...

The area of best convection was southwest of us overnight with more
thunderstorms throwing off a shield of high cirrus tops over us
which at times pushed through the tropopause. Thunderstorms over the
Caribbean waters south of the Dominican Republic were showing
satellite tops of less than minus 70 degrees C. Showers over the
Caribbean south of Puerto Rico did affect southeast Puerto Rico as
far west as Guanica. Parts of Salinas received as much as 2.4 inches
of rain and an urban and small stream flood advisory was issued a
little before 11 PM AST. Showers this morning were generally south
of Juncos and San Lorenzo. Also, the heavy tops of the thunderstorms
were producing light rain that moved over western Puerto Rico with a
few hundredths of an inch and over the Mona channel with 4-5 tenths
of an inch. Temperatures continue to run a few degrees above normal
with overnight lows in the range of 70 to 80 degrees in the lower
elevations and the 60s in the mid and upper elevations.

Low pressure is seen along the northern coasts of Venezuela and
Colombia while a lobe of high pressure spilled into the western
Atlantic between the two lows that have developed there: one off the
coast of Maine and one spinning north of 40 north and just a little
west of 40 west. This has caused winds to relax over the local area
while still holding a generally southeast direction. A frontal
boundary moving southeast through Hispaniola today and a shortwave
at upper levels over South Carolina moving east northeast will
generate a northeastward-moving low pressure in the lower levels
north of Puerto Rico today that will pull moist air out of the
eastern Caribbean, even though drier air is seen to hold just
northeast of the Leeward Islands. This will hold and even increase
precipitable water values to 2.1 inches this afternoon and 2.25
inches Sunday evening. With 500 mb temperatures dipping near to or
below minus 7.5 degrees C, this will keep the possibilities of
thunderstorms in the forecast at least through Monday-some off
shore, but some also over Puerto Rico. Southeast surface flow will
keep showers moving onshore over southeast Puerto Rico with only
very limited activity over the U.S. Virgin Islands. Some showers on
the north coast are also expected from streamers off of Saint Croix
and Vieques moving toward the north coast.

.LONG TERM...Tuesday trough Saturday...

From Tuesday through Saturday, Puerto Rico and the U.S. Virgin
Islands will experience a progression from wet to significantly
drier weather conditions. On Tuesday, a high-pressure system in the
mid-levels will maintain overall stable weather conditions with
drier intrusion at the upper levels and a trade wind cap inversion.
Although instability will be absent, at the surface, a northeasterly
wind flow induced by a perturbation in the central Atlantic will
bring continuous patches of tropical moisture along the CWA. This
setup will result in scattered to numerous showers across the
islands, particularly during the morning in windward areas and in
the interior and southwestern regions of Puerto Rico during the
afternoon.

By Wednesday, the tropical moisture will move out of the region,
making way for a slightly drier air mass with precipitable water
values decreasing to 1.5 inches or less. The islands will see
reduced shower activity under the influence of continued high
pressure aloft. Winds will remain light and variable from the
northeast due to a loosening pressure gradient, and weather
conditions will generally trend toward fair and stable. Thursday
will bring more of the same, as the drier air mass persists and mid-
level stability suppresses significant convective development. Any
showers will be brief and isolated, favoring coastal areas in the
early morning and interior locations during the afternoon. Winds
will continue to be light and variable, with overall calm weather
across the islands.

By Friday and Saturday, a broad surface high-pressure system
building over the central Atlantic will change the surface pattern,
pushing a much drier air mass into the region. This will further
suppress any significant shower activity, leaving the islands with
predominantly clear skies and slightly cooler temperatures,
particularly during the night and early morning. This drying trend
will mark the end of an active start to the week, providing more
tranquil weather for the weekend.


&&

.AVIATION...

(06Z TAFS)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, SHRA/TSRA over southern PR may
cause brief tempo MVFR especially over the Cordillera Central
through 22/06Z and over ern PR from 21/15-20Z. The 21/00Z TJSJ
sounding indicated ESE winds of 10-15 knots up through FL050, but
winds above this had a westerly component. Maximum winds WSW-W 45-55
kt btwn FL420-460.


&&

.MARINE...

Choppy seas continue across the offshore Atlantic waters, and a
Small Craft Advisory (MWWSJU) is in effect though late this
afternoon for these waters. A weak surface high pressure over the
central Atlantic and an approaching front from the western
Atlantic will promote light to moderate east to southeast winds
through the rest of the weekend. Showers and isolated
thunderstorms will also contribute to worsening marine conditions
during the weekend. Moderate to fresh east to northeast winds
return early next week due to a building surface high over the
western Atlantic.


&&

.BEACH FORECAST...

Life-threatening rip currents will continue today across the
beaches from Aguadilla to Fajardo along the northern coast of
Puerto Rico, Culebra, and St. Croix. There is a High Rip Current
Risk (CFWSJU) in effect through this afternoon for these areas. A
moderate risk is expected on Sunday for most east and north facing
beaches of the islands.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through this afternoon for PRZ001-002-005-
     008-012.

VI...High Rip Current Risk through this afternoon for VIZ002.

AM...Small Craft Advisory until 6 PM AST this evening for AMZ711.

&&

$$

SHORT TERM...WS
LONG TERM...LIS/DSR
AVIATION...WS

NWS SJU Office Area Forecast Discussion