Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 260143 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
943 PM AST Wed Apr 25 2018

.UPDATE...The area of moisture that was over the Greater Antilles
islands yesterday continues to move slowly west toward the area. Mid
level moisture has now arrived at Saint Thomas and their last
observation included a ceiling of 6500 feet. Infrared satellite
imagery shows a wide band of mostly lower to mid level moisture
with the strongest activity located in a line that extends from
the arc of the Lesser Antilles toward the northwest. Models show
much of this moisture arriving in eastern Puerto Rico by 26/06Z
although this may be too fast. Expect shower activity to increase
gradually overnight and measurable rain to be likely in eastern
Puerto Rico between 26/08-15Z.

The complete sounding from 26/00Z showed 1.17 inches of
precipitable water since we are still in the trailing edge of a
dry slot in San Juan. Showers that were active over western Puerto
Rico this afternoon have completely dissipated but the showers in
the moisture approaching the USVI are visible and generally light
and moving toward 300 degrees at about 10 knots.

Rain chances (POP) grids have been modified to conform with latest
model data and will be reflected in the recently issued zone and
digital products.

Southeast flow will continue the warm conditions along the north
coast of Puerto Rico, but additional moisture should keep
temperatures contained to the upper 80s or around 90 just inland
in the Greater San Juan Metropolitan area.

Otherwise, no change to the general discussion below.

&&

.AVIATION...SCT/ISOLD SHRA are sprdg west from TNCM/TKPK.
Nevertheless generally VFR conds persist there with a SCT lyr at
FL018-020 and a BKN lyr at FL045-048. Conds imprv slowly thru
26/15Z and then few clds are expected aft 26/16Z. MVFR conds are
not expected too last long if they occur in the USVI with CIGS
when present at arnd FL050. SHRA will sprd across PR begg arnd
26/08Z with mtn obscurations. MVFR is not expected at
TJSJ/TJPS/, but may occur aft 26/16Z at TJBQ/TJMZ. Sfc winds E 5-10
kt til 26/14Z then bcmg 6-16kt with sea breeze influences. Max
winds W 50-65 kt btwn FL380-500 at 26/18Z.

&&

.MARINE...Small craft advisory conditions are not expected this
month. Swell will remain NE-E, with seas in exposed waters from 3
to 6 feet.

&&

.PREV DISCUSSION... /issued 417 PM AST Wed Apr 25 2018/

SYNOPSIS...

An increase in low level moisture is expected tonight and
Thursday, with increasing rain chances tomorrow. A mid to upper
level ridge will build over the area late Thursday and prevail
across the region through early next week with fair weather and an
occasional shower.

SHORT TERM...Tonight through Friday...

Mostly sunny skies prevailed over the U.S. Virgin Islands and
Puerto Rico today with easterly winds ranging from 5 to 15 mph.
Some showers were observed over the Atlantic waters but other than
those isolated showers today has been tranquil. A weakening
surface trough near the Lesser Antilles has pulled low level
moisture into the Caribbean and North Atlantic waters today with
the bulk of the moisture expected to reach the U.S. Virgin
Islands and Puerto Rico tonight and Thursday morning. As a result
an increase in showers with the possibility for isolated
thunderstorms is expected on Thursday. Early Friday a
strengthening upper and mid level ridge will build across the area
and prevail through the weekend with brisk easterly winds of 15
to 20 mph. Drier air and strong trade winds will limit shower
development across the area on Friday through the weekend.

LONG TERM...

A mid to upper level ridge will hold across the forecast area through
the weekend. This feature is expected to quickly erode early the next
workweek as a broad upper level trough establishes across the western
Atlantic and into the eastern Caribbean. The aforementioned trough
will promote moisture advection across the eastern Caribbean Wednesday
onwards. If the long term forecast holds, expect seasonably conditions
Saturday through Tuesday with locally induced afternoon showers
and a few isolated thunderstorms across the western interior and
west areas of Puerto Rico each day. The chance for showers and
thunderstorms are expected to increase by midweek under deep
tropical moisture and plenty of upper level dynamics associated
with the upper level trough.

AVIATION...

Mainly VFR conditions are expected to prevail early in
the forecast period. SHRA expected to increase from the east across
the USVI terminals by 26/06z then by 26/12z across the eastern PR
terminals. This will be followed by TSRA developing after 16z across
the interior and western PR. Tempo MVFR to brief IFR possible with
afternoon activity at TJMZ/TJBQ. SCT-BKN cigs btw FL030-FL060 and
mtns obscd expected across the PR terminals on Thursday. Winds will
continue from the ESE blo FL100 at 8-18 kt with sea breeze
variations at the northern and western terminals aft 26/14z.

MARINE...

Seas are expected to range from 3 to 4 feet across the coastal
waters while the offshore waters and passages will be near 3 to 5
feet. East southeast winds 10 to 20 kts will prevail over the next
several days. There is a moderate risk of rip currents across the
northern beaches of Puerto Rico today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  76  89  76  89 /  50  40  20  20
STT  76  87  76  87 /  40  30  20  20

&&

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

$$

SHORT TERM...TW
LONG TERM....WS


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