Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 241513
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1113 AM AST Fri Nov 24 2017

.UPDATE...
No significant to inherited forecast package and forecast discussion.
Weak easterly perturbation and associated low level moisture field
will continue to exit region today. Mid to upper level ridging will
remain in place for the next several days to limit convective development
across the islands and coastal waters. Local and diurnally induced afternoon
convection along with early morning passing showers will also remain
possible each day.

&&

.AVIATION UPDATE...VFR conds...SCT-BKN lyrs nr FL025...FL050. Few
SHRA en route btw islands and ovr coastal waters. VCSH at all
local terminals til 24/22z. L/lvl wnds fm E-NE 10-15 kts blo
FL250...SFC wnds mainly fm E 5-15 kts...except for local sea
breeze variations.

&&

.MARINE UPDATE...No chg to previous marine discussion. Winds and
seas 15 kts or less and 5 feet or less during the prd across the
local waters and passages.

&&

.PREV DISCUSSION... /issued 452 AM AST Fri Nov 24 2017/

SYNOPSIS...An upper level ridge dominates and moderates activity
in the pattern. A weak trough leaving the area will increase
shower activity-- mainly across northwest Puerto Rico this
afternoon. Drier weather will follow over the weekend, then a
moisture patch will pass through before general moisture
increases considerably Tuesday through Thursday.

SHORT TERM...Today through Sunday
Model guidance indicates moisture will move in from the east and
keep precipitable water values near 2.0 inches today. Later today
a trough of low pressure will move to the west of Puerto Rico
which will shift surface winds to an east southeast to southeast
direction. Isolated showers are expected across the the U.S.
Virgin Islands and Puerto Rico during the afternoon and evening
hours with the strongest showers over portions of northwestern
Puerto Rico. This activity is dependent on the surface winds
becoming more southerly today. At the mid to upper levels a
strong ridge of high pressure will suppress thunderstorm growth
across the region. Water Vapor imagery overlaid with 500 to 200
mb heights shows drier air in the mid to upper levels which also
coincides with San Juan Skew-T sounding last night.

Saturday, the moisture surge will be replaced by drier air, but
scattered showers will be possible across the northwest interior
Saturday afternoon. Sunday, models indicates a small patch of
moisture will grow southeast of the area and cross the island of
Puerto Rico but confidence is low in such fine detail.

LONG TERM...Monday through Saturday... A patch of moisture
develops just southeast of Puerto Rico and moves onto the island
during the day. Then ridging at 700 mb dries the pattern out for
another 12 to 24 hours. Then modest moisture then prevails through
the end of the work week. At upper levels high pressure dominates
the southern Caribbean and a deep polar trough over the United
States shears out and moves across the Atlantic in a greatly
weakened condition, passing just to the north accompanied by an 65
knot jet late Wednesday. The trough passage at upper levels will
enhance shower activity somewhat with good precipitable water
values, but dynamics are weak. The jet will hang just south of the
area in northwest flow until it lifts back across the area on
Saturday. Patchy moisture Thursday through Saturday will bring
showers but mainly in the west and interior portions of Puerto
Rico.

AVIATION...VFR conditions will prevail across all terminal
sites through the rest of the period. But VCSH is possible across
most terminal sites by 23/15Z with MVFR possible at TJMZ due to
isolated VCTS. SFC winds prevailing out of the E at 5 to 10 kts
through the period. FL250 TO FL500 winds out of the west at 15 to
30 kts.

MARINE...7 foot seas relaxed during the day on Thursday, and seas
will follow a downward trend during much of the next 7 days. No
small craft advisories are foreseen through the end of November.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  78  87  75 /  40  40  40  10
STT  85  77  86  74 /  40  40  30  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flash Flood Watch until 3 PM AST this afternoon for Northwest-
     Western Interior.

VI...None.
AM...None.
&&

$$

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



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