Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230843
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
443 AM AST Thu Nov 23 2017

.SYNOPSIS...Trade winds at the surface will continue through the
period. Moisture increases today and Friday as a weak trough
moves through the area, but the weekend will be considerably
drier. Moisture returns Monday through Wednesday as the next cold
front remains well north of the area.

&&

.SHORT TERM...Today through Saturday...
Today moisture returns across the region with precipitable water
values increasing to almost 1.90 inches by mid afternoon. The
ridge of high pressure that has provided tranquil weather the last
few days will continue to influence the weather through Saturday.
This afternoon a surge of low level moisture associated with an
easterly wave will move in from the east. This moisture will
bring an increase in shower activity across the entire forecast
area with the greatest impact along the Cordillera Central and
western and southwestern Puerto Rico. Isolated thunderstorms are
also possible.

The area of low-level moisture will continue across the forecast
area until Friday evening, then mid-levels dry out almost
completely and shower activity will decrease considerably on
Saturday.

.LONG TERM...Sunday through Wednesday...
Colder air will move out of the eastern United States into the
western Atlantic but will make very little progress south toward
the area. At the same time modest patches of moisture will push
through in generally easterly flow at lower levels owing to the
migrating highs that move north of the area and the weak troughing
over the area during the period. 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 80 knot jet late Wednesday. The trough passage at upper
levels will enhance shower activity somewhat but dynamics are
weak.

&&

.AVIATION...VFR conditions are expected across the local flying
area this morning and for most of the period. Increasing moisture
will bring clouds and SHRAs along the Cordillera and spreading to
TJMZ with brief MVFR conds due to CIGS and mtn obscurations from
El Yunque to SW PR. SFC winds out of the ENE to NE at 5 to 10 kts
during the early morning switching to easterly directions by
23/18Z with sea breeze variations. FL250 TO FL500 winds out of the
west at 15 to 30 kts with the strongest winds near FL410.

&&

.MARINE...Seas have were running 8 to 9 feet at the outer bouy
42043 north of the area, but only 4-5.5 feet at the inner buoy.
Seas will begin to subside this morning and the small craft
advisory should be down by 2 PM AST. 7-foot seas are not expected
in the outer Atlantic waters until mid week next week and are not
expected anywhere else in the forecast area this month.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  86  77  87  77 /  50  50  50  40
STT  86  77  86  76 /  50  50  40  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flash Flood Watch until 2 PM AST this afternoon for Northwest-
     Western Interior.

VI...None.
AM...Small Craft Advisory until 2 PM AST this afternoon for Atlantic
     Waters of Puerto Rico and USVI from 10 NM to 19.5N.

&&

$$

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



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