Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 200157 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
957 PM AST Sun Nov 19 2017

.UPDATE...The rain from the shear line is moving off the north
coast of Puerto Rico much as the HIRESnmm had forecast. The next
day, Monday`s forecast, show less coverage but still good showers
to form in southeasterly flow off of El Yunque and into san Juan
and also over northwest Puerto Rico and even the northern U.S.
Virgin Islands. Although exact location of showers cannot be
determined, probabilities are good for this scenario. Precipitable
water will fall but not very much tomorrow, compared with Tuesday
and an axis of divergence aloft will be across western Puerto Rico
and over the U.S. Virgin Islands. No changes have been made to the
short or long term synoptic reasoning or grids.

&&

.AVIATION...VFR conds returning as per previous discussion.

&&

.MARINE...No changes from previous discussion.


&&

.PREV DISCUSSION... /issued 455 PM AST Sun Nov 19 2017/

SYNOPSIS...A shear line across Puerto Rico and north of the U.S.
Virgin Islands will be taken northwest a short distance by
several high pressures invading the Western Atlantic. As mid-level
high pressure forms just north of the area on Wednesday, moisture,
clouds and showers will return to the area Wednesday through
Saturday.

SHORT TERM...Tonight through Tuesday...

GOES 16 satellite imagery shows an area of disturbed weather to
the southwest of Puerto Rico that is moving off to the northeast
and has brought heavy rain, frequent lightning and flooding to
portions of Puerto Rico. Currently the shear line is positioned
across northwest Puerto Rico with converging southerly winds
ahead of the boundary. Although stability will increase this
evening, light to moderate rain will continue, diminishing, and
some inundation of low-lying and poorly drained areas will
continue.

Although moisture begins to decrease in the mid-levels tonight,
lingering low-level moisture will drive another round of showers
and thunderstorms during the day Monday. Tuesday the trough of
low pressure will begin to pull away from the forecast area and
allow an upper level ridge of high pressure to nose in over the
region. This ridge will give the islands a brief reprieve from the
heavy rainfall by limiting the influx of deep tropical moisture
across Puerto Rico and the U.S. Virgin Islands. The GFS is showing
a line of drier air at 700 mb coinciding with a ridge of high
pressure that extenuates from the tropical Atlantic into the
Caribbean and then northwest across Puerto Rico. Model data is
showing precipitable water values falling to between 1.25 to 1.50
inches which is in the 25th percentile. This in contrast with
values that were in the 75th percentile the last four days.

LONG TERM...Wednesday through Sunday

Moisture begins to return around a mid level high pressure
Wednesday and coupled with an approaching trough from the east,
winds will turn somewhat northeasterly. Although a mid to upper
level ridge will hold across the forecast area much of the
forecast period, these easterly winds will continue to bring
patches of low level moisture across the local isles to keep the
chance of passing showers each day. Therefore, expect a seasonable
weather pattern with passing showers across U.S. Virgin Islands and
eastern Puerto Rico as well as limited shower activity across
western Puerto Rico Wednesday through Saturday. The upper
level ridge will begin to erode during the upcoming weekend and
lower levels clouds will allow some more sun and hence the
possibility of increasing thunderstorms in interior and western
Puerto Rico.

AVIATION...

Aviation...SHRA/ and diminishing TSRA expected to continue across
the local area thru 20/02Z, causing VCSH/VCTS across the
terminals with TEMPO MVFR conds possible at TJBQ and TJMZ thru
20/02Z. VFR conds to continue thru 20/15Z with CIGS BLO FL120 and
mtn obscurations. Southerly winds sfc to FL200 gradly shifting to
SE 10 to 20 kt by 20/18Z. Maximum winds WSW 30 kt around FL400
thru 20/18Z.

MARINE...Seas 3 to 5 feet possibly 6 in the outer Atlantic waters
by Wednesday. Seas somewhat lower in the Caribbean. Small craft
advisory conditions not expected during the next 7 days in the
forecast area, but 7 foot seas possible north of the area by
Tuesday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  77  85  78  86 /  30  50  20  30
STT  80  82  80  82 /  40  40  20  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flash Flood Watch through Monday afternoon for Northwest-Western
     Interior.

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

$$

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


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