Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 090846

Area Forecast Discussion
National Weather Service San Juan PR
446 AM AST Mon Dec 9 2019


The threat for isolated to scattered showers continues across the
region, but a few factors will minimize the threat for urban and
small stream flooding. Though some localized flooding is possible,
decent low level wind flow will keep the showers moving, and
unfavorable atmospheric conditions will limit the strength of the
showers. No major weather systems are expected for the next week.
However, a strong Atlantic high pressure system will drive breezy
easterly winds and rough seas.


.SHORT TERM...Today through Wednesday...

An approaching upper level trough will usher in a westerly jet up to
55 knots Monday night through early Tuesday morning. At mid levels,
a cold pool accompanying the upper level trough will approach the
area and pass to our northeast, but not without bringing 500 mb
temperatures of minus 9 degrees to the area on Tuesday. This will
also be the time when the driest mid level air will be encountered.
At the surface, high pressure north of the area near 40 north this
morning will strengthen to as much as 1044 mb in the central
Atlantic by Tuesday morning. This will increase the local pressure
gradient and also the east northeast winds in the lower levels.
Surface winds over the local waters will increase to 15 to 20 knots
and continue through beyond Wednesday. Bands of moisture are
interspersed in this flow over the western tropical Atlantic with
areas of drier air between. One of those bands moved across the U.S.
Virgin Islands overnight and is entering Puerto Rico this morning.
Shower activity is expected to increase from levels seen yesterday.
Because of the stronger winds, showers will be more significant over
the windward slopes of Puerto Rico even during the typically drier
daytime hours. Weak mid level moisture along with rapidly moving
showers will considerably limit amounts of rain received during the
period but amounts up to an inch are possible. Temperatures at 500
mb are expected to fall to minus 8 this afternoon and so
thunderstorms cannot be ruled out in central and western Puerto
Rico, while drier mid levels may inhibit deep convection on Tuesday.

.LONG TERM...Thursday through Tuesday

Uneventful weather conditions are expected to prevail in the
longterm forecast. Near average moisture will be with us, with
precipitable water values mostly between 1.5 and 1.7 inches.
Also, trade winds stay fairly brisk as well, helping to keep
showers moving along and reducing the amount of rainfall
accumulations. No major fronts or tropical waves are forecasted
for the next week. Furthermore, the upper levels of the
atmosphere remain less conducive to thunderstorm activity with
plenty of ridging aloft.

Despite the generally average conditions in the
long-term, there will still be plenty of opportunity for passing
showers. Advective showers will develop in the nighttime and
morning hours, threatening the east coast of Puerto Rico and the
USVI, particularly Thursday and Friday when the highest moisture
content will occur. Furthermore, the usual afternoon shower
activity is possible, particularly for western Puerto Rico.
However, for reasons mentioned in the last paragraph, these
showers will likely have minimal impact. Long range models suggest
drier conditions heading into the next week, with the chance for
some Saharan dust working into the region. However, there is
minimal confidence in that forecast this far out.



VFR conditions are expected to prevail across all
terminals. Sct SHRA will develop in SW PR aft 08/17Z with highest
trrn obscured. Isold TSRA psbl. Brief MVFR may be seen in TJMZ. Sfc
winds ENE 5-15 kt with sea breeze influences and gusts to 25 kt. Max
winds W increasing to 50-65 kt btwn FL380-510--highest near FL460.



The easterly trade winds are increasing as expected, due to a
strong area of high pressure developing in the Atlantic. Seas will
become choppy and hazardous, with Small Craft Advisory conditions
beginning later this evening and lasting at least into Friday.
Isolated to scattered showers are possible across local waters,
with only a small threat for thunderstorms.


SJU  85  75  84  76 /  50  50  70  70
STT  85  75  86  75 /  40  40  40  40


AM...Small Craft Advisory from 8 PM this evening to 8 AM AST Friday
     for Anegada Passage Southward to 17N-Atlantic Waters of
     Puerto Rico and USVI from 10 NM to 19.5N-Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to



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