Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 230146
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
946 PM AST Mon Jan 22 2018

.UPDATE...Fair weather conditions prevailed during the late
afternoon/evening hours across the region. The 00z TJSJ sounding
indicated just 1.21 inches of PWAT as drier air filtered across
the area. However, a band of showers currently affecting the
Leeward islands is moving toward the USVI and will result mainly
in quick passing showers with brief gusty winds at times through
Tuesday morning. This surge of low level moisture will aid in the
development of diurnally induced showers over the islands and
mainly across the western portions of PR on Tuesday afternoon.
Overnight temperatures could reach the mid to high 50s across the
higher elevations. Winds will continue from the east between 5-15
mph.


&&

.AVIATION Update...Mainly VFR conds to prevail across all
terminals. However, -SHRA on trades moving across the Leeward
islands could move briefly across the USVI terminals during the
overnight into Tuesday morning. Latest 23/00z TJSJ upper air
sounding indicated ENE winds at 11-24 kt from SFC to FL100.


&&

.MARINE Update...The northerly swell is subsiding and the regional
seas no longer meet small craft advisory criteria, therefore SCA
were dropped earlier tonight. However, small crafts should
continue to exercise caution across both Atlantic and Caribbean
waters and passages due to seas up to 6 feet and/or winds up to
20 kt. High rip current risk will continue on Tuesday mainly for
the northwestern coast of Puerto Rico. Moderate risk continues
across the rest of the coastal areas of the islands.


&&

.PREV DISCUSSION... /issued 341 PM AST Mon Jan 22 2018/

SYNOPSIS...A surface high pressure across the north Atlantic is
moving east and will strengthen over the course of the next few
days as it absorbs another surface high across the northeastern
Atlantic. Another surface high pressure will move into the
western Atlantic, north of the local area late this week. This
will keep mainly easterly winds across the local area for the next
few days, becoming east northeasterly and rather breezy towards
the end of the week. Upper ridge will continue over the local
islands for the next several days with an upper trough passing to
the north of the local area, digging a bit more to the east of
the Leeward Islands early next week. Frontal boundary is expected
to remain well north of the local islands early next week.

SHORT TERM...Tonight through Wednesday...
Easterly trade winds continue as high pressure retreats and is
absorbed into another high in the northwest Atlantic. The next
vigorous cold front is poised to move into the western Atlantic
Tuesday evening. Its best progress during the week will be made to
the east, so by Thursday it will still be a little over 400 miles
to our north northwest. At first we will see a continuation of
patches of low level moisture that will move through with
scattered showers and a slightly deeper low level moist layer, but
on Thursday flow will begin to turn northeast to bring in cooler
and slightly drier air from the central Atlantic. This will
diminish the showers over most of Puerto Rico, but may leave
scattered showers and cooler weather on the northeast coast.

At upper levels a weak trough is passing through the area and will
coincide with the back edge of the better moisture seen today--
though mainly in the Atlantic waters north of the islands. The
moisture returns Tuesday and Wednesday in spite of the ridge that
builds--albeit weakly. A second weaker trough will move through
Wednesday night and drier air will again move in from the northeast.
Mid levels remain quite dry.

LONG TERM...Thursday through Tuesday of next week...

From previous discussion...
The upper level high pressure ridge will hold across the region through
Friday. However the prevailing easterlies will continue to transport
sufficient moisture across the region to support periods of passing
late evening and early morning showers, as well as isolated to scattered
afternoon convection. Saturday through Sunday, a polar trough will amplify
and spread eastward across the west and central Atlantic causing a slight
erosion of the upper level ridge. By Monday and Tuesday the mid to upper
level ridge will reestablish over the region and again limit shower
development.

In the meantime, the a cold front is forecast to move across the west
Atlantic and reach the local area by the upcoming weekend. Model guidance
continued to suggest a gradual increase in trade winds moisture during
the latter part of the week with more favorable conditions for moisture
convergence and shower development over the weekend, as the cold front
sinks southwards and stalls just north of the region by Sunday. Improving
weather conditions are however expected by Monday and Tuesday with a
return of more typical passing trade wind showers.

AVIATION...VFR conditions are expected at all TAF stations. SHRA in
wrn PR may produce brief MVFR conds til 22/21Z and mtn obscurations.
Winds east 8 to 16 kt with sea breeze influences til 22/21Z then
subsiding to less than 12 kt.  SHRA will increase from east to west
arriving in ern PR arnd 23/09Z. Sea breezes return aft 23/14Z east 8
to 16 kt. Max winds west 35-40 kt FL420-500 diminishing aft
23/00Z.

MARINE...Seas remain hazardous and are expected to remain
hazardous through Tuesday. Wave heights will be up to 7 feet
today, mainly for the Atlantic waters and local passages,
elsewhere, seas will be up to 6 feet. There is a high risk of rip
currents for the north facing beaches of Puerto Rico, moderate
risk for most of the remaining coastlines of PR and the USVI. A
strong northerly swell could invade the local waters this coming
weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  74  84  75  84 /  30  30  50  50
STT  75  84  74  84 /  30  30  40  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 AM AST Tuesday for North Central-
     Northeast-San Juan and Vicinity-Southeast.

     High Rip Current Risk through Tuesday afternoon for Northwest-
     Western Interior.

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

$$

SHORT TERM...DS
LONG TERM....JA


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