Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 070114

Area Forecast Discussion
National Weather Service San Juan PR
914 PM AST Thu May 6 2021

Clouds and shower activity over land areas quickly dissipated or
moved offshore during the evening hours leaving partly cloudy to
mostly clear skies. Passing showers continued to affect mainly parts
of the Caribbean waters and the Mona passage, with lesser activity
noted elsewhere. Recent TJSJ upper air sounding was overall dry
and stable with only shallow moisture trapped below 700 millibars.
Winds were mainly form the E at 5-15 knots below 700 millibars,
then backing and becoming from the north while increasing with
height. No change to the inherited short term forecast package and
reasoning at this time. An overall dry, fair weather day is
expected for Friday except for some locally and diurnally induced
afternoon showers mainly over parts of the interior and west
sections of Puerto Rico. Lesser activity is expected elsewhere
where mostly sunny skies should prevail.


.AVIATION UPDATE...VFR durg prd. Mstly Isold SHRA ovr regional
waters and en route btw islands. SCT ocnl BKN cld lyrs nr
FL025...FL050. Sfc wnds calm to lgt/vrb bcmg fm E-Ne 10-15 kts
with ocnly hir gusts aft 07/14Z.


.MARINE UPDATE...No change to previous marine discussion. Seas
will continue at 5 feet or less with winds of up 20 kts mainly over
the Caribbean waters and portions of the local passages...10-15 knots


.PREV DISCUSSION... /issued 448 PM AST Thu May 6 2021/

Moderate to heavy shower activity will continue for the rest of
tonight, mainly over sections of Puerto Rico and local waters.
The same weather pattern is expected to prevail for the next two
days as a upper level ridge and the associated surface high
pressure dominated the region. Currently, expect mainly afternoon
convection in northwestern and interior sections of the islands
due to the local effects. Tranquil marine conditions will continue
across the regional waters with seas up to 5 feet over the
offshore waters and up to 4 feet over the nearshore waters.

SHORT TERM...Rest of Today through Saturday...

The presence of a mid to upper level ridge will result in fair
weather conditions, mainly in the morning and late evening hours
for the short term period. However, expect afternoon convection
due to the combination of the available moisture and the diurnal
and local effects. As the ridges continue to move more into the
Central Atlantic, a southeasterly component is expected to
dominate the local pattern with a somewhat drier airmass filtering
into the region and slightly improving the weather pattern. That
said, the shower activity that form on Friday and Saturday will
affect northwestern sections of Puerto Rico due to the wind
direction profile. Additionally, this wind flow profile could
result in streamers over the San Juan metro area in the afternoon
hours, leaving the possibility of minor flooding and ponding of
water in roadways and poor drainage areas. However, the local
effects will produce some afternoon shower activity over the
interior and northwest sections of Puerto Rico. If model guidances
are correct, there is a low to moderate flooding threat for
Friday into Saturday.

Additionally, a slight increase in warmer temperatures is expected
due to the southeasterly component in winds over the region in
combination with mostly clear to partly cloudy skies on Friday and

PREV DISCUSSION... /issued 447 AM AST Thu May 6 2021/

LONG TERM...Sunday through Wednesday...

An approaching cold front over the Atlantic will pull air out of
the southeast. The GFS still shows a band of moisture reminiscent
of a tropical wave moving up with it and entering the forecast
area on Sunday. This band is fairly quick moving and will pass
through Puerto Rico before dawn on Monday. Although the airmass
then dries out by Tuesday, precipitable water values will not
return to current levels before the end of the period. 500 mb
temperatures plunge to minus 8 degrees overnight on Sunday and are
still as low as minus 7 degrees on Monday, so there should be
enough instability to generate at least isolated thunderstorm
Sunday night through Monday evening, especially since mid-levels,
once they moisten on Sunday, will not dry out again until the end
of next week. Upper levels are generally suppressing convective
activity and will do so until a trough deepens over Cuba on
Sunday. The trough is blocked by upper level high pressure to our
southeast and is not able to progress into the area. Also the
ridge from the high to the southeast will force anti-cyclonic flow
over the area and push much of the jet coming out of the trough
to our north. This will also help to limit showers and
thunderstorms Monday afternoon or overnight into Tuesday. In fact,
these unfavorable conditions may actually commence on Monday so a
close watch for improvement then will need to be made with
successive model runs.

VFR conditions will persist. Showers will develop across
the local flying area, and some of them will move over terminals but
will be brief. Afternoon convection could impact JBQ/IST/ISX thru
06/23z. Winds will continue from the east at 10 to 20 knots with
local sea breeze variations and higher gusts, then will drop below
10 kt after 06/23z.

Small craft operators should exercise caution over the Caribbean
and Anegada passage due to moderate to fresh easterly winds the
rest of today. For tomorrow, winds are expected to have a slight
decrease and tranquil marine conditions will continue for most of
the local waters will prevail. There is a moderate risk of rip
current across northern and southeastern beaches of Puerto Rico
including some beaches at Culebra and Vieques, and the majority of
the St. Croix beaches as well.


SJU  77  89  76  89 /  20  20  20  50
STT  75  87  76  86 /  30  20  20  20




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