Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 251513

Area Forecast Discussion
National Weather Service San Juan PR
1113 AM AST Sun Oct 25 2020

.UPDATE...Deep clouds have not yet formed, but some small, moderate
showers are moving west southwest to west at 20 to 25 knots while
passing north of Saint John and Saint Thomas. Scattered showers
are still expected across interior Puerto Rico. The latest
sounding from San Juan at 25/12Z shows medium-low instability with
a precipitable water of 1.64 inches. Although the lowest 5 Kft is
moist, relative humidity is less than 46 percent between 5.8 Kft
and 29 Kft, and less than 20 percent near 18-20 Kft. This will
limit convection this morning and early this afternoon. Satellite
total precipitable water images show somewhat better moisture
moving in later this afternoon and shower activity may be
enhanced later this afternoon. Better moisture patches move
through the area tonight, but only eastern Puerto Rico will see
significant accumulations. Minor changes to gridded forecasts, as
little changes are indicated for the synoptic situation.


.AVIATION...VFR conds are expected at all TAF sites thru 26/16Z.
Sfc winds E-SE 10-18 kt with ocnl hir gusts and sea breeze
variations til 25/22Z then becoming less than 12 kt with land
breezes. Sct SHRA/Isold TSRA ovr W and interior PR and VCTY
TJMZ/TJBQ 25/17-22Z. Isold SHRA ovr regional waters and en route
btw local islands thru at least 26/16Z. L/lvl wnds E 18-28 kts BLO
FL120 bcmg fm N and incr w/ht ABV FL260. Max winds fm NNW-N 35-60
kt btw FL380-480 highest at FL420.


.MARINE...Seas continue from between 5 to 6 feet at the outer buoy
41043 to around 4 feet at the inner buoy. Models continue to show
north to northeast swell increasing tonight through Tuesday and
small craft advisories remain in place for tonight for the local
outer Atlantic waters with some seas between 7-8 feet spilling
into the Anegada Passage Monday night or Tuesday. These seas,
begin with swell from the north but transition to wind waves and
swell from the east by mid-week should subside by Friday.


.PREV DISCUSSION... /issued 443 AM AST Sun Oct 25 2020/


Today is expected to be similar to yesterday, with below average
rainfall amounts and plenty of sunshine. However, we will trend
toward wetter conditions this week as moisture streams in from
the east. Wednesday and Thursday look particularly wet, with
showers for eastern Puerto Rico in the mornings and western Puerto
Rico in the afternoons. Rough seas are expected for the next
several days, primarily due to brisk winds from the east.


Mostly clear to partly cloudy skies prevailed overnight with a few
passing showers noted moving over the coastal waters and reaching
parts of the east coastal sections of Puerto Rico. Expect a gradual
clearing and sunny skies by early morning, followed by mostly
fair weather skies across most of the region during the rest of
the day. Some locally and diurnally induced afternoon convection
will however be possible over parts of the interior and west
sections of Puerto Rico. Significant and widespread rainfall
accumulations are so far not anticipated in those areas. Mostly
sunny skies will prevail elsewhere including around the U.S.
Virgin Islands where limited shower activity is expected for today
and should be mostly on the west end and just downwind of the
islands. Slightly warmer than normal daytime high temperatures are
again forecast for the north coastal areas today under a
prevailing east southeast wind flow.

Mid to upper level high pressure ridge will hold across the region
today but will shift farther west Monday through Tuesday as an upper
trough /TUTT east of the area sinks further southwards while becoming
amplified over the northeastern Caribbean. In the meantime overall
dry and stable conditions will be maintained aloft. Model guidance
as well as satellite imagery and derived water vapor products, all
suggest an overall dry airmass in place today except for some
shallow patches of moisture moving across the region.

Afternoon showers and isolated thunderstorm development can
therefore be expected across the interior sections of Puerto Rico,
with the heaviest rainfall focused on the western interior sections
of the island. Expect afternoon shower activity to be short lived
and should quickly stream westwards leaving no significant rainfall
accumulations. However, strong shower activity will be possible in
isolated areas over the west interior. This may lead to ponding of
water on roadways and in poor drainage areas with the heavy rains.
The U.S. Virgin Islands can expect overall mostly sunny skies and
fair weather conditions today.

Tropical moisture transport is forecast to gradually return across
the region later this evening through Monday, along with increasing
trade winds as surface high pressure ridge builds north of the
region and a weak easterly wave crosses the area. This should
increase the chance for more frequent early morning passing showers,
as well as the chance for shower and isolated thunderstorm
development in and around the islands during the afternoon hours.

For Monday and Tuesday expected increasing moisture and instability
aloft as the upper ridge erodes and a tropical wave approaches the
eastern caribbean late Tuesday. This will increase the potential for
showers and thunderstorm development across the forecast area
resulting in better chance for urban and small stream flooding.

LONG TERM...Wednesday through Monday...

Wednesday continues to look wet as a surge of moisture is expected
to arrive associated with an old tropical wave. The moisture will
arrive early in the morning which will likely kick off scattered
showers across the eastern Puerto Rican coastal areas and over the
USVI. Then in the afternoon, the showers will shift to western and
southwestern Puerto Rico under winds between 925 and 700 mb of
around 20 to 25 knots from the ENE. These brisk winds will cause
quick moving showers to help limit rain accumulations. Upper-level
conditions are a bit uncertain, since long-range models are showing
different solutions. However at this point, conditions do not look
overly favorable for strong thunderstorms, but isolated
thunderstorms are likely. This will be closely monitored however,
since there will be troughing at least in the vicinity that could
enhance activity for Wednesday.

Though long-range models seem to disagree about how the upper-level
conditions will play out, there does not appear to be any day in the
long term period where upper-level troughing would act to really
enhance activity. Furthermore, strong high surface pressure is
expected to continue in the central Atlantic during the long term
period, which will keep brisk low-level winds in place to limit
rainfall accumulation. Plentiful moisture will still be in place
for Thursday and Friday with winds more from the ESE, which will
produce showers and isolated thunderstorms for northwestern PR in
the afternoon. Scattered showers are possible in the overnight and
morning hours for both days. Heading into the next weekend,
models again disagree on how wet those days will be. It appears we
will be near a gradient of high moisture to our south and dry air
to our north, thus it is difficult this far out to determine the
weather for those days.


VFR conds expected at all TAF sites durg prd. Sfc winds
lgt/vrb bcmg E-SE 10-18 kt with ocnl hir gusts and sea breeze
variations aft 25/14Z. Sct SHRA/Isold TSRA ovr W interior PR and
VCTY TJMZ/TJBQ 25/17Z-25/22Z. Isold SHRA ovr regional waters and en
route btw local islands til 25/12Z. L/lvl wnds fm E_SE 15-20 kts BLO
FL150 bcmg fm N and incr w/ht ABV. Max winds fm NNW-N 35-50 kt btw


Another northerly swell is expected to arrive today, along with
easterly winds up to 20 knots across the region, so small craft
operators are urged to exercise caution. The winds will also
produce a wind driven swell and lead to confused seas. So far
forecast models have overestimated the wave heights for this
event, so the start time for the recently advertised Small Craft
Advisory for the outer Atlantic Waters has been pushed back until
later this evening. Though the northerly swell will back off by
Tuesday, moderate to fresh easterly winds will continue to produce
rough seas for most of this week.


SJU  90  79  89  77 /  40  60  60  70
STT  89  79  88  78 /  30  40  40  70


PR...High Rip Current Risk through late Monday night for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity-

VI...High Rip Current Risk through late Monday night for St Croix.

AM...Small Craft Advisory from 8 PM this evening to 8 AM AST Tuesday
     for Atlantic Waters of Puerto Rico and USVI from 10 NM to



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