Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 270117 AAA
Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
917 PM AST TUE JUL 26 2016
.UPDATE...Thunderstorms have ended over the local waters although
the sounding still shows CAPE values approaching 4000 J/Kg and LI
of minus 10. A few showers are still moving across northeast
Puerto Rico and the local waters from south of Saint Croix to
south of Ponce. The GFS suggests moisture will diminish somewhat
overnight but then bounce back in the lower levels on Wednesday.
Although precipitable water--which was underforecast from this
evening`s 27/00z sounding--is forecast to be only 1.65 inches at
27/18z, the lifted index is still forecast to be between -8 and
-10 and this should allow for isolated thunderstorms in interior
Puerto Rico and showers with scattered thunderstorms and gusty
winds in northwest Puerto Rico.
.DISCUSSION...Little changes were made to the grids for tonight
and tomorrow except to lower interior temperatures for the minimum
tonight, raise the POPs in northwest Puerto Rico and lower the
POPs almost everywhere else. Dry air is expected to dominate but
it will be warm. MEX MOS had 92 degrees for tomorrows high. This
may be okay for more interior highs but at the airport we are
expecting sea breezes to develop and this should moderate the high
down to 90. This is still a little warmer than the 89 the ASOS
recorded at San Juan just before 4 PM AST today, Monday.
The overall picture for the rest of the week appears thusly:
limited but potent showers for Wednesday, a little more on
Thursday with additional moisture, less on Friday, slightly more
on Saturday and then much more activity with the wave on Sunday.
This wave is not quite as strong in the 26/18z GFS as it was in
the 26/00z GFS, but the difference is minor. We will monitor for a
.AVIATION...VFR conds expected overnight with VCSH due to ISOLD-SCT
-SHRA over the local flying area. Overnight...East winds around 10KT
expected across the USVI and Leeward terminals and winds generally
6KT or less across the terminals in PR. After 27/13Z winds to remain
from the East increasing to 10-15KT with sea breeze variations.
VCSH/VCTS likely at TJBQ and TJMZ after 27/17Z with TSRA possible at
TJMZ after 27/19Z. Leeward island vsbys may drop below 7 miles due
to Saharan dust next 24 hours.
.MARINE...Seas generally remain below 6 feet until Saturday. A
small craft advisory for seas may be required by Sunday.
.PREV DISCUSSION... /issued 142 PM AST TUE JUL 26 2016/
A tropical wave will exit the local area this evening. Followed by
a dry air mass with Saharan Dust Particles on Wednesday and
Thursday. Then another easterly tropical wave is forecast to move
across the islands on Friday. The model guidance are forecasting a
strong easterly wave around Sunday. At the surface, a broad
surface high pressure across the northeast Atlantic will continue
to produce a moderate east wind flow across the region for the
next several days. At the upper levels, a TUTT low will build over
the region Thursday into Friday.
Most of the showers and thunderstorms that affected the eastern
half of Puerto Rico and the U.S. Virgin Islands during the
morning hours dissipated or moved over the Atlantic waters.
However, still expecting more showers and thunderstorms to
develop along the interior and northwestern portions of Puerto
Rico this afternoon. As a result the potential for urban and
small stream flooding, frequent lightning and gusty winds still
possible across these sections.
Satellite imagery and model guidance suggested the arrival of a
dry air mass with Saharan dust particles as this wave moves away
from the CWA. As a result, limit shower/thunderstorm activity is
expected on Wednesday. A TUTT low is forecast to build over the
region by Thursday and Friday. This upper level feature is
expected to enhance the potential for showers and thunderstorms.
Another tropical wave is forecast to enter the Eastern Caribbean,
increasing the tropical moisture across the local islands. Saharan
dust is expected once again on Saturday and on Sunday another wave
is forecast to enter the eastern Caribbean.
AVIATION...VFR conds will prevail across most of the TAF sites,
except in and around TJMZ and TJBQ where mountain obscurations and
brief MVFR conds are likely between 18Z-22Z. Low level winds
prevail E at 10-20 kts with sea breeze variations and gusty winds
near the TSRA.
Small crafts should exercise caution across the Atlantic and
Caribbean waters due to seas up to 6 feet across the region.
The potential for thunderstorms still expected over the western
half of the local waters, as the tropical wave move away this
evening. Periods of torrential rainfall, frequent lightning and
gusty winds are expected. Also, isolated waterspouts can`t be
.PRELIMINARY POINT TEMPS/POPS...
SJU 77 90 80 89 / 30 30 40 40
STT 79 90 80 88 / 30 30 40 40