Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1034 am AST Tuesday Oct 21 2014
Update...minor changes made to short term grids. Mostly sunny to
partly cloudy skies have been observed across all local islands
this morning. A few light passing showers reached portions of
eastern PR and the U.S. Virgin Islands but rainfall amounts were
not significant. For the rest of the day...streamers forming over
the USVI...Vieques and Culebra will create periods of showers over
the surrounding waters with a few of them reaching eastern PR.
Then...afternoon convection over the Cordillera Central will bring
again periods of scattered to numerous showers and thunderstorms
affecting the interior and western quadrant of PR.
Aviation...added tempo groups to tjbq and tjmz as afternoon
convection should focus over the western side of PR. The 21/12z
tjsj sounding indicated a prevailing easterly wind flow with 15 knots
winds between 1-5 kft.
Marine...buoys along the North Shore of Puerto Rico are still
indicating a 3-4 feet north swell at 11-14 seconds. Hazardous rip
currents are still possible during the rest of the day. No changes
to previous forecast.
Previous discussion... /issued 553 am AST Tuesday Oct 21 2014/
Synopsis...a weak short wave will pass through the local area
this afternoon followed by north flow over the area and ridging
north of the area that continues into next week. A trough will
extend across the Leeward Islands from a low in the western
tropical Atlantic. It will shift southwest and enter the eastern
Caribbean early next week.
At middle levels...a ridge from high pressure over the western
tropical Atlantic extends eastward into the Caribbean. High
pressure then continues over the eastern Caribbean through
Thursday. An inverted trough in the western Atlantic will displace
the high to the north as it moves slowly across the Caribbean
south of the local area over the weekend. High pressure spreads
over the Atlantic waters north of Puerto Rico and the U.S. Virgin
Islands through the end of the month after a long wave trough and
cut-off low move east through the western Atlantic north of the
At lower levels...low pressure in the southwest Caribbean and
higher pressure north of the area continue. A weak low pressure
also continues to wobble around in a box bounded by 50 and 60 west
longitude and 20 and 30 north latitude until Thursday. East
southeast trade wind flow will continue across the area over the
weekend while low pressures develop in the western Caribbean and
move northeast over the western tip of Cuba through the end of the
Discussion...a similar situation to yesterday abides again today
with small showers across the local waters moving in the east
northeast to easterly flow and a smattering of small showers
moving across the U.S. Virgin Islands and the northeastern and
eastern coasts of Puerto Rico. Amounts of one to two hundredths
have been noted where rain has fallen. Although there will be
slightly less moisture in the column over the area today...a weak
short wave this afternoon and lifted indices similar to yesterday
will bring another round of showers and thunderstorms...some with
very heavy rain...to interior and western Puerto Rico where Urban
and Small Stream flooding in some areas is expected. Isolated
showers and mostly sunny skies will be noted elsewhere. Northerly
flow aloft and drier air at lower and middle levels will reduce
showers and thunderstorms both Wednesday and Thursday. The added
sunshine and the shift to southeasterly flow in the lower levels
will push temperatures into the lower 90s in the San Juan greater
metropolitan area and keep temperatures in the lower 90s in the
lower elevations on the south and west coasts.
Then...on Friday...moisture will begin to increase and showers will
return in the southeasterly flow...mainly to northwest Puerto Rico
and north of the Cordillera Central. Slightly drier days will
begin again on Sunday and last until Wednesday of next week after
a trough at upper levels passes south of the area and better
conditions are found in its northeast quadrant.
Aviation...VFR conditions are expected through the forecast period.
Winds increase to 8-15 kts from the east after 21/13z with sea breeze
variations. Vcsh possible at tjsj/tisx/tist until early in the
afternoon. Rain showers/thunderstorms and rain are once again expected across the Central
Mountain range of PR between 21/17-22z creating brief MVFR conds
and mountain obscurations... impacting mainly tjmz/tjbq taf sites.
Marine...generally diminishing seas will prevail through Friday.
Over the weekend waves from the southeast will increase in both
the Caribbean and Atlantic waters...but small craft advisories are
not expected through the end of the month.
Preliminary point temps/pops...
sju 88 78 90 77 / 30 30 30 10
stt 88 79 88 79 / 20 30 30 20