Scientific Forecaster Discussion
fxca62 tjsj 240205
Area forecast discussion
National Weather Service San Juan PR
1005 PM AST Mon Apr 23 2018
Update...latest sounding at 24/00z showed a surprisingly moist
environment. Precipitable water came in at 1.90 inches. Showers
have dissipated from Puerto Rico and only a few light showers are
left over the local waters. Flow was east southeast at lower
levels. Although an area of upper level divergence will move into
our local Atlantic waters from the Dominican Republic, it is
expected that little effect will be seen. Models show drier air at
850 mb encroaching on the area after 24/09z and most showers
should end between 24/12-18z east of Puerto Rico. Still expecting
local showers and thunderstorms in western and interior Puerto
Rico Tuesday afternoon but since the upper level trough will have
already passed through all activity should be much weaker than
Updated a few temperatures and local winds, but
changes were minor.
Aviation...VFR conds are expected to continue thru 24/16z. Aft
24/16z local rain showers in wrn and interior PR could provoke MVFR conds
and mtn obscurations due to rain showers/tsra, but brief at taf sites.
Winds east-southeast 6 to 12 kt with land breezes, bcmg 10 to 20 kt aft
24/14z with sea breezes. Max winds west-southwest 75-85 kt btwn fl360-410
diminishing to 55-65 kt by 24/12z.
Marine...seas are continuing to subside. This trend is expected
to continue through Thursday as winds also relax. Small craft
advisories are not expected to be needed in the next 7 days. Also
the high risk of rip currents for Saint Croix will be taken down
at 6 am AST.
Previous discussion... /issued 447 PM AST Mon Apr 23 2018/
a short wave trough aloft interacting with plenty of tropical
moisture will continue to bring periods of showers and
thunderstorms across the islands through late tonight. An upper
ridge is forecast to builds over the region by Wednesday.
Short term...tonight through Wednesday...
considerable cloudiness continued across the region today as a mid
to upper trough became amplified just west of the area and will lift
east northeast across the region. Although this cloudiness
delayed afternoon convection, showers and thunderstorms developed
across the San Juan Metro area producing urban flooding. Although
the strongest activity dissipated, additional showers will remain
possible from time to time through the overnight hours. Another
thunderstorms were detected across the interior and western
sections which produce periods of gusty winds, frequent lightning
and torrential rainfall. This activity is expected to drift
northward til its dissipate later tonight.
In general the proximity of the mid to upper level short wave
trough combined with moisture advection and local effects will
support afternoon convection on Tuesday especially across over
the northern half and west to northwest sections of the island
including parts of the San Juan Metro area as well as downwind
from the Virgin Isles.
Although the moisture is expected to diminish by Wednesday as a
mid to upper level ridge builds aloft, early morning and
afternoon convection will be possible due to the lingering
moisture along with local and diurnal effects. This activity
should be focused over the northern half and west to northwest
sections of the island including parts of the San Juan Metro area.
Long term...Thursday through next Tuesday...
upper levels will be dominated by a ridge which may hold steady
for several days during the long range forecast. The local winds
will be mainly from the east-southeast to east during the same time period,
mainly ranging between 5 and 15mph with occasional areas of up to
20mph over the local waters. The main variation in pattern appears
to be the fluctuations in available moisture. Some days will be a
bit drier than others and therefore the coverage of shower and
thunderstorm activity would be expected to be lower but these
moments of lower moisture may be short lived and for the most part
near normal available moisture is expected with precipitable
water values hovering between 1.4 and 1.6 inches. This means that
for the most part only locally induced showers and thunderstorms
are to be expected in the long range if the models were to verify
the overall pattern. This means that isolated to scattered showers
across the local waters, USVI, and eastern PR are expected in the
overnight and early morning hours, while afternoon convection
would be expected across central and western PR due to the local
effects. Some showers may stream off the Luquillo mountain range
and affect portions of the San Juan Metro as well.
VFR conds at local terminals durg prd. However...til
23/22z...expect isold to scattered rain showers ovr and around the islands
and most terminals with isold thunderstorms and rain fcst in the vcty of
tjsj/tjmz/tjbq. Tempo MVFR and brief IFR cond can therefore be
expected. Except for local sea breeze variation, winds will be
mainly FM east-southeast at 15 to 20 kts with occasionally higher gusts along
the coastal areas and vcty shra/tsra. Wnds bcmg 10kts of less aft
23/23z. Max winds west-southwest 85-95 kt btwn fl350-440 diminishing to 55-65
kt by 24/06z.
winds will continue at 15 to 20 mph across local waters and
passages. Choppy seas at 5 to 7 feet will continue across the
Atlantic offshore waters, therefore a Small Craft Advisory still
in effect until early Tuesday morning. Elsewhere mariners should
exercise caution due to seas up to 6 feet and winds around 18
knots. Seas and winds are forecast to slowly diminish across the
local waters after Tuesday. Isolated thunderstorms will remain
possible over parts of the near shore coastal waters until later
Preliminary point temps/pops...
sju 74 86 75 88 / 40 40 20 20
stt 75 86 75 87 / 30 20 20 40
Vi...high rip current risk until 6 am AST Tuesday for St Croix.
Am...Small Craft Advisory until 2 am AST Tuesday for Atlantic waters
of Puerto Rico and USVI from 10 nm to 19.5n.