National Weather Service San Juan PR
524 AM AST Mon Oct 18 2021
.SYNOPSIS...Light steering winds will promote slow moving storms
this afternoon, enhancing the risk for flooding. Winds are
expected to turn more from the east to northeast through midweek,
and moisture content is forecast to remain at normal to above
normal levels through at least Thursday. A building mid-level
ridge during the end of the week should promote a drying trend,
but diurnally induced afternoon showers are expected each day.
&&
.SHORT TERM...Today through Wednesday...
Good moisture persists across the region. Based on Total
Precipitable Water imagery from GOES-16, moisture levels remain on
the high end of normal or above normal across and in the vicinity
of the CWA; PWATs are around 1.8 to 2.3 inches. Meanwhile, the
steering flow remains light. That said, it will increasingly become
more east-northeasterly today into tomorrow. Local effects and
daytime heating will continue to dominate the weather pattern.
Afternoon convection is likely to begin over the interior of Puerto
Rico, extending into southern portions of the island. Tomorrow, the
peak of activity will likely be over interior and southwestern
Puerto Rico.
Conditions aloft are not very favorable for convection, with mid- to
upper-level ridging. That being said, the moisture will combine with
available instability/daytime heating to cause widespread shower
activity with isolated thunderstorms. Showers will be slow-moving,
leading to locally high rainfall accumulations. Erosion of the ridge
is expected for tomorrow into Wednesday, providing an increase in
instability over the region. As moisture is expected to persist,
active weather is anticipated, remaining the theme through the short
term period. Additional moisture makes its way into the region late
on Wednesday, as a tropical wave approaches from the east. Also on
Wednesday, winds finally pick up. The frontal boundary, and its
trough, will pull away to the northeast, allowing the pressure
gradient to tighten somewhat, as a ridge builds into the western
central Atlantic. Streamers become more likely in the east with this
increase in wind speeds, and showers will not have the same tendency
to linger over an area for an extended period of time. That said, an
increase in activity means that the potential for high rainfall
totals in some areas continues.
There remains an enhanced risk for flooding for portions of the
region through the short term period, as rain falls on already
saturated soils.
.LONG TERM...Thursday through Tuesday...
A tropical wave is expected to cross the region on Thursday, with
good moisture content through 700 mb. At upper levels, a short wave
trough is expected to move over the area, and the 500 mb temperature
is forecast to remain between -6.5 to -7.0 degrees Celsius. The
wave should move across the USVI through the morning hours, and
across PR during the afternoon hours. This will enhance morning
convection across the USVI and later across mainland PR. Trailing
moisture during the evening hours will promote scattered shower
with posible thunderstorm activity across portions of the regional
waters and between the USVI/eastern sections of PR as well.
Therefore, the threat for flooding rains will remain high.
A mid-level ridge is then expected to build over the area from the
north and hold through the rest of the long term period. This will
promote drier air intrusion and warmer 500 mb temperatures. At
least on Fri-Sat, seasonable weather conditions are expected with
sunny to partly cloudy skies during the morning hours with an
occasional trade wind shower moving over portions of the USVI and
eastern PR, followed by diurnally induced afternoon showers with
isolated thunderstorms over the western interior of PR. On Sunday,
a tropical wave is forecast to move across the Caribbean waters,
increasing slightly moisture content and the chances of early
morning/afternoon convection in diurnal pattern across the
islands. Further warming of the 500 mb temps are expected early
next week, and precipitable water content is expected to decrease
between 1.50-1.75 inches. This should favor fair weather
conditions with shallow convection developing during the afternoon
over western PR.
&&
.AVIATION...Generally VFR conds expected next 24 hours across all terminals.
Local effects and daytime heating dominate activity - peak SHRA/TSRA
activity this afternoon expected to be interior into southern PR,
causing VCTS for TJSJ/TJPS. VCSH for most other terminals.
Relatively light winds are likely to be dominated by sea breeze,
with speeds to near 10 knots after sunrise.
&&
.MARINE...Relative tranquil seas will continue for the next
several days. Winds are expected to remain light today, slightly
increasing each day and turning more from the east. Choppy wind
driven seas up to 6 feet are forecast to return late in the
workweek. There is a low risk of rip currents across all the
islands today.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 78 / 40 40 30 40
STT 88 78 89 79 / 60 50 50 50