National Weather Service San Juan PR
505 AM AST Thu Mar 12 2020
.SYNOPSIS...High pressure continues over the Atlantic waters and a
weakening front to the north continues to be a source of clouds
and scattered showers. On Friday low pressure will develop at
lower levels to the northeast of the area and will cause flow to
become northeasterly here. This will send patches of moisture
across the area through much of next week, with night and early
morning showers in the northeast portion of Puerto Rico and
afternoon showers in the south and southwest when sufficient
moisture exists. The islands to the east will generally see
seasonal temperatures and isolated showers.
&&
.SHORT TERM...Thursday through Saturday...
A mid to upper level trough will continue to hold over the
eastern Caribbean through the end of the workweek. On Thursday,
however, dry air is expected to filter in at the mid-levels. In
fact, the latest estimates from GOES- 16 Total Precipitable Water
satellite product shows an area of precipitable water as low as
1.0 to 1.1 inches just to the southeast of the forecast area,
while the guidance are suggesting values of about 1.2 to 1.3
inches. This will translate to a reduction in rain activity
across the area, although a few showers carried by the trade winds
could still move over portions of eastern Puerto Rico and the
U.S. Virgin Islands, especially in the morning hours. However, in
the afternoon, due to local effects, showers should develop over
the northwestern quadrant of Puerto Rico. Additionally,
temperatures at 500 mb are expected to be at around -10 degree
Celsius and since the trough will provide some instability aloft,
isolated thunderstorms cannot be ruled out.
Moving into Friday, moisture increase at the mid-levels once again,
with precipitable water values climbing back to 1.6 inches. Also,
with the axis of the trough at the mid to upper levels closer to
the area, conditions will become more favorable for enhance shower
activity over the area and isolated thunderstorms. The heaviest
activity is expected across the interior and western Puerto Rico
during the afternoon hours, but additional activity will also be
possible over eastern Puerto Rico, including the San Juan metro
area. Urban and small stream flooding will be possible as well.
On Saturday, the upper level trough will begin to move toward the
east, while at the surface and the induced trough at lower levels will
shift the winds from the northeast, with drier air anticipated.
However, small patches of moisture may bring passing showers over
the islands.
.LONG TERM...Sunday through Thursday...
The upper level trough that has been hanging around for several
days will pass through on Saturday and a second weaker trough
will move through on Tuesday. Although the flow aloft will turn
to the northwest after the trough passage on Saturday it will
quickly become westerly late Sunday and continue through Thursday.
The lingering trough aloft will be instrumental in the continuing
development of an area of low pressure to the northeast of the
area at low and mid levels. This low will also have a northeast
southwest oriented trough that will pass just to the southeast of
the area that will cause mid and lower level flow to become
northeasterly and remain so for the rest of the period.
Air to our northeast is cooler and the lower levels will have
patches of moisture that will drive showers across the area for
much of the period. The NBM and the GFS are at extreme odds with
each other as to the probabilities of precipitation during this
period with the NBM forecasting 70-90 percent chances of rain and
the GFS dipping even into the single digits. Drier air will
certainly enter into the forecast area at 700 mb and cause shower
activity to diminish in quantity and intensity, but low level
moisture should not completely disappear and fragments of moisture
are expected to break off of the showers around the low pressure
to the northeast and ride across the forecast area. Expect these
fragments to result in minor but persistent passing showers--
mainly over Puerto Rico. Moisture from the fragments will also
cause shower activity in the south and southwest around the
Cordillera Central to occur if moisture patches coincide with
afternoon heating. On Thursday 500 mb temperatures may again
return to minus 9 degrees, but moisture will be limited,
nevertheless an isolated thunderstorm cannot be ruled out then.
&&
.AVIATION...VFR conditions are expected to prevail through 12/16Z
across all terminals. After that, VCSH and VCTS could cause brief
reductions in VIS to MVFR conditions across TJMZ/TJBQ,
respectively, until 12/22Z. Winds at FL050 will be out of the ESE
at 10 to 16 knots. Maximum winds WSW 75-85 knots btwn FL330-470, .
diminishing after 12/15Z.
&&
.MARINE...Seas have now been mainly driven by winds. These winds
are now tapering off. The outer buoy was running 20 to 22 knots
earlier this morning, but is now generally below 20 knots. Seas
there have come down a foot from the 12 feet they were earlier. At
the inner buoy winds are considerably lower and seas have just
dropped below 7 feet. Models have been fairly consistent in
bringing all seas in the local forecast area to below 7 feet by 2
AM AST tomorrow, (Friday). A new set of northeasterly swell will
enter the area Saturday night, but seas will scarcely rise above
7 feet early next week.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 74 84 75 / 40 50 40 40
STT 84 74 83 73 / 20 30 30 40
&&