National Weather Service San Juan PR
409 PM AST Sun May 21 2017
.SYNOPSIS...Showers and thunderstorms are once again occurring
this afternoon across central and western PR due to local effects
and available moisture. Weak tropical wave is expected to move to
the south of the local islands tonight while the induced trough
continues northeast through tonight before it weakens and
dissipated by Monday and a SFC high develops north of the local
islands. Haze due to Saharan Dust and warm temps are expected
through this week.
&&
.SHORT TERM...Today through Wednesday...
The band of moisture as seen over the area in the MIMIC product is
thinning as it hold over Saint Thomas and southwest between Vieques
and Saint Croix. Although streamers with very light precipitation
(so far) have developed off of these islands little other effect has
been seen. Due to weaker moisture in the area showers are not as
widespread or intense as they were yesterday at 2 PM AST, but are
still developing. The GFS suggests that columnar moisture will peak
around 22/06Z and then drop below levels seen anytime in the last 48
hours, so showers may linger into the evening as well.
As what remains of an eastward moving ridge at upper levels passes
over Puerto Rico Monday night, convergence aloft will begin to help
suppress vertical motion. Northwest flow will have already begun and
This will greatly decrease the amount of shower coverage Monday
afternoon. It may not be enough to completely suppress
thunderstorms, however, as the model anticipates higher CAPE levels
than today or yesterday. This will be reversed on Tuesday when
divergence aloft will increase with the passage of a weak upper
level short wave and moisture temporarily increases Tuesday and
Wednesday. Nevertheless equivalent potential temperature values will
decrease considerably after tonight and remain low till at least
Tuesday afternoon in the presence of Saharan dust that will also
increase considerably between 22/15Z and 23/15Z. This will lead to
hazier skies and weakly developing clouds during the week.
.LONG TERM...Wednesday through Monday...
A mid to upper level ridge will hold across the area through the
forecast period with some weakening expected Sunday and into early
next week. The ridge pattern aloft will promote limited moisture
with precipitable water values remaining below the normal range.
At lower levels, a broad high pressure will dominate the central
Atlantic, resulting in east southeast winds across the eastern
Caribbean Wed-Fri. Latest guidance continues to indicate the
Saharan Air Layer across the eastern Caribbean much of the
workweek, however, it looks particularly dense early in the week.
Therefore, under the influence of east southeast winds and a dry
and stable air mass with dust particles, a fair weather pattern
is expected with warm to hot temperatures and hazy skies much of
the workweek. As the ridge aloft erodes Sunday and into the next
week, an increase in low level moisture is expected with a better
chance for showers and thunderstorms. One more thing to note, the
latest guidance is indicating that the 100-850mb thickness is
expected to drop late this week, which could cause a few degrees
cooler minimum temperatures during the overnight hours if the
skies are clear and the wind is light.
&&
.AVIATION...VFR conds have prevailed across the local area and are
expected to continue except for TJMZ which has already seen some
SHRA/TSRA development and may have CIGS nr BKN030 til 21/21Z. Some
mtn obscurations over hir terrain expected til 21/04Z. Sfc winds
easterly with sea/land breeze variations 5 to 15 kt. Maximum winds
50 to 70 knots from FL390-470 by 22/18Z strongest at FL470.
&&
.MARINE...Seas are expected to continue 2-4 feet through Monday,
then increase gradually on Tuesday and Wednesday with potential of
up to 7 feet by Wednesday. There is a moderate risk of rip
currents for northern Puerto Rico and Culebra today and Monday,
low risk elsewhere.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 79 90 78 91 / 30 30 20 20
STT 79 88 80 89 / 30 20 20 20