Area Forecast Discussion
Issued by NWS San Juan, PR

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169
FXCA62 TJSJ 131036 CCA
AFDSJU

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
636 AM AST Sun Jun 13 2021

.SYNOPSIS...A seasonable weather pattern is expected to prevail
across the area through at least the middle of the week, with
afternoon convection affecting mainly the western half of Puerto
Rico. Marine conditions will continue to remain fairly tranquil
through at least early this week, however, choppier conditions
are expected beginning by the middle portion of the week as winds
increase. Increase in shower activity can be expected by next
weekend.

&&

.SHORT TERM...Today through Tuesday...

A decrease in the Saharan Dust concentration is expected today, so
skies are expected to be less hazy then they were yesterday. Above
normal moisture is still expected today, and the upper trough is
still positioned favorably for thunderstorm development this
afternoon. Showers and thunderstorms are forecast this afternoon,
mainly across western and northwestern PR, while scattered showers
are expected elsewhere in PR and isolated showers for the USVI. The
local wind flow will be mainly from the east to slightly ESE. The
daytime high temperatures are expected to be warmer today than we
have observed the past few days, with highs in the upper 80s to
around 90 across the lower elevations.

Drier air and Saharan Dust return to the local area on Monday
morning, just ahead of a tropical wave that is expected in the late
morning and early afternoon hours on Monday. Even though there is a
tropical wave moving in, and moisture will increase once again on
Monday afternoon, the latest guidance is not suggesting significant
rainfall in the afternoon, which could be due to drier air at the
700mb level and Saharan dust present. That said, scattered showers
and isolated thunderstorms are in the forecast for the afternoon,
but the rainfall accumulations are expected to be modest compared to
what we have observed over the past few days. Then for Tuesday, even
drier air moves in, this time the dry air and Saharan dust will
persist through the day, causing hazy skies and warm to hot
temperatures. The forecast soundings suggest relatively dry layer at
700mb as well, and not much more than  isolated to scattered showers
are expected. In terms of temperatures, the daytime high temps could
reach 90 at the San Juan airport, as the lower elevations are
expected to have temps in the upper 80s to low 90s.

This means that there will be mainly fair weather, with brief
showers during the morning and overnight hours for eastern PR and
the USVI, while showers and thunderstorms may affect central and
western PR each afternoon. Soils are still saturated though, so
significant showers could cause at least some urban and small
stream flooding, which is possibly particularly across the
northwestern quadrant of PR since that is the area that is
expected to have the highest amounts of rain this weekend and on
Monday.

.LONG TERM...Wednesday through Monday...

A surface high pressure across the north central Atlantic will yield
east southeasterly winds across the local islands midweek through
the weekend. The forecast for the long-term period continues mostly
on track with Wednesday and early Thursday looking warm and somewhat
dry with a 500 mb high pressure ridge and hazy skies due to the
presence of Saharan dust particulate. At lower levels, moisture will
reach below normal levels, however, enough will be present for
daytime heating and local effects to produce showers and possibly
some isolated thunderstorms mainly over the northwestern quadrant
of Puerto Rico during afternoon hours.

Slight discrepancies exist between the GFS and ECMWF models by
Thursday onwards. GFS shows two easterly waves affecting the area;
one arriving late Thursday through early Saturday and the other
moving in by Sunday through Monday. Conversely, ECMWF has a more
continuous transition between both easterly waves but much weaker
intensity affecting the area Friday through Monday. Both models
show 500 mb temperatures between -6 to -8 degrees, normal to
slightly above normal levels for this time of year. Dynamically
speaking, upper-level conditions are not extraordinary but enough
to aid in convection development with sufficient available deep
layer moisture. Precipitable water values will increase to more
than 2.0 inches with each easterly wave event, raising moisture to
above normal levels.

Despite model discrepancies, we could expect the wettest days of
the long-term forecast to occur by Friday through the weekend.
Thus, shower and thunderstorm activity may increase with each wave
passage, raising the available moisture. The moisture will
combine with diurnal heating and local effects to cause more
significant shower or thunderstorm development across portions of
Puerto Rico, but also the waves will bring shower activity to the
local area overall, affecting the USVI as well.


&&

.AVIATION...

VFR conds expected across the local terminals today. SHRA/TSRA
activity may be observed near TJBQ this afternoon, causing at least
some VCTS. SCT SHRA is forecast for the TJSJ terminal, so VCSH is
possible this afternoon as well. Winds will be light early in the
morning, becoming e to ese with sea breeze variations after 13/13Z
at around 10-15KT.

&&

.MARINE...Tranquil marine conditions are expected to prevail
across the local waters through at least early this week with
seas of up to 4 feet expected and slightly increasing by tomorrow
night over the offshore Atlantic waters. Winds are expected to
increase during the middle portion of this week, resulting in
choppier seas. There is a moderate risk of rip currents for most
of the north facing beaches of Puerto Rico as well as the eastern
tip of Saint Croix today.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  90  78  90  78 /  40  30  30  30
STT  88  79  88  79 /  30  30  30  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.

&&

$$

SHORT TERM...JA
LONG TERM....CVB
PUBLIC DESK...CVB/JA



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