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Area forecast discussion 
National Weather Service San Juan PR
624 am AST Friday may 29 2015

Synopsis...upper low will remain to the north of the local area
for the next few days as it slowly shifts to the west...then an
upper ridge will dominate over the local area by Tuesday. A
tropical wave is expected to move in later today and increase the
available moisture through the weekend answer into early next week.


Discussion...a rather interesting setup is in store for today and
into the weekend. At the time...confidence is moderate as we seem
to have several questions about the impact be of such as
setup...consider the lack of rainfall that eastern PR and the USVI
has observed in the past several months...the tropical wave moving
in with the upper low to the north of the local area...the high
clouds moving in from the south and the discrepancy from the
different model solutions for today and Saturday. However...the
main forecaster consensus is that an increase in shower and
thunderstorm activity is expected starting today and lasting
through at least Sunday. Occasional strong thunderstorms with
locally heavy rain are expected this weekend across different
sectors in the local forecast area. At least some Urban and Small
Stream flooding is expected with the showers and thunderstorms.

Currently we have an upper low to the north northeast of the local
islands and the tropical wave moving in from the east. The
available moisture will increase today with precipitable water
values jumping to almost 2 inches by this afternoon. When this
moisture advection is combined with the daytime heating and the
local effects...some good showers and thunderstorms normally
develop across the local area. However...satellite infrared is showing
an area of high clouds moving into the local area this morning but
the radar has yet to detect any significant rainfall associated
with the cloudiness over the local Caribbean waters. Having said
that...the Guadeloupe radar does show some shower activity and the
showers are moving rather fast to the north northeast. A few of
those showers look moderate to locally heavy. This cloudiness is
quite large answer may linger over PR and the USVI at least through
the morning hours...which will limit surface heating over the
islands...limiting the thunderstorm development. However...model
guidance is showing some good divergence aloft this afternoon
across the local waters and over PR and the USVI. This is
basically the opposite to what the same model has been saying for
the past 5 days or so. Up until today...the GFS model indicated
convergence for today in the upper levels...strong divergence
aloft would be conducive to shower and thunderstorm development.
The hi- res models such as the WRF 2km and 6km and the nmm are
showing little to no shower activity today over the islands...the
GFS shows some activity but the only model that is rather bullish
on the precipitation is the arw model which shows a significant amount of
precipitation across the Caribbean waters and northwest- PR...with some isolated
areas of precipitation elsewhere. Our forecast was adjusted slightly but
it still shows a steady increase of shower and thunderstorm
activity through the day with the Caribbean waters having numerous
showers and the interior of PR into northwest-PR developing showers and
thunderstorms in the afternoon...scattered showers and isolated
thunderstorms elsewhere.

Saturday still looks like a pretty active day. However...the GFS
model used to be pretty consistent showing upper level divergence
through the day Saturday...the 29/00z run still shows that
divergence aloft but during the evening hours...while the 29/06z
run once again shows that divergence aloft during the day and then
lingering into the evening. Either case...the divergence aloft is
quite strong according to the GFS model...with available moisture
still very good with precipitable water values still hovering at
around 2 inches. The nmm model and WRF are also not liking this
setup very much and are showing a relatively dry Saturday...however
the arw model brings in a lot of showers and thunderstorms to the
local islands...especially the northern half of PR and the USVI.
According to the latest GFS model the available moisture will be
good and the divergence aloft will be strong through at least

So we do expect a rather active weekend...even with some of the
hi-res models indicating little to no precipitation these next few days.
The big picture setup is conducive to shower and thunderstorm
activity across the local least on Friday and
Saturday...however forecast soundings show a profile more
consistent with high cloudiness but not as much precipitation for Sunday
and Monday.

There are questions on the potential impact the showers and
thunderstorms will have over the islands. The discrepancy between
the global and the hi-res models and the fact that eastern PR is
so was decided to hold off on a Flash Flood Watch until
we saw how the models are handling this pattern and how much rain
it actually falls today across the local islands. We have to take
into account not only meteorological conditions but also
hydrological conditions...and right now the hydrological
conditions are that the rivers are running very low and the soil
across eastern PR is in need of some will take a lot of
rain to cause river flooding across eastern PR...not so much for
the western half of PR which has ovserved consistent rainfall for
the past several days.

Stay tuned for possible changes as we gather more information on
this event today.

Weather conditions are expected to improve by Tuesday and into the
rest of the week...with a more typical pattern of morning showers
across the waters and afternoon convection due to local effects.


Aviation...increasing rain showers and thunderstorms and rain en Route from the
Windward Islands will affect the flying area of tncm and tkpk through
the forecast period...spreading through southeastern PR and the USVI by
14z. MVFR/IFR possible and widespread obscurations likely in higher
terrain and northwestern PR after 16z. Southeast winds of 10-15 knots expected
below 5 kft with higher gusts near thunderstorms and rain...SW-W winds above/increasing
W/height. Unsettled weather conditions will continue through the


Marine...winds and seas will continue at 4 feet or less and 10
knots or less respectively today. Deteriorating conditions expected
tonight and Saturday due to increased potential for thunderstorm
development and inclement weather across portions of the regional


Preliminary point temps/pops...
sju 87 78 87 75 / 60 40 80 50
stt 84 77 86 76 / 50 50 70 60


Sju watches/warnings/advisories...



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