Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 241626

Area Forecast Discussion
National Weather Service San Juan PR
1226 PM AST Wed May 24 2017

Today, hazy skies with suspended dust particles were observed
across the islands. In addition, local weather temperatures were
in the upper 80s and low 90s. Heat index calculations across some
of the local weather stations ranged in the upper 90s and low
hundred degrees. A strong temperature inversion was detected by
the 24/12z TJSJ RAOB. Little or no shower activity was observed
with the Doppler Radar. Hazy, dry, warm and limited shower
activity are expected to continue through at least Friday.
However, suspended dust particles concentrations is expected to
decrease through the work week.


VFR conditions will prevail across all TAF sites through the fcst
period. Suspended dust particles are moving across the flying
area, reducing VSBY at 7-9 nm. SHRA can not be ruled out btwn
24/18-23z in form of VCSH at TJMZ/TJBQ. Clear skies expected
overnight with isol -SHRA en route from the Leeward Islands and
USVI. Sfc winds mainly from the E-ESE at 15-20 kts with higher
gusts, decreasing at around 10 knots aft 24/23z.


Local buoy network detected increasing seas between 3-5 feet and
winds at 15 to 20 knots. Winds and seas are expected to increase
between 4 and 6 feet with fresh winds through tonight. No changes
were made to the previous forecast.


.PREV DISCUSSION... /issued 410 AM AST Wed May 24 2017/

SYNOPSIS...Mostly fair and relatively dry weather conditions
with high concentrations of saharan dust particulates will
continue through the end of the week. An upper level ridge is
forecast to hold over the region until at least Friday. Surface
high across the central Atlantic will maintain an easterly trade
wind flow across the region for most of the forecast period. A
wetter pattern is possible early next week as a tropical wave
moves across the local islands.

SHORT TERM...Today through Friday...
Partly cloudy skies prevailed across Puerto Rico and the U.S. Virgin
Islands overnight. No shower activity was observed over land areas.
A very dry airmass will continue to dominate the local region for
the next few days as an upper level ridge continues in total control
of the local weather. Saharan dust will continue across the region
today. As a result, hazy skies can be expected. A southeast wind
flow will promote higher than normal temperatures along the northern
slopes of Puerto Rico this afternoon.

Model guidance suggests that Saharan Air Layer is expected to
continue affecting the local region through at least Friday. This
will maintain hazy skies and warmer than normal temperatures across
the region for the next several days. However, by Friday an increase
in moisture is expected. Therefore, an increase in shower activity
is then expected as this moisture, combined with strong daytime
heating, sea breeze convergence and mountains lifting will induce
showers and possible thunderstorms development over central
interior, western and northwest Puerto Rico Friday afternoon.
Significant precipitation is not expected to affect the U.S. Virgin

LONG TERM...Saturday through Thursday...
An upper level low and an associated trough is forecast to develop
just to the east northeast of the region during the upcoming
weekend. This feature is expected to erode the mid to upper level
ridge that persisted over the region for the last few days.
However, the upper trough axis is forecast to remain east of the
region, maintaining the islands on the subsidence side of the
trough. Surface high pressure across the central Atlantic is
expected to maintain moderate easterly trade winds across the
region until early next week. Optical Thickness model guidance
suggests that the Saharan Air Layer will persist over the
region through most of the work week and into the upcoming
weekend, resulting in hazy and a relatively dry airmass across the
local islands. Latest surface analysis depicted a tropical wave
across the tropical Atlantic. This wave will continue to move
westward and approach to the Lesser Antilles late Monday and
Tuesday. It is then forecast to move over the eastern Caribbean
and over the local islands Tuesday through Wednesday. This feature
should bring an increase in tropical moisture transport and
consequently increase the chance for showers and thunderstorms
over the local islands and surrounding waters next week.

AVIATION...VFR conditions will prevail across all TAF sites through
at least 24/16z. VCSH can be expected across TJMZ and TJBQ with
mountain obscurations. Low level winds will be mainly southeast at
10 to 15 kts with higher gusts.

MARINE...High pressure building north of the region will aid in
slightly increasing the trade winds across the surrounding waters,
resulting in posible seas building up to 6 feet across the Atlantic
waters and local passages through Wednesday. Marine conditions will
then gradually improve through the weekend when seas subsiding to
4 feet or less across most of the waters.


SJU  79  90  77  88 /  10  20  20  20
STT  80  89  77  87 /  10  20  20  40




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