Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170829

Area Forecast Discussion
National Weather Service San Juan PR
429 AM AST Tue May 17 2022

An increase in shower activity is forecast for today, as more
moisture associated with a tropical wave well at the south of the
local Islands. Therefore, minor flooding as a ponding of water in
roadways and in poor drainage areas is possible. Daytime
temperatures are forecast to reach the upper 80s to the lower
90s. A drying trend is forecast starting on Wednesday into the
upcoming weekend, as a much drier airmass with Saharan Air Layer
(SAL) filters into the area.


.SHORT TERM...Today through Thursday...

A surge in moisture, up to 2 inches of precipitable water content
(PWAT) is expected across the local area today, behind the passage
of a tropical wave across the Caribbean Sea. Latest satellite and
MIMIC-Total Precipitable Water imagery is showing this area moving
over the northern Leeward Islands into the Anegada Passage. Weak
ridging at the mid/upper levels should keep marginal stable
conditions aloft with 500 mb temperatures at -5/6C. However, will
include isolated thunderstorms for this afternoon over northwestern
PR due to the weakening of the trade wind cap and above normal PWAT
content. Urban and small stream flooding is possible with this
activity, as well as ponding of water in poor drainage and low-lying
areas across east/southeastern PR during the morning hours.

Conditions become more hostile for organized convection on Wednesday
and Thursday as a Saharan Air Layer(SAL) filters over the region.
Expect hazy skies later in the period, with a significant increase
in the medium range forecast. A low to mid-level ridge will slowly
build from the Tropical Atlantic, promoting east to southeasterly
trades and the SAL intrusion. At the same time, slightly cooler 500
mb temperatures are expected under the influence of a short wave
trof, but as PWAT erodes gradually through the rest of the short
term period mostly shallow diurnally induced afternoon convection is
expected each day over west/northwest PR. Across the USVI and
eastern sections of PR, quick passing showers mixed with hazy skies
are expected to prevail late in the short term period. Heat indices
between 100-105 are possible each day across the northern and
western portions of PR, as well as above normal minimum temperatures
along coastal areas.

.LONG TERM...Friday into Tuesday...

A drying trend is forecast for most the long term period. A
surface high pressure moving northwestern over the western
Atlantic will continue to result in a southeasterly component in
the wind pattern over the local islands. Dragged in this wind
flow, a big pulse of Saharan Dust particles is forecast to move
over the local islands on Friday into the rest of the upcoming
weekend. The bulk of this event is expected to be in place by
Friday into Sunday, diminishing on late Sunday night and turning
less by Tuesday. The presence of the particles and the drier
airmass will inhibit the development of shower activity across all
the forecast area. Additionally, this will induce a reduction in
visibility, as well will enhance heat indices near or above the
100s. Given all the expected conditions haze was introduced for
Friday into Saturday. In terms of shower activity, given the
expected conditions, the unfavorable conditions in the mid to
upper level with a establish ridge over the area, will inhibit the
long-lasting afternoon shower activity, if any, during this
period. Daytime temperatures are forecast to increase during the
Saharan Air Layer (SAL) event, and are expected to fluctuate
between the upper 70s to the low 90s, as well an increase in the
heat indices are forecast rounding between the upper 90s to near
the 100s.

By Tuesday, both model guidance area suggesting a strong upper
level trough just east of the forecast area. No matter the
favorable side for convection will remain at our east, this
instability will induced cloudiness and low level moisture. This
aforementioned moisture will reach the local islands, embedded in
the trade winds. So far, precipitable waters values are not
forecast to be above climatological normal, but with the local
effects will result in the typically afternoon showers activity.


.AVIATION...Mainly VFR conditions are expected to prevail across all
terminals through the forecast period. However, showers are expected
to develop in and around TJBQ from 17/16z-23z, which may cause tempo
MVFR conditions. VCSH expected across the rest of the terminals. Low
level winds will continue ESE at 10-15 kt with higher gusts and sea
breeze variations aft 17/14z. HZ expected to gradually increase into
Wednesday due to suspended Saharan dust, but vsby expected to remain
P6SM. TIST was reporting false OVC018 cigs overnight.



A surface high pressure over the Atlantic will continue to result
in moderate east-southeasterly winds across the region. As a
result winds up to 15 knots are expected across most of the local
waters, except for the nearshore Caribbean waters, where winds
will remain up to 20 knots. Therefore, mariners should exercise
cautions across those waters. Across the coastal sections, there
is moderate risk of rip current for most of the local exposed
beaches, the risk will remain low across south beaches of Rincon.


SJU  89  77  90  77 /  50  40  20  10
STT  85  79  87  77 /  30  50  30  30




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