Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Juan PR
537 am AST Thursday Aug 28 2014

Synopsis...retrogressing TUTT low now centered just northeast of
the northern leewards near 21n 57w will continue to shift further
west northwest during the next few days. At low levels...a broad
surface high pressure centered over the central Atlantic and a
and easterly perturbation/tropical wave moving across the eastern
Caribbean...will help tighten the local pressure gradient and
increase the local trade winds generating a winds surge which will
move across and affect the forecast area today.


Discussion...TUTT low and associated trough will continue to move
westwards to just northwest of the islands over the next few days...
where it will linger and become amplified across the southwest
Atlantic through Saturday. TUTT induced low to middle level trough
and weak tropical wave will bring a slight increase in moisture
across the region today. Although the majority of the accompanying
moisture will remain mainly south of the islands...still expect
sufficient moisture convergence and instability for some early
morning and afternoon convection across parts of the islands and
coastal waters. Most of the afternoon showers and isolated thunderstorm
activity should however be focused mainly across the interior and
west sections of Puerto Rico due local terrain effects.

The next tropical wave is still forecast to enter the eastern
Caribbean late Friday and move across the region over the upcoming
weekend. At this time GFS model guidance suggest a significant
increase in tropical moisture to be lifted northwards across the
region with this wave with precipitable water values to once again exceed two
inches by Saturday and Sunday. So far the GFS continued to initialize
well and if in fact this scenario unfolds...expect the interaction
between the TUTT low and the the tropical wave...will lead to increased
potential for shower development and enhanced thunderstorm activity
across the islands for most of the weekend.


Aviation...a wind surge created by the tightening of the local
pressure gradient in response to an easterly perturbation moving
across the eastern Caribbean...will bring and in increase in the
low level easterly trade winds today. Brief periods of MVFR
expected this morning across tncm...tkpk and tist due to quick
passing rain showers and few-scattered low clouds. Previous tjsj 28/00z sounding
indicated strong easterly winds of 20 kts to up to as much as 45
knots below fl150. Expct brief surface wind gust mainly west/passing rain showers at
all taf sites until 28/15z. MVFR likely in at tjmz after 28/16z with
mountain obscure due to rain showers/isolated thunderstorms and rain until 28/22z.Vcsh near tjbq from


Marine...previously mentioned wind surge moving across the regional
waters today will help to generate rough and hazardous sea conditions
across the local waters at least until the afternoon hours. For this
reason small craft advisories will remain in effect until later today
for parts of the local waters. Elsewhere small craft operators
should exercise caution.


Preliminary point temps/pops...
sju 88 78 88 77 / 20 30 30 50
stt 89 79 88 77 / 30 30 30 50


Sju watches/warnings/advisories...
Coastal waters...Small Craft Advisory until 9 am AST this morning for Anegada
Passage southward to 17n.

Small Craft Advisory until 6 PM AST this evening for Atlantic waters
of Puerto Rico and USVI from 10nm to 19.5n-Caribbean waters
of Puerto Rico from 10 nm to 17n.

Small Craft Advisory until 5 PM AST this afternoon for Mona
Passage southward to 17n.

Small Craft Advisory from 9 am this morning to 5 PM AST this
afternoon for coastal waters of northern Puerto Rico out 10
nm-coastal waters of northwestern Puerto Rico out 10 nm.




National Weather Service Glossary of Abbreviations