Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
311 PM AST Tuesday Oct 21 2014
Synopsis...in the upper levels...a short wave trough extended
south across the region into the east central Caribbean...as an
upper level ridge will slowly build eastward across the local area
over the next few days and into the upcoming weekend before weakening.
Surface high pressure ridge across the western Atlantic and just
north of the region...will remain in place through Thursday...then
slowly retrogress into the eastern Atlantic by Friday as an area
of low pressure and associated cold front moves across the western
Atlantic. A tropical wave with axis now near 52 west and west of
the Windward Islands...will continue westward and move across the
Lesser Antilles by late. This wave will then move across the eastern
Caribbean Friday through Saturday where it is expected to merge with
the broad equatorial trough spread across the eastern Caribbean.
Discussion...latest model guidance and blended total precipitable
water product continued to suggest decreasing precipitable water values now
ranging between 1.50 and 1.70 inches. However....still sufficient
pockets of moisture across parts of the islands to combine with
diurnal and local effects along with the short wave trough trough
moving across the region...to allow for afternoon convection mainly
across the west interior sections of Puerto Rico. Activity should
however diminish by sunset leaving mostly clear to partly skies.
Over the next couple of days...decreasing moisture advection along
with the building high pressure ridge aloft will increase upper level
subsidence and help to suppress and limit daytime convection each day.
However some isolated to scattered showers or a thunderstorm will remain
possible across parts of the islands through Thursday.
The next feature forecast to transport moisture to the region will
be the previously mentioned tropical wave. This feature will interact
with a TUTT which will retrogress westward across the Lesser Antilles...
and enhance convective activity across the eastern Caribbean and
parts of the local islands. The wave is forecast to cross the Windward
Islands late Thursday...then move across the eastern Caribbean Friday
through Saturday. Although expect a slight increase in moisture across
the region during the latter part of the week...present scenario
suggest the brunt of the moisture accompanying the wave should move
mainly south of the area. The dominant ridge aloft is to suppress
any widespread convection across the islands and coastal waters
with most of the showers and thunderstorm development each day
to be focused across the west interior sections of Puerto Rico
over the next several days.
Aviation...after rain showers/thunderstorms and rain this afternoon over western PR. VFR
conditions will prevail through the forecast period with possible vcsh
across tncm/tkpk/tjsj. Light and variable winds overnight.
Marine...buoys along the North Shore of Puerto Rico and across
the Atlantic waters continued to suggest 3 to 4 feet northerly
swell at 11-14 seconds today. However the swell action will
continue to fade during the evening and overnight hours. Expect
seas of 4 feet or less and winds 14 knots or less through Wednesday
with fairly tranquil seas during the end of the work week.
Preliminary point temps/pops...
sju 77 89 76 88 / 30 30 10 20
stt 77 89 79 88 / 30 30 20 20