Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1143 am AST Thursday Nov 26 2015
Update...mostly sunny to partly cloudy skies were observed this
morning across all local islands. High clouds continued over the
forecast area with westerly flow above. The available moisture
will combine with day time heating and local effect to produce
showers and isolated thunderstorms this afternoon over the
interior and western sections of PR. Minor changes made to marine
girds...otherwise no change from previous forecast.
Aviation update...the 26/12z tjsj upper air sounding indicated
east-southeast winds at 5-13 knots below fl100...westerlies above/increasing
W/height. Rain showers/thunderstorms and rain expected across the interior and western
sections of PR...creating mountain top obscured and brief MVFR
conditions...impacting mainly jbq/jmz through 22z. Vcsh likely at
Marine update...increased seas up to 9 feet for tomorrow across
the Atlantic offshore waters as model guidance is currently
underestimating seas by 1-2 feet across the Atlantic. Rincon and
San Juan buoys are indicating seas near 6 feet...and 2-4 feet
south of the islands. Small craft advisories will start in effect
this evening across much of the waters exposed to the northerly
Previous discussion... /issued /
Synopsis...trough pattern will establish over the western
Atlantic today. The upper level trough is forecast to become a
cutoff low by Friday...digging to the south into the Bahamas and
the Windward Passage during the weekend. Broad low over the
central Caribbean will induce a wet southeasterly flow across the
Discussion...a fairly dry air mass between 850-400 mb will limit
the organized convection across the islands through Saturday.
However...available moisture will combine with local effect to
produced scattered convection over portions of the interior and
northwest PR each afternoon.
Conditions will deteriorate and become more favorable for
organized convection from Sunday through early next week as the
aforementioned upper level low will deepen and relocate over
Hispaniola by Sunday. Under the influence of this broad upper low
and wet southeast flow... active weather will be likely across the
local islands early next week. There is some uncertainty in the
forecast at this time...as Euro model European model (ecmwf) shows the upper low
far to the west. Because the position of the upper low is critical
for the development of organized convection across the region...we
will continue to monitor closely the model solutions to see how
this pattern unfolds.
Aviation...mainly VFR conditions are expected to prevail across all
terminals through the forecast period. However...a few morning showers
possible across tist... the rest of the
morning. Afternoon showers are expected to develop over the interior
and northwest PR from 26/18z-22z...impacting mainly jbq/jmz. Southeast winds at
10 to 15 knots with sea breeze variations will prevail at low levels.
Marine...a long period northerly swell will gradually build across
the local Atlantic waters and Caribbean passages through Friday.
Seas are forecast to peak on Friday night...reaching 7-8 feet across
the Atlantic waters. Small Craft Advisory now in effect for the Atlantic
waters and Caribbean passages from tonight through at least Saturday
Preliminary point temps/pops...
sju 87 75 88 77 / 40 20 40 40
stt 86 76 86 77 / 40 30 30 40
PR... Small Craft Advisory from 2 PM Friday to 2 am AST Saturday for
Vi...Small Craft Advisory from 8 PM this evening to 2 am AST Saturday
for St. Thomas/St. John/adjacent islands.
Am...Small Craft Advisory from 8 PM this evening to 2 am AST Saturday
for Anegada Passage southward to 17n-Atlantic waters of Puerto
Rico and USVI from 10nm to 19.5n-coastal waters of northern
Puerto Rico out 10 nm-coastal waters of northern USVI and
Culebra out 10 nm-coastal waters of northwestern Puerto
Rico out 10 nm-Mona Passage southward to 17n.
Small Craft Advisory from 8 am Friday to 2 am AST Saturday for
Caribbean waters of Puerto Rico from 10 nm to 17n-coastal
waters of southern USVI Vieques and eastern Puerto Rico out