Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 230827
Area Forecast Discussion
National Weather Service San Juan PR
427 AM AST Thu Feb 23 2017
.SYNOPSIS...Mid to upper level high pressure ridge will build overhead and
remain in place through Friday, then gradually weaken and shift eastward
on Saturday and Sunday as a polar trough will move across the western
Atlantic. Surface high pressure will continue to build and spread eastward
into the north central Atlantic by Friday as an area of low pressure
and associated cold front will enter the southwest Atlantic today while
lifting east northeast across the west Atlantic. This overall pattern
will cause the local winds to become more easterly today then southeasterly
by Friday as the surface high becomes reinforced across the north central
and northeast Atlantic.
.DISCUSSION...The prevailing northeast winds will continue to
transport patches of shallow low level moisture and occasional
showers to the regional waters and portions of the north and east
coastal sections of the islands during the rest of the morning
hours. By late morning cloudiness and shower activity should diminish
resulting in mostly sunny fair weather skies. During the afternoon
hours, expect the prevailing winds to become more southeasterly
and gradually increase. This along with local and diurnal effect
will lead to increasing cloudiness and isolated to scattered
shower development across parts of the central and northwest
sections of Puerto Rico as well as around parts of the San juan
metro area. For the rest of the islands, expect limited shower
activity with only a few quick quick passing showers in isolated
spots as mostly sunny skies should prevail during the day.
On Friday and into the weekend similar weather conditions expected with
brief passing early morning showers forecast to affect mainly the east
coastal sections of the islands. This will be followed by mostly isolated
afternoon showers across portions of the northern half of Puerto Rico.
Mostly fair weather skies expected for the USVI. Under a prevailing
southeast low level wind flow on Friday and Saturday, expect slightly
warmer daytime high temperature along the north half of Puerto Rico
and the San Juan metro area.
By Sunday and Monday, the formation of a weak prefrontal trough will
create sufficient low level moisture convergence across the region
to increase chances of early morning and afternoon convection mainly
over parts of the interior sections of Puerto Rico. By Tuesday of
next week ...the Atlantic surface high is forecast to build once
again north of the region and consequently tighten the local pressure
gradient. This in turn will increase the east to northeast trade winds...
which will be also be supported by increasing subsidence aloft and a
gradual drying trend across the region once again.
.AVIATION...Mostly VFR conditions expected across the local flying
area for the next 24 hours. However ISOLD-SCT SHRA expected in
the vicinity of most of the terminals this morning except for
TJMZ. Passing SHRA ovr the coastal waters and en route between
islands til 23/14z. Few tops nr FL220. Low level winds are
expected to become more from the E to ESE after 23/12Z. ISOLD to
SCT SHRA is expected this afternoon, with VCSH possible across
the local terminals through the rest of the day.
.MARINE...The wave watch model guidance continued to suggest
building seas across the local Atlantic water during the day.
Recent data from buoy 41043 showed seas now up to 7 feet in the
past few hours. Therefore small craft advisory will go into effect
for the Atlantic outer waters from 23/18Z. Small craft operators
should exercise caution across the rest of the Atlantic waters and
local passages. Winds are also expected to increase and become
more east to southeast today and through Friday.
.PRELIMINARY POINT TEMPS/POPS...
SJU 87 73 87 74 / 20 20 20 10
STT 85 73 82 74 / 30 20 20 20
AM...Small Craft Advisory from 2 PM this afternoon to 8 AM AST
Saturday for Atlantic Waters of Puerto Rico and USVI from
10 NM to 19.5N.