Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 221434

Area Forecast Discussion
National Weather Service San Juan PR
1034 AM AST Sun Apr 22 2018

.UPDATE...No change to the inherited forecast package at this time.
Recent TJSJ 22/18z suggest a gradual erosion of low level moisture
with precipitable water now down to around 1.35 inches compared to
1.70 inches yesterday. Low level winds have become more northeasterly
today which suggest any afternoon convection that form across the
interior should steer more west southwest towards coastal areas.
In addition isolated thunderstorms will be likely due to the erosion
of the upper ridge and the proximity of an upper short wave trough
to out west.


.AVIATION...No change to previous AVN discussion. Overall VFR conds.
Aftn SHRA/Isold still fcst ovr ctrl Mtn Range and W sections of PR
til 22/18z.


.MARINE UPDATE...Overall seas continued between 4 to 6 feet and winds
of 15 to 20 kts across the coastal waters. Small craft should exercise
caution across most coastal waters and passages. A slight increase
in winds and seas is expected across the offshore Atlantic overnight
through Monday due to a surface high pressure north of the region and
the arrival of a northerly swell.


.PREV DISCUSSION... /issued 449 AM AST Sun Apr 22 2018/

SYNOPSIS...Strong surface high pressure across the central Atlantic
will continue to dominate the local area through the rest of the
weekend. An increase in low level moisture is expected early next
week. Upper level trough will dominates the western Atlantic
Monday through Wednesday next week.

SHORT TERM...Today through Tuesday...

Today looks like it will be a bit less active than the past few
days. The drier air moving in and ENE wind flow will cause a few
brief showers to affect NE-PR and the USVI in the morning but the
accumulations will be minimal. Then in the afternoon, the shower and
possible thunderstorm development should be mostly across the
southwestern quadrant of Puerto Rico with some showers also expected
across the higher elevations of eastern and central PR, but the
chance of thunderstorms is higher across SW-PR. This drier air will
be short lived as higher moisture will be moving in for Monday and
Tuesday, at the same time an upper trough will be just to our west,
which should have an impact in the local instability. So the shower
and thunderstorm activity and coverage should be more significant on
Monday and Tuesday, especially for Puerto Rico as the local effects
combine with the upper level dynamics to produce persistent
thunderstorms in the afternoons. The USVI and the rest of the local
forecast area should observe an overall increase in shower activity
but thunderstorms are more probable over Puerto Rico.

LONG TERM...Wednesday through Monday...

Upper level ridge will build across the northeast Caribbean
through this period. However, for mid week an increase in low
level moisture is forecast by computer models. As a result, better
chances for shower and thunderstorm development can be expected
Wednesday and Thursday. For the rest of the period, seasonable
weather conditions are expected.

AVIATION...VFR conds expected through the forecast period across
the local terminals. VCSH for TJSJ, TIST, and TISX through 22/14Z,
then after 22/17Z, SHRA/TSRA expected in the vicinity of TJPS with
possible TEMPO MVFR at TJMZ until about 22/22Z. Winds will be mainly
from the E to ENE at about 10-15KT with occasional gusts.

MARINE...Seas between 4 to 6 feet and winds of 15 to 20 kts are
expected to prevail across the coastal waters. Small craft should
exercise caution across most coastal waters.


SJU  86  76  87  75 /  40  60  60  60
STT  87  76  86  75 /  20  40  40  40


VI...High Rip Current Risk until 6 PM AST this evening for St Croix.



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