Area Forecast Discussion
Issued by NWS San Juan, PR

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270
FXCA62 TJSJ 241512
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1112 AM AST Mon Sep 24 2018

.UPDATE...Passing trade winds clouds and showers continued to move
across the regional waters and affect portions of the islands during
the morning hours. High pressure ridging north of the area and
the approaching trough associated with the remnants of what was
once TD eleven will help create moderate easterly winds and also
induce low level convergence across the region. This along with
local and diurnal effect will continue to support late morning and
afternoon convection across the islands as previously mentioned.
Some of the rainfall will lead to urban and small stream flooding,
especially over portions of the interior and west sections of PR
including parts of the San Juan metro area. No changes made to the
inherited forecast package at this time. Therefore expect increasing
moisture and instability across the region overnight through Tuesday
associated with remnants of TD eleven and interaction of the westward
moving Tutt low. Still looking for moist and unsettled weather pattern
by the end of the work week and into the weekend as previously mentioned.

&&

.AVIATION UPDATE...No change to previous AVN discussion. -SHRA/SHRA
ovr the regional waters ...SCT-BKN lyrs nr FL025...FL050. Aftn
convection still expected to develop over ctrl and west sections
of PR, and mainly downwind of the adjacent islands at least til
24/22z.

&&

.MARINE UPDATE...No change to previous marine discussion at this time.
Recent data for the surrounding buoys continued to suggest the arrival
of a northerly swell across the offshore Atlantic waters and the Anegada
passage. Expect the swell action to continue to spread across the rest
of the passages and near shore Atlantic waters during the rest of the
day through Tuesday. Moderate to locally fresh easterly winds will continue
to create choppy condition across the local waters and passages. Small
craft operators are therefore urged to continue to exercise caution.

&&

.PREV DISCUSSION... /issued 435 AM AST Mon Sep 24 2018/

SYNOPSIS...Trough pattern aloft will support thunderstorm
development each day. The remnants of TD#11 are expected to move
across the area on Tuesday and Wednesday. Another surge in low
level moisture is expected late in the week in association with TC
Kirk. A large long period northerly swell is forecast to affect
the regional waters during the weekend.

SHORT TERM...Today through Wednesday...

Mostly clear skies prevailed earlier this morning with winds of 5
to 10 kts. Doppler radar shows a few passing showers mostly over
the local waters. A TUTT low currently east of Culebra will
continue to meander westward through Wednesday. A weak surface
trough is forecast to develop near hispaniola and remain through
the middle of the week. Diurnally induced afternoon showers and
thunderstorms are forecast to develop today across the interior
and western areas of Puerto Rico. Tuesday through Wednesday
showers and thunderstorms are forecast to develop area-wide. These
showers and thunderstorms will be helped by the increasing
instability at the low, mid and upper levels of the atmosphere.
The SJU-GFS shows decreasing 500 mb temperatures and 200 mb
heights. The low level instability will be generated by the
surface trough and the increasing low level moisture provided by
the now dissipated TD#11. Moisture is forecast to increase each
day with the bulk of the moisture arriving late Tuesday or early
Wednesday. Therefore, urban and small stream flooding is likely
each day this week, as a result of favorable environmental
conditions for continued thunderstorm development. This is also
confirmed by the Galvez- Davidson Index (GDI) which measures
stability. The GDI shows values of 25 to 35, and increasing to 35
to 45 on Tuesday and Wednesday. The 35 to 45 values means that
showers and thunderstorms are capable of producing heavy rainfall
that could cause urban and small stream flooding, and ponding of
water on roadways.

LONG TERM...Thursday through Tuesday...

Trough pattern aloft across the northeastern Caribbean and in the
central Atlantic will support shower and thunderstorm development
through the long term period. Mainly across the interior and
western sections of Puerto Rico. A patch of drier air from an
upper level ridge over the southwestern Atlantic should move from
the north briefly between Thursday and Friday. However, a surge in
low level moisture is expected to return quickly on Friday and
during the weekend in association with the weak Tropical Storm
Kirk. Based on the latest forecast, Kirk is expected to have
fluctuations on intensity and there is still high uncertainty on
the local impacts that it could have once it enters the eastern
Caribbean by the end of the week.

AVIATION...VFR conds will continue through the reminder of the
prd. SHRA/VCTS are possible due to afternoon convection, which
could cause brief MVRF conds across terminal sites TJBQ, TJMZ and
the western areas of Puerto Rico. Elsewhere, showers are possible
across the eastern areas of Puerto Rico during the afternoon. Sfc
winds out of the east ranging from 10 to 15 kts with higher gusts.

MARINE...A northerly swell will continue to affect the Atlantic
waters and passages through Wednesday. This will cause seas to
range between 4-6 feet and occasionally up to 8 feet. A moderate
to high risk of rip currents will prevail across the north facing
beaches of the islands for much of the work week. A larger
northerly long period swell is forecast to affect the regional
waters during the weekend. Winds will continue from the east at
10-15 knots. Shower and thunderstorm activity is expected to
increase between Tuesday and Wednesday across the forecast area.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  90  78  87  78 /  40  60  70  50
STT  89  79  89  78 /  40  70  70  60

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late tonight for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

VI...None.
AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....RAM



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