Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 231502
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1102 AM AST Mon Oct 23 2017

.UPDATE...The latest sounding from 23/12z showed the increased
moisture expected by the GFS. Have updated the POPs to reflect a
better chance of precipitation--but this will be mainly over
western Puerto Rico, as showers are likely to be swept inland from
the coasts by a moderate sea breeze. No changes to the synoptic
thinking. Although isolated thunderstorms are possible, upper
layers of the atmosphere are dry and somewhat hostile. Where
thunderstorms do develop, however, strong wind gusts may ensue.

&&

.AVIATION...No changes from below.


&&

.MARINE...No changes from below.


&&

.PREV DISCUSSION... /issued 353 AM AST Mon Oct 23 2017/

SYNOPSIS...A developing mid to upper level trough will continue
to become amplified while sinking southwards across the northern
Leeward islands and into the eastern Caribbean through Wednesday.
Strong surface high pressure will remain anchored across the west
and central Atlantic to maintain a moderate east to southeast
trade wind flow through the end of the work week. A weak tropical
wave is forecast to enter and move across the eastern Caribbean by
mid week.

SHORT TERM...Isolated trade winds passing showers will continue to
affect the USVI and eastern sections of Puerto during the morning
hours until at least mid week. Mostly fair weather conditions
expected elsewhere over land areas across Puerto Rico and the U.S.
Virgin Islands. An upper level trough just to the northeast of
Puerto Rico will sink slowly southward. However, Puerto Rico and
the USVI are expected to remain under the upper level convergent
pattern of the trough. This will continue to limit the thunderstorms
development for the next couple of days. Some shower development
is expected each afternoon with a couple of weak thunderstorms
possible across western and interior sections of Puerto Rico. A
weakening upper level ridge will also aid to inhibit the development
of significant showers and thunderstorms over the region for the
next couple of days. In facts, latest TJSJ upper air sounding as
well as the satellite derived precipitable water products both
suggest continued dry and stable airmass to persist through at
least mid week. No significant weather feature or impacts are
anticipated at this time. An increase in moisture is expected by
the end of the week.

LONG TERM...The prevailING easterly trades will bring occasional
patches of shallow moisture to the region with a seasonal weather
pattern expected through Wednesday. The local region should remain
on the convergent side of the amplifying upper trough through Wednesday.
Thereafter, high pressure ridge will build aloft and spread westward
across the area on Thursday and Friday. An approaching tropical wave
along with weak easterly pertubations propagating westward in the
prevailing easterlies will provide sufficient moisture to the region
to support daily showers and isolated thunderstorms development each
day through the end of the week. Occasional passing showers can therefore
be expected across the USVI and eastern Puerto Rico during the early
morning hours followed by locally induced showers and thunderstorms
each afternoon. The main activity should be across the interior and
west sections of PR with occasional shower activity in and around
the San Juan metro area during the afternoon hours. An induced surface
trough is expected to develop and shift westward across the region
Friday through the weekend. This will favor a fairly moist and unstable
airmass and consequently support a better chance for showers and thunderstorms
development across the islands and coastal waters through the next weekend.

AVIATION...Mostly VFR condition expected across the local flying
area during the forecast period. Only vicinity SHRA are possible
over the Leeward, USVI and eastern PR TAF sites sites until 23/15Z.
Isold-Sct SHRA and a couple of -TSRA expected from 23/16z-23/22z
over central mountain range of PR affecting mostly TJMZ and TJBQ.
SFC winds will continue light and variable until 23/12Z, becoming
from E-SE at 10 to 15 kts with some higher gust along coastal areas
after 23/13z-23/23z.

MARINE...Seas generally between 2 to 5 feet and winds 10 to 15 knots
are expected to prevail across the regional waters and local passages
today. Overall tranquil marine conditions are expected to prevail much
of the week.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  87  77  88  76 /  30  40  20  30
STT  87  77  87  78 /  30  20  10  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...WS
LONG TERM....WS


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