Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 241607

Area Forecast Discussion
National Weather Service San Juan PR
1207 PM AST Sat Mar 24 2018

.UPDATE...A band of precipitation over the Atlantic spread minor
amounts over the northwest coast of Puerto Rico but has mostly
dissipated. A second band formed just east of the east coast of
Puerto Rico and moved west through Juncos, Rio Grande and Las
Piedras. The band spread west south west through some of the
south central coast municipalities as far west as Ponce. Urban and
small stream flooding is possible there and some areas have
received more than one inch. Showers are still expected to
develop over the southwest municipalities this afternoon. The
24/12z sounding showed moisture steadily increasing during the
last 24 hours and precipitable water came in at 1.67 inches this
morning. Winds are now mostly east northeast up to 15 kft. Winds
are westerly above this.

It appears that the focus to the boundary that was approaching us
has reformed over southeast Puerto Rico and current models suggest
that it could move somewhat northwest during the day today and
also tonight. Therefore expectations of showers and possibly a
thunderstorms over the mainland of Puerto Rico continue. Have
decided that grids still match expectations for placement and
probability of rainfall therefore will hold.


.AVIATION...SHRA/TSRA will shift to more central portions of
Puerto Rico with mtn obscurations and MVFR/IFR conds. However TAF
sites where MVFR is psbl are limited to TJMZ. SHRA are also
possible arnd TIST/TISX. Winds will increase to 10-15kt  with sea
breeze variations. Maximum winds WSW increasing to 80 kt at FL400
by 25/12Z.


.MARINE...No changes from previous discussion. Seas of 8 feet or
more are expected to return Wednesday after noon.


.PREV DISCUSSION... /issued 500 AM AST Sat Mar 24 2018/

SYNOPSIS...A dissipating and diffuse frontal boundary/shear
line will result in periods of showers with isolated thunderstorms
through the weekend. A drier and stable air mass is expected
by mid week next week.

SHORT TERM...Today through Monday...

A dissipating and diffuse frontal boundary/shear line across the
region will continue to provide abundant low level moisture across
the local island and surrounding waters through at least Monday.
This moisture in combination with daytime heating, local effects
and the aid of an amplifying polar trough across the western
Atlantic will provide sufficient instability across the forecast
area through the forecast period. Low levels winds are expected to
turn more from the east northeast today. As a result the development
of scattered to numerous showers and isolated thunderstorms are
expected to be more prevalent across portions of central and
southwest sections of Puerto this afternoon. Rainfall will be
locally heavy at times especially over those sections of Puerto
Rico. Urban and small stream flooding will therefore be possible
this afternoon and again Sunday Afternoon. Most of the thunderstorm
activity is expected to diminish early Saturday evening, leaving
partly cloudy skies with isolated to scattered shower activity
tonight through early Sunday morning. However, for Sunday
afternoon, similar conditions are expected.

By Monday, low level moisture will begin to decrease very slowly.
However, the prevailing east northeast low level wind flow will
combine again with daytime heating and the proximity of an upper
level trough to induce again the development of shower and
thunderstorms activity over central and southwest sections of
Puerto Rico as well as across the U.S. Virgin Islands.

LONG TERM...Tuesday through Sunday....

As a broad mid to upper level trough across the western Atlantic
shifts eastward into the central Atlantic, a ridge pattern aloft
will slowly build across the forecast area Wednesday and hold
through the upcoming weekend. Under the aforementioned building
ridge, low level moisture is expected to erode across the eastern

Therefore, there is a high chance for passing showers with
isolated thunderstorms across the northern half of Puerto Rico
and the USVI Tuesday morning followed by showers and thunderstorms
developing over and south of the Cordillera Central Tuesday
afternoon. Still expect some lingering moisture on Wednesday, as
a result, expect another round of afternoon showers along the
southern slopes of Puerto Rico. Limited shower activity with
pleasant temperatures is expected Thursday onwards.

AVIATION...Mostly VFR conditions are expected across the local
flying area through 24/16Z. SHRA/TSRA will begin to develop after

24/16Z, which could affect TJBQ and TJMZ. SHRA are also possible
across the leeward, USVI, TJPS and TJSJ taf sites after 24/16Z.
Winds will be easterly up to 10 knots until 24/13z, increasing to
10- 15kt after 24/13Z with sea breeze variations.

MARINE...A northerly swell is expected to invade the local waters
later today to result in hazardous seas particularly across the
offshore Atlantic waters tonight and Sunday. Seas building 5 to 7
feet with occasional seas up to 9 feet by tonight.


SJU  85  75  84  75 /  40  50  50  40
STT  84  74  84  74 /  70  50  50  40


PR...High Rip Current Risk from 6 PM AST this evening through Sunday
     afternoon for Northwest-Western Interior.

AM...Small Craft Advisory from 8 PM this evening to 8 PM AST Sunday
     for Atlantic Waters of Puerto Rico and USVI from 10 NM to



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