Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 240856
Area Forecast Discussion
National Weather Service San Juan PR
456 AM AST WED AUG 24 2016
.SYNOPSIS...At upper levels...Weak high pressure will accompany
the strong tropical wave as it crosses the area today and
Thursday. Then northerly flow will persist until Sunday when the
effects of a trough moving toward the area will be felt. The
trough will cross through the U.s. Virgin Islands Monday and
followed by a modest ridge that will cause flow to strengthen
considerably late next week.
At mid levels...The mid level troughing accompanying the tropical
wave will be followed by very weak high pressure that intervenes
between the Caribbean and the potential Hurricane Gaston over the
weekend. During the up coming week, high pressure builds north of
the forecast area until a trough moves west over the area on
Friday of next week.
At lower levels...A strong tropical wave has only traces of
circulation indicated and will pass over the area today. Then
gradients will remain relatively weak over Puerto Rico and the
U.S. Virgin Islands as Gaston intervenes between high pressure
just west of the Azore islands and the local area. High pressure
and moderate trade wind flow returns next week and continue
through the end of that week.
.DISCUSSION...Scattered showers and isolated thunderstorms
affected Puerto Rico north of the Cordillera Central and the
northern U.S. Virgin Islands overnight. Showers have become a
little more numerous in the pre-dawn hours and rain is expected
across the Northern tier of Puerto Rico this morning, with showers
continuing into the afternoon as the wind switches from northeast
to southeast rather abruptly later this morning.
Very vigorous displays of lightning have been seen north and
northeast of Guadeloupe and this area is slowly moving west
northwest. Isolated lighting strikes have occurred in the forecast
area and midway between saint Croix and Saint Kitts. Thunderstorm
activity will increase area-wide today.
Models appear to be in good agreement with the timing and
intensity of the rainfall and flash flooding potential
continues--especially for the eastern half of Puerto Rico this
morning and early afternoon. Drying was seen late this afternoon
over the southeast half of the forecast area, but another influx
of moisture is seen pushing into the area early Thursday morning
with showers likely again Thursday morning in the southeast half
of Puerto Rico.
The GFS continues with deep moisture through at least Sunday so
that fair weather is not expected in the area for almost the next
7 days. The upper level low northeast of the forecast area moves
closer during the week, but then dissipates and high pressure
replaces it north of the forecast area. Nevertheless the upper
level trough transiting the Caribbean on Monday should increase
the chance of showers and thunderstorms in the local area.
.AVIATION...Strong tropical wave across the eastern Caribbean
will continue to move west-northwest impacting all terminals.
SHRA/TSRA associated to the wave will result in MVFR to IFR conds.
Showers and thunderstorms already affecting TNCM/TKPK are
expected to become more frequent through the early morning hours
across the northern/eastern terminals of PR and the USVI,
spreading across the rest of the terminals by 16z. Low level winds
expected to continue ENE at 7-15 knots thru 12z...veering ESE
afterwards. Gusty winds between 25-35 kts possible with TSRA.
.MARINE...Winds have been confined to the area northeast of a
northwest-southeast line through the low and apex of the wave.
Hence it appears that high seas and strong winds will be mostly
confined to the northeast corner of the forecast area. Winds of 18
knots sustained have been noted at buoy 41043. Recent gusts have
reached 25 knots, but 31 knots was noted around 24/01z. Although
the low/apex of the wave is very close, current forecasts are
showing these winds to remain north of Puerto Rico and the U.S.
Virgin Islands at this time.
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 77 87 76 / 80 80 70 40
STT 83 78 84 79 / 80 80 70 40
PR...Flash Flood Watch through this evening for all of Puerto Rico
including Culebra and Vieques.
VI...Flash Flood Watch through this evening for St Croix-
St.Thomas...St. John.. and Adjacent Islands.
AM...Small Craft Advisory from 8 AM this morning to 8 AM AST
Thursday for Atlantic Waters of Puerto Rico and USVI from
10 NM to 19.5N. And the northern end of the Anegada passage.