Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 030803
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
403 AM AST Wed Mar 3 2021

.SYNOPSIS...A drier air mass will continue through at least
Friday over the region. A few areas of low-level clouds and light
showers may move at times across the islands, but overall fair
weather conditions should prevail through the workweek. Moisture
increases over the region in a southeasterly wind flow by late
Friday into Saturday. Drier conditions are expected on Sunday and
moisture gradually increases early next week. Winds are forecast
to increase once again from the east to northeast during the
first part of next week.


&&

.SHORT TERM...Today through Friday...

A mid to upper level ridge will continue to hold today over the
local area through early Thursday. This is causing a trade wind cap
around 850 mb, while the moisture content remains trapped under this
layer. At the surface, a generally dry air mass is evident on
satellite imagery and in the forecast soundings, with precipitable
water values around 1.0 inch or less. A few trade wind showers could
be advected into the eastern sections of Puerto Rico and over the
U.S. Virgin Islands during this period, but no significant rainfall
accumulation are expected due to the lack of mid and upper level
support.

As a polar trough and a cold front exit the eastern coast of the
United States later today and the surface high pressure moves toward
the eastern Atlantic, the wind flow will response by shifting more
from the east-southeast today, then more from the south-southeast
for the end of the workweek. Additionally, the trade wind cap will
gradually weaken, allowing deeper moisture to flow into the
region. So far, the model guidance are showing a moisture field
being pulled from the south by late Friday, increasing
precipitable water values to 1.2 to 1.4 inches. As a result,
shower activity is expected to increase by then over the islands.
The layer of southeasterly winds is expected to extend into the
mid-levels of the atmosphere, resulting in temperatures increasing
as well, with highs reaching the upper 80s, especially near the
coastal sections.


.LONG TERM...Saturday through Wednesday...

Polar trough and frontal boundary over the western Atlantic are
expected to remain just north of the region, while an induced
surface trough west of the area promotes a moist southeasterly
wind flow across the region on Saturday. Therefore, expect showers
to move across the USVI and east/southeast sections of PR during
the early morning hours, followed by periods of possible locally
heavy rainfall over the interior and northwestern PR. On Sunday,
trade winds are forecast to return to around 15-20 kts and
limit shower development during the afternoon hours over western
PR. Across the USVI, fair weather conditions should prevail and
normal to above normal temperatures are expected in general across
the islands.

For early next week, another induced surface trough is forecast
to develop to our northwest, and increase moisture content in
general across the local area on Monday. However, models suggest
today that the best areas of moisture are to remain over
Hispaniola, and a broad surface high pressure will move over the
western Atlantic...promoting a moderate to fresh northeast wind
flow across the islands through midweek. This should favor a cool
advective weather pattern, with trade wind showers moving over
the USVI and east/northeast sections of PR during the night/early
morning hours...followed by shallow convection developing during
the afternoon hours over west/southwestern PR.


&&

.AVIATION...Mainly VFR conditions expected to prevail through the forecast
period. Winds will be from the east at 10 to 15 knots, shifting from
the ESE around 03/15Z. VCSH are possible after 15Z for TJBQ, which
could result in brief MVFR conditions due to reduced VIS and low
ceilings.


&&

.MARINE...Small crafts should exercise caution due to seas up to 6
feet and east winds up to 20 knots across most of the regional
waters. There is a moderate risk of rip currents for all the
islands. A long period northerly swell will move briefly on
Thursday and Friday. Winds will gradually turn more from the east
to southeast later today and become light to moderate by the end
of the week.


&&

.FIRE WEATHER...There is an elevated fire danger for the southern
coastal plains of Puerto Rico today. Other areas across the
interior and western PR may see relative humidities in the low
40%s, but light afternoon showers are expected to develop this
afternoon across these areas.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  85  72  85  73 /  30  30  40  10
STT  83  74  84  74 /  30  20  20  30

&&

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

$$

SHORT TERM...ERG
LONG TERM...DSR



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