Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 040158 AAA

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
958 PM AST Wed Mar 3 2021

.UPDATE...Sounding has partially come in, but continues to show
the dryness of the air moving through. Showers are still expected
later Thursday morning, but drier air will also follow. No change
to the discussion or reasoning and no changes were made to the
grids, so the zone package will not be re-issued.


.AVIATION...VFR conds will prevail. Some mtn topping is expected
btwn 04/10-04/15Z mainly in eastern Puerto Rico. Shra psbl aft
04/17Z in nwrn PR, but tops expected to remain blo 16 kft. Sfc
winds less than 6 kt in land breezes, bcmg 10-15 kt from the SE
with sea breeze influences. Max winds WNW-NW 30-45 kt btwn


.MARINE...Seas are gradually subsiding, Rip current risk is no
higher than moderate and exercise catuion statements have been
taken down.


.PREV DISCUSSION... /issued 444 PM 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...Tonight through Friday...

A drier airmass and mid to upper level ridge continues maintaining
a fair weather pattern over the local area. The TJSJ 12Z upper air
sounding shows a strong trade wind cap around 850 mb with
precipitable water values of less than an inch. This weather
pattern is expected to prevail until at least Thursday with a few
areas of low-level clouds and light showers moving at times across
the islands, but no significant rainfall accumulation are
expected due to the lack of mid and upper level support.

Winds will continue shifting from the southeast tonight through
Friday. The wind shift is generated by the eastward movement of
the Atlantic surface high pressure which is being pushed by a
polar trough and cold front exiting the eastern coast of the U.S.
Deeper moisture pulling due to the southeasterly wind flow will
cause a rapid increase in precipitable water content. Both GFS and
ECMWF models are in accordance with this event except on timing.
ECMWF has the moist plume arriving earlier on Friday than GFS
which has it reaching over the forecast area during the afternoon
into evening hours. Additionally, a prefrontal trough northwest of
Puerto Rico could affect the dynamical conditions over the local

Nevertheless, model guidance suggests an enhancement in moisture
convergence for the end of the workweek. If this forecast
materializes, environmental conditions will become favorable to
support enhanced shower activity by late Friday over the islands.
Light to gentle southeasterly wind flow could cause light to
moderate rainfall accumulations, especially over southeastern
Puerto Rico.

LONG TERM...Saturday through Wednesday...

FROM PREV DISCUSSION... /issued 403 AM AST Wed Mar 3 2021/

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.

VFR conds expected for the next 24 hours. BKN sky at around FL060
this afternoon at TJPS and TJBQ but will improve by this evening.
ESE winds up to 15 knots with sea breeze variations through 03/22Z,
becoming lighter tonight. Winds will be more SE after 04/12Z up to
10kts or so, but variations will be observed after 04/15Z, at TJBQ
winds may be from the NNE and TJSJ may observe winds from the East.
Little SHRA activity is expected, VCSH may be possible but no major
weather hazard is expected.

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


SJU  72  87  73  87 /  30  40  10  30
STT  74  84  74  84 /  20  20  20  50




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