Area Forecast Discussion
National Weather Service San Juan PR
503 AM AST Tue Jan 25 2022
An area of increased moisture over the area will continue to promote
passing showers across the waters this morning, with a few pushing
onshore from time to time. Scattered to numerous showers are also
expected this afternoon in the west. Generally fair weather is
likely to prevail through much of the week, with a persisting
advective weather pattern bringing some showers to the area. A
frontal boundary could bring an increase in shower activity to the
region early next week into midweek.
.SHORT TERM...Today through Thursday...
A mixture of sunshine and clouds will prevail today across most
portions of the islands. But keep in mind that a trade wind
perturbation will increase moisture across the islands today. Values
detected with the GOES-E derived Total Precipitable Water ranged
between 0.99 and 1.37 inches, and this air mass is moving across the
region. The mid to upper-level ridge will continue over PR and USVI,
promoting dry air aloft and a trade wind cap limiting rain
activity`s intensity and vertical development. Despite this, do not
rule out periods of moderate rain, producing ponding of water in
roads and poorly drained areas, especially during the afternoon
hours across western PR. East-southeasterly winds will continue in
response to the shear line associated with an old frontal boundary
moving eastward across the Western Atlantic, off to the northwest of
Moisture content will rapidly erode late this afternoon into the
evening as the trade winds push a somewhat cooler, dry air mass,
limiting rain activity by Wednesday. However, the advective pattern
dominating the region will continue on Thursday, and cool air
advection will promote isolated to scattered showers late Wednesday
night into Thursday. Clouds and showers will increase even more by
the morning and afternoon of Thursday when scattered to numerous
showers could be possible.
.LONG TERM...Friday through Tuesday...
A seasonally typical pattern, with patches of moisture streaming
through the area from time to time, will persist through at least
the end of the week. The ridge persists at the mid- and upper-
levels, and will continue to inhibit significant shower development,
especially during the afternoons. A frontal boundary is expected to
slowly approach the region from the northwest, stalling out north of
the region. Monday into Tuesday, there will be some moisture
convergence over the area, as a relatively large patch of moisture
makes its way into the region from the southwest and then
interacts with the frontal boundary to the north. It appears
likely that the frontal boundary will push towards, if not over,
the islands later Tuesday as it begins to deteriorate.
Surface and low-level winds will start the forecast period generally
out of the east to east-northeast, becoming more southeasterly, and
then tending towards southerly Monday. Depending on whether or not
the frontal boundary actually makes it over the area, winds will
shift again - winds will likely become more northerly and then
northeasterly behind any frontal passage. Above around 700 hPa,
though, winds will be more westerly, especially starting early next
week, which will make shear a likely inhibitor of strong shower
The mid- to upper-level ridge will begin to weaken, first at the
lower mid-levels and then progressively upward, in response to a
polar trough swinging off of the coast of North America. Stability
will decrease somewhat because of this. By Tuesday, the deep-layer
trough is expected to extend to just west or maybe northwest of the
area. The decreasing stability will support an increase in shower
activity from whatever moisture is available. However, as mentioned
previously, shear could counteract this somewhat. On Monday, a jet
is expected to be affecting the area at around 200 hPa, lasting into
early Wednesday, which could provide increased ventilation,
supporting additional convective potential then.
Expecting showers in a typical pattern. Friday will be among the
wetter days during this forecast period, considering the decreasing
stability, the patch of moisture that is expected to be in the
vicinity of the area, and the fact that the vertical wind shear is
not expected to be as strong, with winds staying more easterly to
northeasterly to around 600 hPa or so, based on guidance from the
GFS, with similar suggested by the Euro. Typical to drier-than-
normal conditions are expected for the weekend into early next week.
And another increase in shower activity is likely by Tuesday, due to
moisture convergence, the approaching deep-layer trough, and the
influence of the upper-level jet.
Forecast confidence is normal to below-normal. Uncertainties
continue to be largely based on the moisture patches - both the
timing and strength, as well as the frontal boundary. Frontal
passage is still far from certain, and its influence on the
conditions over the region is likely to be felt for much of the
first half of the week next week.
VFR conditions will persist across the local flying area. SHRA/-SHRA
will spread from the east over E-PR/IST/ISX, then across the
interior and western sections by the afternoon. Cloudiness will
diminish across the USVI by mid-morning. On the other hand, it
should increase across PR. Surface winds will continue calm to light
and VRB thru 25/13z, then, increasing at 10-15kt from the ESE and
with sea breeze variations.
Winds continue out of the east at up to 15 knots, helping to sustain
seas of up to 5 feet, especially for the offshore Atlantic waters,
where a fading weak swell continues to affect the area. These
conditions will continue, with gradual improvement possible in the
second half of the week.
There remains a high risk of rip currents for many local beaches,
including all of the north coast of Puerto Rico (including Aguada
and Rincon), northern and northwestern portions of the northern
USVI, beaches of Culebra, as well as beaches of northern, western,
and eastern St. Croix.
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 73 84 73 / 50 20 20 60
STT 83 72 83 73 / 20 20 10 50