National Weather Service San Juan PR
427 AM AST Wed Jan 12 2022
.SYNOPSIS...A patchy weather pattern will prevail, with
intermittent patches of shallow moisture and relatively drier air
streaming across the area. Somewhat favorable upper-level
conditions during peak higher moisture and/or daytime heating
periods will enhance shower and possible thunderstorm activity.
Thus, increasing the potential for ponding of water to localized
urban and small stream flooding. A southerly component in the
general wind flow will result in above-normal seasonal
temperatures by the end of the workweek into early next week.
&&
.SHORT TERM...Today through Friday...
The mid- to upper-level ridge will continue to erode as a short wave
trough crosses the west Atlantic just north of the area. This will
create an unstable divergent pattern aloft today through Thursday.
This increased instability along with cooler 500 millibar
temperatures will aid in enhancing the advective morning showers
over the regional waters and also during the afternoon hours in
isolated areas.
A stalled cold frontal boundary will continue to cross into the west
central Atlantic and gradually weaken, as a high pressure ridge will
build across the west and southwest Atlantic. This pattern will
induce a weak surface trough across the northeast Caribbean and
consequently induce a light northeasterly wind flow. This in turn
will continue to transport occasional patches of shallow moisture
across the region from time to time during the next several days.
However, the local steering winds are forecast to become more east-
southeast later today, then south-southeasterly by Thursday into
Friday.
The available pockets of moisture along with local effects and the
instability aloft, will aid in afternoon shower development mainly
over the central and west to northwest sections of Puerto Rico.
There is also a chance isolated thunderstorms later in the afternoon
and therefore areas of locally heavy rains may cause minor ponding
of water on roads and in poor drainage areas today.
Increased potential of early morning and afternoon shower activity
is expected on Thursday into Friday, resulting in scattered to
numerous showers over the region. There is also increased potential
for thunderstorm activity due to the proximity of the upper trough,
with isolated thunderstorms possible each afternoon. With winds more
east-southeasterly, expect the most likely affected areas to be the
north central to northwest and interior sections of Puerto Rico,
with a chance of minor Urban and small stream flooding in isolated
areas. Some brief afternoon showers will also be possible in and
around the U.S. Virgin islands, but significant accumulations and
flooding is so far not anticipated over the islands. By Friday, with
the low level steering flow forecast to become more south-southeast,
isolated to scattered showers will be likely in and around parts of
the San Juan metro areas as well, However widespread rainfall
accumulations and significant flood impacts are still not
anticipated at this time.
.LONG TERM...Saturday through Thursday...
The most recent model guidance suggests a generally east to
southeast wind flow dominating the long-term forecast period,
supported by a surface high pressure extending into the
northeastern Caribbean. A slightly drier pattern is also
suggested, with low-level moisture convergence driving patches of
moisture across the area and model-estimated precipitable water
vapor ranging between 1.2 and 1.4 inches through late Monday
night. After that, a set of easterly disturbances will increase
the available moisture, with precipitable water values peaking
around 1.5 inches by Monday night and gradually increasing to 1.6
inches by Wednesday into Thursday.
The bulk of the resulting shower activity should remain over the
local waters, but evening-early morning showers will over southern
and eastern sections of the local islands at times. Afternoon
convective development should be limited. However, somewhat
favorable conditions aloft, with 500 mbar temperatures between -7
and -10 degrees Celcius, during peak daytime heating and/or
higher moisture periods could result in enhanced showers and
possible isolated thunderstorm activity across portions of the
interior into western and northern sections of the local islands.
Thus, an increase in the potential of ponding of waters to
localized urban and small stream flooding is likely, especially
with the afternoon activity during the weekend and Wednesday
onwards. A southerly component in the general wind flow could
result in above-seasonal temperatures early next week.
&&
.AVIATION...VFR conds at all terminals with SCT ocnl BKN lyrs nr
FL025...FL050...and wdly sct SHRA ovr regional waters and en route
btw the local islands til 12/14Z. Mtn top obscr psbl ovr E PR til
12/14Z due to low clds and -SHRA. SHRA expected ovr wrn PR and VCTY
TJMZ/TJBQ/TJSJ fm 12/16Z-22Z. Sfc winds calm to lgt/vrb bcmg fm E-NE
10-15 kts with some hir gusts and sea breeze variations aft 12/14Z.
Low/lvl wnds fm NE BLO FL200...then backing and bcmg fm WSW and incr
w/ht. Max wnds of 60-70 kts btwn FL330-450.
&&
.MARINE...Light to moderate east to northeast trade winds will
prevail today, becoming light and more southerly to southeasterly
on Thursday and Friday. Seas will remain at 5 feet or below
through the workweek, but a small northerly swell will cause
choppy seas up to 6 feet during the upcoming weekend. Brief
isolated to scattered showers will affect the local waters at
times.
There is a high risk of rip currents for beaches across the north
coast of Puerto Rico, while a moderate risk remains in place for
most of the remaining local beaches.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 83 72 84 74 / 30 40 30 30
STT 85 74 85 75 / 30 50 50 40
&&