National Weather Service San Juan PR
518 AM AST Fri Oct 23 2020
.SYNOPSIS...Variable weather conditions are expected during the
next few days with the passage of a weak tropical wave today,
followed by relatively drier air on Saturday and an easterly
disturbance along with a surge in moisture by Sunday into early
next week. Thereafter, a generally moist and unsettle weather
pattern is expected with increased deep tropical moisture and
favorable conditions aloft. Improving marine conditions are
expected today, but yet another swell will create hazardous marine
conditions by the end of the weekend into early next week.
&&
.SHORT TERM...Today through Sunday...
The southerly wind flow will bring in moisture associated with a
weak tropical wave that will pass by the area to the south later
today. Precipitable water values are forecast be near normal. At the
upper levels, an upper-level trough strengthens and sets to our
northeast will help provide instability aloft. Thus, the combination
of these features will help to support afternoon convection across
northwestern PR and the development of streamers down from El Yunque
and the U.S. Virgin Islands.
As the tropical wave moves away from the forecast area, leftover
moisture, favorable upper leve dynamics and local effects will
result in scattered shower activity across northwestern PR. Isolated
activity is expected elsewhere. A drier airmass will quickly approach
the area late in the afternoon.
Model guidance suggest an increase in low-level moisture associated
with and easterly perturbation across the islands on Sunday. This
will result in more frequent passing showers, especially across the
windward sections during the morning hours. Afternoon convection is
expected over the northwestern sections of Puerto Rico. This will
generate showers with isolated thunderstorms that could lead to
minor flooding and ponding of water.
.LONG TERM...Monday through Saturday...
Recent model guidance now suggests that a moist and unsettle
weather pattern will dominate the local weather conditions through
the long term forecast period. First, a trade wind or easterly
disturbance will enhance moisture advection with frequent overnight
and early morning trade wind showers, and the potential for
increased locally and diurnally induced activity on Monday. In the
meantime, an amplifying mid- to upper-level trough will spread
southward across the eastern Caribbean, with weak cutoff low and
surface-induced trough lingering over the region through early
Thursday. Another, but strong cutoff upper level low will quickly
follow, sinking south just northwest of Puerto Rico by Thursday
evening, then drifting westward through early Friday.
To support this scenario, deep tropical moisture with model-
estimated precipitable water above 2 inches by 12Z on Tuesday and
18Z on Wednesday along with favorable conditions aloft with 500
mbar temperatures randing between -6 and -8 degrees C will aid in
the development of shower and thunderstorm activity. Lastly, a
tropical wave is forecast to reach the local islands between
Friday and Saturday. Overall, a deepening surface pressure will
maintain a generally moderate to fresh easterly wind flow through
the period.
This forecast follows closely GFS model guidance. However, there
is medium confidence in this scenario since the ECMWF suggest a
different scenario with relatively drier with ridging aloft by
the end of the workweek.
&&
.AVIATION...Mainly VFR conds are expected to prevail durg the 24 hr
prd. Afternoon convection will produce VCSH for TJBQ/TIST/TISX btwn
23/18-22Z. SFC wnds mainly light 5-10 kts fm S-SE...bcmg lgt/vrb to
calm aft 23/22Z. No sig operational wx impacts is expected over the
forecast area durg prd.
&&
.MARINE...Swell generated by Hurricane Epsilon continues to
diminish, resulting in improved seas. In fact, seas at bouy 41043
have dropped to 5 feet at 11 seconds, while seas at the inner
bouy just north of San Juan, 41053, have dropped to 6 feet at 13
seconds. While this improvement is expected to continue today,
Small Craft Advisories will remain in place through this afternoon
for western-most marine zones. A second pulse is expected to
arrive by the end of the weekend, spreading across the local
waters from the east and creating hazardous seas through early
next week. A high risk of rip currents continues for most north-
and east-facing beaches of Puerto Rico, Culebra, and the US
Virgin Islands.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 91 79 92 78 / 50 50 30 20
STT 89 79 89 79 / 50 50 20 30